Notched strength of fabric laminates. II: Effect of stacking sequence

Composites Science and Technology 44 (1992) 13-20

Notched strength of fabric laminates. II: Effect of stacking sequence P. S. S h e m b e k a r & N. K. Naik Aeronautical Engineering Department, Indian Institute of Technology, Powai, Bombay--400076, India (Received 9 August 1990; accepted 8 April 1991) The effect of stacking sequence on the tensile strength of notched and unnotched woven fabric (WF) composite laminates has been experimentally investigated. Comparison is made with unidirectional (UD) tape laminate results that are available in the literature. The fabric structure is a governing parameter in the failure mechanisms of WF composites. The failure mechanisms of WF composites, therefore, differ from UD tape composites. The notch sensitivities of different lay-ups are compared by using the characteristic dimensions do and ao of the Whitney and Nuismer fracture model. The ratio of predicted notched strength to unnotched strength, (ON)PRE/O0, is used to study the effect of stacking sequence and d/W ratio on the notch sensitivity of the WF composites. Results are presented for unbalanced plain weave Eglass/epoxy composites of eight different lay-ups. The criteria for choosing the optimum stacking sequence are discussed.

Keywords: notched strength, woven fabric, stacking sequence, unnotched strength, characteristic dimension, notch sensitivity

NOTATION ao

d do

v, W or~

(ON)pRE (O )oT Oo

associated failure criteria have been the subject of extensive research. The fracture behaviour of composites depends on a number of intrinsic and extrinsic variables, which makes the problem complex. Although a considerable insight into the problem has been gained, a comprehensive evaluation of the effect of different variables on the fracture behaviour of composites is still lacking. The effect of some of the variables on the notched strength of composites has been discussed by Awerbuch and Madhukar.1 Because of their higher structural efficiency, unidirectional (UD) tape laminates are being widely studied. The use of woven fabric (WF) composites in structural applications is also increasing, but relatively few data and little work are available on woven fabric composites. They offer advantages such as dimensional stability, deep draw shapeability, enhanced toughness and increased impact resistance. Because of the fabric structure, the failure mechanisms of WF composites differ from those of UD tape composites. The woven fabric consists of two sets of interlacing threads, the warp and the weft (fill), as shown in Fig. 1. The

Characteristic dimension (Whitney and Nuismer (WN) fracture model, average stress criterion) Hole diameter Characteristic dimension (WN fracture model, point stress criterion) Fibre volume fraction Width of specimen Notched tensile strength of a finite width plate---experimental Predicted notched tensile strength of a finite width plate Notched tensile strength of an infinite width plate, based on orthotropic finite width correction factor Unnotched strength

INTRODUCTION The problem of the stress distribution around a circular hole in a composite plate and the Composites Science and Technology 0266-3538/92/$05-00 © 1992 Elsevier Science Publishers Ltd. 13

14

P. S. Shembekar, N. K. Naik /

LITE Fig. 1. Woven fabric structure--unbalanced plain weave• fabric's properties depend on its structure. The parameters involved in determining the fabric structure are the weave, the fineness of yarn, the fabric count, the characteristics of the warp and weft threads, and factors introduced during weaving, such as yarn crimp. The weave is the pattern of repeat in the warp and weft directions. Depending upon the number of repeats, the fabric structure is denoted as plain weave, twill weave or satin weave. The weight of yarn per unit length, i.e. the linear density, is a measure of yarn fineness and is given by the tex number (g/km). The fabric count is the number of yarns per unit length along the warp or weft direction. The fabric structure can be balanced or unbalanced depending on the number of counts in the warp and weft directions, degree of undulation, and fineness of yarn. These parameters should be the same in both directions for a balanced fabric. The fabric structure shown in Fig. 1 is unbalanced, as the warp and weft counts are different. Ishikawa and C h o u 2-4 proposed three analytical models to describe the elastic behaviour of

fabric composites under in-plane loading along the warp or weft direction: (1) the mosaic model gives the upper and lower bounds of elastic stiffness; (2) fibre continuity and undulation are considered in the fibre undulation model, which is particularly suitable for predicting the elastic properties and the knee behaviour of plain weave fabric composites; (3) the bridging model enables determination of the stiffness of general satin composites. Some studies are also available on the notched strength of WF composites. Bailie et al. 5 studied the effect of holes on the tensile strength of graphite fibre/epoxy fabric laminates. The ratios of notched to unnotched strengths or the residual strength ratios of WF composites were found to be greater than their tape laminate counterparts. The residual strength ratio for the fabric laminates increased as the lay-up became progressively closer to quasi-isotropic, whereas it decreased for tape laminates. Lagace 6 found that the notch sensitivity of WF laminates under tension is generally the same as tape laminates of similar configuration. The splitting that often occurred in UD tape laminates did not occur in

Notched strength of fabric laminates. H WF laminates. This increased resistance to fracture resulted in a higher notched strength for some of the laminates. Chang et al.7 studied the notched strength of WF composites with drilled and moulded-in holes. Specimens with mouldedin holes were found to exhibit higher failure strengths than those with drilled holes. For certain lay-ups, the failure strengths with moulded-in holes were found to be higher than those of unnotched laminates. Naik and coworkers 8-1° studied the failure behaviour of WF composites. The notched strength was found to exceed the unnotched strength in some off-axis tension tests, owing to fibre reorientation along the loading direction. 9"1° Although WF composites are receiving increased attention, our understanding of the failure behaviour of WF composites with or without holes is incomplete. Owing to the increased number of parameters, the problem for WF composites is more complex than that for UD composites. Stacking sequence is one of the intrinsic variables found to affect the notch sensitivity of UD tape laminates significantly. 1 Pagano and Pipes H sought to predict the detailed stacking sequence of specific layer orientation that leads to optimum protection against delamination under uniaxial static and fatigue loading. Whitney and Kim ~2 studied the effect of stacking sequence on the unnotched and notched tensile strengths of quasi-isotropic graphite/epoxy UD tape laminates. They found that the unnotched tensile strength of laminates was reduced as a result of the presence of interlaminar tensile stresses at the straight free edge, while notched strength was independent of stacking sequence for notch sizes that produced tensile failure prior to delamination at the straight free edge. Herakovich 13 showed a relationship between delamination and the mismatch of engineering properties among adjacent layers of laminated composites: the interlaminar stresses were primarily a function of the mismatch in the coefficients of mutual influence and Poisson's ratio. For more efficient design, interspersed + 0 layers were recommended by virtue of reduction in mismatch. Daniel et al.14 carried out experimental studies on the effect of material and stacking sequence on the behaviour of UD tape laminates with holes. They found that stacking sequence variation can alter the mode of failure from catastrophic to non-catastrophic. Such studies on the influence of stacking sequence on the

15

unnotched and notched strength of WF composites are not available. In this paper, the effect of stacking sequence on the unnotched and notched strength of WF composites is studied. A comparison is made of the failure behaviour of WF composites and UD tape composites. The effect of stacking sequence on the notch sensitivity is investigated as a function of characteristic dimensions of the WN fracture model. The predicted values of the notched tensile strength of finite width plates for different stacking sequences are compared.

EXPERIMENTAL WORK Material system In this work, an unbalanced plain weave fabric of E-glass was used. The fabric was de-sized and treated with epoxy-compatible coupling agent. Thickness of the fabric was 0.17 mm. The warp thread count was 17/cm and weft thread count was 13/cm. The properties of the E-glass fibres used were: tensile modulus = 72 GPa; actual tensile strength = 2.1 GPa; and density = 2.54g/cm 3. The epoxy resin system used was LY556, with hardener HY951 supplied by Cibatul, India. Layers of size 350 × 350 mm 2 were cut from the fabric roll. Laminates were made by compression-moulding and cured at ambient temperature. Eight different lay-ups were considered. The laminate configurations are given in Table 1. The notations used for woven fabric laminates are discussed in Part 1.15 The fibre volume fraction was 0-45 and the average thickness of the laminates was 1-32 mm.

Table 1. Laminate nomenclature

Number

1 2 3 4

Notation

Laminate configuration Contracted

Expanded

SS1 SS2 SS3

(0)4s (45, 0)2s (45, 02, 45)~

(0, 0, 0, 0, 0, 0, 0, 0).r (45, 0, 45, 0, 0, 45, 0, 45)T (45, 0, 0, 45, 45, 0, 0, 45)x (45, 45, 0, 0, 0, 0, 45, 45)-r (0, 45, 0, 45, 45, 0, 45, 0)1(0, 0, 45, 45,45, 45, 0, 0)-r (0, 45, 45, 0, 0, 45, 45, 0).r (45, 45, 45, 45, 45, 45, 45, 45)T

5

ss5

SS4

(452, 02)s

6

SS6

(02,452) s

7

SS7

(0, 452, 0)s

8

SS8

(45),~

(0, 45)~

16

P. S. Shembekar, N. K. Naik

Test methods Test specimens were cut from the laminates as specified by ASTM Test D3039. Test coupons of size 50 x 250 mm 2 were cut for notched strength study. For the notched specimens, the hole diameters were d = 5, 10, 15, 18 and 20 mm. The drilling was carried out with carbide-tipped drills, with optimum drilling conditions. To avoid delamination while drilling, backing plates were used. Tensile tests were carried out to study the tensile behaviour of the unnotched and notched specimens. The tests were performed on an Instron 1195 machine. The specimens were tested at ambient temperature (27°C) at a cross-head speed of 2 mm/min. A total of 300 test specimens was manufactured and tested. Elastic properties, stress-strain behaviour and strength properties were noted. The elastic and strength properties for all of the lay-ups are given in Part I.~5

RESULTS A N D DISCUSSION Unnotched strength The stress-strain behaviour of typical laminates is shown in Fig. 2. Nonlinear behaviour is inherent in woven fabric composites, as a result of their fabric structure, and it can be seen that all of the laminates exhibited nonlinear behaviour. The degree of nonlinearity is dependent

on the number of (45) plies and their location. For the (0)4s laminate (SS1), the nonlinear behaviour is less pronounced, whereas for the (45)4s laminate (SS8), it is more pronounced. For the quasi-isotropic lay-ups (SS2-SS7), the degree of nonlinearity depends on the arrangement of the (45) layers. The (0)4s laminates failed catastrophically. After a great deal of deformation, equivalent to yielding in the case of metals, the (45)4s laminates also failed suddenly, and necking was observed in these laminates prior to failure. Failure was gradual for the quasi-isotropic lay-ups. The (0) plies failed first and the (45) plies failed later, owing to their higher strain limit. After failure of the (0) plies, a 'post-failure strength' is provided by the (45) plies. This phenomenon is especially important for the damage tolerance of composite laminates. The post-failure strength is also a function of stacking sequence. A typical lay-up that gives higher post-failure strength is (02,452)~, as can be seen from Fig. 2. The (0, 45)2~ laminate has lower post-failure strength because of the dispersed (45) plies. Figure 3 shows the strength hierarchy. The scatter in the data is also shown in the figure. The stacking sequence has a considerable effect on the unnotched strength. Among the quasiisotropic laminates, stacking sequences (45, 0)2, (45, 02, 45)~, and (452, 02)s have higher unnotched strength than the (0,45)2~, (02,452)~, and 500

~

t-O0 // /6

-- 55~

MAXIMUM AVERAGE MINIMUM

E-Gloss /Epoxy Unbalanced plain Vf : 0.45

/.00- -,~

E - Glass / E p o x y Unbalanced plain

"A

[ ] - Unnotched strength, DNotched strengfh,0N d=15mm

weave

300

I

vf : 0.45

/

~II/ I I / "I /

~"

///

=~ 200

-555 556

551 -- ( 0 )4s 555 -- (0.~5)2s 5 5 6 - (02, ~.52)s

weave

300

"/

17

//I~

m //

!"

o 200- " A

/

/

",

i~.

$58 -- (1.5)~s

/

7 / ,//

// //

//

/ "/ ,/

/

/,

cm

/ ,/

100

100

/

¢

/

4 0 0 O0

~iI~ ~ 0.05

LLL~LLI ,:li/~:lllltlll ,,, llILIL 0.I0 0.15 0.20 0.25 Longitudinal strain

Fig. 2. Stress-strain behaviour of unnotched composites that have different stacking sequences.

SSI

SS2

SS3

SS~ SSS SS6 Stacking sequence

SS7

SS8

Fig. 3. Unnotched and notched strengths for different stacking sequences.

Notched strength of fabric laminates. H (0,452, 0)~ materials. It can be noted that the stacking sequences of the first group have the (45) plies outside whereas the stacking sequences of the second group have the (0) plies outside. It is also important to note that the stacking sequences (45, 0)2s and (0, 45)2~, in which the (0) and (45) plies are dispersed, have higher unnotched strength in their respective groups. To further study this phenomenon, measurements were carried out on single (45) and (0) plies. Under uniaxial tension, the (45) ply was found to twist, while no twisting was observed in the case of the (0) ply. The coupling between stretching and twisting can be attributed to the antisymmetry of the (45) ply that results from the unbalanced structure of the fabric. When the (45) plies are inside, the outer (0) plies tend to twist because of the twisting of the (45) plies and delamination is caused; hence, the laminate strength is reduced. By contrast, when the (45) plies are outside, the inner (0) plies remain together until failure; hence, the laminate strength is higher than in the previous case. It was also observed that when the (45) plies were staggered the strength was higher. When the (45) plies are clustered together, the cumulative effect of twist can be predominant, leading to a lower strength. Failure modes of the unnotched laminates are shown in Fig. 4. (O)~s

(4S,O)~s

( 4S,01,4S )s

(45~ ,Oz) s

/

17

Using linear elastic fracture mechanics, Herakovich 13 studied the effect of stacking sequence on graphite/epoxy quasi-isotropic UD tape laminates. He found that when the +45 ° or - 4 5 ° plies were interspersed between the 0 ° and 90 ° plies, the mismatch between the coefficients of mutual influence is reduced, which leads to a reduction of the interlaminar shear stress, l:yz. This increases the resistance to initiation of delamination and thereby "increases the unnotched strength. The interface moment is less when the 90 ° or 0 ° plies are placed outside. This also leads to more resistance to the initiation of delamination and thereby increases the strength. Herakovitch concluded that for higher unnotched strength, the +45 ° and - 4 5 ° plies should be dispersed and the interface moment should be minimized. In the case of WF composites, the warp and weft fibres are held together not only by in-plane shear but also mechanical coupling. Thus, the effect of mismatch of coefficients of mutual influence for the (45)2s laminate is less than for the equivalent (+45)s laminate made from UD tapes. In the case of unbalanced plain weave fabric composites, twisting of the (45) plies is a dominant factor that governs the initiation of delamination and the failure strength. Although this study uses unbalanced plain weave quasiisotropic laminates, similar behaviour is expected for twill and satin weave quasi-isotropic laminates, since these fabrics have an antisymmetric structure. In general, the unnotched strength of the quasi-isotropic lay-ups can be improved by placing the (45) plies outside and dispersing the (0) plies with the (45) plies.

Notched strength J

IO, 45 )2S

( 0 l , 45~ )S

( O, 45~ ,0) s

(65)t, s

)

E - Glass / Epoxy ; UnbQlanced plain weave ; Vf = 0.45

Fig. 4. Failure modes of the unnotched unbalanced plain weave fabric laminates under uniaxial loading.

Figure 3 shows the notched strengths with a 15-mm diameter hole for different stacking sequences. It should be noted that the hierarchies for the unnotched and notched strengths are not the same. This is because of the differences in failure modes. The failure modes of the notched laminates are shown in Figs 5 and 6. The failure was catastrophic for all of the notched laminates. The crack initiates at 90 ° to the loading direction and has a tendency to propagate along the direction of least resistance. Since the number of counts is less along the weft direction, the crack/damage zone has a tendency to propagate along the warp direction. Here, 'damage zone' is

P. S. Shembekar, N. K. Naik

18 (O)4s

• ....

~

( t,5, 0 )~s

I

~

L. . . .

J

__

( t.5,02,/.5 ) s

~52, 0z )s

d

~- = 0 1

0.2

0 '3

0.36

0 't.

Fig. 5. Failure modes of the notched unbalanced plain weave fabric laminates under uniaxial loading: SS1-SS4.

(O,45)zs

( 0 2 , ~5 z )s

O, ~-5 2 , 0 ) s

I (~.5)~s

d

-

w = 0.1 -

0.2

0.3

036

0 4

Fig. 6. F a i l u r e m o d e s o f t h e n o t c h e d u n b a l a n c e d p l a i n weave fabric laminates under uniaxial loading: SS5-SS8.

a collective term for matrix cracking, fibre pull-out, fibre breakage, delamination, etc. The damage zone area is larger for smaller holes than for larger holes. The two halves of the specimens have shifted with respect to each other after failure, as a result of the difference in the number of counts along the warp and weft directions. The highest notched strength was obtained for (0, 45)2s and (45, 0)2s lay-ups in which the (45) and (0) plies are dispersed. This is true for all hole diameters considered. The notched strengths are given in Part 1.15 Comparing the stacking sequence (45, 02, 45)~ with (452, 02)~ and (02,452)~ with (0,452, 0)s, it can be seen that clustering of the (45) plies near the mid-plane gives higher notched strength. This observation is based on all of the d / W cases considered. Though the ( 4 5 ) 4 s laminate had the lowest notched strength, it had the least notch sensitivity and greatest resistance to crack/damage zone propagation. In the notched (45) plies, the reorientation of fibres takes place near the hole edge, which facilitates the redistribution of stresses. 9J° The (45) plies, therefore, offer more resistance to crack/damage zone propagation. Since the strain limit of the (0) plies is lower than the (45) plies, the crack initiates in the (0) plies in a quasi-isotropic laminate. When the (0) plies are dispersed between the (45) plies, the crack has to initiate in the (0) plies individually. Therefore, the dispersion of the (0) plies with the (45) plies leads to higher resistance to damage propagation, thus offering higher notched strengths. By the same reasoning, when the (45) plies are clustered near the mid-plane, a higher notched strength is obtained. However, too many (45) plies stacked near the mid-plane would lead to delamination, owing to the dominant effect of the twisting of the (45) plies, and, consequently, the lower notched strength. Thus, the (02,452)s laminate in which all the (45) plies are clustered near the mid-plane has a lower notched strength than the (0, 45)2~ laminates. Daniel et a1.14 carried out experimental studies on the notched strength of quasi-isotropic boron/epoxy laminates made from UD tape. The lay-up [0/+45/90]s was found to have lower strength than the alternate stacking sequence [+45/90/0/-45]s. Dispersing the +45 ° and - 4 5 ° plies with the 0° and 90 ° yielded higher notched strength, owing to the presence of compressive interlaminar stresses, - o z . The lay-up [02/ -t-45/0]s was found to have higher strength than

Notched strength of fabric laminates. H the stacking sequence variant [+45/02/0]~. The transverse compression in the 0 ° outer plies results in beneficial compressive interlaminar stresses near the boundary. The situation is reversed for the [+45/02/0]~ lay-up, resulting in decreased strength. In other words, having the 0 ° ply outside results in a higher notched strength. In this case, the direction of interlaminar normal stress is a governing factor for the notched strength, an observation that cannot be extended to unbalanced plain weave, twill weave and satin weave fabric laminates because of the twisting phenomenon. In general, dispersing the (0) plies with (45) plies and placing the (45) plies near the mid-plane gives higher notched strength in quasi-isotropic WF composites.

12

19

E - Glass / Epoxy

10-

c:

.o_ 8 J=

•"" 6 t_ o~ m

2-

0-

SS1

SS3

SS2

SS4

SS6

SS7

SS5

SS8

Stacking s e q u e n c e

Laminate evaluation

- ao,

Although the (0)4~ laminate has the highest unnotched strength among the eight lay-ups considered, it does not give the highest notched strength. For smaller diameter holes, the notched strengths of the quasi-isotropic (45,0)2~ and (0, 45)2~ lay-ups are higher than that of the (0)4~ laminate. This means that the optimum stacking sequence is different for unnotched and notched composite laminates. Therefore laminate evaluation involves notch sensitivity and unnotched strength. The notch sensitivity is inversely related to the ratio of notched to unnotched strength: higher notch sensitivity indicates a lower ratio of notched to unnotched strength. The characteristic dimensions of the WN fracture model can be used as a measure of notch sensitivity: ~ higher ao and do indicate lower notch sensitivity. The characteristic dimensions a0 and do were found by using the ASC and PSC models. 15 Since the characteristic dimensions are based on all of the d / W cases, an overall comparison becomes possible. The notch sensitivity hierarchy based on these dimensions is shown in Fig. 7. It can be seen that the (45)4s laminate is the least notch-sensitive and the (0)4s laminate has the highest notch sensitivity; notch sensitivities of the quasi-isotropic lay-ups lie in between. The (0, 45)2~ laminate, having dispersed (0) plies with (45) plies and clustered (45) plies near the mid-plane, possesses lower notch sensitivity than the (45, 0)2s laminate, which has dispersed (0) with (45) but (0) clustered near the mid-plane. This indicates that clustering of the

I

~ -

do

Fig. 7. Characteristic dimensions o f the WN fracture model for different stacking sequences.

(45) plies near the mid-plane plays a more important role in reducing the notch sensitivity than the dispersion of (0) with (45). Also by comparing (02,452)s with (452,02), it can be shown that clustering (45) near the mid-plane provides lower notch sensitivity. It has been shown that the agreement between the experimental notched strength, aN, and the predicted notched strength, (ON)pRE, is very

~ A

0.8 o

558 ~ . . , .

E-Glass/Epoxy Unbalanced plain weave =

"~ 0.v IM

e~

0.8

ssT----~..._ ~ \ ss6 - - , - - - . . . ~ s s ~ ~ ss2~ " - - . ~ SS3--~

~0 0.5 0

.o 0.4

0

Z 0.3

0.{

0.1 0.2 0.3 Diameter to width ratio, (d/W)

0.4

Fig. 8. Variation of the strength ratio, (aN)PRE/OO, for differentstackingsequences.

20

P. S. Shembekar, N. K. Naik

good. 13 Comparison of notch sensitivities based on (ON)PRE/O0 is a more appropriate measure, since the effect of d / W can also be studied. Figure 8 shows the variation of (ON)PRE/O0 with d / W for all of the lay-ups. The notch sensitivity hierarchies based on (ON)PRE/O0 and the characteristic dimensions ao and do are essentially the same. The hierarchy based on the characteristic strength ratio [(aN)OT/OO] also follows the same trend. 15 For larger d / W ratios, variation in stacking sequence does not affect notch sensitivity to any great extent. The notch sensitivity of (45)4s and (0, 45)2s increases rapidly with d / W ratio. Except for (0)45, the curves for all the lay-ups tend to converge at higher d / W ratios. In general, the notch sensitivity of quasiisotropic W F composite laminates can be reduced by placing the (45) plies near the mid-plane and dispersing the (0) with the (45) plies.

CONCLUSIONS The stacking sequence has a significant effect on the unnotched and notched strengths of W F composite laminates. The fabric structure governs the failure m o d e s of W F composites, the more so when the fabric structure is antisymmetric. Therefore, the stacking sequence effect on the strength of W F composites is not the same as in the case of their U D tape laminate counterparts. The ratio of predicted notched strength to unnotched strength, (aN)PRE/Cr0, is found to be convenient for the study of the effect of stacking sequence and d / W ratio on the notch sensitivity of W F composites.

ACKNOWLEDGEMENTS The work reported in this paper was financially supported by a Grants-in-Aid project s p o n s o r e d by the Structures Panel, Aeronautics R & D Board, Ministry of Defence, G o v e r n m e n t of

India. Project No. A e r o / R D - 1 3 4 / 1 0 0 / 1 0 / 8 7 88/483. The authors gratefully acknowledge their support.

REFERENCES 1. Awerbuch, J. & Madhukar, M. S., Notched strength of composite laminates: predictions and experiments---a review. J. Reinf. Plast. Comp., 4 (1985) 3. 2. Ishikawa, T., Anti-symmetric elastic properties of composite plates of satin weave cloth. Fibre Sci. Technol., 15 (1981) 127. 3. Ishikawa, T. & Chou, T. W., Stiffness and strength behaviour of woven fabric composites. J. Mater. Sci., 17 (1982) 3211. 4. Ishikawa, T. & Chou, T. W., One-dimensional micromechanical analysis of woven fabric composites. AIAA J., 21 (1983) 1714. 5. Bailie, J. A., Duggan, M. F., Fisher, L. M. & Yee, R. C., Effects of holes on graphite cloth epoxy laminates tension strength. J. Aircraft, 19 (1982) 559. 6. Lagace, P. A., Notch sensitivity of graphite/epoxy fabric laminates. Comp. Sci. Technol., 2,6 (1986) 95. 7. Chang, L. W., Yau, S. S. & Chou, T. W., Notched strength of woven fabric composites with moulded-in holes. Composites, 18 (1987) 233. 8. Naik, N. K., Shembekar, P. S. & Verma, M. K., On the influence of stacking sequence on notch sensitivity of fabric laminates. J. Comp. Mater., 24 (1990) 838. 9. Shembekar, P. S., Hosur, M. V., Verma, M. K. & Naik, N. K., On the failure behaviour of woven fabric composites. Advances in Structural Testing, Analysis and Design, ICSTAD Proceedings. Tata McGraw-Hill, New Delhi, 1990, p. 237. 10. Naik, N. K., Shembekar, P. S. & Hosur, M. V., Failure behavior of woven fabric composites. J. Comp. Technol. Res., 13 (1991) 107. 11. Pagano, N. J. & Pipes, R. B., The influence of stacking sequence on laminate strength. J. Comp. Mater., 5 (1971) 50. 12. Whitney, J. M. & Kim, R. Y., Effect of Stacking Sequence on the Notched Strength of Laminated Composites. ASTM STP 617, 1977, p. 229. 13. Herakovich, C. T., On the relationship between engineering properties and delamination of composite materials. J. Comp. Mater., 15 (1981) 336. 14. Daniel, I. M., Rowlands, R. W. & Whiteside, J. B., Effects of material and stacking sequences on behaviour of composite plates with holes. Exp. Mech., 14 (1974) 1. 15. Naik, N. K. & Shembekar, P. S., Notched strength of fabric laminates. I: Prediction. Comp. Sci. Technol., 43 (1992) 401.