mechanical properties of fiber glass and kevlar woven fabric reinforced composites

Pergamon Composites Engineering, Vol. 5. No. 9. pp. 1159-I 175, 1995

Copyright 0 1995 Elsevier Science Ltd Primed in Great Britain. All rights reserved

0961.9526195 59.50+ .OO

0961-9526(95) 00100-6

MECHANICAL PROPERTIES OF FIBER GLASS AND KEVLAR WOVEN FABRIC REINFORCED COMPOSITES

Youjiang Wang, Jian Li and Dongming Zhao School of Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295,

U.S.A.

(Received 8 September 1994; final version accepted 26 October 1994)

Abstract-An experimental study was carried out on woven fabric/epoxy composites focusing on their mechanical properties under uniaxial tensile, flexural, compressive, short beam shear and Mode I interlaminar fracture loading conditions. Composites with fiber glass and Kevlar 49 woven fabrics and with different fabric constructions and microfiber additives were investigated. The interlaminar fracture behavior was characterized using double cantilever beam (DCB) test and the fracture mechanism was analyzed by means of scanning electron microscopy (SEM). The mechanical properties were evaluated in uniaxial tension, three-point flexure, compression and short beam shear. The effects of the reinforcing fabric structure and microfiber additives to the matrix on the mechanical behavior and failure mechanisms of the composites were analyzed.

1. INTRODUCTION

Woven fabric reinforced composites are among the most important and widely used forms of textile structural composites (Chou and Ko, 1989). Woven fabrics as an attrac- tive reinforcement provide excellent integrity and conformability for advanced structural composite applications. An understanding of the effect of fiber type, fabric construction, and microfiber inclusion on the performance of such composites would therefore provide very useful information in applications. Like most laminated composites, woven fabric reinforced composites also have poor interlaminar properties, such as low interlaminar strength and toughness in comparison with their in-plane properties. Delamination is con- sidered one of the most common failure forms in such composites. Research efforts to improve the interlaminar fracture behavior and mechanical properties of such composites have generally focused on the reinforcement, e.g. the development of three-dimensional reinforcement; the matrix material, e.g. the use of thermoplastics and toughened ther- mosets; and the fiber/matrix interface, e.g. improved coupling agents for controlled inter- facial properties. Many methods to enhance the toughness of polymeric composites using matrix additives have been reported in the literature (McKenna, 1975; Bucknall, 1989; Scott and Phillips, 1975; Bascom et al., 1980; Jang, 1991; Larsen, 1971; McGarry and Mandell, 1972; Dulgin, 1971; Phillips and Tetelman, 1972).

The study reported here aimed at characterizing the mechanical behavior of fiber glass and Kevlar woven fabric reinforced epoxy composite laminates under various conditions. The fabrics used included fiber glass and Kevlar woven structures with different weave patterns, and the microfibers chosen as additives to the matrix included calcium sulfate whiskers, processed mineral fibers and aramid fibers. Double cantilever beam (DCB), uniaxial tension, three-point bending, compression and short beam shear tests were carried out. Elastic properties, failure strength, toughness, and failure modes and mechanisms were analyzed.

2. EXPERIMENTAL PROCEDURE

2.1. Materials

EPON 815 with curing agent U (weight ratio of resin/curing agent = 4: l), a room temperature curing epoxy resin from Shell Chemical Co., was used as the matrix material.

Two types of fiberglass fabrics, style 1597 and style 1800, and two types of Kevlar 49 fabrics, style 354 and style 383, from Clark-Schwebel were used as reinforcing media. All

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1160 Y. Wang et al.

the fabrics contained an epoxy-compatible finish. Glass 1597 is a triple plain weave fabric, glass 1800 and Kevlar 354 are plain weave fabrics, and Kevlar 383 is a five harness (SH) satin weave fabric. The fabric structures are shown in Fig. 1. The number of layers of fabric used to make the test panel, ranged from 4 to 18, was determined such that the final panel thickness was about 5 mm, that the fiber volume fraction was about 45%, and that the number is even.

Three types of microfibers were used as additives to the matrix in order to evaluate the effectiveness of toughening by the addition of such microfibers. The fibers and their typical dimensions are: (1) Franklin H-30 fiber from USG, a calcium sulfate whisker fiber, 2 ,um wide, 60 pm long; (2) 204BX PMF fiber from Sloss, a processed mineral fiber, 1-1Opm wide, 275pm long; and (3) Kevlar@ UltrathixTM from DuPont, a Kevlar pulp fiber, 0. l-l .2pm wide, 800 pm long. The weight ratios used relative to the matrix weight for these fibers were 3%) 2%) and I%, respectively. When the microfibers were added to the matrix resin, the resin viscosity increased significantly. Therefore only low percentages of microfibers were added to the matrix material. Four layers of fiberglass fabrics style 1597 were used as the reinforcement in the samples with microfibers. As seen from Fig. 2, these microfibers have significant variations in lengths and widths.

2.2. Fabrication

Seven types of composite panels (labeled A-G) were fabricated, including four (A-D) with the four types of fabrics in neat epoxy matrix, and three (E, F and G) with fiberglass fabric style 1597 in matrix containing different microfibers. The fabrication procedure is briefly described below.

The woven fabric was first cut into pieces of 305 mm long and 102 mm wide, with the fabric warp direction parallel to the sample longitudinal direction. The correct number of layers were aligned and stacked together, and put inside an aluminum mold whose interior dimensions match those of the preform (305 x 102 mm). A piece of plastic film (60 x 102 mm) was placed at one end between the two middle fabric layers to introduce an initial crack in the samples. The preform was then impregnated with the matrix mixture. Uniform sample thickness (approximately 5 mm) was maintained after the mold was tightly closed using C-clamps. After about 24 h curing at room temperature, the composite laminate panel was removed from the mold and ready for cutting into test specimens.

The volume fractions V, of the composite samples were calculated from the preform basic weight (weight per unit area) and they ranged between 40% and 47%.

2.3. Specimen preparation

Each composite panel was cut into three straight sided DCB specimens of about 254 mm long (L) and 25.4 mm wide (b), with 55 mm precrack length (a& shown in Fig. 3. The DCB test was first carried out in which a DCB specimen was separated into two strips of 2.55 mm thickness. Then the strips were cut into different sizes for the various tests: 254 x 25.4 mm for tension, 60 x 25.4 mm for three-point bending, 100 x 25.4 mm for compression, and 25.4 x 25.4 mm for short beam shear tests (Fig. 4). A diamond tipped abrasive saw mounted on a horizontal milling machine was used for the specimen cutting.

2.4. The DCB test

Three straight sided DCB specimens from each type of composite laminate were tested at the ambient condition (approximately 20°C and 65% RH) using a Monsanto tensile test machine with a 1 kN load cell. A pair of hinges were adhered to the loading end of the split beam specimen to allow the load to be applied without introducing bending moment at the end, as shown in Fig. 3. The loading and unloading speeds were kept at 30mm/min. Instantaneous load and crosshead displacement measured by a linear variable differential transformer (LVDT) were recorded by a computerized data

Woven fabric reinforced composites

Fig. 1. Photographs of reinforcing fabrics: (a) E-glass fabric Style 1597, triple plain weave, (b) E-glass fabric Style 1800, plain weave, (c) Kevlar 49 fabric Style 354, plain weave, (d) Kevlar

49 fabric Style 383, five-harness satin weave.

1162 Y. Wang et al.

Fig. 2. SEM graphs of microfibers: (a) Franklin H-30, (b) 204BX PMF, (c) Kevlar pulp.

Woven fabric reinforced composites 1163

Fig. 6. SEM graphs of fracture surfaces after test: (a) sample A (glass fabric 1597, epox (b) sample D (Kevlar fabric 383, epoxy).

1164 Y. Wang et al.

Fig. 10. Photographs of failed flexural test specimens: (a) glass fabric 1597/epoxy, (b) Key fabric 383/epoxy (top: tensile surface, bottom: compressive surface).

Jar


Fig. 11. Side view of a failed Kevlar fabric 383Iepoxy (sample D) flexural test specimen.

Fig. 15. Photographs of failed compressive test specimens: (a) glass fabric 1597/epoxy, (b) Kevlar fabric 383/epoxy.

1166 Y. Wang et al.

Fig. 17. Photographs of failed short beam shear test specimens: (a) glass fabric 1597/epoxy, (b) Kevlar fabric 383kpoxy.


Applied load P

1167

Fig. 3. Double cantilever beam (DCB) test (lengths in mm).

acquisition system. The strain energy release rate, G,, can be calculated based on the load-displacement response from

G,=-;g

where U is the total strain energy stored in the DCB specimen, a is the delamination crack length and b is the specimen width. The value of G, at which the delamination crack starts to propagate is the critical energy release rate or the interlaminar fracture toughness, G,, . For large deflections, the calculation of G,, involves the use of nonlinear analysis of

(a) Applied load t

W

Biaxial Extensometer (gage length=25.4)

Hydraulic grips

Applied load

_i

(a) Applied load

40 -_----------_--~ * _!z_+

Fig. 4. Test configurations (lengths in mm): (a) uniaxial tensile test, (b) compression test, (c) three-point flexural test, (d) short beam shear test.

1168 Y. Wang et al.

cantilever beams (Devitt et al., 1980). The basic steps after the DCB test are: (1) measure the bending stiffness of the DCB, (2) identify P and 6 at crack propagation from test curve, and (3) evaluate G,, using the nonlinear model.

2.5. Uniaxial tensile test

Two tensile specimens for each type of composite laminate were tested at the ambient condition on a computer controlled MTS 810 Tester (220 kN capacity) at 2 mm/min, as shown in Fig. 4(a). Hydraulic grips with surfalloy faces were used for specimen loading. The specimens were tested without tabs as these surfalloy faces, unlike the serrated ones, cause little damage to the specimen surfaces. A biaxial extensometer was used to monitor the specimen longitudinal and transverse strains. Instantaneous load P and displacements were recorded at a rate of one set per second. The elastic modulus (E), the Poisson’s ratio (v) were calculated using data regression, and the tensile strength was calculated from the maximum load and the actual specimen cross-sectional area.

2.6. Three-point flexural test

Five bending specimens for each composite type were tested on an Instron testing machine using a three-point bending fixture (Fig. 4(b)). The radius of the loading rollers was 5 mm and the test speed was 2.54 mm/min. The span length L used was kept at 40 mm, and therefore the span length to thickness ratio was about 16 : 1. Instantaneous load P and crosshead displacement 6 measured by a linear variable differential transformer (LVDT) were recorded by a computerized data acquisition system at one second intervals. The flexural modulus (E) and strength were calculated following ASTM D 790.

2.7. Compression test

The compression test was carried out to determine the compressive strength and modulus using surfalloy-faced hydraulic grips without tabs (Fig. 4(c)). The specimen gage length (between grips) was about 25.4 mm. Five specimens were tested for each sample. The displacement during the test was monitored using a clip gage transducer and a load-displacement curve was obtained for each specimen. From the test record, the compressive modulus and the compressive strength corresponding to the maximum load at failure could be determined.

2.8. Short beam shear test

Six short beam shear specimens from each composite type were tested using a modi- fied three-point bending fixture on an Instron tester (Fig. 4(d)). The radius of the loading edges was about 0.5 mm and the test speed was 2.54 mm/min. The span length was set at 12.7 mm, corresponding to a span length/thickness ratio of about 4 : 1. Instantaneous load P and crosshead displacement 6 measured by the LVDT were recorded by a computerized data acquisition system at a rate of one data pair per second. From the maximum load, the apparent interlaminar shear strength was calculated according to ASTM D 2344, and the maximum bending stress was also calculated.

3. RESULTS AND DISCUSSION

Composite samples tested and the experimental results are summarized in Table 1, including

(1) reinforcement, matrix material, and fiber volume fraction (V,), (2) interlaminar fracture toughness (4,) from the DCB test, (3) elastic modulus (E), Poisson’s ratio (v), and ultimate strength (a,) from the

uniaxial tensile test, (4) flexural modulus (EJ and strength (o,,) from the three-point flexural test, (5) compressive modulus (E,) and strength (0,) from the compression test, and (6) the maximum bending stress (or,) and apparent interlaminar shear stress (r) corre-

sponding to the maximum load from the short beam shear (SBS) test.


Table I. Specimen description and experimental results for DCB fracture, uniaxial tensile, 3-point flexural, compressive, and short beam shear (SBS) tests

Tensile Flexural Compressive SBS DCB

v, Gr, E V E E Sample Description (%) (kN/m) (GPa) ({Pa) (GPa) (hZa) (GPa) &Pa) (;:a) (Mka)

A-D: Reinforcement fabric style and number of layers as described. Matrix: Epon epoxy. A Glass 1597 45 0.82 20.4 280 0.18 17.8 424 16.0 192

4 layers B Glass 1800 43 1.67 21.3 321 - 17.4 402 16.5 214

16 layers C Kevlar 49 354 40 1.85 22.1 388 - 14.3 356 12.6 95

18 layers D Kevlar 49 383 47 1.64 23.8 425 0.11 15.0 326 14.0 98

12 layers

386 36.3

349 35.8

240 24.1

222 23.2

E-G: Reinforcement: Glass 1597, 4 layers. Matrix: Epon epoxy with microfibers. E Franklin H-30 45 1.44 19.1 263 0.18 15.5 392 15.3

3% F 204BX PMF 45 1.71 18.7 274 0.17 15.2 382 15.7

2% G Kevlar pulp 45 1.56 19.2 295 0.17 15.7 438 15.9

1%

203 365 36.6

208 362 35.8

204 383 37.8

3.1. In teriaminar fracture properties

Typical load versus displacement (P-8) curves for the woven glass fabric/epoxy (sample A) and woven Kevlar fabric/epoxy (sample D) from the DCB test are plotted in Fig. 5. In the P-6 curves, several critical points with sudden load drops corresponding to crack propagation can be identified. Therefore several G,, values for different crack lengths can be obtained for each specimen, and the values reported in Table 1 represent the overall average.

The toughening mechanisms in such composites and the effect of microfiber additives on the interlaminar toughness have been studied in detail and reported elsewhere (Wang and Zhao, 1995). It appeared that the weave pattern had a significant effect on the interlaminar fracture toughness, and there was a general trend of increasing G,, with the number of fabric layers, irrespective of the fiber type. The fiber type also showed a strong influence on the response of the composite system. From Fig. 5 it was noticed that for the glass fabric composite the crack propagation could be easily identified with sudden load drops, and each load drop corresponded to a large increase in crack length. In contrast, for the Kevlar fabric composite the crack extended gradually without sharp load drops. It can also be seen from the DCB test curves that the load-deflection responses for fiber glass composites are quite linear with little hysteresis, and that the opposite is true for Kevlar composites. All the composites seemed to show plastic deformations, identified from the residual displacement after full unloading.

The measured fracture toughness values (ranged from 0.8-l .9 kJ/m2) for the fabric reinforced epoxy composites were significantly higher than that for neat epoxy resins (typically 0.2 kJ/m2), indicating that the fracture resistance was not from the resin toughness alone. From SEM photographs in Fig. 6, some mechanisms contributing to the composite fracture resistance can be identified, including (1) fiber rupture, (2) fiber pull-out, (3) fiber debonding, (4) Kevlar fiber fibrillation, (5) multiple fracture of matrix resin, and (6) possible matrix plastic deformation. The toughness values of the composites tested, reflected the extent of the presence of these fracture mechanisms during the fracture test.

Large resin rich areas in composites could not only lead to low fracture toughness but also result in other undesirable effects on the mechanical properties. To improve the properties of textile composites, it is therefore desirable to reduce the resin rich areas in size and fraction, and to enhance the resin performance so as to reduce its weakening

1170 Y. Wang et al.

60

Applied Load (N)

70

60

50

40

30

20

10

0

Sample A

.

0 20 ‘40 60 80 100 120 140 160 180 200

Displacement (mm)

04

Applied Load (N)

0 20 40 60 80 100 120 140 160 180 200

Displacement (mm)

Fig. S. Typical load-displacement curves for the DCB test: (a) sample A (glass fabric 1597, epoxy), (b) sample D (Kevlar fabric 383, epoxy).

effects, by, for example, microfiber inclusion. As indicated by results in Table 1, the composite samples containing microfibers exhibited a 75-108% increase in fracture energy compared with similar samples (sample A) without microfibers. Micrographical analysis indicated that microfiber rupture, pull-out, fibrillation and crack deflection contributed to the overall fracture energy. It suggests that matrix toughening by microfibers could be an effective way to improve the interlaminar toughness of composites.

3.2. Mechanical properties

The split half specimens from the DCB test were tested in uniaxial tension, three- point bending, compression and short beam shearing for elastic modulus and strength. Testing the split specimens had the advantages of eliminating the variance from manufac- turing, and saving time and materials. The damage and plastic deformation from the DCB test to each half specimen were unnoticeable from observation.


Applied Load (kN)

1171

35

0 0.5 1 1.5 2 2.5 3 3.5 4

Displacement (mm)

Fig. 7. Typical load-displacement curves for the uniaxial tensile test for sample A (glass fabric 1597, epoxy) and sample D (Kevlar fabric 383, epoxy).

3.2.1. Tensile responses. Figure 7 illustrates typical load-displacement (P-d) responses of glass fabric and Kevlar fabric reinforced samples. The Kevlar reinforced samples showed a higher elastic modulus and higher strength in tension than the glass fabric reinforced samples. This is likely because Kevlar fibers are stronger and stiffer than glass fibers. The Kevlar reinforced samples also exhibited a fairly linear response, while the glass fabric reinforced samples exhibited significant nonlinearity in their tensile response. This suggests that the progressive matrix damage and debonding and/or failure of fibers lying in the transverse direction play an important role. In the glass fabric reinforced samples, the instantaneous (tangent) modulus decreases as transverse fibers debond. In Kevlar fabric reinforced samples, however, the transverse fibers do not con- tribute significantly, even at very low loads, due to the highly anisotropic nature of the Kevlar fiber (i.e. Kevlar fiber is extremely weak and soft in the transverse direction).

Figure 8 compares the experimental tensile modulus values with those predicted by the Classical Lamination Theory (CLT) (Jones, 1975), neglecting the fiber crimp and fabric pattern effects. Very good predictions for glass fabric composites were obtained, indicating that the effect of fiber crimp and fabric patterns on the tensile modulus is very small for such composites. The CLT overpredicted the modulus for Kevlar composites, indicating possibly a stronger effect due to fiber crimps in Kevlar fabric reinforced composites.

3.2.2. Fit?XUFal responses. Figure 9 compares the typical load-displacement curves from the three-point bending test for the two types of sample (glass and Kevlar). The glass fabric composites showed a slightly higher flexural strength than the Kevlar fabric composites. Failure of the tensile side is the dominant failure mode for glass composites,

Sample A

Sample B

Sample C

Sample D

0 5 IO 15 20 25 30 35

Tensile Modulus (GPa)

Fig. 8. Experimental results and theoretical predictions on tensile modulus.

1172 Y. Wang ef al.

Applied Load (kN)

1.2

1.0

0.8

0.6

0.4

0.2

0 1 2 3 4 5 6 7 8 9 10

Displacement (mm)

Fig. 9. Typical load-displacement curves for the three-point flexural test for sample A (glass fabric 1597, epoxy) and sample D (Kevlar fabric 383, epoxy).

and of both sides for Kevlar laminates, as seen from the pictures of failed specimens shown in Fig. 10. Below mid-plane delamination was noticeable for some Kevlar samples as can be seen in Fig. 11. The glass fabric composites tended to fail at the tensile surface in a brittle manner, while the Kevlar fabric composites generally failed gradually at larger deflections, indicating increased energy absorption and better damage tolerance. This difference is likely because the Kevlar fiber has poor transverse and shear properties and therefore Kevlar composites show poor strength in compression, as the compressive test results confirm. Fiber crimps in laminates also tend to lower the composite compressive strength as they make it easier for kink bands to form.

3.2.3. Compressive responses. The typical compressive responses of glass fabric and Kevlar fabric reinforced samples are shown in Fig. 12. The compressive moduli for the Kevlar samples are much lower than their respective tensile values, as shown in Fig. 13. This is due to the highly anisotropic nature of the Kevlar fiber (poor in transverse

Applied Load (kN)

15

10

5

0

,-

I --

T-

0 0.2 0.4 0.6 0.8 1

Displacement (mm)

Fig. 12. Typical load-displacement curves for the compression test for sample A (glass fabric 1597, epoxy) and sample D (Kevlar fabric 383, epoxy).


Modulus (GPa)

25

20

15

10

5

0

•In Tensile tj Compressive

Sample A Sample B Sample C Sample D

Fig. 13. Comparison between compressive and tensile moduli.

direction). The compressive strengths are lower than their tensile and flexural values, as seen in Fig. 14, due to fiber crimps and, for Kevlar composites, fiber anisotropy. It is also interesting to note that the flexural moduli of the Kevlar samples are lower than their tensile ones, as opposed to the glass samples. This correlates well with the low compressive strength for Kevlar samples. Figure 15 shows two failed specimens in compression, and shear failure was observed to be the predominant mode.

3.2.4. Short beam shear test responses. When a beam is subjected to three-point bending, the maximum shear stress T (interlaminar shear stress) occurs in the beam mid- plane and the maximum bending stresses ob (tensile and compression) occur at the beam upper and lower surfaces. The ratio r/or, increases as the beam span length to thickness ratio decreases, and thus the beam is more likely to fail in shear. An isotropic material in bending will fail in shear if r/q, exceeds 0.58 according to the Von Mises criterion. Anisotropic UIaterialS may fail in shear at a lOWt?r t/q, ratio. %-KC there is no gUari?inteC

that the specimen in a short beam shear test will fail in shear, the calculated r is referred to as the apparent interlaminar strength, which is a lower bound estimate to the interlaminar shear strength.

Figure 16 shows the load-displacement responses of composite samples in the short beam shear tests. The glass fabric composite samples showed similar responses to those observed in bending. However, the Kevlar composite samples behaved in a plastic manner after the initial peak load. It is interesting to note that the maximum bending stresses from the SBS test were all about 90% of the bending strength from the flexural test for glass fabric composites, and 67% for the Kevlar fabric composites. Pictures of failed glass and

Strength (MPa)

450 -r B Compressive •n Tensile ??Flexural

400

350

300

250

200

150 100

50 0

Sample A Sample B Sample C Sample D

Fig. 14. Comparison between compressive, tensile and flexural strengths.

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Applied Load (kN) 4

Y. Wang et al.

(a) (b)

0 0.4 0.8 1.2 1.6 2

Displacement (mm)

Fig. 16. Typical load-displacement curves for the short beam shear test for sample A (glass fabric 1597, epoxy) and sample D (Kevlar fabric 383, epoxy).

None

3% Franklin

2% PMF

1% Kevlar

0 5 10 15 20 25

Tensile Modulus (GPa)

cc)

None

3% Franklin

2% PMF

1% Kevlar

0

(d)

50 100 150 200 250 : Tensile Strength (MPa)

None

3% Franklin

2% PMF

1% Kevlar

0 5 10 15 20

Flexural Modulus (GPa)

(e)

--I 0 5 10 15 20

Compressive Modulus (GPa)

IO

None

3% Franklin

2% PMF

1% Kevlar

0 100 200 300 400 500

(0

None

3% Franklin

2% PMF

1% Kevlar 1 0 100 200 300 400 500

Compressive Strength (MPa)

Flexural Strength (MPa)

Fig. 18. Effect of microfiber addition in glass fabric 1597/epoxy composites on: (a) tensile modulus, (b) tensile strength, (c) flexural modulus, (d) flexural strength, (e) compressive modulus,

(f) compressive strength.


Kevlar composite samples are shown in Fig. 17. The glass sample did not reveal interlaminar failure. The Kevlar sample showed large plastic deformation throughout the short beam. There was no significant damage to either side of the Kevlar sample. Visible delamination was observed at locations close to the mid-plane. All these observations suggest that the glass fabric reinforced samples failed in bending in the SBS tests, and that the Kevlar fabric reinforced composites failed in shear. A possible explanation is that Kevlar fibers themselves have very weak transverse and shear resistance, and therefore Kevlar fabric laminates would have a lower interlaminar strength than glass fabric laminates, others being equal.

3.2.5. Effect of microfibers. Figure 18 compares the test results for sample A (without microfibers) with those for samples E, F and G (with microfibers). It appears that the presence of microfibers in the composite system did not have any strong effect on the elastic properties and the strengths of glass fabric reinforced composites. This is the result of low microfiber fraction in the composites.

4. CONCLUSIONS

An experimental study was carried out to evaluate the effects of fiber type, fabric pattern, and microfiber addition on the mechanical properties of woven fabric composites. Fiber glass and Kevlar 49 fabrics were used as the reinforcement. Fiber type showed strong effect on the mechanical behavior, particularly the load-deformation responses of the composites under tension, flexure, compression, short beam shear, and double cantilever beam test conditions. Because Kevlar fibers have poor transverse and shear strength, Kevlar composites exhibited a high degree of nonlinearity and lower strength in flexure, compression, and short beam shear compared with glass fiber reinforced composites. An addition of l-3% of microfibers to the epoxy matrix increased the interlaminar fracture toughness of glass fiber composites by 75-108%, but it showed no significant effect on other mechanical properties. It was observed that at similar fiber volume fractions, the structure of the reinforcement had a stronger influence on the toughness than fiber type.

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Bascom, W. D., Bitner, J. L., Moulton, R. J. and Siebert, A. R. (1980). The interlaminar fracture of organic- matrix, woven reinforcement composites. Composites lO( l), 9.

Bucknall, C. B. (1989). Approaches to toughness enhancement. In Advanced Composites (Edited by I. K. Partridge). Elsevier Applied Science Publishers, London.

Chou, T. W. and Ko, F. K. (eds) (1989). Textile Structurul Composites. Elsevier, New York. Devitt, D. F., Schapery, R. A. and Bradley, W. L. (1980). A method for determining the mode I delamination

fracture toughness of elastic and viscoelastic composite materials. J. Compos. Mater. 14, 270. Dulgin, D. (1971). Reinforcement of rubbers and plastics by particulate fillers. Composites 2. Jang, B. Z. (1991). Fracture behavior of fiber-resin composites containing a controlled interlaminar phase

(CIP). Sci. Engng Compos. Mater. 2(l), 29. Jones, R. M. (1975). Mechanics of Composite Muteriuls. McGraw-Hill, New York. Larsen, J. V. (1971). Fracture energy of CTBN epoxy-carbon fiber composites. SPI 26th Annual Technical

Conference, RPD 1971, Section 10-D. McGarry, F. J. and Mandell, J. F. (1972). Fracture toughness of fibrous glass reinforced plastic composites. SPI

27fh Annual Technical Conference, RPD 1972, Section 9A. McKenna, G. B. (1975). Interlaminar effects in fiber-reinforced plastics-A review. Polymer-Plasfics Tech-

nology & Engineering 5(l), 23. Phillios. D. C. and Tetelman. A. S. (1972). The fracture toughness of fiber composites. Composites 3, 216. Scott: J: M. and Phillips, R. d. (1975): Carbon fiber composites with rubber toughened materials. J. Muter. Sci.

10, 551. Wang, Y. and Zhao, D. (1995). Characterization of the interlaminar fracture behavior of woven fabric rein-

forced polymeric composites. Composires 26, 115.

mechanical properties of fiber glass and kevlar woven fabric reinforced composites

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