IMPLEMENTATION OF A COMPOSITE AIRCRAFT

IMPLEMENTATION OF A COMPOSITE AIRCRAFT DEICER WITH A SHAPE MEMORY ALLOY

by Javier Herrero Sup. Eng., Polytechnic University of Madrid, 1997

Submitted to the College of Engineering and the faculty of the Graduate School of Wichita State University in partial fulfillment of the requirements for the degree of Master of Science

Spring 2000

IMPLEMENTATION OF A COMPOSITE AIRCRAFT DEICER WITH A SHAPE MEMORY ALLOY

I have examined the final copy of this report for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of Master of Science with a major in Aerospace Engineering.

________________________________ Dr. Roy Myose, Co-Advisor

________________________________ Dr. Walter Horn, Co-Advisor

ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Roy Myose, for his guidance and patience during my research over the past two years. I also would like to thank my co-advisor, Dr. Walter Horn, for giving me the opportunity to work on this project and for his assistance and direction. I would like to express my thanks to Youngkeun Hwang, for all the things he taught me during the time we shared on this project, and to Yeow Ng and Pierre Harter for their support and valuable suggestions that made possible the manufacture of the prototypes. Finally, I would like to thank my good friend, Jose Janer, who inspired and encouraged me during his last months in this world.

ii

ABSTRACT

This research explored issues associated with the practical implementation of an innovative low power deicing system for aircraft leading edges. The basis of the system was the shape memory effect exhibited by a certain family of metallic alloys. An analytical model and a feasibility evaluation of this type of device were documented in previous WSU research of the Shape Memory Alloy deicing project. This previous research was motivated by the fact that existing deicing systems for general aviation aircraft show limitations in terms of effectiveness, power consumption and durability. The current research attempted to use the conclusions and analytical model from the previous research to implement a wing scale device based on this technology. For that end, several methods of electrical connection were investigated, and several activation patterns were tried in both deicing conditions as well as and room temperature with a six inch specimen. A wing scale specimen two feet long was built and subjected to both deicing tests and cyclic activation tests.

iii

TABLE OF CONTENTS

CHAPTER

I.

II.

PAGE

INTRODUCTION .......................................................................................

1

1.1

Objective ..........................................................................................

1

1.2

Review of results from previous WSU deicing project to be considered in the current research ..........................................................................

2

1.3

Device description and terminology ................................................

4

1.4

Engineering aspects of the SMA that affect the deicing application

5

RESEARCH ON THE ELECTRICAL CONNECTION OF THE SMA ELEMENTS ..............................................................................................

12

2.1

Introduction ......................................................................................

12

2.2

Conductive epoxy connection ..........................................................

12

2.3

Flat specimen experiments ...............................................................

14

2.4

Continuous wire specimen ...............................................................

16

2.4.1

Continuous wire six inch specimen: laying-up and tooling .

17

2.4.2

Effects of thermo-mechanical processing during the manufacture

2.5

..............................................................................................

19

2.4.3

Continuous wire six inch specimen: power consumption tests

24

2.4.4

Continuous wire six inch specimen: alternative activation tests

26

2.4.5

Continuous wire six inch specimen: deicing tests ...............

28

2.4.6

Artificial ice formation and ice thickness control ................

29

Pressure plate connection .................................................................

32

2.5.1

Pressure plate connection: alternating activation deicing tests

33

2.5.2

Ice-specimen interface .........................................................

35

2.5.3

Pressure plates connection: simultaneous activation deicing test

36

iv

CHAPTER

III.

PAGE

IMPLEMENTATION OF A WING SCALE SPECIMEN .........................

41

3.1

Introduction ......................................................................................

41

3.2

Determination of the Minimum Intensity of Current (MIC) required to fully activate a single Flexinol™ wire. Wire activation experiments

41

3.2.1

Previous considerations .......................................................

42

3.2.2

Experiment 1- epoxy coated wire ........................................

42

3.2.3

Experiment 2- bare wire at room temperature .....................

44

3.2.4

Experiment 3- bare wire at icing condition temperature .....

45

3.2.5

Experiment 4- intensity vs. time curves of the activation of a bare wire at room temperature .....................................................

3.2.6

3.3

3.4

3.5

46

Experiment 5- intensity vs. time curves of the activation of a bare wire at icing condition temperature .....................................

49

Design of the electrical connection ..................................................

49

3.3.1

Previous considerations .......................................................

49

3.3.2

Calculations..........................................................................

51

Manufacturing aspects of the two-foot specimen ............................

52

3.4.1

Lay-up procedure and curing cycle......................................

52

3.4.2

Specimen demolding ............................................................

55

3.4.3

Electrical connection elements ............................................

55

Tests conducted with the two-foot specimen ...................................

55

3.5.1

Strain and temperature monitoring ......................................

56

3.5.2

Two-foot specimen: deicing test ..........................................

60

3.5.3

Discussion of the deicing test ..............................................

63

3.5.4

Two-foot specimen: cyclic activation test ...........................

64

3.5.5

Discussion on this tests series ..............................................

65

v

CHAPTER

IV.

PAGE

CONCLUSIONS AND RECOMMENDATIONS ......................................

69

4.1

Conclusions for the six inch specimen.............................................

69

4.2

Conclusions for the two foot specimen............................................

70

4.3

Recommendations on the thermo-mechanical coupling in the deicing performance .....................................................................................

72

4.4

Proposal of solution to the loss of pre-strain capability...................

74

4.5

Structural responsibility of the deicer-skin ......................................

77

REFERENCES

..............................................................................................

vi

78

LIST OF TABLES

TABLE

PAGE

1-1

Transformation temperatures of the Nitinol alloy used in this project ........

2-1

Flat specimen temperature cycling test. Comparison of different connection

8

methods and wire types ................................................................................

16

2-2

Continuous wire six inch specimen: power consumption tests ...................

25

2-3

Continuous wire six inch specimen: alternative activation tests .................

27

2-4

Continuous wire. Alternating activation deicing tests .................................

31

2-5

Pressure plate connectors: results from alternating deicing tests.................

34

2-6

Pressure plates connectors: results from simultaneous deicing tests ...........

37

3-1

SMA deicing project previous results ..........................................................

42

3-2

Parameters for the activation of a 0.015 in diameter bare wire at room temperature (source: Dynalloy Inc.) ................................................................................

42

3-3

Epoxy coated wire: experiment conditions ..................................................

43

3-4

Epoxy coated wire: experiment results ........................................................

43

3-5

Bare wire: experiment conditions ................................................................

44

3-6

Bare wire: experiment results ......................................................................

44

3-7

Bare wire at icing conditions: experiment conditions..................................

45

3-8

Bare wire at icing conditions: experiment results ........................................

46

3-9

Intensity vs. time curves: experiment conditions .........................................

47

3-10

Strain gaging properties ...............................................................................

59

vii

LIST OF FIGURES

FIGURE

PAGE

1-1

Deicing leading edge: schematic configuration ...........................................

5

1-2

Transformation temperatures of the Flexinol TM .........................................

7

1-3

Typical pseudoelastic stress-strain curve .....................................................

9

1-4

Typical temperature dependence of the pseudoelastic behavior..................

10

1-5

Typical temperature dependence of the transformation temperatures .........

11

2-1

Conductive epoxy arrangement ...................................................................

13

2-2

Actual flat specimens ...................................................................................

15

2-3

Concept of the continuous wire specimen ...................................................

17

2-4

Continuous wire tooling. Actual tooling view .............................................

18

2-5

Effects of thermo-mechanical processing during the manufacture..............

21

2-6

Bolted Nitinol Specimen ..............................................................................

25

2-7

Electrical connection scheme for alternative activation ..............................

26

2-8

Continuous wire deicing tests: electrical connection scheme ......................

28

2-9

Freezer with spraying nozzle and instrumentation on the left .....................

29

2-10

Points where the ice thickness was checked before every test ....................

30

2-11

Pressure plate connector ..............................................................................

32

2-12

Pressure plate: electrical scheme for alternating activation deicing tests ....

33

2-13

Pressure plates connection: electrical scheme for simultaneous activation deicing tests

..............................................................................................

36

3-1

Single wire activation test. Intensity vs. time ..............................................

48

3-2

Single wire activation. Intensity vs. time. Rwire = 2.54 ohms, (in the fridge @ -11 0F) ................................................................................

50

3-3

Electrical connection of the two-foot specimen...........................................

52

3-4

Schematic view. Filament winding of the two-foot specimen .....................

53

3-5

Cross section of the mandrel ........................................................................

54

viii

FIGURE

PAGE

3-6

The two-foot specimen instrumented and mounted in the test bed .............

56

3-7

Instrumentation of the two foot specimen, front view .................................

57

3-8

Two foot specimen inside the test fridge .....................................................

61

3-9

Deicing test two-foot specimen. Summary of data ......................................

61

3-10

Two foot specimen: deicing test ..................................................................

62

3-11

Two foot specimen: cyclic activation test ....................................................

67

4-1

Surface mounted actuators proposal ............................................................

75

4-2

BF Goodrich boot deicer inflation cycle ......................................................

76

ix

CHAPTER I INTRODUCTION 1.1

Objective This project will discuss the main issues associated with the practical

implementation of a low power aircraft deicing system, based upon the shape memory effect of certain alloy. The feasibility of this type of device was documented in the previous WSU graduate research in this field known as the Shape Memory Alloy deicing project [1]. The shape memory metallic alloys have been successfully applied in recent innovations corresponding to other fields, such as surgical implants, pipe couplings, fastener rings, and circuit breakers [2]. The unusual property that characterizes the shape memory alloys is the occurrence of a large sudden contraction in the grain direction of the material, when it is heated and a specific temperature is reached. An aircraft must be designed to fly in varying meteorological conditions including low temperature conditions together with the presence of high humidity in the surrounding environment. During such flights, the frequent build-up of ice on some external surfaces of the aircraft can cause hazardous situations including decrease of lift, weight changes, loss of engine power, and damage to turbine engine blades. In addition, loss of forward vision can occur due to ice forming on windshield panels, or obstruction of the pressure holes resulting in false readings of airspeed and altitude. Therefore, for most aircraft, protective systems must be incorporated to ensure their safety and that of the occupants under icing conditions.

1

2

The concept of the subject de-icing device is based on the geometric modification of the iced surface. The systems of this type are designed to modify the external shape of that element of the aircraft undergoing ice accretion, so that the ice is cracked and released to the air stream. The wing leading edge is the element of the aircraft considered for this research. The deicing device is the leading edge itself, manufactured with composite material housing wires of Nitinol alloy embedded in a parallel pattern orthogonal to the span-wise direction of the wing. These wires would actuate the whole structure to modify its external shape thanks to the contraction properties of the alloy. Although important behavioral aspects of this type of structure were characterized in the previous WSU graduate research in this field [1] and [3], the practical implementation of a complete aircraft deicer presents technological issues and problems, some of which are discussed and documented in this project. 1.2

Review of results from previous WSU deicing project to be considered in the current research The SMA chosen for the project was Flexinol™ of Dynalloy Inc., which is a

variety of NiTiNOL (Nickel Titanium Naval Ordnance Laboratory, nickel-titanium alloy in varying proportions). Its behavioral properties concerning this application were then explored. In the context of this project, the SMA element is a set of wires, embedded in a parallel pattern orthogonal to the span-wise direction of the leading edge. The heating procedure consists of an electrical current running through the wires.

3

By means of an analytical model for the geometrical modification of the hybrid structure [3] and some partial lab characterizations, the previous steps of this research determined a number of valuable parameters necessary for the design of the first prototype. Namely: 

Minimum amount of strain required to crack the ice by mechanical means only: 0.5 percent of tensile strain. However, this result turns out to be of secondary importance, since the device developed in the context of this project deices not exclusively by mechanical means. Also, circumferential strain is not the same deicing mechanism as that used to obtain this value, which is shear strain.



Maximum recovery force in a wire upon electrical activation: 3 lbs (length of the wire: 9 inches).



Maximum amount of circumferential strain, under safe value of current: -0.45%.



Temperature necessary to fully activate a wire of a 0.005 inch diameter: AF = 200 oF, AS = 170 oF.



Minimum current required to activate a single Nitinol wire (0.005 inch diameter): 1.7 Amps. A deicing leading edge 6 inches long was built and tested under the following

configuration: 

Wire level connection: bundles or segments of 20 wires in parallel, connected with conductive nickel epoxy sandwiched between two brass strips.



Bundle level connection: groups of three connected in series for alternative activation.



Power source: AC, 70 volts

4

The deicing test results indicated that deicing was achieved [1], but heating was the likely cause of ice removal and not the surface strain of the leading edge. In addition, the electrical connection used at the wire level proved to be unreliable, since it showed an unexpected drop in resistivity, lacking repetitiveness, and being a possible cause of permanent damage to the specimen. Alternative activation of bundles located alternatively was suggested from that experiment for future development. 1.3

Device description and terminology The terms defined below will be used to describe the different configurations

considered in the project. 

Specimen: a prototype portion of leading edge with de-icing capability.



Wire element: a semi-circular fiber of Nitinol wire embedded in the specimen, and running parallel to each other from one edge to the other, see Figure 1-1.



Segment: a one-inch wide portion of the specimen. In all the cases it will have a density of 20 Nitinol wires. Inside a segment, the wire elements can be connected in series or in parallel, however that type of wire connection will be common to all the segments in the same specimen.



Wire level connection: the type of connection used to provide electrical continuity from a wire to another. It can be series or parallel.



Segment level connection: the type of connection used to provide electrical continuity from a wire segment to another. It can be series or parallel.



Activation: refers to the sequence in which the segments receive the electrical current. It can be alternative or simultaneous.

5

As outlined above, the specimens prepared for the experimental testing have a semi-cylindrical shape. Although the leading edge of an actual wing is not necessarily semi-circular, this shape was chosen for ease of manufacture. Each specimen consisted of a composite laminate with embedded Nitinol wires running parallel to each other, and perpendicular to the span-wise direction of the wing. This configuration is shown in Figure 1-1. The composite material layers were prepreg fabric of fiberglass-epoxy.

nitinol wire element

circumferential direction axial direction

fiberglass/epoxy glass/epoxilaminate laminate fiber

Figure 1-1 Deicing leading edge: schematic configuration

All the specimens manufactured for this research follow the general design described above, differing in the technology used to achieve the electrical connection. However, a different design will be proposed in subsequent chapters to solve two problems found. 1.4

Engineering aspects of the SMA that affect the deicing application As outlined above, the shape memory effect (SME) is the property that some

alloys possess according to which, after being deformed at a certain temperature, they

6

recover the original shape upon being heated to a second temperature. This effect was first discussed by the Russian metallurgist Kurdjumov. In 1951, Chang and Read described it for an In-Tl alloy, however most of the current properties came to light after the development of the nickel-titanium alloy by the Naval Ordnance Laboratory (NiTiNOL) in 1968. Two separate effects characterize the response of the SMA: pseudoeasticity due to stress induced martensite (SIM), and strain memory effect (SME). The following points describe those engineering aspects regarding the SMA that directly affect the applications of this project. Aspect 1- Internal microstructure of the alloy: A brief metallurgic description of the alloy is necessary to understand the effects mentioned above. In the environmental conditions under which the [Ni-50% wt Ti] alloy has been utilized for this project, it presents two internal states or phases. First is austenite, the high-temperature phase, a crystalline structure which is cubic with body-centered symmetry, of the type known as CsCl structure. The other one is martensite which has internal structure of the type cubic with body-centered Ni atom and orthorombic crystal. The alloy is originally fabricated in the austenitic state, and an internal growth of crystals of martensite can be promoted by two causes: a decrease in the temperature of the material, or the application of a tensile stress. If either of these two actions is applied and kept up to a certain boundary, a state can be reached in which all the internal crystals of the material are of martensite. This internal transformation can be reversed by withdrawing the causing action at which point all of the material will revert to the austenite phase.

7

Figure 1-2 describes both direct and inverse transformations when the material is cooled from a high temperature austenitic phase and reaches a certain temperature MS, it starts transforming to the martensitic phase. If the cooling is maintained, the material will reach a temperature MF at which all of it is martensite. On the other hand, if the alloy is heated from a cool martensitic state, it will reach a temperature AS at which the martensite starts transforming into austenite. If the heat addition is maintained, it will reach AF, the temperature at which all the material is again austenite.

As

Mf Martensite Fraction (%)

100%

Af

Ms 0% 113

131

149

167

Temperature (F) Transformation temperatures of the Flexinol(TM) alloy (Ni-50% wt Ti)

Figure 1-2 Transformation temperatures of the Flexinol TM [Ni-50% wt Ti]

For the alloy [Ni-50% wt Ti] the values of these four temperatures are indicated in Table 1.1, taken from reference [4]. The Nitinol family of alloys has found wide technological applications, and adjustments in the composition can be made to produce MS temperatures between –273 oC and 100oC.

8

Note that all four transformation temperatures described (martensite and austenite start and finish) can be affected by the state of stresses in the material [2].

Source: Dynalloy Inc. Alloy: Flexinol™ [Ni-50 % wt Ti] Austenite

Martensite

Start As

Finish AF

Start Ms

Finish MF

65 oC

75 oC

55 oC

45 oC

149 oF

167 oF

131 oF

113 oF

Table 1-1 Transformation temperatures of the Nitinol alloy used in this project

Aspect 2- One way shape effect: The alloy is manufactured at a high temperature austenitic state, and cooled down to room temperature at which it is all martensite. If a permanent strain is imparted to the alloy at a temperature below MF (for the alloy used [Ni-50% wt Ti] room temperature is below MF ) and subsequently it is heated, the reverse transformation (from martensite into parent austenite) recuperates the original austenitic crystalline structure, and this crystal transformation absorbs the permanent strain and the shape before deformation is recovered. Aspect 3- Stress Induced Martensite (SIM) and pseudoelasticity: As outlined above, the martensitic transformation can be stress induced, reverting back to the parent phase upon unloading the material. Figure 1-3 shows this effect for a Cu-Zn-Sn alloy which has a MS of -48 oC. A tensile test at constant temperature of 24oC, which is 76 oC above MS, was performed. At point A, the stress-induced martensite phase starts to form. At point B, the martensitic transformation has been completed and any straining beyond this point will produce irreversible plastic deformation or fracture.

9

B

STRESS MPa

200 C

A

100

D

0 0.02

0.04

0.06

0.08

STRAIN Pseudoelastic stress-strain curve for a single-crystal Cu-Zn-Sn alloy at 24 deg. C

Figure 1-3 Typical pseudoelastic stress-strain curve (extracted from [4])

Upon unloading, the martensite reverts to the parent phase between C and D. Further unloading results in the return to the original length of the specimen. The pseudoelastic strain exceeds 6%. It is not sufficient for the testing temperature to be above MS to obtain the pseudoelastic effect, if it is below AS the martensite will not revert totally back to austenite upon unloading as shown in the middle and left hand graphs of Figure 1-4. In this case, the deformation is irreversible, and some strain remains. This is the situation in which the Shape Memory Effect takes place as described in the previous point where additional heating is required to revert to martensite, since the deformation temperature is below AS. Aspect 4- Stress rate, variation of the transformation temperatures with applied stress: A different experiment can illustrate the thermo-mechanical behavior of the SMA under a different approach. In the experiment discussed in the previous section, the alloy was cycled in stress, while the temperature was kept constant. For the current scenario,

10

the plateau of the stress-strain curves (such as shown in Fig 1-3) reveals the stress needed to start forming stress-induced martensite for a given temperature that remains constant

STRESS MPa

throughout the test.

300

300

300

200

200

200

100

100

100

0

0

0 0.02

0.04 0.06 0.08

STRAIN Ttest= -50C Ttest > Af = -90C

0.02

0.04 0.06 0.08

0.02

0.04 0.06 0.08

STRAIN

STRAIN Ttest= -98C Ttest ~ Af = -90C

Ttest= -113C Ttest < Af = -90C

Temperature dependence of stress-strain curve for a Cu-Zn-Sn alloy

Figure 1-4 Typical temperature dependence of the pseudoelastic behavior (extracted from [4], Ttest stands for test temperature)

The experiment associated with the dependence of transition temperature on stress, consists of measuring the temperature necessary to start forming temperatureinduced martensite while a constant stress is applied. The results reveal that the four transformation temperatures vary in a quasi-linear pattern with stress, as illustrated in Figure 1-5. The transformation temperatures (e.g. AS, and AF) are changed at different stress levels.

11

Temperature C

100

50 Af As Ms Mf

0

-50 100

200

300

400

Stress (MPa) Variation of Ms, Mf, As, and Af in a Cu-Zn-Al-Mn alloy thermally cycled at different stress levels

Figure 1-5 Typical temperature dependence of the transformation temperatures (extracted from [2])

CHAPTER II RESEARCH ON THE ELECTRICAL CONNECTION OF THE SMA ELEMENTS 2.1

Introduction The composite leading edge with deicing capability is a hybrid structure in which

the Flexinol ™ fibers can be activated by the application of an electrical current. Thus, the electrical connection of the wires becomes a technical aspect of central importance in the implementation of the system. This chapter deals with this issue, considering the following points: 1. How the wires will be physically connected to each other to attain electrical nodes and create the groups defined as segments in section 1.3. 2. What type of electrical scheme will be followed to connect the segments to each other: parallel or series, and number of wires in each group. 3. In what sequence the SMA wires will be activated in order to optimize the deicing action. The wire level connection types studied and tested are described in the following sections of this chapter 2.2

Conductive epoxy connection In the previous feasibility study of the WSU-SMA deicing project, a first series of

6 inch leading edge prototypes was manufactured following the initial design. A manufacturing process was developed involving automatic winding of the Flexinol™ wire over the lower plies of composite host laminate. The material lay-up was carried out

12

13

on a cylindrical mold from which two specimens were obtained. This procedure implies the cutting of the wires to separate both semi-cylindrical specimens. The solution adopted for the electrical connection in these specimens was as follows. The ends of the wire elements were physically cut during the manufacturing, and the wires were totally embedded inside the laminate. They were electrically connected in parallel by using conductive epoxy matrix in the area close to the tips of the wires. This matrix achieves its conductive properties due to its richness in metal particles. The connection to the circuit in the exterior of the laminate is achieved by means of two brass strips. These strips are in contact with the conductive epoxy in a sandwich fashion (see Figure 2-1) and show a grid of circular holes in order to ensure the electrical contact between the brass and the conductive epoxy.

brass strips

conductive epoxy nitinol wire glass fiber/epoxy laminate

Figure 2-1 Conductive epoxy arrangement

14

Although the first prototype achieved the goal of quick de-icing, they presented a number of technological problems: 

High variability in the electrical resistance in different sections of the specimen



Requirement of high voltages leading to an increase of the electrical power required for deicing



Occurrence of irreversible damage of the structure by charring and permanent deformation of the host material



Lack of reliability in the connection associated with low repeatability in the testing conditions The conclusions derived from the first prototype forced the consideration of a new

connection solution, under a different configuration. The main difference with respect to the conductive epoxy, was a new wiring pattern of the SMA. Only a semi-cylinder mold would be used, and the wire would not be cut at its tips, and a loop not embedded was allowed. With this procedure, all the alloy strips would be connected in a single wire. It was a denominated "Continuous Wiring" versus the wiring of parallel separated segments, which required a subsequent electrical connection by means of a secondary operation 2.3

Flat specimen experiments For a preliminary study of the effectiveness of the Continuous Wiring a series of

flat specimens with both connection methods were manufactured and tested. The specimens built were one inch wide, as a first inexpensive approach. A view of the actual specimens is shown in Figure 2-2. Two different specimens of the conductive epoxy type were built: using nickel epoxy and silver epoxy. The only difference is that the second

15

one contains an epoxy rich in silver particles, instead of nickel particles. This formulation is intended to provide an improved electrical contact with respect to the previous one. The third specimen consisted of a single continuous loop of Flexinol™ wire. A special tooling similar to the one described later on, was used to fabricate this specimen. A fourth specimen consisting of a single continuous wire of copper wire was also fabricated. This specimen was used as a comparison element to determine the power required for deicing exclusively by thermal heating. Several other specimens consisting of copper wire strips were also built. However, the copper wire strips in these other specimens had values of electrical resistance that situated their parameters out of the range of interest of the experiment.

Figure 2-2 Actual flat specimens (left to right: two with conductive epoxy, continuous Flexinol™, continuous copper, and copper strips).

The power required to achieve SMA activation was determined by testing the flat specimens in below-freezing conditions. For the purpose of this test, the power required

16

to raise the specimen surface temperature to 140oF was measured. This surface temperature was close enough for wire activation, and low enough so that the host fiberglass epoxy laminate did not exceed the glass transition temperature. The ambient temperature corresponding to icing conditions was obtained inside an environmental chamber set up for the experiments that is described in section 2.4.6. Several test runs were performed, and the average test results are presented in Table 2-1 [12]. The continuous wire connection method required substantially less power to reach 140oF compared to the other methods. Furthermore, no charring or permanent deformation was evident using this method. It was therefore decided that a new deicing device should be fabricated using this continuous wire method.

Wire type

Electrical connection

Ambient temp.

Specimen final temp.

Voltage used (V DC)

Current (Amps)

Power (Watts)

Flexinol

Nickel epoxy

-5oF

144oF

4.0

13.2

52.8

Flexinol

Silver epoxy

-14oF

141oF

2.1

10.5

22.1

Flexinol

Continuous wire

-7oF

141oF

25.6

0.69

17.7

Copper

Continuous wire

-5oF

142oF

3.0

10.0

30.0

Table 2-1 Flat specimen temperature cycling test. Comparison of different connection methods and wire types

2.4

Continuous wire specimen The results from the flat specimen tests led to the manufacture of a 6 inch

cylindrical leading edge specimen with its wires interconnected with the same procedure

17

used for the flat continuous wire. A wire fiber is laid along the laminate and when it reaches the end it is wrapped around a threaded rod to return and form the next parallel wire fiber. This new system constitutes a second generation of specimens under the concept shown in Figure 2-3. nitinol wire element

fiber laminate fiberglass/epoxi glass/epoxy laminate

threaded rod

Figure 2-3 Concept of the continuous wire specimen

2.4.1

Continuous wire six inch specimen: laying-up and tooling A new tooling was designed and built for the second generation of leading edge

specimens. The first generation is the set of specimens prepared in previous phases of the SMA deicing project, based on the conductive epoxy arrangement. A view of the actual tooling is shown in Figure 2-4. Based on the experience of the first family of prototypes and the demands of the new design, a new manufacturing process was developed. It was implemented with a high simplicity and repeatability of steps. This way it was assured

18

that the supply of specimens of the new generation have identical properties and a low variability in the electrical and mechanical properties. The ends of the wire elements are not cut during the manufacturing process, and all the wires have continuity forming physically a single wire. To achieve this lay-up, once a wire element is positioned the Flexinol™ is forced around a threaded rod forming a loop that allows to place the following wire element onto the laminate. The lay-up sequence and tooling arrangement is shown in Figure 2-4 (continued). A remarkable feature of the tooling is the possibility of applying tension to the Flexinol™ wires by means of its movable threaded rod. This operation was performed before laying up the last two layers of the laminate in order to place the wires in a parallel pattern. Once the laminate was totally laid up, it was bagged for subsequent curing in autoclave oven, under temperature and pressure cycles.

Figure 2-4 Continuous wire tooling. Actual tooling view

19

Surface Bagging

+45/-45 fiberglass/epoxy adhesive epoxy Flexinol TM +45/-45 fiberglass/epoxy +45/-45 fiberglass/epoxy

Flexinol Wire

Threaded Rod

ADJUSTABLE TENSION ON WIRE

Figure 2-4 (continued) Continuous wire tooling: schematic view

2.4.2

Effects of thermo-mechanical processing during the manufacture Section 1.4 of chapter 1 described how temperature cycling and stress cycling

affect the behavioral characteristics of the SMA. The manufacturing operations described in the previous point (tension application for wire positioning and autoclave oven curing), precisely carry out both types of cycling on the SMA. This section tries to determine the final effects of this processing and its impact on the specimen in terms of deicing performance. Namely, in terms of the SMA wire activation and amount of contraction reached. For the SMA working under one way shape memory effect it is a function of the

20

available pre-strain before activation. Figure 2-5 describes all the steps undergone by the alloy during the manufacturing and subsequent deicing service. The actual manufacturing of the specimen starts at the point that the graphic shows as No. 4. In Step No. 4 the hybrid laminate was laid up, and the Flexinol™ is positioned with no tension at all. The wire is 100% martensite, and no strain or tension has been applied yet. In Step No. 5 the wires were aligned by displacing both adjustable threaded rods, see Figure 2-5. They were moved symmetrically an equal amount of a quarter of an inch. Considering that the initial length of a single loop of wire was approximately 10 inches, the existing strain after Step 5 is:



L 0.5   0.05  5% L0 10

In Step No. 6 the specimen is cured following an autoclave cycle with an initial

moderate ramp that extends up to a temperature of 270 oF where a curing plateau of 90 minutes commences. AS (158 oF) is reached during that ramp. Thus, some austenite starts to form from the existing martensite (100% of the material at this point). In Step No. 7 the proportion of austenite increases in the material, and AF is

reached. If the wire was free from constrains, at this point 100% of the material would be austenite. However, there is a stress on the alloy due to the tension applied from the movable threaded rods for alignment purposes. The effect of applying a stress on the alloy is described in section 1.4 (aspect 3) of chapter 1. It is the formation of stress induced martensite, or SIM.

22

Due to having stress induced martensite at this point the composition of the alloy may not be 100% austenite. This effect can also be interpreted as an upwards shift of the AS and AF temperatures, as described in section 1.4 (aspect 4). To quantify the percentage of extra martensite obtained due to this effect, quantitative measures like the ones shown in Figures 1-4 and 1-5 are needed. However, no such data is available in the literature surveyed, for the alloy used (Flexinol™ which is Nitinol [Ni-Ti 50% wt]). Therefore, the data indicated from this point are estimations based on the facts observed in the resulting specimen. In Step No. 8 the curing plateau of the cycle is reached. Since no increase in

temperature or stress is carried out on the alloy the percentage of extra martensite existing at the start of the plateau will remain the same until its end. That extra martensite has some pre-strain due to the alignment operation that was never neutralized because that martensite never returned to the parent austenite phase. In Step Nos. 9 and 10 the curing plateau has finished and the specimen is cooled

down again to room temperature. During this cooling, those grains of the material that were austenite at the curing temperature transform into temperature-induced martensite. Those grains have neutralized the pre-strain that came from the alignment operation and macroscopically manifest as a portion of the wire, which is shorter than it was when the autoclave curing began. In Step Nos. 11 and 12 the autoclave cycle is totally finished and the specimen

cooled down to room temperature. A severe bending was observed on the threaded rods imposed by the Flexinol™ wires. This bending was quantified as approximately a quarter

23

inch in the most displaced point, which was at the middle section of the rods. At the fixed ends of the rods the constraints from the tooling forced a zero displacement of the rods at these points. The conclusion from these observations is that the wires in the middle part of the specimen neutralized all their pre-strain while those closer to the lateral sides of the specimen practically kept it all. The intermediate wires did show a distribution varying smoothly between those two extrema. It can be estimated that from the initial 5% of prestrain only an amount of 2% to 3% was available in the specimen after the manufacturing process. Deicing Service Cycle: In Step Nos. 1 to 4: now the wires are not constrained by the tooling. They are

heated up to a maximum allowed temperature during the experiments, which was 160 oF measured at the surface to avoid damage on the host composite material. This temperature may not exceed AF, and therefore not all the martensite may be reverted to parent austenite during the deicing test with the continuous wire specimen. In Step Nos. 5 and 6 the specimen is cooled down again to service temperature

(freezing conditions) and the austenite obtained by the activation transforms again into martensite free of applied stresses and with its pre-strain neutralized. As an example number, it can be assumed that half of the martensite reverted to austenite during the activation, neutralizing half of the existing pre-strain. In Step No. 7 the elastic recuperation of the laminate returns the hybrid structure

to the configuration before activation and pre-strains the wire back to the approximate 3%. This pre-strain is necessary to provide contraction in the next deicing action

24

required. This model of behavior of the hybrid structure has to be validated by the experimental observations. 2.4.3

Continuous wire six inch specimen: power consumption tests

These test were similar to those conducted on the flat specimens. The time, minimum voltage required, and intensity during the process of heating the specimen up to 140 oF were measured. The following remarks describe some details of the tests. 1. The specimen temperature was measured on its outer surface. 2. The voltages were kept in the order of magnitude of a light aircraft battery. 3. A second specimen was built for comparison purposes: identical to the main one but with copper wires instead of Flexinol™. Since the copper of course does not show any shape memory effect, this specimen provides an estimation of the energy consumed for the same deicing action exclusively by thermal heating. The results of this series of test are shown in Table 2-2. The last test of the series (marked with *) was conducted after bolting together the loops of wire exterior to the laminate as shown in Figure 2-6. This operation was done in order to diminish the resistance of that extra amount of material that in fact does not perform any deicing action. The conclusions from these experiments are: 1. For the voltages applied, the resistance of the continuous wire specimen makes it impossible to reach a temperature of 140 oF or greater, which is necessary to activate the SMA wires. Solutions for this are either to increase the voltages applied or to consider a different electrical connection scheme.

25

2. The action of bolting the loops of the wires to reduce the total resistance was not very effective, and the results are similar to those with no bolting.

Wire type

6” specimen resistance (ohms)

Ambient temp. (F)

Specimen final temp. (F)

Time (sec)

Voltage used

Current (Amps)

Power (Watts)

(V DC) Copper

4

-10.2

122.2

75

25.6

6.27

157

Flexinol

339

-4.6

15.5

342

24.9

0.07

1.74

Flexinol

340.2

-10.6

23.8

260

38.7

0.11

4.40

Flexinol*

318.3

-10.2

19.3

288

24.9

0.08

1.94

(*) Bolted loops in the specimen Table 2-2 Continuous wire six inch specimen: power consumption tests

Flexinol wire loop

Figure 2-6 Bolted nitinol specimen

Point of bolting

26

2.4.4

Continuous wire six inch specimen: alternative activation tests

In the feasibility study from the previous SMA deicing research it was suggested to activate the wires by segments in an alternative sequence to create a shear stress in the interface between the ice and the laminate due to deformation gradients in the structure. To apply that concept and take into account the result of the previous test regarding applied voltages, the tests now described were performed. The alternative activation tests were conducted with the electrical connection scheme shown in Figure 2-7, with an AC (alternating current) power supply able to supply from 10 to 120 volts. The power supply was an adjustable autotransformer. The experiments started with the specimen at icing conditions, with no ice formed on it. The times to reach 140 oF were measured as well as the surface temperature of the specimen. When one of the groups of segments reached this temperature, the current was manually switched to the other group by means of the switcher schematically shown in Figure 2-7.

20 wires 1 inch segment






Figure 2-7 Electrical connection scheme for alternative activation

27

The tests were conducted only with the Flexinol™ specimens, and the data were collected for the two groups of three segments. The results corresponding to one of the groups are shown in the Table 2-3.

Wire type

Resistance group of 3 segments (ohms)

Ambient temp. (F)

Specimen final temp. (F)

Time (sec)

Voltage used

% of autotransformer voltage

Flexinol

170

-11.2

140

95

34.2

30

Flexinol

170

-10.3

148

42

24.9

40

Flexinol

170

-11.0

152

26

38.7

50

Flexinol*

170

-10.9

150

12

49.3

60

(V AC)

(*) Bolted loops in the specimen Table 2-3 Continuous wire six inch specimen: alternative activation tests

Totally symmetric values were obtained for the other group of three segments. The conclusions from this experiment are: 1. Total activation can be obtained with the continuous wire specimen with the price of increasing the voltage of the power supply. 2. Similar behavior is obtained by activating the hybrid structure with alternating current as with direct current. 3. No technical obstacle in the specimen has been found when performing alternative activation of different groups of segments if that is required for optimal deicing.

28

2.4.5

Continuous wire six inch specimen: deicing tests

From the conclusions of the previous experiments, these decisions were made: 1. The voltage of the power supply would be kept in values similar to the batteries installed on board light aircraft, for which this system is understood to be valid. 2. For the same reason, the power supply should provide direct current rather than alternating current. 3. A different electrical connection scheme should be used for the deicing experiments, in order to obtain full activation of the SMA under the voltages mentioned above. In order to produce alternative strain in different areas of the specimen, two groups of three segments need to be activated alternatively. However, this time the segments of a group are connected in parallel rather than in series following the scheme shown in Figure 2-8. The results of this deicing test are discussed in the section 2.4.6.

20 continuous wires 1 inch segment






Figure 2-8 Continuous wire deicing tests: electrical connection scheme

29

2.4.6

Artificial ice formation and ice thickness control

To artificially achieve the ice accretion on the specimen the environmental chamber shown in Figure 2-9 was used. In the same picture, the nozzle used to spray the ice, that is can be seen magnetically adhered to the side of the chamber. In the first trial of spraying, the exterior air ingress into the chamber caused an overall increase of thermal kinetic energy. The effects of this increase were an increase in the interior temperature, and a condensation of the sprayed water droplets under liquid form with no ice accretion. To solve this problem the solution adopted was to spray water at a temperature slightly above the freezing point. This solution made possible the successful creation of ice on any specimen with all the repeatability required. Other details of the chamber are the three small fans installed inside in order to homogenize quickly the temperature of the interior air, and its plastic transparent window to visualize the tests.

Figure 2-9 Freezer with spraying nozzle and instrumentation on the left

30

An important variable of the tests was the thickness of the ice layer obtained after spraying. This was determined after spraying, and any thickness greater that 0.5 inches was not tolerated. Thickness was checked at a set of points along the profile of the specimen as indicated in Figure 2-10.

A B

C

D E

Figure 2-10 Points where the ice thickness was checked before every test

The results from the successful deicing tests carried out are shown in Table 2-4. The symbol (*) indicated that the test was carried out after neutralizing a segment of 20 wires that was damaged during a failed test between the first and the second tests of this table. The following conclusions can be obtained from the deicing tests: 1. In accordance with the manufacturer information, the full activation of the Flexinol™ takes place in a few seconds at the most. Since the times registered in these deicing tests is in the order of minutes, it can be concluded that the deicing is performed by a combined action of mechanical contraction and thermal heating from the Flexinol™.

31

Resistance group 1 of 3 segments (ohms)

Resistance group 2 of 3 segments (ohms)

Ambient temp. (F)

Time (min:sec)

Activation cycles to deice

Voltage used

Maximum ice thickness (inches)

18.4

19.0

-10.2

8:15

7

38.2

0.35

18.0

19.5

-10.6

15:10

21

38.8

0.38

17.5

18.1

5.5

9:30

10

34.2

0.25

19.3

3.9 (*)

-10.9

3:50

3

25.5

0.24

(V DC)

Table 2-4: Continuous wire. Alternating activation deicing tests

2. If that is the case, before dislodging the ice a liquid water interface is created between the ice layer and the specimen. With the presence of that layer the shearing effect pursued with the alternating activation is practically nullified. 3. Deicing times turned out to be very high. This can be due to an incomplete activation of the wires for the temperatures allowed in the specimen (up to 150 0F). In order to assure full activation the arrangements for further research will have to look for higher currents going through the wires.

32

2.5

Pressure plate connection

Based on the conclusions from the last point and with the intention of increasing the current through each wire, a different connection method was implemented. Starting from the continuous wire specimen, every 20 wires (one segment) were connected to form an electrical node using a connector with bolted steel plates (see Figure 2-11). Every connector had a clear area of 1 inch in order to accommodate 20 wires. Due to the stiffness of the Flexinol™ wire, it is necessary to tighten up the bolts with a certain amount of torque to keep them positioned and ensure electrical conductivity. The connection did not turn out to be effective until sufficient pressure was reached in the tightening process. For this reason, this type of connection is referred to as pressure plate connector. Plate Connector Plates Connector

Flexinol wire loop

Figure 2-11 Pressure plate connector

33

With this connection, the twenty wires of each segment run in parallel and the total resistance of the specimen is greatly reduced. For this reason the operation under this type of connection involves higher values of intensity per wire. 2.5.1

Pressure plate connection: alternating activation deicing tests

The electrical connection implemented for these tests is indicated in Figure 2-12.

20 wires in parallel






1 inch segment

1 inch segment

1 inch segment

1 inch segment

1 inch segment

1 inch segment

Figure 2-12 Pressure plate: electrical scheme for alternating activation deicing tests

The surface temperature of the specimen was kept under 180 oF during the experiments in order to avoid any damage to the host fiberglass/epoxy laminate, but looking for the full activation of the Flexinol™ wires. Damage to three wires resulted during the continuous wire tests and were neutralized afterwards. For that reason the resistance values for the two groups of segments are slightly different. The results of the tests are shown in the following tables.

34

0.28" TEST No. 1

0.32"

Alternating

Activation 0.40"

0.38" 0.34"

Measured resistance Group 1:

1.7 ohms

Group 2::

1.1 ohms

Measured supply voltage

12.23 V

Estimated intensity

Ice thickness

Group 1:

7.19 amps

Group 2:

11.12 amps

Time to deice

3 min 10 sec

Max. temperature reached

162 F

Power consumption

111.97 watts

0.30"

TEST No. 2

0.35" Activation 0.50"

0.40" 0.35" Ice thickness

Alternating

Measured resistance Group 1:

1.5 ohms

Group 2::

0.9 ohms


12.34 V

Estimated intensity Group 1:

8.23 amps

Group 2:

13.71 amps

Time to deice

2 min 40 sec


183 F

Power consumption

135.37 watts

Table 2-5 Pressure plate connectors: results from alternating deicing tests

35

Although the deicing times have been clearly reduced with respect to those obtained with the continuous wire specimen they still are of the order of minutes. As stated before this means that the deicing action is the combined effect of surface deformation and heating. Although the ice is never sheared because of an unavoidable water interface, the physical separation of the surfaces provided by the shape memory effect actually accelerates the release of the ice layer. This happens due to the increasing gap created, which breaks the water film destroying the adherence due to the atmospheric pressure effect. The latter conclusion is based on the fact that in all the successful deicing tests, it has been observed that the whole ice crust is dislodged as a single piece. This indicates that no cracking of the ice takes place in the deicing. 2.5.2

Ice-specimen interface

To illustrate more clearly, the process mentioned previously, a single deicing test was performed out of the chamber and the interface between the ice and the specimen was observed. To have a clear vision of the interface, the activation was carried out simultaneously over all the segments of the specimen. The segments still count on 20 wires each connected in parallel by means of the pressure plate connectors. All the segments were interconnected in series in order to have the same current going through each of them. During the first three seconds of the test, the deicing follows this sequence: 1. A thin interface of melted water is created between the ice and the surface of the specimen in the area surrounding the stagnation point (point at 90 0 from the test bed).

36

2. Centered on the stagnation point, the shape memory effect creates an increasing gap between the surfaces of the solid ice and the specimen. This gap is occupied by liquid water. In the areas close to the clamped ends of the laminate the ice remains bonded with no liquid interface. These bonded areas are about one inch wide. In the subsequent process, the deicing time increases up to four minutes. That lag is due to time necessary to thermally melt the ice in the areas still bonded, since the shape memory activation has been completed. Besides this, since the test chamber provides static deicing conditions, some time is also needed to enlarge the liquid interface up to the point in which the gravity can compensate the atmospheric pressure effect, which prevents the ice from falling precisely because of the presence of the liquid water film. 2.5.3

Pressure plate connection: simultaneous activation deicing tests

Based on the previous observations, the simultaneous activation was tested in deicing tests. The electrical scheme of the test is shown in Figure 2-13.







1 inch segment

1 inch segment

1 inch segment

1 inch segment

1 inch segment

1 inch segment

Figure 2-13 Pressure plate connection: electrical scheme for simultaneous activation deicing tests

37

The surface temperature of the specimen was kept under 180 oF during the experiments in order to avoid any damage to the host laminate, but looking for the full activation of the SMA wires. The results of the tests are shown in the following tables.

Ice thickness: 0.30"

TEST No. 1 0.35" 0.50"

0.40" 0.35"

Activation

Simultaneous

Measured total resistance of the specimen

3.2 ohms


12.84 V

Estimated intensity through the whole specimen

4.01 amps

Time to deice

3 min 10 sec


162 F

Power consumption

111.97 watts


TEST No. 2 0.30" 0.53"

0.40" 0.33"

Activation

Simultaneous


1.9 ohms


12.34 V


6.5 amps

Time to deice

5 min 40 sec


180 F

Power consumption

80.21 watts

Table 2-6 Pressure plate connectors: results from simultaneous deicing tests

38


TEST No. 3 0.42" 0.58"

0.39" 0.42"

Activation

Simultaneous


1.5 ohms


24.70 V


16.5 amps

Time to deice

2 min 40sec


180 F

Power consumption

407.55 watts


TEST No. 4 0.35" 0.50"

0.45" 0.48"

Activation

Simultaneous


1.8 ohms


24.5 V


13.6 amps

Time to deice

4 min 15 sec


180 F

Power consumption

332.2 watts

Table 2-6 (continued) Pressure plate connectors: results from simultaneous deicing tests

39


TEST No. 5 0.34" 0.51"

0.39" 0.41"

Activation

Simultaneous


1.6 ohms


24.9 V


15.56 amps

Time to deice

4 min 2 sec


180 F

Power consumption

387.44 watts


TEST No. 6

0.35" 0.50"

0.40" 0.35"

Activation

Simultaneous


1.5 ohms


24.1 V


16 amps

Time to deice

5 min 10sec


180 F

Power consumption

385.6 watts

Table 2-6 (continued) Pressure plate connectors: results from simultaneous deicing tests

40

The following conclusions can be drawn from these results: 1.

As it was anticipated, due to the creation of a liquid interface between ice and

specimen, there is no clear difference in the deicing performance from alternating activation to simultaneous activation. 2.

The time to deice tends to increase as the number of deicing tests performed grows.

This trend indicated that the deicing tends to be based more on thermal effect rather that combined shape memory contraction with thermal heating. The Nitinol has been used up in terms of the properties it showed directly from the vendor, which correspond to the one-way shape memory effect. During the manufacturing process the wires gained some pre-strain (estimated to be about 3% in section 2.4.2). This initial pre-strain can be ‘spent’ by the one-way shape memory effect during the first deicing performance. Thus, an external agent must provide the pre-strain in order to have deicing capability in the subsequent deicing actions. The specimen design relies on the composite laminate for this mission: when the Nitinol is no longer activated, the elastic recuperation of the composite that was compressed would strain the wires again to its original prestrain. This mechanism proved to be successful up to approximately 10 deicing performances, after which the composite became stiffer due to cyclic thermal loading and the pre-strain application capability was lost.

CHAPTER III IMPLEMENTATION OF A WING SCALE SPECIMEN 3.1

Introduction The final phase of the SMA Deicer Project deals with the construction of a large-

scale specimen of a length comparable to the characteristic span-wise length of a light aircraft wing. This specimen simulates a section of the activated deicer leading edge. It can also be considered as a section activated sequentially after other sections of the same size have been activated in case of alternative activation along the leading edge of a fullscale wing. This chapter describes the tests conducted to characterize the wire activation, the design of the specimen, some technological aspects about its manufacturing, and the results of the tests to which it was subjected. 3.2

Determination of the Minimum Intensity of Current (MIC) required to fully activate a single Flexinol™ wire. Wire activation experiments. The parameter Minimum Intensity of Current to fully activate a single wire has

been of central importance in the design of the large-scale specimen since it will determine the scheme to follow in the electrical connection of the whole system. One of the experiments conducted during the previous research of the SMA Deicing Project together with an analytic estimation documented this issue. However additional research was conducted in the current research to characterize the activation and ensure the safe operation of the large-scale prototype. Some important facts to be considered in these experiments are listed in the following section.

41

42

3.2.1

Previous considerations The previous research of the SMA Deicing Project and the manufacturer of the

alloy Flexinol ™ reported the following data:

Minimum intensity required for the activation of a 0.005 in diameter bare wire at room temperature

0.45 amps.

Intensity required to heat up a bare 0.015 diameter wire up to 200 oF, initial wire temperature –30 oF

1.7 amps.

Table 3-1 SMA deicing project previous results

Minimum intensity

2.75 amps.

Activation time

1 sec.

Total deactivation time

15 sec.

Wire resistance

0.2 ohms/in.

Operation note

Currents which heat the wire in 1 second can be left on without over-heating it. Intensity values greater than the specified may deteriorate the internal structure of the alloy resulting in less activation after several hundred cycles. If not overheated, the degradation will appear after several thousand cycles.

Table 3-2 Parameters for the activation of a 0.015 in diameter bare wire at room temperature (source: Dynalloy Inc.)

3.2.2

Experiment 1- epoxy coated wire The deicing application is based on a SMA hybrid composite. Thus, the

temperature at which the epoxy resin undergoes irreversible damage limits the temperature reached by the wire during its activation. To determine this upper boundary, wire activation tests were conducted on a single wire coated with epoxy resin of the same type as the composite host laminate. A single wire test device was used, adding four additional isolation layers of plastic material at the clamps, to ensure the electrical

43

isolation of the wire from the test bed. Before the measurements, a first heat cycle was applied to neutralize any previous pre-strain existing in the wire. Afterwards, a prescribed 2% of pre-strain was applied by tensioning the wire using the tooling. The pre-strain amount was measured by means of the same strain gauge used subsequently to measure the recovery strain of the wire upon electrical activation. The test conditions are shown in Table 3-3. Current in excess of 2 amps resulted in damage as shown in Table 3-4.

Ambient temperature

Room temp.

Wire diameter

0.015 in

Wire length

9 in

Wire initial resistance

2.5 ohms

Time of current application

200 seconds (3 minutes and 20 secs)

Strain gage calibration

0.065 in/volt (of output increment)

Wire applied pre-strain

2%

Table 3-3 Epoxy coated wire: experiment conditions

Test No.

Wire intensity (stabilized value)

Strain %

1

1.18

1.45

2

1.85

1.55

3

1.97

1.50

4

2.15

1.65

5

2.32

Epoxy darkens

6

2.41

Epoxy chars

7

2.62

Epoxy burns

Table 3-4 Epoxy coated wire: experiment results

44

Thus, 2.32 amps constitutes an upper boundary for the parameter MIC. 3.2.3

Experiment 2- bare wire at room temperature A series of tests measured the recovery strain under different activation currents

on a bare wire (no epoxy coating) at room temperature as shown in Table 3-5. Ambient temperature

Room temp.

Wire diameter

0.015 in

Wire length

9 in


2.45 ohms






2%

Table 3-5 Bare wire: experiment conditions

Test No.

Applied voltage (v)

Wire intensity (amps, stabilized value)

Strain %

1

1.64

0.64

0.21

2

2.82

1.13

1.35

3

4.65

1.86

1.49

4

5.35

2.14

1.65

5

6.08

2.43

1.66

6

6.91

2.76

1.55

7

8.12

3.25

1.54

8

8.63

3.45

1.61

Table 3-6 Bare wire: experiment results

45

From the results given in Table 3-6 the following conclusions can be drawn. First, for currents close to 2 amps a recovery strain of about 1.5% is obtained. A subsequent increase in the current up to 3.45 amps does not produce an important increment in the recovery strain that tends to reach the value of the applied pre-strain of 2%. Thus, the wire can be considered as fully activated at room temperature for any current greater than 2 amps. 3.2.4

Experiment 3- bare wire at icing condition temperature A series of tests measured the recovery strain under different activation currents

on a bare wire (no epoxy coating) at a temperature similar to those for icing conditions (see table 3-7). The set-up used is the same as the one described for the static icing tests of Chapter two.

Ambient temperature

-12.oF (Freezer chamber)

Wire diameter

0.015 in

Wire length

9 in


2.5 ohms






2%

Table 3-7 Bare wire at icing conditions: experiment conditions

46

Test No.

Applied voltage (v)

Wire intensity (amps, stabilized value)

Strain %

1

1.56

0.60

0.54

2

2.96

1.14

0.63

3

4.63

1.78

0.54

4

4.80

1.92

1.60

5

5.56

2.14

1.65

6

6.50

2.60

1.95

Table 3-8 Bare wire at icing conditions: experiment results

From the results shown in Table 3-8, similar conclusions to those from experiment 2 can be obtained. For currents close to 2 amps a recovery strain of about 1.5% is obtained. A subsequent increase in the current up to 2.60 amps does not produce a significant increment in the recovery strain. Thus, the wire can be considered as fully activated at freezing temperature for any current greater than 2 amps. The main difference noted in this test with respect to the one at room temperature appears in the wire activation times. While these times are about one second at room temperature, in the freezer conditions they can take up to 3 seconds for the lower voltages. 3.2.5

Experiment 4- intensity vs. time curves for the activation of a bare wire at room temperature The activation of the Flexinol ™ wire relies on the austenitic transformation

undergone by the internal grain of the alloy under an increment of temperature. This transformation implies a variation in the rest of the properties of the material such as

47

density, stiffness, and electrical resistivity. For instance, the progress of the austenitic transformation can be monitored by measuring the resistivity of the alloy as a function of temperature. Thus, it is convenient to ask whether it makes sense to look for a single value of intensity of current to fully activate the wire or a set of them. A series of tests were conducted activating the bare wire electrically to measure the intensity through the wire as a function of time by means of a multi-tester in series with the wire. The activation of the wire was held for 200 seconds (3 minutes and 20 seconds).

Ambient temperature

Room temp.

Wire diameter

0.015 in

Wire length

9 in


2.45 ohms



Table 3-9 Intensity vs. time curves: experiment conditions

From the results of this test (curves shown in Figure 3-1) the following conclusions can be drawn. The intensity or resistivity value tends to quickly stabilize, after a first peak associated with the activation and austenitic transformation of the wire. By adjusting the power to obtain the stabilized value, the operation can be considered as safe, since the value at the peak does not differ greatly from the stabilized value. Besides this, the difference with respect to the stabilized value only takes place during a short time.

48

2.5

2 5.8 volts 9.0 volts 4.5 volts

strain (%)

1.5

1

1.5 volts 0.5

0 0

20

40

60

80

100

120

140

time (sec)

Figure 3-1 Single wire activation test. Intensity vs. time (described in table 3-9)

160

180

49

3.2.6

Experiment 5- intensity vs. time curves of the activation of a bare wire at icing condition temperature The curves obtained under freezing conditions turn out to behave the same way as

at room temperature. A certain lag in the activation time was expected, however it is hard to detect. Again, stabilization with time and smooth transitions allow a single value of current for the wire activation. See Figure 3-2. 3.3 3.3.1

Design of the electrical connection Previous considerations Every connector (under a certain physical implementation: brass bolts, plates, or

conductive epoxy) constitutes an electrical node in the circuit formed by the specimen, the power supply, and the wiring elements. Two data are considered as fixed specifications for this design: 1.

Total voltage applied to the specimen: 24 volts, provided by a typical light

aviation battery or two automobile batteries in series. This is fixed value since the SMA Deicer is a system to be potentially installed in general aviation light aircraft. For this type of airplane, the typical power source on board is a 24 volt battery similar to two batteries used by automobiles. 2.

Current circulating through a wire to attain its activation: 2.0 amps. This

value has been determined with the experiments shown in the previous section in this chapter. A certain number of wires will be connected in parallel by means of electrical nodes to constitute a segment. The segments will be connected in series, thus they all will receive the same current.

50

2 1.8 4.63 volts

1.6 5.78 volts

1.4 8.73 volts

strain (%)

1.2 2.96 volts

1 0.8 0.6 0.4 0.2 0 0

50

100

150

200

time (sec)

Figure 3-2 Single wire activation. Intensity vs. time. Rwire = 2.54 ohms, (in the fridge @ -11 F)

250

51

The total number of wires for the large-scale specimen after its manufacture turned out to be 388 wires. 3.3.2

(1)

Calculations



Unknowns: only n, number of wires to be connected in a node, to form a segment.



Equations: four equations have to be applied: Ohm’s law for the whole specimen:

I total  (2)

Vtotal 24  Rtotal Rtotal

Kirchoff’s law of intensities for given node in n wires: I total  n  I wire  n  2.0

(3)

Composition of resistances in series: Rtotal  Rsegment  (number of segments)  Rsegment 

(4)

388 n

Composition of resistances in parallel: 1 Rsegment

 n

1 Rwire

R segment 

2.5 n

Substituting 2, 3, and 4 into 1, it yields:

24 2n   388  2.5    2 n  

n  80.83  81 wires per node    Rtotal  0.16    I total  150 A

Since 388 is not divisible by 80 or 81, the closest solution is to create 5 nodes having 3 of them 78 wires, and 2 of them 77 wires. This arrangement will provide a current per wire very similar to the desired value of 2.0 amps. See Figure 3-3.

52






24 volts

Figure 3-3 Electrical connection of the two-foot specimen

3.4

Manufacturing aspects of the two-foot specimen

The manufacturing process of the two-foot specimen differs from that of the continuous 6 inch specimen described in section 2.4. It was obtained by filament winding of the Flexinol

TM

wire on a cylindrical mandrel with the lower layers of the laminate

layed-up previously, as performed by Chau Huynh [1]. The total lay-up is the same as followed for the 6 inch specimen shown in Figure 2-4. 3.4.1

Lay-up procedure and curing cycle

The mold allowed the winding over two symmetrical shells of one feet each located at both sides of the cylinder. See Figure 3-4 in the next page. A margin of one inch was left during the lay-up between the borders of the two shells. This is needed to assure a length of free wire outside the laminate that allows the

53

placement of the elements for electrical connection. See Figure 3-5. Following the electrical scheme designed in the previous section, five segments of 78 or 77 wires each were connected in series. The 78 or 77 wires go in parallel since the threaded rods form a common electrical node for them.

glass/epoxy laminated shells

Figure 3-4 Schematic view. Filament winding of the two-foot specimen

The total resistance of the two-foot specimen turned out to be 1.5 ohms, which is a value one order of magnitude greater than the value expected for the total specimen from design (0.16 ohms, but the theoretical calculations did not include the effect of the threaded rod connection elements). The winding tension on the Flexinol TM wire was kept between 5 and 10 pounds. A lower value would allow severe misalignment in the SMA fibers, while a value over 10 would apply a pre-strain in the wire beyond the 5% estimated for the lay-up phase (see Figure 2-5 to check this estimation).

54

Figure 3-5 Cross section of the mandrel

Right before the curing operation the mandrel and specimen set was removed from the automatic winding machine and surface bagging was performed on it. To extract the set from the winding machine the tension was removed from the wire, which still is a continuous filament at this point. No additional fixture was necessary to keep the tension on the wire at this moment since the curing cycle was carried out immediately thereafter. However it would be different if the part is going to be kept on the shelf for some hours before curing. The curing cycle in terms of temperature consisted of a ramp of ten minutes, a 270 oF constant temperature plateau of forty minutes followed by a cooling down ramp of ten minutes to room temperature. The curing pressure in the chamber was 80 psi, and the vacuum port to compress the bagging was actuated at 30 psi. This curing cycle is the same as that used for the previous specimens, and has proven to produce high quality composite laminates with SMA fiber completely embedded in them.

55

3.4.2

Specimen demolding

After curing, the continuous wire was cut to obtain two shells from the mold. No caution was taken to avoid some resin flow to the area or margin reserved for the free wire tips. The result was that some wires showed impregnation with cured resin and had to be cleaned practically one by one. No further production of any specimen should be started without slightly blocking this flow by some means. 3.4.3

Electrical connection elements

As it has been stated before, it is important to form the electrical connection of the SMA wires externally to the laminate. This is the only way in which the pressure needed to achieve the full electrical contact can be applied on the wires. Other ways of connection such as electrically welding wire to wire have not been attempted. Soldering has been attempted in the past and has proved to be ineffective. Furthermore, local overheating of the SMA would imply undesired strain recovery and unpredictable response The elements used to achieve the optimal connection were pairs of brass threaded rods. The rods were held under pressure by means of hose clamps placed on the tips of the rods. See Figure 3-6. The geometry of the rods allows only one point contact, however it is sufficient if pressure is applied on the rods. 3.5

Tests conducted with the two-foot specimen

This section describes three issues: the arrangement used to test and measure strains and temperatures on the specimen and the two tests conducted with it, namely deicing test and cyclic activation test.

56

3.5.1

Strain and temperature monitoring

The instrumentation equipment used for the test is the same as that shown in Figure 2-8. For the test currently discussed, five channels of measurements were used for strain monitoring and three for temperature monitoring with type K thermocouples. See Figures 3-6 and 3-7. The following issues were considered regarding the instrumentation of the specimen.

Figure 3-6 The two-foot specimen instrumented and mounted in the test bed

57

Thermocouple

18"

Full Wheatstone Bridge

6" 0.5"

1.5"

1.5"

Left half

Right half 9" 21"

Figure 3-7 Instrumentation of the specimen, front view

58

A) Strain measurement: at every measurement station a full Wheatstone bridge

was installed. This option shows several advantages when compared to half or fourth bridge: 1. Allows measurement of both circumferential and axial strain. 2. Provides temperature compensation. Since the specimen will undergo a wide range of temperatures during the test (from chamber –10oF to wire activation 120oF), the temperature compensation is a critical issue here. 3. Provides compensation for local bending effects that could appear due to uneven activation from wire to wire. 4. Provides the maximum sensitivity. B) Sensitivity factor of the full Wheatstone bridge: since the structure is a

laminate with a particular value of Poisson’s Ratio, the sensitivity factor provided by the manufacturer is not applicable (calculated with Poisson’s Ratio of 0.3, valid for a number of isotropic materials). To recalculate it, the equations of the Wheatstone bridge are needed. For an applied circumferential strain of :

V  I

I R ( 1   ) 2 2V

 R  R ( 1   )  2    k 

where V is the input voltage of the bridge

V is the output voltage from the bridge,

59

R is the increment of resistance of each strain gage in the bridge due to the amount of strain  k is the gage factor of the strain gages

 is the Poisson’s Ratio of the material tested Bong-Ho Kim’s work [3] provides an average Poisson’s ratio of 0.5 for the laminate in the range of temperatures of the test. The gage factor of the strain gages used is 2.1%. The sensitivity has to be evaluated for a typical operation point, which in this case is 1000. Using these values, the sensitivity of the full Wheatstone bridge turns out to be 1.5 mV/V @ 1000. The strain gages used have a very low transverse sensitivity as desired for this full Wheatstone bridge application. Table 3-10 summarizes the conditions.

Strain Gages Type

MM-EA-00-060CN-120

Resistance (ohms)

120

Gage Factor

2.1%

Transverse Sensitivity

0.2%

Full Wheatstone Bridge Sensitivity

1.5 mV/V @ 1000

Table 3-10 Strain gaging properties

C) Strain gage bridge operation voltage. The original arrangement to measure

strain used a stabilized power supply of 24 volts for every channel. However this voltage caused some heat dissipation from every strain gage and made the strain gages reach a temperature close to 100 oF. To avoid an undesired local activation of the SMA wires, the

60

strain gages have to be kept cool. When strain gage temperature is an issue the manufacturer recommends a reduction of the power voltage to half or more to obtain a cool operation of the strain gages. The cold chamber arrangement includes a group of inner fans to homogenize the internal temperature. These fans are powered by an external stabilized power supply that provides voltages of 12 and 5 volts. The fans and a portion of the power supply can be seen on the right upper corner of Figure 3-8. Both voltage options were tried to power the strain gage bridges, and the 12 volt option was still too hot. Thus, the 5 volt option was used with satisfactory results. 3.5.2

Two-foot specimen: deicing test

The deicing test of the large-scale specimen was performed under simultaneous activation of the five segments of the specimen, with the power supplied by two automotive batteries connected in series, to obtain 24 volts which is the typical voltage level of a light aircraft. Although both shells obtained from the manufacturing process were connected together as a single specimen, there was a physical gap between them in the test bed to avoid the interference of their respective edges during the activation (see Figure 3-8). A quarter gallon of cold distilled water was sprayed over both shells, and the ice layer obtained is described in Figure 3-9. The SMA activation was carried out in temperature ramps limited by the security boundary of 120 0F. The deicing action occurred during the first ramp, after 36 seconds on the right hand shell. Two more ramps were completed pursuing the deicing in the left shell. The deicing was not achieved on that shell, and the test was terminated. Figure 3-10 shows the values of strain and temperature recorded

61

during the test. All the plots included in this section show strain in absolute value. In all the cases it is a contraction, that is a negative circumferential strain.

Figure 3-8 Two foot specimen inside the test fridge

0”

Two foot specimen DEICING TEST 0.22" 0.35"

0.38" 0”

Ice thickness

Activation

Simultaneous

Temp. in chamber

-10.5 F

Measured resistance

1.5 ohms


24.09 V

Estimated intensity

16.06 amps

Time to deice

36 sec


113 F

Power consumption:

386.89 watts

Figure 3-9 Deicing test two-foot specimen. Summary of data

62

1

140 DEICING EVENT

120

0.8

TEMP @ station 6"

0.6

80

STRAIN @ station 9"

0.4 60 STRAIN @ station 22.5"

0.2

STRAIN @ station 21"

40

0

20

TEMP @ station 0.5" STRAIN @ station 7.5"

-0.2

0 0

50

100

150 time (sec)

Figure 3-10 Two-feet specimen: deicing test

200

250

circunferential strain %

TEMP @ station 18"

100

temp F

STRAIN @ station 19.5"

63

3.5.3

1)

Discussion of the deicing test

The deicing was not achieved on the left hand shell due to an insufficient activation

on some areas of this half. Also, some irreversible deformation was observed in two areas of the specimen of an approximate width of one inch each. This brings as a conclusion that in some areas of the specimen the temperature was actually higher than the value at the points monitored with the thermocouples, and in others it was lower. The temperature readings are lacking sufficient evidence to support this conclusion since both readings at 6” and 18” stations are very similar (see Figure 3-10). The reading at 0.5” could not be validated since the head of the thermocouple was found separated from the surface of the specimen due to some moisture found in the adhesion. 2)

Thus, the electrical connection system with brass threaded rods has not been able to

deliver a uniform current distribution to all the FlexinolTM wires. However, there has been activation in the entire specimen and this system can be easily improved with more specialized procedures of union. This caused uneven activation in different areas of the specimen and the circumferential strain values fluctuated from station to station (see Figure 3-10 strain at the 7.5” and 21” stations). 3)

The fluctuations in strain occurred in both halves of the specimen. That is, not all the

low strains took place in the half that did not deice as it might be thought at first glance. The uneven activation causes a fluctuation in the strains at several places of the specimen. 4)

Strains of about 0.5 % have been enough to deice in the half that underwent average

enough activation. Bong-Ho Kim’s [3] predicted a strain about 0.3 % at the monitored

64

line for this proportion of alloy in the structure. The results obtained can be considered as a fairy acceptable validation of these predictions. 5)

The circumferential strain recorded from the specimen station at 19.5 inches belongs

to one of the areas of the specimen that showed over-activation due to non uniformity in the current distribution. The other area is located about the 14” station. These two areas probably reached temperatures slightly beyond 120 0F, and present irreversible strains in the composite laminate. 6)

From the previous point it can be concluded that any local overheating over 120 0F

can jeopardize the subsequent operation of the whole specimen. Therefore, any durable implementation of the system requires a connection system able to deliver current within a narrow tolerance margin. From these experiments, and those in the previous chapter, it looks like the most uniform system is the plate connectors.

3.5.4

Two-foot specimen: cyclic activation test

This second test conducted with the two-foot specimen was intended to determine the durability of the SMA specimen under cyclic loading. The results presented in section 2.5.3 seemed to indicate that the specimen undergoes a loss of strain capacity after several cycles of use. This effect can be due to the fact that the elastic recuperation of the composite laminate is unable to reapply completely the pre-strain needed by the SMA for its activation. Thus, the structure would “spend” gradually the residual pre-strain that it had from manufacturing since the pre-strain “reloaded” by the composite when the current is withdrawn is less than used for the activation. Ten loading cycles were

65

performed between 50 0F and 100 0F , and under the same conditions described for the previous experiment. Figure 3-12 shows the strains recorded during this test. Again the area about the specimen station at 9 inches is more activated than the rest, due to the non uniformity in the distribution of current. However, that zone did not suffer irreversible strain as the one located at 19.5 inches (its trace appears underneath the horizontal axis). In the Figure 3-12 it can be seen that the strains were very similar at all the stations monitored except for the one at 9 inches where more activation than the others occurred due to an irregularity in the current distribution. The temperature was kept under 100 0F in some monitored stations of the specimen (see Figure 3-12 continued). However, at the 19” station, it reached higher values for sure. These values are below 120 0

F since the maximum amount of strain in this test is smaller than that of the previous test

that reached full activation. 3.5.5

1)

Discussion on this test series

From the plots shown in Figure 3-12 it can be concluded that those areas close to

their full activation undergo a decrement in the strain capacity while the ones kept in moderate activation seem to show high repeatibility. 2)

Further research has to be done to completely confirm these conclusions. That

research should include a large number of cycles (of the order of hundreds) in two modes: from zero activation to 100 0F, and between 50 0F and 100 0F or a similar interval. 3)

The trends shown in Figure 3-12 seem to indicate that the operation at a low or

medium activation level can be the solution to go around the problem of the “cyclic

66

fatigue” of the specimen (although this has to be confirmed by testing a much larger number of cycles). However, a medium activation can result in ineffective deicing performance. Perhaps the solution can be the activation of the specimen using alternating current of a very low frequency at the same voltage that allows a vibration of the laminate able to generate the deicing action.

67

0.45 0.4 STRAIN @ station 9" 0.35

circumferential strain %

0.3 0.25 STRAIN @ station 22.5"

0.2 0.15

STRAIN @ station 21"

0.1 0.05 STRAIN @ station 7.5"

0 0

100

200

300

400

500

600

700

-0.05 STRAIN @ station 19.5" -0.1 time (sec)

specimen: cyclic activation test test FigureFigure 3-11 3-12 Two Two foot feet specimen: cyclic activation

800

900

1000

68

140

120

100

temperature (F)

80

TEMP @ 18” TEMP @station station 60 TEMP @ station 6"

TEMP @ station 6”

40

20

0 0

200

400

600

800

1000

-20 time (sec)

Fig(continued) 3-12 (continued) specimen: cyclic activationtest test Figure 3-11 Two Two foot feet specimen: cyclic activation

1200

1400

69

CHAPTER IV CONCLUSIONS AND RECOMMENDATIONS

4.1

Conclusions for the six inch specimen The plate connector system has proven to be the best way of distributing the

current uniformly to every Flexinol TM wire until industrial procedure presents something more reliable. No research has been carried out on a possible welding or soldering of the SMA for this purpose. Due to the formation of a liquid interface between ice and specimen, there is no clear difference in the deicing performance from alternating activation to simultaneous activation. With the six inch plate connector specimen, the time to deice increased as the number of deicing tests performed grew. This trend indicated that the deicing tends to be based more on thermal effect rather than combined shape memory contraction with thermal heating once the specimen’s pre-strain was removed with additional deicing cycles. The Flexinol

TM

has been used all along with the properties it showed directly

from the vendor, which corresponds to one-way shape memory effect. During the manufacturing process the wires gained some pre-strain (estimated to be about 3% in section 2.4.2). This initial pre-strain can be ‘spent’ by the one-way shape memory effect during the first deicing performance. Thus, an external agent must provide the pre-strain in order to have deicing capability in the subsequent deicing actions. The specimen design relies on the composite laminate for this mission: when the activation of the

69

70

Nitinol is withdrawn, the elastic recuperation of the composite that was compressed would strain the wires again to its original pre-strain. This mechanism proved to be successful up to approximately 10 deicing performances, after which the composite became stiffer due to cyclic thermal loading and the pre-strain application capability was lost. 4.2

Conclusions for the two foot specimen Deicing was not achieved on the left hand shell due to insufficient activation on

some areas of this half. Also, some irreversible deformation was observed in two areas of the specimen (about one inch wide each). This brings as a conclusion that in some areas of the specimen the temperature was actually higher than the value at the points monitored with the thermocouples, and in others it was lower. Thus the electrical connection system with brass threaded rods has not been able to deliver a uniform current distribution to all the FlexinolTM wires. However, there has been activation in the specimen as a whole and this system can be easily improved with industrial procedures. Strains of about 0.5 % have been enough to deice in the areas that underwent full activation. Bong-Ho Kim [3] predicted a strain of about 0.3 % at the line of stagnation for this proportion of alloy in the structure. The results obtained can be considered as a fairy acceptable validation of these predictions. The circumferential strain recorded from the specimen at the 19.5” station belongs to one of the areas of the specimen that showed over-activation due to non- uniformity in the current distribution. Two areas quite likely reached temperatures slightly beyond 120 0

F, and irreversible strains in the composite laminate resulted. The trace shown by the

71

strain measurement at 19.5” reveals how the SMA is unable to overcome the strain of the composite. From the previous point it can be concluded that any local overheating over 120 0F can jeopardize the subsequent operation of the whole specimen. Therefore, any durable implementation of the system requires a connection system able to deliver current within a narrow tolerance margin. From these experiment, and those in the previous chapter, it looks like the most uniform system is the plate connector system. From the previous plot it can be concluded that those areas close to their full activation undergo a decrement in the strain capacity while the ones kept in moderate activation seem to show high repeatibility. Further research has to be done to completely confirm these conclusions. That research should include a large number of cycles (of the order of hundreds) in two modes: from zero activation to 100 0F, and between 50 0F and 100 0F or a similar interval. The trends shown in the tests seem to indicate that the operation at a low or medium activation level can be the solution to go around the problem of the “cyclic fatigue” of the specimen. However, a medium activation can result in ineffective deicing performance, and the solution could be the activation of the specimen using an alternating current of a very low frequency at the same voltage that allows a vibration of the laminate able to generate the deicing action.

72

4.3

Recommendations on the thermo-mechanical coupling in the deicing performance As soon as a repeatable de-icing capability is achieved, the question of what

extent the deicing is achieved by mechanical cracking of the ice and not by thermal melting of the ice must be answered. This issue arose independently from attendants in both SAE and NIAR presentations of this research project. To provide an answer on this point, a special specimen with its shape memory effect neutralized can be manufactured and tested for pure thermal deicing. It can be implemented following the concept of placing the Nitinol fibers in lay-up lines parallel to the span-wise direction of the leading edge. Besides this, a NASTRAN thermal model of this specimen can be attempted to complement this point. A finite element analytical model developed for this case might be necessary if the variation of resistivity in the Nitinol with the temperature is to be considered in the simulation. To determine this last point, it would be necessary to carry out some single wire tests to determine the coefficients of the law of variation [8]:

   rt 1   T  Trt 

where rt stands for room temperature. Also requiring consideration in the simulation is the incremental variation of other properties, in order to make it closer to the real events. Namely the constitutive behavior of the Nitinol depending on the temperature:    ( , T )

73

The expected results for a research on this point is to conclude that, although the ice is never sheared because of an unavoidable water interface, the physical separation of the surfaces provided by the shape memory effect actually accelerates the release of the ice layer. This happens due to the increasing gap created, which breaks the water film destroying the adherence due to the atmospheric pressure effect. This anticipation is based on the following facts: In all the successful deicing tests, it has been observed that the whole ice crust is dislodged as a single piece. This fact indicates that no cracking of the ice takes place in the deicing. From an observation performed out of the chamber, it was concluded that, during the first three seconds of the test, the deicing follows this sequence (under the configuration decided in the former point: i = 2 amps, simultaneous activation): 1.- An extremely thin interface of melted water is created between the ice and the surface of the specimen in the area surrounding the stagnation point. 2.- Centered on the stagnation point, the shape memory effect creates an increasing gap between the surfaces of the solid ice and the specimen. This gap is occupied by liquid water. In the areas close to the clamped ends of the laminate the ice layer remains bonded with no liquid interface. These bonded areas are about one inch wide. From that point, the deicing time increases up to the order of minutes. That lag is due to time necessary to thermally melt the still bonded areas. Besides this, since the test chamber does not simulate any air stream, some time is also needed to enlarge the liquid interface to the point that the gravity can make the ice crust fall. This is due to the

74

atmospheric pressure effect, which prevents the ice from falling precisely because of the presence of the liquid water film. A possible explanation for this 'clamped end effect' is that the surface strain of the laminate is prevented due to the support conditions of the fixture of the test bed. To provide some light on this point three tests are proposed: 1.- Determination of the circumferential temperature distribution. 2.- Determination of the circumferential strain distribution with special attention on the support areas. 3.- Exploration of different support conditions for the attachment to the test bed. 4.4

Proposal of solution to the loss of pre-strain capability The following describes a design that could provide a solution for two problems

found: some unbroken bonding or ‘ice ligaments’ in areas like the clamped ends of the specimen, and incompetent laminate as external agent of pre-strain application on the SMA. The implementation of this solution would attempt to improve the deicing capability of the six inch specimen by providing more effectiveness through mechanical means. It would consist of a simple mechanism using surface mounted SMA actuators, as suggested in [9], to provide a contraction of the leading edge in its width, as well as a positive elongation in the direction of its axis of symmetry, see Figure 4-1. This sequence of deformation is similar to that followed by pneumatic deicers, as shown in Figure 4-2. The leading edge skin is still a fiberglass/epoxy laminate with embedded FlexinolTM wires, and it performs the first activation.

75

fiberglass-epoxy / nitinol

nitinol wire element

spar

FIGURE 1 SURFACE MOUNTED ACTUATORS SYSTEM surface mounted actuator

(activated nitinol in red)

Figure 4-1 Surface mounted actuators proposal

76

The modification proposed in Figure 4-1 uses the strain in the first activation to separate the surface from the ice in the neighborhood of the stagnation point, as well as to pre-strain the linear actuators mounted outside the laminate. During the second activation, the ice bonding in the area closer to the spar is broken, and the laminate is prestrained simultaneously. This way, each SMA element is intended to perform deicing as well as recuperating the pre-strain in the other element. This idea intends to substitute the two-way memory effect by two elements of one-way effect, acting in opposing action.

flexible hose

pressurized air intakes from pump

FIGURE 2 BF Goodrich boot deicer infaltion cycle

Figure 4-2 BF Goodrich boot deicer inflation cycle

Another alternative design would be of a system that stores mechanical energy. Several existing deicing systems are based on the application of an impact, a force with high acceleration and short displacement. This principle can be proposed for a device that uses the SMA strain as a source of energy before the deicing moment. The SMA would be a linear actuator that is activated for a slow displacement that sets a mechanical spring to a compressed position. When the deicing is required, the spring would be released,

77

returning the energy received from the SMA as an impact on the internal face of the skin that forms the leading edge. The spring release itself would take care of the cold prestrain of the Nitinol. This design approach could have two versions: internally and externally mounted. 4.5

Structural responsibility of the deicer-skin This point affects another possible limitation of the SMA deicer: since the Nitinol

is embedded in the laminate, there has to be a limit to the stiffness so that the surface strain is possible. However, the leading edge as a structural element requires a minimum stiffness. Those minimum and maximum values set a design range and could conflict.

78

REFERENCES

[1] Huynh, C. Feasibility study of a composite leading edge deicing system with embedded Nitinol wires, Master’s thesis, Wichita State University, Fall 1997. [2] Wayman, C. M. and Duerig, T. W., An introduction to Martensite and Shape Memory, Engineering aspects of shape memory alloys, Duerig et al, ButterworthHeinemann, London 3-20, 1990. [3] Kim, B. Geometric modification of laminated cylindrical shells with embedded Nitinol, Ph. D. dissertation, Wichita State University, Spring 1997. [4] Meyers, M. and Chawla, K., Mechanical Metallurgy, 1984 Prentice-Hall. [5] Birman, V. (Univ. of Missoury-Rolla), Review of mechanics of shape memory alloy structures, ASME Appl. Mech Rev vol. 50, no.11, part 1, November 1997. [6] Thomas, S., Cassoni, R., and MacArthur, C., Aircraft anti-icing and de-icing techniques and modeling, Journal of Aircraft, vol. 33, no. 5, September-October 1996. [7] Gerardi, J., Ingram, R., Catarella, R., Wind tunnel test results for a shape memory alloy based de-icing system for aircraft, International Icing Symposium ’95 Montreal, Canada, September 1995. [8] Lagoudas, D., Boyd, J., and Bo, Z., Micromechanics of active composites with SMA fibers, Journal of Engineering Materials and Technology, vol. 116, July 1994. [9] Schetky, L. and Wu, M., The properties and processing of shape memory alloys for use as actuators in intelligent composite materials, ASME 1991 [10] Bond T, J Shin, Advanced ice protection systems test in the NASA Lewis Icing Research Tunnel, NASA Technical Memorandum 103757, 1991.

79

[11] Brailovski, Trochu, and Lebouef, Design of shape memory linear actuators (Ecole Polytechnique de Montreal), SMST-97, Asilomar, California, 1997. [12] Myose, R., Horn, W., Hwang, Y., Herrero, J., Huynh, C., Boudraa, T., Application of Shape Memory Alloys for Leading Edge Deicing, SAE Technical Papers Series (199901-1585), April 1999.

IMPLEMENTATION OF A COMPOSITE AIRCRAFT

IMPLEMENTATION OF A COMPOSITE AIRCRAFT

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