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ScienceDirect Procedia CIRP 66 (2017) 249 – 253
1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017
Sensing and Actuating Functions by Shape Memory Alloy Wires Integrated into Fiber Reinforced Plastics Björn Senfa*, Thomas Mädera, Iñaki Navarro y de Sosab, André Buchta, Marcus Knoblocha, David Löpitza, Welf-Guntram Drossela,b a
Fraunhofer Institute for Machine Tools and Forming Technology IWU, Noethnitzer Strasse 44, 01187 Dresden, Germany b Technische Universität Chemnitz, Reichenhainer Straße 70, 09126 Chemnitz, Germany
* Corresponding author. Tel.: +49-351-4772-2310; fax: +49-351-4772-32310. E-mail address:
[email protected]
Abstract Lightweight design based on fiber reinforced plastics (FRP) has potential for improvement by integration of sensors and actuators made of smart material filaments. Regarding FRP with integrated actuating shape memory alloy (SMA) wires, this paper presents important characteristics of such an adaptive composite and its components for design purposes. Beyond that, the first successful pultrusion processing of sensing SMA wires is proposed to address lightweight design mass production for safety-related applications. Measurements of this smart composite structure with strain sensor functionality proved high sensitivity compared to conventional sensors. © Authors. Published by Elsevier B.V. This ©2017 2017The The Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing Keywords: shape memory alloy wires; SMA; fiber refinorced plastics; FRP; lightweight design; smart composite structure; pultrusion
1. Introduction Smart materials respond to changing environmental conditions in a suitable way for a certain application. This smart behavior requires a sensing as well as an actuating functionality [1]. This paper addresses the self-sensing and the actuating functionality of shape memory alloy (SMA) wires in separate ways. The first part focusses on SMA wires which are embedded as actuators in FRP to realize shape changing composites. In addition to this technology, the second part is focused on SMA wires with inherent strain sensor effect. Both examples demonstrate the great potential to improve lightweight design with structural integrated functions. 2. Material Even though both examples are presented from a different point of view, the adaptive and the sensing composite are both made from glass fiber reinforced thermoset plastic with integrated Nickel-Titanium (NiTi) alloy (Table 1) and the respective findings are applicable for both differing
manufacturing processes. The basic properties of the applied SMA wires are listed in Table 1. The phase transformation temperatures (As, Af, Ms, Mf) of the NiTi actuator wire are above and those of the NiTi sensor wire are below the assumed ambient temperature of 20 °C in initial unloaded state. The phase transformation temperatures are critical material properties. They need to fit to the purposed application and to the surrounding polymer matrix to prevent accidentally actuation and heat damage. Table 1. Properties of the applied SMA wires. NiTi sensor
NiTi actuator
Mass fraction of Ni in %
55.9
54.8
Austenite start temperature As in °C
-30
73
Austenite finish temperature Af in °C
10
88
Martensite start temperature Ms in °C
-83
37
Martensite finish temperature Mf in °C
-120
24
Diameter in mm
0.150
0.500
Heat treatment
straight annealed
straight annealed
2212-8271 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.03.291
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oxidic
oxidic
3. Adaptive composite Thermal SMA wires perform as actuators with high energy density when twinned martensite phase is present at low temperature [2]. Shape changing lightweight composite structures are feasible by integrating NiTi actuator wires. For this purpose, vacuum infusion manufactured glass fiber reinforced epoxy resin is herein applied. The resulting composite structure is assumed to be thermally activated by ambient temperature to adapt to environmental conditions. Alternatively, the activation could be performed through resistance heating of the SMA wire by an electric power supply. An exemplary application for ventilation purposes or temperature control is shown in Fig. 1.
Fig. 1. Adaptive ventilation flap made of FRP and integrated SMA actuators.
The working principle of such an adaptive composite structure is based on the reversible mechanical interaction between the structural FRP stiffness and the uniaxial SMA wire forces [3]. A SMA volume fraction of at least 1 % is necessary to deform a glass fiber reinforced plastic. The adaptive ventilation flap (Fig. 1) has a thickness of 2 mm and a width of 30 mm. Four NiTi actuator wires with a diameter of 0,5 mm are capable to induce a significant bending deformation. Since the SMA wires are thermally activated, such an adaptive composite needs to be designed considering both heat transfer and force transmission. The bending deformation is a consequence of the heat induced tension by the off-center integrated actuator wires. According to [4], the anisotropic and temperature dependent viscoelastic behavior of composite structures can be simplified to anisotropic linearelastic behavior for short-term loads and low humidity environments. This simplification can be applied to glass fiber reinforced and temperature-resistant epoxy resin, which are used here. Besides the direction-dependent mechanical behavior of the FRP, the temperature-dependent stress-strain curve of the NiTi actuator wire is necessary to describe the adaptive composite behavior. The force transmission between the wires and the polymer matrix is not achievable by the interface shear strength and has to be assured by additional tight fit [5]. The mechanical interaction is also influenced by the different thermal expansion coefficients of the components [6,7]. Another aspect is the load dependency of the SMA phase transformation temperatures. Those values increase
approx. 10 K per 85 MPa stress increment [8]. On the one hand, in case of direct resistance heating of integrated NiTi actuator wires, the thermal conductivity of FRP depends on temperature and fiber volume fraction but not on the layup. On the other hand, the heat loss is driven by convection and thermal radiation of the hole composite. Regarding the wire surface condition there is not recognized any effect on the interface heat resistance between wire and FRP. [9] The specific heat capacity of the composite components, the glass transition temperature of the polymer and the phase transformation enthalpy of SMA wire can be determined by differential scanning calorimetry. Those values are essential to determine the amount of thermal energy required to activate the adaptive composite and to avoid overheating damages of the FRP and SMA. 4. Sensing composite SMA wires are also capable to work as strain sensors. Along with the stress-induced phase transformation (from austenite to detwinned martensite and vice versa under constant temperature), the ohmic resistance undergoes a large variation [10]. Consequently, the bijective correlation between changes in electric resistance and changes in mechanical strain of the NiTi sensor wire qualifies this material for the implementation in strain gages. The electrical resistance of metallic sensor wires is determined by geometry and structural change (also known as piezoresistive effects) [11]. According to [12] the temperature influence can be compensated and thus will be neglected here. The wire geometry (diameter and length) changes when an axial load is applied. Poisson’s ratio Ȟ represents the changing diameter in relation to changes in length. Conventional strain gauges apply this geometrical effect as measurement principle (term one in equation 1). Piezoresistive effects (term two in equation 1) are usually negligible for most metals [13].
ΔR ª ΔL º ªΔρ º = «(1 + 2 ⋅ν ) » + « » R ¬ L ¼1 ¬ ρ ¼ 2
(1)
The strain measurement sensitivity can be expressed by the gauge factor k (equation 2). If the piezoresistive effects are also negligible for NiTi sensors, the expected k-factor would be between 1.6 and 1.9 for typical Poisson’s ratios of NiTi given in literature (0.3 < Ȟ < 0.45 [14]).
ΔR ΔL =k R L
(2)
Strain dependent electrical resistance measurements of the NiTi sensor wire were carried out with the commercial digital multimeter Agilent 34401A and the material testing machine Zwick Z020. A nearly straight proportional behavior and the typical temperature dependent stress-strain-hysteresis of NiTi is recognizable (Fig. 2). The temperature influence on the
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800
40
700
35
600
30
500
25
400
20
300
15
200
10
100
5
0 1
2
3
bobbin rack resin reservoir curing tool
cutting device
0 0
related applications such as airplane stringers, rotor blade or sports equipment components.
alternating pullers
electric resistance change in %
mechanical stress in MPa
electrical resistance change is also shown. Because NiTi alloys exhibit a stress-strain asymmetry under tension and compression [15] and because of the low slope of the resistance-strain curve below approx. 0.5 % the NiTi sensor wire should be applied with a prestrain above 0.5 % to obtain a constantly high sensitivity and to examine the possibility to measure tension and compression strain of the surrounding polymer matrix when integrated. Compression strain will then unload the prestrained NiTi sensor wire causing changes in electrical resistance with a negative slope.
4
Fig. 3. Schematic view of the applied pultrusion machine.
The appropriate location of the NiTi wires in the pultrusion profile is fundamental. To enhance the sensing capabilities concerning bending deformation, the wires have to be placed with a maximum distance to the neutral plane. (xz-plane in Fig. 4). In case of two possible bending axes during application, the sensor wire must be located with no distance to one axis of the cross sectional area (e.g. x-axis in Fig. 4) to avoid any influence of the perpendicular bending axis. Two sensor wires need to be integrated to distinguish between multiple bending axes.
strain in %
SMA wire
stress 22°C
stress 40°C
stress 60°C
resistance 22°C
resistance 40°C
resistance 60°C
Fig. 2. Characteristic curves of the relative electric resistance change depending on NiTi sensor wire strain, temperature and mechanical stress.
The k-factor of the applied NiTi sensor wire is 6.7 at 22°C and 7.2 at 40°C and 60°C corresponding to measurement values shown in Fig. 2. Therefore, the piezoresistive effects are significant (term two in equation 1) and causing a promising strain gauge sensitivity. This paper presents a novel application of NiTi strain sensors integrated into pultrusion profiles. Pultrusion is a continuous process for the production of straight and low weight profiles made of FRP. Hence, fibers and optional additional fabrics are pulled from bobbins into a liquid thermosetting resin. The wet reinforcing fibers are then pulled through a heated tool where the curing process of the plastics starts. By increasing the temperature in different heating zones inside the tool, the thermosetting plastic cures completely within seconds. Two alternating pullers grasp and move forward the cured FRP profile to an automated cutting device (Fig. 3). The test profiles were produced with type eglass fibers (4 800 tex) and an unsaturated polyester resin resulting in a fiber volume fraction of 58 %. The state of the art process was modified by integrating a prestrained NiTi sensor wire. This continuous processing of sensing composite profiles allow new solutions for mass production of safety-
neutral plane z
y h
x l w
Fig. 4. Schematic sketch of the pultrusion profile with integrated sensor wire.
Three point bending tests based on ISO 178 with a support distance of 80 mm were performed to investigate the behavior of a NiTi strain sensor wire integrated into a rectangular pultrusion profile (12.5 mm x 25.0 mm). Fig. 5 visualizes the test setup. A laser triangulation sensor (optoNCDT1607-4) quantified the displacement of the specimen in the middle between the bearings at the opposite of the compression die. Simultaneously, the electrical resistance change of the NiTi sensor was measured.
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4. Conclusion
compression die pultrusion specimen specimen support electrical resistance measurement connection laser displacement sensor
Fig. 5. Three point bending test of the sensing pultrusion specimen.
Measurement results are shown in Fig. 6 for two specimens. The strain values are calculated according to equation 2. The gauge factor of k = 6.7 was obtained from the characteristic resistance-strain curve at 22 °C temperature shown in Fig. 2. Both specimens have different mean distances of the NiTi sensor wire to the neutral plane (z1 = 3.85 mm for P1 and z2 = 5.38 mm for P2). As previously mentioned, the distance of the wire sensor affects the measurement effectiveness that is better for P2 with the larger distance. With the knowledge of the height of the pultrusion profile, it is possible to extrapolate the maximum strain on the surface of the specimen. By means of the Young’s modulus of the FRP, the maximum stress could be determined. Therefore it is also possible to detect any critical load exceeding the strength limit. Both specimens are measured with the NiTi sensor wires at the upper/compression side (P1.1 and P2.1) and after turning the specimens at the lower/tension side (P1.2 and P2.2). The curves symmetry confirms the possibility to measure tension and compression of the pultrusion profile when the NiTi sensor wire is prestrained. 0,15
Acknowledgements This study was performed within the Federal Cluster of Excellence 1075 “MERGE Technologies for Multifunctional Lightweight Structures”. The authors gratefully acknowledge the financial support of the German Research Foundation (DFG). References [1]
[2]
[3]
0,1 0,5 0,05 0
0
strain in %
electrical resistance change in %
1
SMA wires with its high specific actuation energy density and its capability to be applied as strain sensors are integrated in a polymer matrix. The design of such adaptive composites with actuating functionality depends on anisotropic linearelastic and temperature-sensitive stress-strain behavior, thermal expansion coefficients, SMA phase transformation temperatures and on contact surface interaction of the composite components. It is preferable to use ambient heat energy for actuation. Sensing composites with integrated NiTi strain sensors offer a high gauge factor compared to conventional strain gauges. The pultrusion processing of sensing SMA wires offer lightweight design mass production with structural health monitoring capabilities. Stress and strain measurement during multiaxial bending loading is possible using NiTi SMA. A promising approach to lower the inertial mass of lightweight structures is the integration of functions.
[4]
-0,05 -0,5 -0,1
[5]
-0,15
-1 0
0,1
0,2
0,3
0,4
0,5
displacement in mm P1.1
P1.2
P2.1
P2.2
[6]
Fig. 6. Relative electrical resistance changes and strain curves depending on pultrusion profile bending displacement.
[7]
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