Effects of Layer Waviness on the Compression Fatigue Performance of Thermoplastic Composite Laminates

8/19/2019 Effects of Layer Waviness on the Compression Fatigue Performance of Thermoplastic Composite Laminates

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ffects of laye r w avine ss on the com pression

fa t igue per formance of thermoplast ic

composi te l aminates

D a nie l O H a r e A d a m s * a n d M . W . H y e r t

*Department of Aerospace Engineering and Engineering Mechanics, Iowa State

University, Ames, Iowa 50011, USA

tDepartment of Engineering Science and Mechanics, Virginia Polytechnic Institute

and State University, Blacksburg, Virginia 24061, USA

Received 14 M ay 1993; revised 9 July 1993)

The influence of layer waviness on the compression fat igue response of carbon/polysulphone composite

laminates was studied. Specimens with a moderate level of layer waviness as well as wave-free

control specimens were cycled to fai lure at a variety of maximum stress levels to establish

S N

curves.

A one and a hal f decade loss of compression fat igue l i fe was observed for moderate layer wave

specimens as compared wi th the cont rol specimens. Brooming fai lure, character ized by through- the-

th ickness splaying of the layers and by num erous delaminat ions, was the comm on fai lure mode. T he

stress level corresponding to the 106 cycle run-out for these layer wave specimens was reduced to

approxima tely 45 of the s tatic compression s t rength of the wave-free laminate, as com pared wi th a

reduct ion to 75 for the cont rol specimens. M odera te layer wave specimens cycled to the

106

cycle

run-out showed n o evidence o f delaminat ion in the v icin i ty of the layer wave. Specimens wi th a m i ld

layer wave fai led in the gr ips away f rom the wave and exhibi ted fatigue li fe comp arable to the wave-

free specimens.

Keywords: defects; compression; thermoplastic)

L a y e r w a v i n e s s i s a m a n u f a c t u r i n g i m p e r f e c t i o n m o s t

c o m m o n l y o b s e r v e d i n t h i c k - s e c ti o n c o m p o s i t e l a m i -

n a t e s . T h i s i m p e r f e c t i o n i s c h a r a c t e r i z e d b y t h e o u t -

o f - p la n e u n d u l a t i o n o f a l a y e r o r a g r o u p o f l a y e rs w i t h in

a m u l t i d i r e c t i o n a l l a m i n a t e . W h i l e m o s t c o m m o n l y

o b s e r v e d i n c y l i n d r i c a l s t r u c t u r e s , l a y e r w a v i n e s s h a s

a l s o b e e n f o u n d i n t h i c k , f l a t l a m i n a t e s . T h e c a u s e s

o f l a y e r w a v i n e s s h a v e i n s o m e c a s e s b e e n i d e n t i f i e d ,

a n d t h e d e g r e e o f w a v i n e ss r e d u c e d b y a l t e r in g t h e

m a n u f a c t u r i n g p ro c e s s . H o w e v e r , i n m a n y a p p l i c a ti o n s ,

s o m e d e g r e e o f l a y e r w a v i n e s s r e m a i n s a n d m u s t b e

t o l e r a t e d w i t h i n t h e c o m p o s i t e s t r u c t u r e .

L a y e r w a v i n e s s h a s b e e n s h o w n t o r e d u c e s ig n if i-

c a n t l y t h e s t a t i c c o m p r e s s i o n s t r e n g t h o f c o m p o s i t e

l a m i n a t e s . G a r a l a 1 t e s t e d 1 5 m m ( 0 . 6 i n ) t h i c k c a r b o n /

e p o x y c y l i n d e r s u n d e r e x t e r n a l h y d r o s t a t i c p r e s s u r e

l o a d i n g . I n s o m e c a s e s f a i l u r e s w e l l b e l o w t h e d e s i g n

p r e s s u r e w e r e b e l i e v e d t o b e d u e t o la y e r w a v i n e s s .

A b d a l l a h e t a L E t e s t e d 2 5 m m ( 1 .0 i n ) w i d e c o m p o s i t e

t in g s u n d e r e x t e r n a l h y d r o s t a t i c p r e s su r e l o a d in g . T h e

l o w e s t f a i l u r e p r e s s u r e s a n d s t r a i n s w e r e a s s o c i a t e d

w i t h r i n g s c o n t a i n i n g t h e g r e a t e s t d e g r e e o f l a y e r

w a v i n e s s . F a i l u r e i n t h e s e r i n g s o c c u r r e d a t l o c a t i o n s

o f s e v e r e l a y e r w a v i n e s s. G a s c o i g n e a n d A b d a l l a h 3

f u r t h e r i n v e s t i g a t e d t h e s t r a i n f i e l ds in t h e v i c i n i ty o f

l a y e r w a v i n e s s u s i n g m o i r 6 i n t e r f e r o m e t r y . L a r g e

i n t e r l a m i n a r s h e a r s t r a i n s , n o n e x i s t e n t i n a w a v e - f r e e

r i n g , w e r e f o u n d n e a r t h e l a y e r w a v e i n f l e c t i o n p o i n t s .

S e v e r a l i n v e s t i g a t o r s h a v e a c c o u n t e d f o r w a v i n e s s

i n m o d e l s f o r s t at ic c o m p r e s s i o n s t r e n g t h . A m a j o r i t y

o f t h e m o d e l l i n g e f f o r ts h a v e f o c u s e d o n f ib r e w a v i n e s s

i n u n i d i r e c t i o n a l c o m p o s i t e s 4 -1 °. S w a n s o n 11 m o d e l l e d

i n p l a n e f i b r e w a v i n e s s i n m u l t i d i r e c t i o n a l l a m i n a t e s b y

i n c l u d i n g t h e l a t e r a l s u p p o r t o f f e r e d b y a d j a c e n t p l i e s

i n t h e l a m i n a t e . S h u a r t 12'13 i n c o r p o r a t e d l a y e r w a v i n e s s

i n t o a m i c r o b u c k l i n g m o d e l f o r m u l t i d i r e c t i o n a l l a m i -

n a t e s . B o t h i n p l a n e a n d o u t - o f - p l a n e w a v i n e s s w a s

c o n s i d e r e d . P e e l

e t a l .

14 e x t e n d e d t h i s s t u d y t o c o n s i d e r

a d d i t i o n a l o u t - o f - p l a n e l a y e r w a v e c o n f i g u r a t i o n s . B o g -

e t t i e t a l . 15.16 d e v e l o p e d a l a m i n a t e d p l a t e t h e o r y b a s e d

m o d e l t o p r e d i c t s ti f fn e s s a n d s t r e n g t h r e d u c t i o n s d u e

t o l a y e r w a v i n e s s. H y e r a n d B r o w n 17 a n d T e l e g a d a s

a n d H y e r l s,1 9 i n v e s t i g a t e d l a y e r w a v i n e s s e f f e c t s i n

t h i c k c r o s s p l y c o m p o s i t e c y l i n d e r s u s i n g f i n i t e e l e m e n t

ana l ys i s .

A l t h o u g h l a y e r w a v i n e ss e f f e c t s o n s t a ti c c o m p r e s s i o n

s t r e n g t h h a v e r e c e i v e d c o n s i d e r a b l e a t t e n t i o n , t h e

e f f e c t s o n c o m p r e s s i o n f a t i g u e p e r f o r m a n c e h a v e

n o t . H o w e v e r , c o m p r e s s i o n f a t i g u e t e s t i n g h a s b e e n

p e r f o r m e d o n c o m p o s i t e l a m i n a t e s w i t h o t h e r t y p e s o f

s t r e ss c o n c e n t r a t i o n s , i n c l u d i n g p l y d r o p - o f f s2° , i mpact

d a m a g e 2 1- 23 , a n d o p e n h o l e s 2 4- 27 . S l a u g h t e r a n d

F l e c k E S d e v e l o p e d m i c r o b u c k l i n g m o d e l s f o r c o m -

p r e s s i o n f a t i g u e l o a d i n g w h i c h i n c l u d e i n i t i a l f i b r e

m i s a l i g n m e n t .

I n a r e l a t e d p u b l i c a t io n , A d a m s a n d H y e r 29 r e p o r t e d

o n t h e s t a t i c c o m p r e s s i o n t e s t i n g o f t h e r m o p l a s t i c

c o m p o s i t e l a m i n a t e s w i t h a n i n t e n t i o n a l ly f a b r ic a t e d ,

0142-1123/94/060385-07

© 1994 Butterworth Heinemann Ltd

F a t i g u e , 1 9 9 4 , V o l 1 6 , A u g u s t 3 8 5



Thermoplastic composite laminates: D. O Hare Adams and M. W. Hyer

isolated layer wave. The laminate chosen was a 22 ply

[902/02/902/02/902/02w]S laminate with the layer wave

fabricated into the central 0° layer. The wavy layer is

designated as 02w, the overbar indicating this layer is

not repeated in the symmetric stacking sequence. A

three-step procedure was used to fabricate the isolated

layer wave into the composite laminates. Layer wave

geometries up to 1.5 layer thicknesses in amplitude

and as short as 9 layer thicknesses in length were

tested. The more severe wave geometries were shown

to produce reductions in static compression strength

as high as 35 , although the wavy 0 ° layer accounts

for only 20 of the load carrying capacity of the

laminate.

In the present study a similar methodology was

used to investigate the effects of layer waviness

on compression fatigue response. Specimens with a

moderate layer wave geometry were cycled to failure

at a variety of maximum stress levels to establish an

S - N curve for this particular level of layer waviness.

In addition, wave-free control specimens were used to

develop an

S - N

curve corresponding to the case of

no layer waviness. Thus, the reductions in compression

fatigue life associated with a specific layer wave

geometry were established. An overview of the results

of this study follows.

COMPRESSION FATIGUE TESTING

The wavy layer specimens were fabricated by the

method described in ref. 29. The fabrication of

specimens with a known and controlled level of

waviness is an interesting issue in its own right. The

reader is encouraged to consult ref. 29 for details.

Experimental setup

Compression fatigue testing on the wavy layer

was performed using a test fixture designed and

manufactured at NASA Langley Research Center3°.

This fixture, shown in Figure 1 consists of two massive

steel blocks with U-shaped cavities and which are

aligned by four rods and linear bearings. The com-

pression fatigue specimen is placed between end

loading plates within the cavity of each block. The

thickness of each end loading plate is machined to be

slightly less than the specimen thickness. Gripping

surfaces on the rear of the cavities and on the two

cover plates are lightly serrated. Four bolts are used

to secure each cover plate. Thus, the ends of the

specimens are supported along a portion of their

SerrateO

g t l p p m g

stlrface ,,

over

D~ates

AhgmHeli{t~) ls

End

l o a d i n g

pla les

I

100 mm

Figur e 2 Com pr ess ion f a t igue t e s t spec ime n configu r a t ion

length and loaded th rough the end loading plates. The

serrated gripping surfaces allow for limited load transfer

through shear-loading of the supported portions of the

specimen faces. In Figure 1 the top cover plate is in

place whereas the bottom cover plate is removed,

revealing the bottom end loading plate.

The specimen configuration used in compression

fatigue testing is shown in Figure 2. The 102 mm (4.0

in.) long by 25 mm (1.0 in.) wide specimens were cut

using a water-cooled diamond saw. Specimen ends

were cut as fiat and parallel as possible to ensure that

the compression load was introduced uniformly. The

layer wave was centred along the length of each

specimen. Use of 102 mm (4 in) long specimens

resulted in an unsupported gage length of 25 mm (1.0

in). With specimen thicknesses of approximately 4.3

mm (0.17 in), the length-to-thickness ratio for the

gauge section was less than 6. As a result buckling

was not an issue.

A total of nine 152 mm (6 in) square T300/P1700

laminates were fabricated, all of which were 22 ply

[902/02/902/02/902/02w]S laminates. Six laminates were

fabricated with layer waviness and t hree were fabricated

wave-free. All laminates were ultrasonically C-scanned

to ensure laminate quality prior to cutting into test

specimens.

The layer wave geometry was characterized separ-

ately for each specimen. The wavy region of each

specimen was photographed under a microscope at

low magnification. From photographic enlargements,

the wavelength, h, and wave amplitude, B, were

measured as shown in F igure 3 . In this figure, t

denotes the thickness of the two-ply wavy layer (ca.

0.4 mm, 0.016 in). The layer wave 'severity' parameter

B/h was calculated for each specimen. Intuitively, a

more 'severe' wave implies a shorter wavelength, h,

coupled with a larger wave amplitude, B, and thus a

larger value of B/X.

The variation in wave geometries among the six

laminates was less than the variation obtained in the

laminates used for the static tests 29. In fact, layer

wave geometries with B/h ratios between 0.05 and

0.06, namely moderat e waves, were produced in five

of the six laminates. A representative layer wave

v

Figur e 1 Com pr ess ion f a t igue t e s t f i x tu r e

F igur e 3 Def in i t i on o f laye r wave geome t r y pa r ame te r s

3 8 6 F a ti g u e 1 9 9 4 Vo l 1 6 Au g u s t




Ta b l e 1 C o m p re ss i o n fa t i g u e t e s t r e su l t s f ro m c o n t ro l sp e c i me n s

M a x . s t r e s s C y c l e s t o Fa i l u re

L a m i n a t e S p e c i m e n M P a / k s i ) f ai l ur e l o c a t io n

1 1 552/80 297 886 Gr ip

2 552/80 183 507 Gr ip

3 586/85 18 734 Gr ip

4 5 8 6 / 8 5 1 8 4 6 G r i p

2 1 586/85 448 Ga uge sec t ion

2 4 8 3 / 7 0 3 7 2 0 0 9 G r i p

3 5 1 7 / 7 5 9 0 7 5 G r i p

4 483/70 429 864 Gr ip

5 4 4 8 / 6 5 5 7 4 0 9 1 G r i p

3 1 552/80 18 248 Gr ip

2 483/70 19 783 Gr ip

3 414/60 1000 000 -

4 5 1 7 / 7 5 1 4 7 8 4 G r i p

5 448/65 43 285 Gr ip

F i g u r e

4 M o d e r a t e la y e r - w a ve g e o m e t r y

geometry from this group is shown in Figure 4. The

specimens from these laminates became the primary

group for the purpose of generating fatigue life (S-

N) data. The layer wave geometry in the remaining

laminate was much less severe, with a G/h ratio of

approximately 0.02. This layer wave geometry, referred

to as a mild wave, is shown in Figure 5. The specimens

from this laminate became the secondary group for

fatigue testing.

Testing was performed under load control using an

Instron servo-hydraulic load frame. Specimens were

placed in the fatigue fixture and preloaded with a

small compression load prior to installing the cover

plates. The four inner bolts were tightened to 81 N

m (60 ft lbs) whereas the outer bolts were tightened

to 41 N m (30 ft lbs). Displacement limits on the

actuator stroke of the test machine were set as tightly

as possible to avoid post-failure crushing of specimens.

All fatigue testing was performed with a stress ratio,

R, of 10. Thus, the maximum compression stress

during each loading cycle was 10 times the minimum

compression stress. Specimens were cycled at a fre-

quency of 5 Hz using a sinusoidal waveform. This

cycling frequency resulted in only a 2°C (4°F) increase

in surface temperature o f the specimen. All specimens

were cycled either to failure, or to 106 cycles, a value

which was considered a run-out in this study.

The static compression strength of the 22 ply [902/

02/902/02/902/02w]s laminate with no waves was deter-

mined using the IIT RI compression tes t 29. Specimens

were cut from two additional laminates to the same

dimensions as those for compression fatigue testing.

However, glass/epoxy tabs were bonded to the IITRI

test specimens, resulting in a 25 mm (1.0 in) guage

length. Based on nine tests, the static compression

strength was determined to be 608 MPa (88.2 ksi).

each control specimen is presented in Table 1. These

results are presented as an

S N

diagram in

Figure 6.

The maximum applied stress, trmax, shown in Figure 6

is nondimensionalized by dividing by the average static

compression strength of laminates with no waves,

namely -608 MPa (-88.2 ksi). The numbers shown

with the data in Figure 6 correspond to the laminate

number from which the specimens were cut. Results

indicate a 106 cycle run-out strength of approximately

75 of the average static compression strength. As

can be seen, considerable scatter was present in the

data, partly due to laminate-to-laminate variations.

For example, specimens from laminate 1 showed

consistently better fatigue resistance than specimens

from laminates 2 and 3. Failures within a grip occurred

in all but one of the control specimens cycled to

failure.

Specimens with layer waviness were separated into

two groups for testing. The primary group, totalling

24 specimens from laminates 1 to 5, had G/h ratios

ranging from 0.05 to 0.06, the moderate wave geometry.

These specimens were cycled at maximum stress levels

ranging from -552 MPa (-80 ksi) to -276 MPa (-40

ksi) in an effort to produce failures in the range of

103 to 106 cycles. The secondary group, consisting of

Experimental resul ts

A total of 44 specimens were tested in fatigue; 14

control specimens with no layer waviness and 30

specimens with layer waviness. The 14 control speci-

mens were tested at maximum stress levels ranging

from -586 MPa (-85 ksi) to -414 MPa (-60 ksi) in

an effort to produce failures in the range of 103 to

106 cycles. The maximum applied compression stress

and the corresponding number of cycles to failure for

Fi g u re 5 M i l d l a y e r -w a v e g e o m e t ry

0.8

.6

t a t i c / 0 . 4

0.2

Fi g u re 6

i 2 1

•

2 0 3 • •

0 0 3

0 3 3 2 W 2

• 2 0

3 • . ~

Numbers : Lam ina te Number , Bracke t: Range o f S ta t i c Da ta )

I I I I I I

101 102 103 104 10 s 10 s 107

N, number o f c yc les

C o mp re ss i o n fa t i g u e re su l t s o f c o n t ro l sp e c i me n s

F at ig u e 1994 V o l 16 Au g u st 387



Thermoplastic composite laminates: D. O Hare Ada ms and M. W. Hyer

f ive spec imens f rom lamina te 6 , had

~/h

ra t io s o f

a p p ro x i m a t e l y 0 .0 2 , t h e m i l d wa v e g e o m e t ry . T h e s e

spec imens w ere cyc led a t max im um s t ress leve l s rang ing

f r o m - 5 5 2 M P a ( - 8 0 k s i ) t o - 4 4 8 M P a ( - 6 0 k s i ) .

R e s u l t s o f c o m p re s s i o n f a t i g u e t e s t i ng fo r s p e c i m e n s

wi th l ayer wav iness a re p resen ted in

Table 2.

T h e

l a y e r wa v e s e v e r i t y p a ra m e t e r g/h , t h e m a x i m u m

comp ress ion s t ress , num ber o f cyc les to fa i lu re , and

loca t ion o f fa i lu re is p res en te d fo r each spe c imen . A

'wave ' fa i lu re loca t ion re fers to a fa i lu re wi th in the

ac tua l l ayer wave , whereas a ' g r ip ' fa i lu re re fe rs to a

fa i lu re beyond the gauge sec t ion , wi th in the g r ipped

por t ions o f the spec im en . G enera l ly , these g r ip fa ilu res

o c c u r r e d w i t h in a s h o r t d i s t a n c e b e y o n d t h e e n d s o f

t h e g a u g e s e c t i o n . A l l b u t o n e s p e c i m e n f ro m t h e

p r i m a ry g ro u p w i t h m o d e ra t e l a y e r wa v e g e o m e t r i e s

( s p e c i m e n s f ro m l a m i n a t e s 1 -5 ) f a i l e d a t t h e l o c a t i o n

o f t h e l a y e r wa v e . F o u r o f t h e f i v e s p e c i m e n s f ro m

t h e s e c o n d a ry g ro u p w i t h m i l d l a y e r wa v e g e o m e t r i e s

(spec imens f rom lamina te 6 ) fa i l ed in the g r ip . The

la t te r resu l t ind ica tes tha t th i s mi ld l eve l o f wav ine ss was

of equa l o r l es s sever i ty than the s t ress concen t ra t ion

p ro d u c e d b y t h e g ri p s. T h u s , t h e s e c o n d a ry g ro u p o f

spec imens was o f l imi ted use fo r assess ing the e f fec t s

o f th e m i ld l a y e r wa v e g e o m e t ry

8 / h

= 0 .02) on

compress ion fa t igue l i fe .

R e s u l t s f ro m t h e p r i m a ry g ro u p o f s p e c i m e n s w i t h

m o d e ra t e l a y e r wa v e g e o m e t r i e s a r e p r e s e n t e d a s a n

S - N d iag ram in Figure 7. On c e a g a i n , t h e n u m b e r o f

t h e l a m i n a t e f ro m wh i c h e a c h s p e c i m e n wa s c u t i s

Tabk 2 Compress ion fatigue test results ~om layer-waviness

specimens

Max. stress Cycles to Failure

Laminate Specimen 8/h MPa ksi) failure location

1 1 0.0 51 552/55 241603 Wave

2 0.053 552/60 34907 Wave

3 0.054 586/50 199856 Wave

4 0.052 586/70 14605 Wave

5 0.053 547/79 1 Wave

2 1 0.054 586/40 1000000 -

2 0.053 483/70 67946 Grip

3 0.053 517/80 444 Wave

4 0.055 483/60 96641 Wave

5 0.0 51 448/70 551 Wave

3 1 0.055 552/50 219081 Wave

2 0.055 483/60 35812 Wave

3 0.054 414/80 33 Wave

4 0.053 517/65 14172 Wave

5 0.050 448/60 46068 Wave

4 1 0.053 552/60 316814 Wave

2 0.058 483/40 1000000 -

3 0.059 414/60 164323 Wave

4 0.054 517/50 762264 Wave

5 0.0 51 448/70 40556 Wave

5 1 0.044 552/70 20736 Wave

2 0.052 483/50 321993 Wave

3 0.055 414/80 120 Wave

4 0.056 517/70 5258 Wave

5 0.056 448/60 10367 Wave

6 1 0.020 552/80 52 Wave

2 0.019 483/70 494 792 Grip

3 0.018 414/75 288 086 Grip

4 0.020 517/70 147 963 Grip

5 0.022 448/60 925 664 Grip

0 . 8

( . ~ _ , o 6

laave I

\ s t a t i c / 0 . 4

0 2

3 z x r - i5 Z ~ 2 ~ .

2

,0,

S 0 0 n

3 , ~

0 2 4 4 •

1 3 3 1

2 4

oderate Mild

Layer Wave J LayerW a v e

I 7

10.05< 8 /) ~ < . 0 6 I ~ = 0 . 0 ~ C

rol

F a t i g u e l z ~ I n

ta t ic a v e . ) I • I

N u m b e r s n d ic a t eL a m i n a t eN u m b e r )

. . . . . . _ 1 . . . . . . . i . . . . . . _ 1 . . . . . . . i . . . . . . _ 1 ~ m l . . . - I i I l l ' '

1 0 1 1 0 2 1 0 3

104 105 10 s 107

N , n u m b e r o f c y c le s

igure 7 Compress ion fatigue results of control and layer-wave

specimens

i n d i ca t e d . R e s u l t s f ro m t h e c o n t ro l s p e c i m e n s a n d t h e

s e c o n d a ry g ro u p o f s p e c i m e n s w i t h m i l d l a y e r wa v e

g e o m e t r i e s a r e p r e s e n t e d fo r c o m p a r i s o n . T h e a v e ra g e

s t a t i c c o m p re s s i o n s t r e n g t h f ro m t wo s p e c i m e n s w i t h

va lues o f

8/h

b e t we e n 0 .0 5 a n d 0 .0 6 t h a t f a i l e d a t t h e

layer wave i s a l so shown on the d iag ram (so l id

t r i ang le) . Resu l t s c lear ly show a reduct ion in the

c o m p re s s i o n f a t i g u e l i f e d u e t o t h e m o d e ra t e l a y e r

wa v e . R e l a t i v e t o t h e c o n t ro l s p e c i m e n s , a o n e a n d a

ha l f decade loss o f fa t igue l i fe was observed in the

range of 103 to 106 cycles . The 106 cycle s trength of

t h e s p e c i m e n s w i t h m o d e ra t e l a y e r wa v i n e s s wa s

re d u c e d t o a p p ro x i m a t e l y 4 5 o f t h e s t a ti c c o m p re s s i o n

s t r e n g th o f th e c o n t ro l s p e c i m e n s . On c e a g a i n, a

c o n s i d e ra b l e a m o u n t o f s c a t te r wa s s e e n i n t h e d a t a ,

due in par t to l amina te- to - lamina te var ia t ions . The

grea tes t fa t igue l i fe was assoc ia ted wi th spec imens

f ro m l a m i n a t e 4 , wh e re a s s p e c i m e n s f ro m l a m i n a t e 1

e x h i b i t e d b e l o w a v e ra g e f a t i g u e l i f e . T h e s e c o n d a ry

g ro u p o f l a y e r wa v e s p e c i m e n s w i t h m i l d l a y e r wa v e

g e o m e t r i e s e x p e r i e n c e d f a t i g u e l i f e c o m p a ra b l e t o t h e

con t ro l spec imens . Again , th i s resu l t sugges t s tha t the

p re s e n c e o f t h e m i l d l a y e r wa v e wa s n o wo r s e t h a n

the in f luence o f the g r ips .

T wo f a i l u r e m o d e s we re o b s e rv e d i n th e c o m p re s s i o n

fa t i g u e s p e c i m e n s , e a c h m o d e c o r r e s p o n d i n g t o a

d i f fe ren t fa i lu re loca t ion . Spec imens fa i l ing a t the

l o c a t i o n o f t h e l a y e r wa v e e x p e r i e n c e d a b ro o m i n g

m o d e , a s s h o wn i n

Figure 8.

T h e s e f a i l u r e s we re

charac te r ize d by th rough- the- th ick ness sp lay ing o f the

l a y e rs a n d n u m e ro u s d e l a m i n a t i o n s . T h e a c t u a l d e g re e

o f o u t -o f -p l a n e b ro o m i n g v a r i e d g re a t l y a m o n g s p e c i -

m e n s , a n d wa s b e l i e v e d t o b e d e p e n d e n t o n t h e

a m o u n t o f po s t - f a il u r e a c t u a t o r m o t i o n o f t h e l o a d

f r a m e a n d s u b s e q u e n t c ru s h i n g o f th e s p e c i m e n p r i o r

t o s t o p p a g e . An a n g l e d f r a c t u re s u r f a c e t h ro u g h t h e

th ickness was observed in near ly a l l o f the fa i l ed

s p e c i m e n s . T h e s e f r a c t u re s u r f a c e s we re fo u n d t o h a v e

prefer red o r ien ta t ions , pass ing th rough an in f lec t ion

p o i n t o f t h e wa v y l a y e r a n d a n g l i n g a wa y f ro m t h e

cen t ra l wave t rough , as i l lu s t ra ted in

Figure

9 . In mos t

s p e c i m e n s , e a c h 0 ° l a y e r wa s f r a c t u re d i n o n l y o n e

loca t ion . Spe c ime n fa i lu re a t the loca t ion o f the l ayer

wa v e wa s s u d d e n a n d c a t a s t ro p h ic . I n n o i n s ta n c e wa s

3 8 8 F a t ig u e 1 9 9 4 V o l 1 6 A u g u s t




Figure Post test condition of moderate layer wave specimen

igure

9 Pre ferred angular orientations of failure planes in layer-

wave specimens

the in i t i a t ion o f fa i lu re de tec te d aud ib ly , v i sua l ly , o r

by ac tua to r s t roke o r load l imi t s such tha t t es t ing

c o u l d b e s t o p p e d a n d t h e s p e c i m e n e x a m i n e d fo r

f a i lu r e i n it ia t io n . T h e re fo re , t h e s e q u e n c e o f e v e n t s

lead ing to f ina l fa i lu re cou ld no t be read i ly de te rmined .

On l y i n f e r e n c e s we re p o s s i b l e b a s e d o n p o s t - f a i l u r e

o b s e rv a t i o n o f s p e c i m e n s , p a r t i c u l a rl y t h o s e s p e c i m e n s

d i s pl a y in g a m i n i m a l d e g re e o f b ro o m i n g . Ho we v e r ,

no cons i s ten t s imi la r i t i es in de lamina t ion pa t te rns

e x i s t e d a m o n g t h e r e l a t i v e l y f e w s p e c i m e n s wh i c h

e x p e r i e n c e d l i m i t e d b ro o m i n g .

T h e r e m a i n i n g l a y e r wa v e s p e c i m e n s , a n d a l l b u t

one con t ro l spec imen , fa i l ed wi th in the g r ip and

e x p e r i e n c e d a f a i l u r e m o d e c o n s is t in g o f l o c a li z e d

buck l ing o f eac h o f the 0 ° layers . The buck led 0 °

l a y e rs fo rm e d a n g l e d f r a c t u re b a n d s t h ro u g h t h e

t h ic k n e s s o f th e s p e c i m e n , a s s h o wn i n

Figure 10.

In

some ins tances , a s ing le f rac tu re band

Figure lOa)

wi t h i n t h e g r i p p ro d u c e d a d e t e c t a b l e i n c re a s e i n t h e

a c t u a t o r s t ro k e d u r i n g t e s t i n g , e x c e e d i n g t h e s t ro k e

l imi t and s topp ing the t es t . In o ther spec imens ,

s u b s e q u e n t f r a c t u re b a n d s a p p e a r t o h a v e fo rm e d

wi th in the g r ip reg ion

Figure lOb).

I f l o a d i n g wa s

n o t s t o p p e d p r i o r t o t h e f r a c t u re b a n d s r e a c h i n g t h e

u n s u p p o r t e d g a u g e s e c t i o n , a b ro o m i n g f a il u r e o c c u r r e d

a t t h e e d g e o f t h e g r i p , a s s h o wn i n Figure 10c.

Six spec imens fa i l ing wi th in the g r ip , th ree l ayer

wa v e s p e c i m e n s a n d t h r e e c o n t ro l s p e c im e n s , a n d t h e

t h re e I06 c y cl e ru n -o u t s p e c i m e n s we re s u b s e q u e n t l y

X- ra y e d t o i n v e s ti g a t e t h e e x t e n t o f d a m a g e . T h e s e

s p e c i m e n s we re t r e a t e d w i t h a z i n c i o d i d e p e n e t r a n t

p r i o r t o X- ra y t o e n h a n c e d a m a g e d e t e c t i o n . I n t e rn a l

d a m a g e r e g i o ns r e a c h e d b y t h e p e n e t r a n t p r o d u c e d

d a rk r e g i o n s i n t h e p h o t o g ra p h s . R e s u l t s f ro m X-

ray ing the s ix spec imens fa i l ing wi th in the g r ip a re

Figure 10 Post- test condition of specimens failing at the grip: a)

single failure band ; b) m ultiple failure band s; c) multiple failure

bonds, brooming failure

p re s e n t e d i n

Figure 11. The

gr ip reg ion fa i lu res ,

c h a ra c t e r iz e d b y d a rk b a n d s e x t e n d i n g a c ro s s t h e w i d t h

of the spec imen , a re c lear ly v i s ib le in each spec imen .

E a c h d i s ti n c t b a n d c o r r e s p o n d e d t o a l o c a li z e d b u c kl i ng

fa i lu re wi th in on e o f the 0 ° l ayers . Fo r the th r ee w avy

layer spec imens which fa i l ed a t the g r ip , there was no

e v i d e n c e o f d a m a g e i n t h e g a u g e s e c t i o n a s a r e s u l t

o f t h e l a y e r wa v e . T h u s i t wo u l d a p p e a r t h a t f a i l u re

i n t h e wa v y r e g i o n , wh e n i t d o e s o c c u r , i s s u d d e n a n d

i s n o t p r e c e d e d b y d a m a g e a c c u m u l a t i o n . R e s u l t s f ro m

X-ray ing the th re e 106 cyc le run -o u t spe c imen s , two

l a y e r wa v e s p e c i m e n s a n d o n e c o n t ro l s p e c i m e n , a r e

p re s e n t e d i n

Figure 12.

No ev ide nce o f fa i lu re in i ti a t ion

wa s s e e n i n a n y o f th e s e t h r e e s p e c i m e n s e i t h e r. T h e

t wo l a y e r wa v e s p e c i m e n s s h o we d n o v i s u a l e v i d e n c e

o f d e l a m i n a t i o n o r o t h e r d a m a g e i n t h e v i ci n it y o f t h e

l a y e r wa v e .

In a l l n ine o f the spec imens X-rayed , se t s o f th in

p a ra l l e l l i n e s we re o b s e rv e d b o t h p e rp e n d i c u l a r a n d

para l le l to the load ing d i rec t ion . These l ines were

s u b s e q u e n t l y d e t e rm i n e d t o b e t r a n s v e r s e c r a c k s w i t h i n

the ind iv idua l l ayers o f the l amina te . T ran sverse c rack

dens i t i es in the 90° l a y e r s p e rp e n d i c u l a r t o t h e l o a d i n g

d i rec t ion ) we re no t iceab ly h igher than in the 0 ° l ayers

p a ra l l e l t o t h e l o a d i n g d i r e c t i o n ) . T h e p re s e n c e o f

t h e l a y e r wa v e d i d n o t a p p e a r t o a f f e c t t h e c r a c k

dens i ty .

F at ig u e 1994 V o l 16 Au g u st 389



T h e rm o p l as t ic c o m p o s i t e l a m i n a t e s : D . O H a r e A d a m s a n d M . W . H y e r

372 009 19 783 14 784 67 946 5258 I 0 367

Co ntro l specimens .... ~ ~ La ye r wav e specimens

Num bers i nd i ca te cyc les t o f a i l u re

Figure 11 X-ray results for spe imens exhibiting grip failure

G r i p r e g i o n

Gauge sec t ion

G r i p r e g i o n

~:~,

G r i p r e g i o n

Gauge sec t ion

G r i p r e g i o n

Con t ro l

specimen

Laye r wave spec im ens

Figure 12 X-ray results for run-out specimens (10~ cycles)

C O N C L U S I O N S

C o m p re s s i o n f a t i g u e s p e c i m e n s f a b r i c a t e d f ro m T 3 0 0 /

P 1 7 0 0 a n d w i t h m o d e ra t e l a y e r wa v e s (8 / k r a t i o s

b e t w e e n 0 .05 a n d 0 .0 6 ) e x h i b i t e d a o n e a n d a h a l f

d e c a d e l o s s o f c o m p re s s i o n f a t i g u e l i f e a s c o m p a re d

wi t h s p e c i m e n s w i t h o u t l a y e r wa v i n e s s . T h e s t r e s s

leve l co rresp ond ing to the 106 cyc le run -ou t fo r these

l a y e r wa v e s p e c i m e n s wa s r e d u c e d t o a p p ro x i m a t e l y

4 5 o f t h e s t a t ic c o m p re s s i o n s t re n g t h o f t h e wa v e -

f r e e l a m i n a t e . C o m p re s s i o n f a t i g u e s p e c i m e n s w i t h

m o d e ra t e l a y e r wa v e s f a i le d a t t h e l o c a t i o n o f t h e

l a y e r wa v e in a s u d d e n , u n d e t e c t e d m a n n e r . B ro o m i n g

fa i lu re , charac te r ize d by th rough - the- th ickn ess sp lay ing

o f t h e l a y e r s a n d b y n u m e ro u s d e l a m i n a t i o n s , wa s t h e

c o m m o n f a i l u r e m o d e . S p e c i m e n s w i t h a m i l d l a y e r

wa v e (8 / k = 0 .0 2 ) f a i l e d i n t h e g r i p s a n d e x h i b i t e d

fa t ig u e l if e c o m p a ra b l e t o t h e c o n t ro l s p e c i m e n s . L a y e r

wa v e s p e c i m e n s c y c l e d t o t h e 106 c y c l e ru n -o u t s h o w e d

n o e v i d e n c e o f d a m a g e a c c u m u l a t i o n o r d e l a m i n a t i o n

i n th e v i c i n it y o f t h e l a y e r w a v e . F o r m o re i n fo rm a t i o n

o n t h e r e s u l ts o f t h e s t u d y , t h e r e a d e r i s r e f e r r e d t o

ref. 31.

A C K N O W L E D G E M E N T S

T h i s wo rk w a s s u p p o r t e d b y t h e V i rg i ni a In s t i t u te fo r

M a t e r i a l S y s t e m s , t h e C u n n i n g h a m F e l l o ws h i p P ro g ra m

a t V i rg i n ia T e c h , a n d O f f i c e o f Na v a l R e s e a rc h G ra n t

N00614-90-J -1688 , the Un ivers i ty Research In i t i a t ive

P ro g ra m . T h e f in a n c ia l s u p p o r t o f t h e s e s o u rc e s is

a p p re c i a t e d .

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