Acoustic Fatigue Strength of Fiber-Reinforced Plastic Panels

23 Acoustic Fatigue Strength of Fiber-Reinforced Plastic Panels T. FUJII and T. FUKUDA Department of Mechanical Engineering, Osaka City University, Sumiyoshi, Osaka 558, Japan

Downloaded by RUTGERS UNIV on May 29, 2018 | https://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch023

S. IIDA and M. SANO National Aerospace Laboratory, Chofu, Tokyo 182, Japan

Composite m a t e r i a l s are used f o r many aircraft p a r t s , i n c l u d i n g structural members which a l l o w f o r the p o s s i b l e development o f an all composite aircraft ( 1 ) . The increased power of aircraft engines over the past three decades has r e s u l t e d in a major social and scientific problem - a e r o n a u t i c a l n o i s e . A c o u s t i c f a t i g u e of the aircraft s t r u c t u r e occurs due to the m e r c i l e s s hammering of the f l u c t u a t i n g sound pressures (2). In Japan f a t i g u e s t u d i e s o f composite m a t e r i a l s is limited, although a program is in progress at the U n i v e r s i t y of Southhampton, ISVR, which is concerned w i t h the a c o u s t i c f a t i g u e of carbon fiber r e i n f o r c e d panel-type s t r u c tures (3). Consequently, an immediate need e x i s t s to develop fundamental data based upon a c o u s t i c f a t i g u e t e s t s f o r composite materials. Four kinds of FRP panels w i t h three l a y e r s c o n s i s t i n g of r o v i n g g l a s s c l o t h and/or g l a s s - f i b e r mat r e i n f o r c e d unsaturated p o l y e s t e r r e s i n are a c o u s t i c a l l y e x c i t e d and f a t i g u e t e s t data obt a i n e d . An observation was made from the t e s t s that the g l a s s f i b e r mat r e i n f o r c e d t e s t panel may be more i s o t r o p i c i n the f a i l ure p a t t e r n than the r o v i n g g l a s s c l o t h r e i n f o r c e d t e s t panels. The experimental r e s u l t s i n d i c a t e that the a c o u s t i c f a t i g u e s t r e n g t h of FRP t e s t panels i s considerably lower than the f a t i g u e s t r e n g t h of FRP m a t e r i a l s t e s t e d under constant s t r e s s . Making c e r t a i n assumptions the a c o u s t i c f a t i g u e l i f e of t e s t panels can be p r e d i c t e d from the experimental data of f a t i g u e s t r e n g t h of FRP obtained under constant s t r e s s t e s t i n g . Measuring any increase of a c o u s t i c f a t i g u e s t r e n g t h r e q u i r e d bonding CFRP on the middle p a r t of a panel or l a m i n a t i n g carbon f i b e r woven tapes simultaneously during panel f a b r i c a t i o n . The l a t t e r s t i f f e n i n g method proved more e f f e c t i v e . A c o u s t i c Fatigue Tests Test Panel. As shown i n Figure 1, the shape of each t e s t panel i s r e c t a n g u l a r and i d e n t i f i e d w i t h one bay of the a i r c r a f t body. Table I shows p r o p e r t i e s of t e s t panels which are laminated 0-8412-0567-l/80/47-132-305$05.00/0

©

1980 American Chemical Society

May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

306

RESINS F O R A E R O S P A C E


Strain gage .

416

ό- o 1 r I

Î

1 Γ

CM

* 2

»•

_

356

ί Η

Journal of the Japan Society for Composite Materials

Test panel (5)

Figure 2, The rehtion between rms stress and sound pressure level (5): (Φ) R3, (A) RMR, (A) MRM, (Ο) M 3 , (Q, Δ, Δ, Θ) estimated at 159 dB

Γf Η

c 1 L -o?

Figure 1.

*

Τ"

m-

Ï

Test panel

0

1

140 150 160 130 Sound Pressure Level (dB) Journal of the Japan Society for Composite Materials


23.

Fujii E T AL.

Acoustic

Fatigue

307

Strength

by hand l a y - u p method a n d c u r e d f o r 24 h o u r s a t 60°C. The f o u r k i n d s o f t e s t p a n e l s a r e R3, RMR, MRM, a n d M3 w h e r e R i s t h e g l a s s r o v i n g c l o t h , M t h e g l a s s - f i b e r mat, 3 t h e number o f l a y e r s , a n d s p e c i f i c i d e n t i f i c a t i o n r e p r e s e n t s the sequence o f l a y e r s . I n o r d e r t o m o n i t o r a n d measure s t r a i n r e s p o n s e , a s t r a i n gage i s mounted a s i n F i g u r e 1 w h e r e maximum s t r e s s w i l l o c c u r i n t h e t e s t pane1.


Table I

Test panel

t

Ε

R3

1.0

3240

46.5

206

RMR

1.0

3050

55.3

206

MRM

1.2

2480

52.5

217

M3

1.1

1600

57.6

165

W

f

f

n

R; r o v i n g g l a s s c l o t h E; t e n s i l e m o d u l u s M; g l a s s - f i b e r mat (kg/mm ) t ; t h i c k n e s s (mm) W^; g l a s s c o n t e n t f ; fundamental by weight n a t u r a l f r e q . (Hz) 2


R e l a t i o n Between rms S t r e s s a n d Sound P r e s s u r e L e v e l . The r e l a t i o n s h i p between l o a d and s t r e s s ( o r s t r a i n ) i n t h e a c o u s t i c f a t i g u e t e s t i s d i f f e r e n t from that observed i n the u s u a l f a t i g u e t e s t u n d e r c o n s t a n t s t r e s s . The a c o u s t i c l o a d i s a random n o i s e whose i n t e n s i t y i s e x p r e s s e d b y a s o u n d p r e s s u r e l e v e l ( S P L , u n i t : dB) a n d m e a s u r e d b y t h e rms v o l t - m e t e r , w h i l e t h e s p e c t r u m o f s t r a i n r e s p o n s e o f t h e t e s t p a n e l shows m u l t i p l e r e s o n a n c e s c h a r a c t e r i z e d b y v i b r a t i o n mode o f t h e p a n e l . The r e s o n a n c e whose i n t e n s i t y peaks i n the spectrum s i g n i f i c a n t l y c o n t r i b u t e s t o t h e f a t i g u e damage o f t h e p a n e l . F i g u r e 2 shows t h e r e l a t i o n s h i p b e t w e e n rms v a l u e o f s t r e s s and t h e s o u n d p r e s s u r e l e v e l f o r e a c h t e s t p a n e l . I t should be n o t e d t h a t rms s t r e s s a t SPL o f 159dB i s e s t i m a t e d b y e x t r a p o l a t i n g l i n e a r i t y b e t w e e n l o a d a n d s t r e s s s i n c e t h e s t r a i n gage i s damaged a t SPL v a l u e s o f more t h a n 148dB. R e s u l t s o f T e s t and D i s c u s s i o n s F a i l u r e P a t t e r n . F i g u r e s 3-6 d e p i c t t h e f a i l u r e p a t t e r n s ob s e r v e d o n d i f f e r e n t p a n e l s when e x p o s e d t o s t r o n g a c o u s t i c n o i s e . F i g u r e 3 shows t h e f a i l u r e o f a n u n s a t u r a t e d p o l y e s t e r r e s i n 3mm in thickness. F a i l u r e o f t h e i s o t r o p i c p a n e l o c c u r s a f t e r a few minutes exposure; t h e c r a c k s t a r t i n g a t p o i n t A o r Β and p r o p a g a t -



308


Figure

3.

Figure

4.

Failure

pattern for unsaturated

Failure

polyester

resin (5)

pattern for test panel with a glass-fiber

mat layer



23.

FUJII E T A L .

Acoustic

Figure

Figure

5.

6,

Fatigue

Failure

Failure

Strength

pattern for RS test panel (5)

pattern for M3 test panel (5)


309



310

Figure 7.

Failure pattern for RC-B test panel

Figure 8.

Failure pattern for RC-L test panel



23.

FUJII E T A L .

Acoustic

Fatigue

Strength

311

i n g a l o n g a n e l l i p t i c a l l o c u s . F i g u r e 4 shows t h e e f f e c t o f a d d i n g one l a y e r o f g l a s s - f i b e r mat a s r e i n f o r c e m e n t . The e f f e c t o f t h e f i b e r s r e i n f o r c i n g t h e m a t r i x i s e v i d e n t f r o m how a n d where t h e cracks develop. F i g u r e s 5 a n d 6 show t h e f a i l u r e p a t t e r n s o f R3 a n d M3 t e s t panels. The r e s u l t s i n d i c a t e t h a t g l a s s - f i b e r mat r e i n f o r c e d p a n e l s a r e more i s o t r o p i c t h a n g l a s s c l o t h r o v i n g r e i n f o r c e d o n e s . The d i f f e r e n c e s i n f a i l u r e p a t t e r n s i s p r o b a b l y due t o d i f f e r e n c e s i n r e i n f o r c i n g mechanisms b e t w e e n g l a s s c l o t h r o v i n g a n d g l a s s f i b e r mat. A d d i t i o n a l l y , t h e M3 t e s t p a n e l i s p r o n e t o c a t a s t r o p h i c f a i l u r e compared t o t h e R3 t e s t p a n e l . Although photographs f o r RMR a n d MRM t e s t p a n e l s a r e o m i t t e d , t h e i r f a i l u r e p a t t e r n s a r e a l m o s t i n t e r m e d i a t e b e t w e e n R3 a n d M3. The i n v e s t i g a t i o n o n a c o u s t i c f a i l u r e i n c l u d e d methods t o i n c r e a s e t h e a c o u s t i c f a t i g u e s t r e n g t h o f FRP. A simpler s t i f f e n e r was made b y b o n d i n g CFRP o n t h e m i d d l e o f a p a n e l (RC-B) o r l a m i n a t i n g f i b e r woven t a p e s d u r i n g t h e m o l d i n g p r o c e s s ( R C - L ) . T h e i r r e s p e c t i v e f a i l u r e p a t t e r n s a r e shown i n F i g u r e s 7 a n d 8 . The b e s t s t i f f e n i n g method was i n RC-L where d e l a m i n a t i o n could n o t t a k e p l a c e w h i l e t h e s t i f f e n e r f o r RC-B debonded when e x p o s e d to acoustic noise. Acoustic Fatigue Strength. Test r e s u l t s f o r acoustic fatigue s t r e n g t h o f each p a n e l are p l o t t e d i n F i g u r e 9 along w i t h f a t i g u e s t r e n g t h under constant s t r e s s o f specimens h a v i n g s i m i l a r s t a t i c s t r e n g t h and g l a s s f i b e r c o n t e n t s . I n F i g u r e 9 the l e f t s i d e o r d i n a t e i s i n rms s t r e s s w h i l e t h e r i g h t s i d e i s i n rms p e a k s t r e s s By m u l t i p l y i n g t e s t d a t a b y t h e \J2 to a c h i e v e rms p e a k s t r e s s , t h e a c o u s t i c f a t i g u e s t r e n g t h o f R3 a n d M3 c a n b e compared o n t h e same c h a r t as t h e i r f a t i g u e s t r e n g t h under constant s t r e s s . C y c l e s t o f a i l u r e p l o t t e d on the a b s c i s s a are c a l c u l a t e d s o t h a t they w i l l be a p p r o x i m a t e l y e q u a l t o t h e v a l u e s o f f u n d a m e n t a l n a t u r a l f r e quency m u l t i p l i e d b y f a i l u r e t i m e o f e a c h t e s t p a n e l . I t i s f o u n d t h a t t h e a c o u s t i c f a t i g u e s t r e n g t h f o r R3 a n d M3 i s c o n s i d e r a b l y lower than the f a t i g u e s t r e n g t h obtained from con s t a n t s t r e s s t e s t i n g . I t i s considered that t h i s f a c t i s caused by m u l t i p l e r e s o n a n t phenomenon o f t h e p a n e l a t v e r y h i g h c y c l i c f r e q u e n c i e s where t h e f a t i g u e s t r e n g t h o f FRP, i n g e n e r a l , r e d u c e s due t o t h e g e n e r a t i o n o f h e a t ( 4 ) . In order t o estimate t h ea c o u s t i c f a t i g u e l i f e o f t e s t pan* e l s , i t i s assumed t h a t t h e a c o u s t i c l o a d u s e d i s a random Gaus sian noise. Peak v a l u e s o f s t r e s s e s t a k i n g p l a c e i n p a n e l s a r e supposed t o be R a y l e i g h d i s t r i b u t e d h a v i n g the p r o b a b i l i t y d e n s i t y function 2

f ( o ) = (2σ/σ )e" \ / \ ' rms

a 2 / o

rms

(1)

The s e c o n d a s s u m p t i o n i s c o n c e r n e d w i t h t h e f u n d a m e n t a l S-N c u r v e obtained from the f a t i g u e t e s t w i t h constant s t r e s s . I n t h i s


312

c a s e , we

RESINS

can

FOR

AEROSPACE

get m

Na = C

(2)

where C and m a r e m a t e r i a l c o n s t a n t s . I f t h e m o d i f i e d M i n e r ' s r u l e i s assumed t o be a p p l i c a b l e t o FRP m a t e r i a l s , t h e c u m u l a t i v e damage d u r i n g f a t i g u e o f FRP i s g i v e n by Σ(η/Ν) = (N /C)/* r ο

o f{a)do m


f

=

(HJC)a

D(m) v

m

v

9

( 3 )

,

f ' rms ' where means t h e number o f c y c l e s t o f a i l u r e o f FRP p a n e l s u n d e r a c o u s t i c l o a d i n g s and ο i s t h e rms p e a k s t r e s s i n E q . ( l ) . And D(m) i s c a l l e d t h e damage f u n c t i o n i n t h e f o r m o f t h e f o l l o w i n g integral D(m)

= / " V ^ V ^ d o

= r(l+m/2)

(4)

,

0

where Γ i s t h e Gamma f u n c t i o n . T h u s , u s i n g E q . ( 3 ) , we can p r e d i c t t h e a c o u s t i c f a t i g u e s t r e n g t h o f FRP f r o m t h e e x p e r i m e n t a l d a t a o f f a t i g u e s t r e n g t h o f FRP o b t a i n e d f r o m c o n s t a n t s t r e s s t e s t i n g . Computational r e s u l t s a r e g i v e n i n F i g u r e 9 w h i c h have t h e good agreement w i t h t h e i r e x perimental data. Improvement o f A c o u s t i c F a t i g u e

Strength

Acoustic fatigue strength for glass cloth roving reinforced FRP, whose i n h e r e n t s t r e n g t h i s t h e h i g h e s t among t e s t e d p a n e l s , i s i m p r o v e d by m a k i n g t h e f o l l o w i n g t y p e l a m i n a t e . RC-B p a n e l i s made by b o n d i n g CFRP as a s a n d w i c h on b o t h s i d e s o f an R3 p a n e l . RC-L p a n e l i s l a m i n a t e d so t h a t a c a r b o n f i b e r woven t a p e i s c e n t e r e d b e t w e e n two l a y e r s o f g l a s s c l o t h r o v i n g . T a b l e I I l i s t s d a t a on RC-B and RC-L t e s t p a n e l s i n c l u d i n g w i d t h and t h i c k n e s s dimensions f o r the s t i f f n e s s . F i g u r e 10 shows t h e i n c r e a s e o f a c o u s t i c f a t i g u e due t o s t i f f e n i n g e f f e c t s . I t s h o u l d be n o t e d t h a t t h e s l o p e s o f p l o t t e d d a t a f o r RC-B and RC-L a r e g r e a t e r t h a n t h e s l o p e o f t h e S-N c u r v e f o r R3 u n d e r c o n s t a n t s t r e s s .

Table Thickness (mm) RC-B

0.23

RC-L

0.5

χ 2

II

Width (mm)

T e n s i l e modulus (kg/mm)

10.0

6000

20.0

8700


23.

Fujii ET AL.

1

1

Acoustic

r| ι ι πι

•

I |

Fatigue

313

Strength

ι ι ι I

15 ·


= 10

ε I 10

Constant stress^"*""'--.^ \ S-Ncurv«

=.

«Λ

ω 5 M3 —CD

10

Miner's prediction theory

Q

10*

5

0 ΙΟ 7

Cycles to failure


Figure 10.

Figure 9. Experimental and predicted results of acoustic fatigue strength of test panels in comparison with S-N curves under constant stress (5): experimental: (Φ) R3, (A) RMR, (A) MRM, (Ο) M3

Increase of acoustic fatigue strength by CFRP

stiffeners: (O)

f X ) RC-B, {•) RC-L


R3,

314

RESINS FOR AEROSPACE


Conclusions ο

Acoustic fatigue strength of FRP panels i s considerably lower than their fatigue strength obtained from constant stress testing.

ο

Making a few assumptions acoustic fatigue strength can be predicted from experimental fatigue strength data with reasonably good agreement.

ο

The better method for panel stiffening i s lamination of carbon fiber woven tapes.

Abstract Four kinds of FRP panels with three layers consisting of glass cloth roving and/or glass mat reinforced unsaturated poly ester resin were acoustically excited and fatigue tested. From the experiments, it i s observed that the laminated structure of the test panel may characterize the failure pattern and the glass mat reinforced test panel may be more isotropic than the glass cloth roving reinforced test panel. In order to predict the acoustic fatigue life of test panels, it i s assumed that the acoustic load i s a random noise with narrow frequency band-width and the modified Miner's rule i s applicable to FRP materials. Using these assumptions, the acoustic fatigue life of test panels is estimated with good agreement from experimental data of FRP fatigue strength obtained under constant stress. Simple s t i f f e n ing methods are investigated to achieve improved acoustic fatigue strength. L i s t of Symbols C, m D( ) Ε f( ) f^ M Ν, R t W^r Γ( ) σ Orms Σ(η/Ν)

Material Constants Damage Function Tensile Modulus Probability Density Function Fundamental Natural Frequency Chopped Strand Glass-Fiber Mat Cycles to Failure Roving Glass Cloth Thickness of Panel Glass Content by Weight Gamma Function Stress Amplitude rms Peak Stress Cumulative Damage


23.

FUJII E T A L .

Acoustic

Fatigue

Strength

315

Acknowledgments The a u t h o r s t h a n k Mr. K. I t a m i f o r h i s h e l p i n p r e p a r a t i o n s o f t e s t p a n e l s a n d e x p e r i m e n t s . They a r e a l s o i n d e b t e d t o N i p p o n G l a s s F i b e r Co. f o r g l a s s - f i b e r s , T a k e d a C h e m i c a l I n d u s t r i e s L t d . f o r r e s i n s and T o r a y I n d u s t r i e s I n c . f o r CFRP. Literature Cited 1. 25, (277).

Fujii,

T.

J.,

Society

of

Materials

Science,

Japan, 1976,


2. Richards, E. J., Mead, D. J., Ed. "Noise and Acoustic Fatigue in Aeronautica"; J o h n W i l e y & Sons Ltd., L o n d o n , 1968, p. vi. 3.

W h i t e , R. G., Memo of ISVR, U n i v . o f S o u t h a m p t o n , 1974, 5 5 2 .

4.

Fujii, T., F u k u d a , T., Yoshida, Symp., J a p a n , 1978, 59-62.

5.

Fujii, T., F u k u d a , T., for C o m p o s i t e Materials,

Η., Preprint of the 7th FRP

Iida, S., Sano, M. J., J a p a n 1978- 4, (4).

RECEIVED February 15, 1980.


Society

Acoustic Fatigue Strength of Fiber-Reinforced Plastic Panels

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