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NASA Technical mPaper - 3108

i-

A Novel Method of Testing the Shear Strength of Thick Honeycomb Composites

,,_±

i

IL

i

A. J. Hodge and

......

i

A. T. Nettles

NASA Technical Paper 3108 1991

A Novel Method of Testing the Shear Strength of Thick Honeycomb Composites

A. J. Hodge and

National Aeronautics and Space Administration Office of Management Scientific and Technical Information Division

A. T. Nettles

George

C. Marshall

Marshall

Space

Space

Flight

Flight

Center,

Center Alabama

TABLE

OF CONTENTS

Page I. II.

INTRODUCTION SPECIMEN i.

DESCRIPTION

Materials

2. Specimen Ill.

TEST l.

IV.

V.

Double

TEST

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

1 1

Fabrication

3

METHODS

Impact

1

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

Testing

2. Four-Point 3.

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

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

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

4

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

Bend

Testing

4

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

Lap Shear Testing

RESULTS

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

4

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

5

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

7

I. Impact Testing ...................................................................................... 2. Four-Point Bend Test ..............................................................................

7 7

3. Double

7

Lap Shear Test ............................................................................

CONCLUSION

APPENDIX-Photographs REFERENCES

........................................................................................ of Testing

Techniques

8

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

9

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

10

iii

I_AGEjLj_

INTENTIONALLY'_AI_

PRECED;NG

PAGE

BLANK

NOT

FILMED

LIST

OF ILLUSTRATIONS

Title

Figure 1.

Lap shear

2.

Fabrication

3.

Dimensions

4.

Method

5.

DLS test fixture

6.

Shear

Page

test ......................................................................................... of lap shear of four-point

of bonding

stress

specimen

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

bend tests ..............................................................

specimens

for DLS tests

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

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

versus

impact

energy

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

2 3 4 5 6 7

iv h

TECHNICAL

A NOVEL

PAPER

METHOD

OF TESTING

OF THICK

HONEYCOMB

THE

SHEAR

STRENGTH

COMPOSITES

I. INTRODUCTION

Determination

of the

shear

strength

of a honeycomb

core

and carbon-fiber

facing

composite

is

normally performed utilizing the American Society for Testing Materials (ASTM) standard ASTM C273, "'Shear Test in Flat Sandwich Constructions or Sandwich Cores." ASTM C393, "Flexure Test of Flat Sandwich

Constructions,"

recommended. thickness

The

and

specimens

ASTM

a width

may

can

also

be used

standard

calls

not less than twice

be too large

to fabricate

to determine

the shear

for a specimen

length

the specimen's

thickness.

and fit into laboratory

strength

[I],

not less than Thus,

scale

testing

but

ASTM

12 times

for thick

C273

the specimen's

honeycomb

machines.

is

The

cores,

honeycomb

core evaluated in this study, which was being tested for the Advanced Launch System (ALS) program, was 3.6-cm (1.4-in) thick and required a specimen length of at least 42.7 cm (16.8 in). Materials available

limited

the specimen

Previous there

studies

can still be damage

age in the form

of dents

length [I,2]

to 30.5

have

shown

to the core.

Cross

or delaminations

cm (12 that

in).

at low-impact

sections

of these

on the face

energies low-impact

sheets.

However,

with

no visible

energy

surface

specimens

a slight

ductile

damage

show buckle

no damoccurs

in

both types of core under the point of impact. Therefore, it is possible that an impact which causes no visible damage to the composite may damage the core sufficiently to reduce the composite's shear stress. The residual

shear

strength

of impacted

honeycomb

composites

with carbon

fiber

facesheets

and

aluminum and glass-phenolic cores was to be determined by two methods: four-point bend and lap shear tests. Since ASTM C273 could not be used to specifications, lap shear testing was attempted with smaller lengths.

When

this was performed,

(fig. 1). An alternate was devised.

method

the facesheets

of lap shear

testing

II. SPECIMEN

of the composite was

needed.

Thus,

tended

to peel

the "double

away

from

lap shear"

the core

(DLS)

test

DESCRIPTION

1. Materials Only tured tured

one carbon

fiber prepreg

system,

T300/934,

was used in the tests.

by Amoco and preimpregnated with Fiberite 934 resin. by Hexel were compared: glass/phenolic and aluminum.

(2 lb/ft3),

a cell size of 4.76

mm (3/16

in), and a thickness

The T300

was manufac-

Two types of honeycomb cores manufacBoth cores have a density of 314.3 N/m 3

of 35 mm [2]. The crush

strengths

ously determined to be 1,158 kPa for the aluminum and 896 kPa for the glass/phenolic [I]. film adhesive used to bond the honeycomb cores and the facesheets was EA9684 produced

were previThe epoxy by Hysol.

Lap Shear Test Configuration Honeycomb, ASTM C273

Face sheet peels

F

Figure

I. Lap

shear

test.

for

2. Specimen

Fabrication

The T300/934 figurations. A layer bond the face sheets programmable

platen

180 °C (355

°F),

When in) in length These

were release

figure

making

was made

into eight-ply

The composite cooled

lay-ups

to be

was heated

to room

the specimens

and 7.6 cm (3.0

The specimens--made the sarne dimensions. center,

press.

and then

fully cured,

beams

specimens,

prepreg

with (0,45,

- 45,90),

quasi-isotropic

con-

of epoxy adhesive film was placed between the facings and honeycomb in order to to the core [4]. The composite was vacuum bagged and pressurized at 80 kPa from a

film was placed

The beam's

bend between

tested

bend length

tested

was in the "L" direction

following

the honeycomb

were cut into beams

ASTM

C393

12.7 cm (5.0 in) apart.

for 2 hours

at

The outer

honeycomb

29.2

cm ( I 1.5

of the honeycomb.

guidelines.

core and the faceplate

with one eight-ply facing and one (0,90)_ four-ply Two incisions were then made on opposite facings

the incisions

cured

at 2.8 °C (5 °F) per minute.

to be four-point

in) in width. four-point

at 1.7 °C (3 °F) per minute,

temperature

For

as shown

the

DLS

in figure

2.

facing--were cut into beams of each 6.35 cm (2.5 in) from the was removed

as can be seen

in

2.

Release

Film

/

Incision Side

up to face

sheet

view

\ _..,,Incision 8.9 cm

-'_"-i -

up to face sheet 12.7 cm

c -!11111k1111 One Half of Lap Shear Specimen

Figure

2.

Fabrication

of lap shear

specimen.

III. TEST

1. Impact

Testing

Samples a Dynatup system.

were impacted

8200

drop

The impacting

pneumatically

2.

Four-Point

at various

weight tup's

impactor,

in place

Bend

Testing were tested

of Flat Sandwich Constructions." and the outer contact points were in) from

energy

levels

ranging

and data were

obtained

mass was t,2t

clamped

The specimens

(2.5

METHODS

the center--halfway

by two aluminum

for residual

plates

shear

stress

using

the Dynatup

of I. 27 cm (0.50

with

holes

utilizing

of 7.62

ASTM

between

the center of contact

and outer

force

to help distribute

730 data

acquisition

in). The specimens cm (3.0

standard

contact

were

in) in diameter.

C393,

3.8cm 25.8 cm

3.

Dimensions

of four-point

bend

points.

the force

.f

__

Figure

was done with

"'Flexure

Test

The inner force contact points were 3.8 cm (1.5 in) from the center, 12.7 cm (5.0 in) from the center. The impact points are located 6.35 cm

3.8 crn (1.5 in), were placed on the area crushing would not occur (fig. 3).

Impact Damage

kg with a diameter

from 0 to 8.6 J. hnpacting

tests.

Impact

Aluminum

so that

Damage

localized

tabs, core

3.

Double

Lap

Shear

Lap shear

testing

Testing of honeycomb

ever, due to the thickness Because the shear strength ing. The during

combination

the

testing

composites

and thick

core

was thus devised

in the test fixture the strength The specimen

component

of stress

men and the weaker the actual

strength

caused

to test the residual

nique was termed the DLS test. After two specimens eight-ply facing, they were bonded with the four-ply to find

ASTM

standard

C273.

a peeling

of figure

5 and pulled

of the individual should

be under

involved.

between

the facings

and core

shear

However, using

strength

of the composite.

were impacted at the same energy levels on the facesheets together (fig. 4). The sample was then

in an Instron

1125. The strength

obtained

was divided

stress

completely,

since two halves fails,

while

were

the average

C273

actually

has a tensile

bonded

observed

strength

one DLS speci-

should

be lower

C273.

damage

Four ply face

sheets

|

V

1

bonded together Two halves

_Impact

Figure

4.

by 2

or compressive

to make

[

ply face

This tech-

sample.

half of the DLS specimen if tested

shear

Impact

Eight

How-

I).

A new technique

placed

done using

of the honeycomb, the specimens could not be made to the proper length. after impact was needed, the original thin facings were used during the test-

of the thin facings (fig.

is normally

Method

of bonding

specimens

I

damage

for DLS

tests.

sheets

than

"Double Lap Shear"TestSetup All dimensions in inches. J

4.0

0.50\

I

3.0 0.50

±I

Dia 0.50

l 1

4.75

0 0

.25

0 0

:0

0

5.0

m

0

0

t 2.5

0 - -__t_ -

-

0.75

m

--..o _._

0.50

Figure

5.

DLS

test

fixture.

0

_t

4.5

IV.

1.

Impact

of impacted

Small

dents

could

occur

until

1.90

cores

samples rather

2.

be felt but not seen

energy

impacted

at that energy

failing

through

Bend

As expected, failed

glass/phenolic

3.

Damage

all failed

at 810 kPa and steadily specimen,

Lap

Four

sustained

however,

impact

strength

decreased

energy

J--failed

Shear

undamaged

increased

J, while

was

levels,

penetration

first

visible

damage

did not

with glass/

first occurring

than the glass/phenolic.

to the extent core crushed

facing.

damage

felt in samples

with visible

less damage

of the

at

At 5.2 J, for

that a hole the diameter of the beneath. The aluminum core

less face sheet and core damage,

with the core buckling

fracture.

the specimens

damage--5.2

Double

level

brittle

to actual

Test

of 1 to 2 J of impact

greatest

energy

received

visible

as low as 0.66

cores.

impact

the aluminum

none

levels

aluminum

the same

levels

from

at energy

with

at approximately

Four-Point

range

ranged

the glass/phenolic core samples had fiber breakage tup was punctured into the facing with the brittle

than

samples

specimens

J in the samples

1.95 J. At higher example, impacting

RESULTS

Testing

Damage

phenolic

TEST

damage,

due to core

decreased

failed

in shear

failure.

strength

The

undamaged

aluminum

core

to 750 kPa at 8.6 J. The undamaged

A sharp

decrease

(fig. 6), with a drop in strength

of over

at 530

at 700 kPa.

shear

in strength 100 kPa.

was

noticed

The specimen

in the with the

kPa.

Test DLS

aluminum

the strength

core

specimens

steadily

decreased

to 590 kPa at 4.2 J. Three

undamaged

failed

at an average

as exhibited

value

of 720 kPa.

in the four-point

glass/phenolic

specimens

bend

were tested

The

and failed

at an average value of 640 kPa, and again a sharp decrease in strength was observed at an impact of about I to 2 J. The composites's strength gradually decreased to 450 kPa at 5.4 J.

900

With

tests.

energy

o ooo

o

o

800 o

700

o

[] [] •

I I

600

[]

m 13

•

500

[]

•

O •

AI 4-pt bd GI/Ph DLS

•

AI DLS

• • •

I

2

0

Figure

GI/Ph 4-pt bd

[]

•

400

n

6.

• I

"

Shear

stress

!

I

4 6 Impact Energy versus

8

10

impact

energy.

(J)

7

V.

Although

much

scatter

observed.

The honeycomb

the impact

energy

a rapid decrease phenolic

core The

stress

in the data obtained

composites

increased.

with aluminum

The composites

in strength specimens

existed

between continued

DLS tests gave

lower

in the DLS tests is probably

CONCLUSIONS

cores

tended

with glass/phenolic

i and 2 J of impact to gradually values

for the shear

in shear

stress

tests,

to gradually

cores,

energy.

decrease

strength

With

a general

decrease

on the other hand, higher

impact

trend was

in strength tended

energies,

as

to have the glass/

in strength. than the four-point

due to the fact that two samples

bend

are bonded

tests.

together

The

lower

shear

and the weaker

of

the two fails. The DLS specimens also have less strength added by shear stress in the face sheets than the four-point bend specimens. The curves for impact energy versus shear stress, however, followed the same general

trend for both tests. Thus,

the modes

of failure

were similar,

and the tests are comparable.

APPENDIX PHOTOGRAPHS

OF TESTING

Four-Point

DLS

TECHNIQUES

Bend

USED

Test

Test 9

ORIGINAL BLACK

AND

WHITE

PAGE PHOTOGRAPH

REFERENCES

I

,

,

Nettles, Phenolic

A.T., Lance, D.G., and Hodge, and Aluminum Honeycomb Core

Nettles,

A.T.,

and

Graphite/Epoxy .

,

Face

Hexel International California, 1989. Loken, Seminar,

10

Hodge,

H.Y.:

"DuPont

SAMPE,

A.J.:

"Impact

Sheets.'"

35th

"'Bonded

Honeycomb

Aramids

Midwest

Testing

International

for Advanced

Chapter,

A.J.: "An Composite

March

Examination of Impact Damage in Glass/ Panels." NASA TP 3042, August 1990. of Glass/Phenolic

SAMPE

Sandwich

Construction."

Composites." 9,

1982.

Symposium,

Presented

Honeycomb April Hexel

Panels

2-5,

1990.

TSB

124,

at the Industrial

With

Dublin,

Composites

Report Documentation

National Aeronautics and Space Administration 1.

4.

Report

No.

2.

NASA

TP-3108

Title

Subtitle

and

A Novel Thick

7.

Method

of Testing

Honeycomb

Government

the Shear

Accession

Strength

Page

No.

Recipient's

5.

Report

Composites

Performing

12.

Sponsoring

Space

Agency

National

Performing

Organization

Code

8.

Performing

Organization

Report

and

No.

M-658

Flight

Center,

11.

Contract

13.

Type

or Grant

Alabama

35812 of Report

and

Technical

and Space

No.

Center

Address

by Materials

strength

in two ways; (DLS)

Administration

and Processes

composites before

14.

Laboratory,

the use of common

yielded

similar

Words

(Suggested

Composite Epoxy

Science

Sponsoring

and Engineering

Period

Covered

Paper Agency

Code

Directorate.

to the supplier's

values

The assessment

of sandwich

beams

was developed scale

fabrication

slightly for shear

by Author(s))

lower

core were

of shear

strength

and by a novel

tested

for

was performed

"double

specimens

and testing

possible.

The two techniques

values

of the two methods

shear

strength

could

lap shear"

so smaller

be used

thus

strength.

18.

Distribution

Statement

Unclassified--Unlimited

Tolerance

Classif.

(of this

Subject

Impact report)

Unclassified 1626

laboratory

data. The DLS test gave

damage:

honeycomb

Materials

Instrumented

FORM

technique

and glass/phenolic

Resins

Damage

Security

bend testing

testing

making

Key

of aluminum

and after impact

by four-point

test. This novel

but were closer

NASA

Unit

Abstract

shear

19,

Work

DC 20546

Sandwich

17.

No.

Notes

Prepared

16.

Space

Aeronautics

Supplementary

1991

and Address

Flight

Name

Washington, 15.

Name

C. Marshall

Marshall

Date

and A.T. Nettles

Organization

George

No,

6,

10.

9.

Catalog

March

of

Author(s)

A.J. Hodge

3.

Category:

24

Testing 20.

Security

Classif.

(of this

Unclassified

page)

21.

No.

of pages

16

22.

Price

A03

OCT 86

For sale by the National Technical

Information Service, Springfield, VA 22161-2171 NASA-L(mgley,

1991