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Flexural properties of stitched GRP laminates
.-
Compositr.c Part A 27A (1996) 525-530 Copyright r(“’1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359~83SX!96/$15.00
S1359-835X(96)00010-3
I.I.SEVIER
Flexural properties laminates
of stitched
GRP
A. P. Mouritz Department of Defence, DSTO, Aeronautical P.O. Box 4331, Melbourne, 3001 Australia (Received 70 April 1995; revised 2 January
and Maritime
Research Laboratory,
1996)
The flexural properties (elastic moduli, yield strength and maximum strength) and flexural failure processes of GRP laminates reinforced through-the-thickness with Kevlar ’-49 stitches were studied under four-point bending at strain rates between about 10e6 and 3 x 10e2 s-‘. The flexural properties of the non-stitched laminate were larger than the stitched laminates over the range of strain rates studied. Scanning electron microscopy revealed that some of the failure processes in the non-stitched laminate differed from those of the stitched laminates, which occurred at lower flexural stresses. It appears that the lower flexural properties of the stitched laminates result from damage caused by stitching as well as by the stitches being stress concentration sites during bending. Copyright cl 1996 Elsevier Science Limited (Keywords: glass/polymer
laminates; stitching; four-point bending; flexural properties; failure processes)
INTRODUCTION A problem for many fibre reinforced polymer laminates used in aircraft and maritime structures is their relatively low mechanical strength in the through-thickness (translaminar) direction, which is determined mainly by the strength of the polymer matrix’. This can result in the relatively easy growth of delamination cracks between the fibres plies under impact loading, which usually reduces the strength and stiffness. However, it has been shown that the impact delamination resistance of carbon fibre reinforced polymer (CFRP) laminates2-‘, glass reinforced polymer (GRP) laminates8-‘0 and Kevlar ” reinforced epoxy laminates”.” can be improved by stitching through-the-thickness using a high tensile strength thread made from carbon, glass or Kevlar ’ “. Stitching improves the delamination resistance by raising the Mode I interlaminar fracture resistance of the laminate, which makes it more difficult for a delamination crack to propagate between the fibre plies4.‘4-‘6. The improved delamination resistance often results in stitched laminates having higher mechanical strength after impact loading compared with the non-stitched material. While stitching improves the mechanical properties in the through-thickness direction, it can degrade the in-plane properties of the laminate. In the stitching process a needle and thread are forced through the laminate, and this can damage the microstructure by breaking. spreading and crimping the fibres around the
stitch holes and by forming polymer-rich regions within the holes7.‘3.‘4. This damage has been found to reduce the in-plane tensile5”0”6.‘8 and compressive33’~~7~‘6~~‘9 strengths. The flexural properties also appear to be affected by stitching, but in some cases stitching appears to improve the flexural strength’.’ ’while in other cases it is reduced5,‘0,‘2. The cause for this discrepancy remains unclear, and therefore the aim of this paper is to study changes to the flexural properties and micro-fracture mechanisms during the bending of GRP laminates stitched with KevlarR thread. The changes to the mechanical properties of stitched laminates at high strain rates has not yet been studied, and therefore another aim of this paper is to measure the strain rate sensitivity of the flexural properties of stitched GRP. This work forms part of an evaluation study’0-‘4 into the possible use of stitching to improve the damage resistance of GRP used in composite naval ships, particularly small patrol boats and mine countermeasures vessels.
EXPERIMENTAL
DETAILS
The GRP laminates were fabricated by resin transfer moulding (RTM) using E-glass fibres and a vinyl ester polymer. and were stitched through-the-thickness with Kevlar R thread. The RTM procedure and materials have been described by Mouritz”. The glass preforms were made from an alternating stacking sequence of
525
Flexural properties
of stitched
5mm
GRP laminates: A. P. Mouritz
Figure 2 shows the dimensions
STITCH
SPACING
KNOT Figure 1 A schematic diagram of a modified lock stitch. The knots were at the back surface (tensile side) of the stitched laminates, and the stitches were spaced 5 mm apart
woven roving (WR) and chopped strand mat (CSM) plies to a total of 14 layers. This sequence was used because many GRP ships are made from an alternating stacking sequence of WR and CSM fibreglass. The preforms were stitched with Kevlar@-49 yarn using an industrial sewing machine by the Cooperative Research Centre-Aerospace Structures (Victoria, Australia). The Kevlar@ was sewn into the preform using a modified lock stitch with a stitch spacing of 5mm. Figure I presents a schematic diagram of the stitch geometry, and the stitch knots (or loops) were located along the bottom surface of the preform. The stitching was performed in parallel rows along the length of the preform, which was 1 m long and 0.5 m wide. The separation distance between the stitch rows was about 7 mm or 3.5 mm, and this produced laminates with average stitch densities of 3 and 6 stitches per cm2, respectively. The Kevlar@ was sewn into the glass preform with a slight but variable residual tensile stress. The magnitude of this stitch tension could not be accurately controlled, and therefore some variability exists between stitches. Examination of the stitched preforms revealed that where a stitch appeared to have high residual stress, the fibre greatest disturbance, the showed architecture particularly around the stitch knots. The non-stitched and stitched glass preforms were impregnated with the vinyl ester resin (known commercially as Derakane@ 41 l-45) using vacuum-assisted RTM, and then cured under ambient conditions. Panels of the cured laminates (1 m x 0.5 m) were about 6.1 mm thick, and the non-stitched GRP had a polymer content of 42% (by weight) while the stitched GRPs had polymer contents of about 38%. The polymer content was reduced slightly because of compaction to the preform caused by stitching. The flexural properties of the laminates were determined using the four-point bend method according to ASTM D790M-8425. Five types of laminate were tested:
1) non-stitched GRP; GRP stitched along the length of the specimen -7-Y (parallel direction) with 3 stitches per cm2; 3) GRP stitched along the length of the specimen with 6 stitches per cm2; 4) GRP stitched across the specimen (transverse direction) with 3 stitches per cm2; 5) GRP stitched across the specimen with 6 stitches per cm2.
526
of the flexural test specimens, and the stitching pattern for laminates with a parallel and transverse stitch direction. The specimens with a parallel stitch direction were cut from the resin transfer moulded panels along the same direction as the stitch rows, whereas those specimens with a transverse stitch direction were cut normal to the direction of the stitch rows. The four-point loading fixture used in the flexural tests had a load span and support span of 70 and 140 mm, respectively. The specimens were placed on the loading fixture with the top surface, being woven roving glass, on the compressive side and the bottom surface, being chopped strand mat, on the tensile side. Consequently, the stitched specimens were placed with the knots on the bottom (tensile) surface. Testing was performed using an Instron 1121 tensile machine driven at cross-head speeds between 0.05 and 1000 mm mini, which generated strain rates between about 10m6 and 3 x lop2 s-l. For each loading rate, the flexural tests were repeated five times on each type of stitched and non-stitched GRP to determine an average value and standard deviation for bending modulus, yield strength and maximum flexural strength. The development and growth of microstructural damage resulting from bending the non-stitched and stitched laminates was studied using scanning electron microscopy (SEM). The laminates were bent at a strain rate of lop4 s-’ under a range of flexural stresses which resulted in increasing amounts of damage, and then unloaded. A separate specimen was used for the different flexural stress levels. To observe the damage in crosssection, the four-point bend specimens were carefully sectioned, polished and etched in dilute hydrofluoric acid. The SEM examination was performed in the backscattered image mode to distinguish between the glass, polymer and Kevlar% phases in the GRP microstructures.
EXPERIMENTAL
RESULTS
Figure 3 shows examples
of flexural stress against displacement curves for the non-stitched and a stitched laminate measured in four-point bending at a strain rate of 1O-4s-i. In this case the laminate was stitched in the parallel direction with a high density (6 stitches per cm2), however the stress-displacement curves for the other stitched laminates were similar to this. The curves show a linear increase up to a stress (gv), when plastic yielding became noticeable. Beyond the yield stress, the curves increase up to a maximum stress (c,,,), before decreasing incrementally due to the progressive failure of the glass plies. Table 1 lists the elastic moduli, flexural yield strengths and flexural breaking strengths of the laminates tested at a strain rate of 10e4 s-l. The flexural properties of the stitched laminates are significantly lower (between 15 and 30%) than those of the non-stitched laminate.
Flexural
k34
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I-lT
I I I I I
I II dtitch
I I I I d
Rows
Non-Stitched
A. P. Mouritz
I I I I I
I II II IIII
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I I I I I
Laminate
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-4l3.5 mm
7 mm
GRP laminates:
I ll
I I I I 1
--II-
of stitched
400
I I I I I-
ll
properties
(b)
(21)
Stitched
Laminate
100
50
0
5
I
I
1
10
15
20
Maximum Displacement
(mm)
Figure 3 Flexural stress-displacement curves for the non-stitched laminate and the laminate stitched in the longitudinal direction with a density of 6 stitches per cm-. mY and omax are the yield and flexural strengths. respectively. The open circles on the curves represent the stresses to which the laminates were loaded before being unloaded and examined for microstructural damage
-a-
--_
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C--
-------
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(c) Figure flexural stitches stitches
Cd)
2 Diagrams showing the dimensions (in millimetres) of the test specimens stitched in the parallel direction to (a) 3 and (b) 6 per cm- and stitched in the transverse direction to(c) 3 and (b) 6 per cm’
Although there is no statistical difference between the flexural properties of the different stitched laminates, the average values of elastic modulus and yield strength of the laminates stitched in the parallel direction are slightly
lower than those stitched in the transverse direction. Furthermore, the average elastic modulus values were about 20% higher in the two laminates with the low stitch density compared with the high stitch density. Figure 4 shows the variation in the flexural strengths of the non-stitched laminate and the laminate stitched in the parallel direction with a high density when the strain rate was increased from ~10~~ to 3 x lo-’ s ’. Both materials show the same rate of increase in strength with strain rate, however, the strength of the non-stitched laminate remained about 15-30% higher than the stitched laminate. The elastic modulus of the nonstitched and stitched laminates was not affected by changes in the strain rate. The failure mechanisms involved during four-point bending were studied by loading the laminates up to the flexural stresses shown in Figure 3 (represented by the open circles), and then unloading and examining by SEM. No damage to the non-stitched GRP was observed until a flexural stress of about 300MPa, when the first sign of damage was the growth of short cracks (less than 20pm in length) between the glass fibres, as shown in Figure Sa. As shown, these cracks only developed along the tensile side of the four-point flexural specimens. Raising the flexural stress above 300 MPa caused these short cracks to grow, and by about 350MPa delaminations developed between the plies, as shown in Figure 5h. At the maximum flexural stress, the nonstitched laminate failed by the rapid growth of delaminations combined with the compressive buckling of the
527
Flexural properties Table 1
of stitched
GRP laminates: A. P. Mouritz
Flexural properties of the non-stitched and stitched laminates; the values given are based on five measurements Flexural yield
Flexural
Elastic
Laminate
strength (MPa)
strength (MPa)
modulus (GPa)
Non-stitched GRP
253 (f32)
386 (f15)
20.9
Parallel high density stitched GRP
180 (f29)
324 (k37)
12.3
Parallel low density stitched GRP
189 (f31)
327 (f12)
15.5
Transverse high density stitched GRP
214 (f16)
334 (f28)
15.7
Transverse low density stitched GRP
220 (133)
330 (It15)
19.4
500
C 4 S 400 $ iz Td S ; u
Stitched Laminate
g 300 E ‘ii ;”
200 10’7
IO.’
1o-3 Strain Rate
I 0-l W’ )
The effect of strain rate on the flexural strengths of the nonstitched laminate and the laminate stitched in the longitudinal direction to a high stitch density
Figure 4
glass plies. When the laminate was bent beyond this load, the delaminations spread along the specimen and extended through the thickness while the plies continued to buckle on the compressive side and fracture on the tensile side until complete failure. In comparison, the first sign of damage in the stitched laminates was observed at a bending stress of about 250 MPa, when cracks developed in the polymer matrix and debonding occurred along the fibre/polymer interfaces leading to small delaminations as shown in Figure 6a. It was interesting to find that this damage was only observed in the GRP microstructure immediately surrounding some of the stitch knots along the tensile side, as illustrated in Figure 6. However, not all of the knots were surrounded by this damage; but rather the damage only occurred around the knots which had caused considerable disturbance to the fibre architecture due to the higher stitching stress. When the flexural stress was increased above 250 MPa, the damage surrounding the knots spread into the microstructure, as shown in Figure 6b, and some of the glass fibres near the knots began to break. In addition, the plies on the compressive side began to buckle and delaminations developed, particularly along the stitch lines. At the maximum
528
W
cr= 350 MPa
Figure 5 A schematic representation of a non-stitched flexural specimen showing the location of bending damage, and scanning electron micrographs of (a) fine-scale cracking near the tensile surface when loaded to a flexural stress of 306 MPa and (b) delamination cracking between glass plies when loaded to 360 MPa
flexural stress, the stitched laminates failed by similar processes to the non-stitched material, with the Kevlar@ threads breaking across the widest delaminations. When bent beyond the maximum stress, the stitched laminates
Flexural properties
(a)
(r =
250MPa
Figure 6 A schematic representation of a stitched flexural specimen showing the location of bending damage, and scanning electron micrographs showing cracking and delaminations surrounding a stitch knot near the tensile surface of a laminate stitched in the transverse direction with 3 stitches per cm’ loaded to (a) 261 MPa and (b) 323 MPa
usually failed along one of the stitching rows across the specimen. DISCUSSION The flexural properties of the GRP laminate were reduced considerably when stitched through-thethickness with Kevlar@ thread. This finding is consistent with three-point bending studies by Harris et al.’ and Cholakara rt al.” who found the flexural strength and stiffness of graphite/epoxy and Kevlar’%/epoxy laminates were reduced by stitching. The results in Table I also show that the yield and flexural strengths of the different stitched laminates were statistically similar, indicating that they were not affected by stitch direction (parallel or transverse) or stitch density (3 or 6 stitches per cm2). This observation differs from flexural studies by Kang and Lee” on a glass/polyester laminate which was stitched with Kevlar ’ using a plain or chain stitch. With the plain stitch, the flexural strength initially increased with stitch density but decreased at high stitch densities. In contrast, with the chain stitch the flexural strength decreased with increasing stitch density. Kang and Lee concluded that the variation in flexural strength with stitch density is dependent on the type of stitching. The GRP laminates studied here were stitched with a modified lock stitch.
of stitched
GRP laminates: A. P. Mouritz
which has a different geometry to the plain and chain stitches used by Kang and Lee”, and this may account for the flexural strength of the GRP being similar for the two stitch densities. The reduction in the flexural properties may be caused, in part, by the damage introduced into the glass preform during stitching. Examination of the microstructure of the stitched GRP laminates revealed breakage of glass fibres surrounding the stitch holes, and polymer-rich regions within the stitch holes. This type of damage is typical of stitching, and has been proposed to cause the reduction in the tensile and compressive strengths of stitched laminates”. Examination of the GRP microstructures following flexural loading revealed some differences in the damage processes between the non-stitched and stitched laminates. The first signs of damage in the stitched laminates occurred in the vicinity of those stitch knots which had disturbed the fibre architecture. It appears that during bending these knots are stress concentration sites, which promote the formation of defects at a lower stress (about 250MPd) than in the non-stitched laminate (about 300MPa). When the non-stitched and stitched laminates were subjected to higher loads up to their maximum strengths, they experienced similar failure mechanisms, including delaminations, and compressive buckling and tensile fracture of the fibres. However, complete failure of the stitched laminates was localised to along the stitch rows while the non-stitched laminates failed in a more wide-spread region by delamination and fibre failure through the thickness. Kang and Lee” and Cholakara et al.” suggest that stitch holes can be stress concentration sites during three-point bending. and thereby reduce the flexural strengths of laminates. The study presented here, however, shows that damage is nucleated near the stitch knots and then spreads around the stitch holes, which ultimately leads to lower flexural properties. It is expected that laminates which are stitched in ‘1 way to produce stress concentration sites, such as the knots in lock and chain stitches, will exhibit a similar failure sequence to that observed here for the modified lock stitch. Figure 4 shows that the relationship between llexural strength and strain rate for the non-stitched and stitched laminates was the same, indicating that the strain rate sensitivity of GRP is not affected by stitching for strain rates below 3 x 10 -’ ss’. No work has been reported on strain rate sensitivity of stitched laminates, although Boukhili et 01.~~measured an increase in the flexural strength with strain rate of non-stitched polyester- and epoxy-based GRP laminates which was similar to the non-stitched GRP studied here. The data presented in Figure 4 shows that the Kevlar’ stitches and the damage arising from stitching does not affect the strain rate sensitivity of GRP.
CONCLUSIONS The flexural properties of GRP laminates are reduced
529
Flexural properties
of stitched
GRP laminates: A. P. Mouritz
when stitched through-the-thickness with Kevlar@ at strain rates between low6 and 3 x 10m2s-’. This reduction was partly caused by the damage introduced during the stitching process, including breakage of glass fibres and formation of polymer-rich regions at the stitch holes. The reduction was also caused by the stitch knots and holes, which are stress concentration sites in the GRP microstructure, and thereby promote the growth of damage at relatively low flexural stresses. The yield and flexural strengths of the stitched laminates were not affected by either the orientation or density of the stitches, however, the elastic modulus was higher for laminates stitched in the transverse direction and/or with a low stitch density.
8 9
10 11 12 I3
50,305
14
Hull, D. ‘Introduction to Composite Materials’, University of Cambridge Press, Cambridge, 1981 Mignery, L.A., Tan, T.M. and Sun, C.T. in ‘Delamination and Debondina. ASTM STP 876’ (ed. W.S. Johnson). American Society f& Testing and Materials, Philadelphia;’ 1985, pp. 371-385 Dow, M.B. and Smith, D.L. in ‘21st Int. SAMPE Tech. Conf.‘, 25-28 September, 1989, pp. 595-605 Pelstring, R.M. and Madan, R.C. in ‘34th Int. SAMPE Symp.‘, 8-11 May 1989, pp. 1519-1528 Harris, H., Schinske, N., Krueger, R. and Swanson, B. in ‘36th Int. SAMPE Symp.‘, 15-18 April 1991, pp. 521-535 Farley, G.L., Smith, B.T. and Maiden, J. J. Rein. Plusf. Comp. 1992, 11, 787 Portanova, M.A., Poe, C.C. and Whitcomb, J.D. in ‘Composite Materials: Testing and Design (Tenth Volume), ASTM STP 1120’ (ed. G.C. Grimes), American Society for Testing and Materials, Philadelphia, 1992, pp. 37-53
530
Su, K.B. in ‘Advances in Thermoplastic Matrix Composite Materials, ASTM STP 1044’ (ed. G.M. Newas), American Society for Testing and Materials, Philadelphia, 1989, pp. 279-300
15 16
17
REFERENCES
Liu, D. J. Rein. Plast. Comp. 1990, 9, 59 Adanur, S. and Tsao, Y.P. in ‘26th Int. SAMPE Tech. Conf.‘, 17-20 October 1994, pp. 25-34 Kang, T.J. and Lee, S.H. J. Comp. Mat. 1994, 28, 1574 Chang, W.C., Jang, B.Z., Chang, T.C., Hwang, L.R. and Wilcox, R.C. Mat. Sci. Eng. 1989, A112, I57 Cholakara, M.T., Jang, B.Z. and Wang, C.Z., in ‘34th Int. SAMPE Conf.‘, 8-l 1 May 1989, pp. 2153-2160 Dransfield, K., Baillie, C. and Mai, Y. W. Comp. Sci. Tech. 1994,
18 19 20 21 22 23 24 25
26
Morales, A. in ‘22nd Int. SAMPE Tech. Conf.‘, 6-8 November 1990, pp. 1217-1230 Dexter, H.B. and Funk, J.G. in ‘27th Conference on Structures, Structural Dynamics and Materials’, 19-21 May 1986, pp. 700709 Hertzberg, I. and Bannister, M.K. in ‘Proc. 5th Aust. Aero. Conf.‘, 13-15 September 1993, pp. 213-218 Du, X., Xue, F. and Gu, Z. in ‘Proc. Int. Symp. Comp. Mat. and Struct.‘, Technomic, Lancaster, PA, 1986, pp. 912-918 Ogo, Y. MSc thesis, University of Delaware, Newark, 1987 Mouritz, A.P. Comp. Sci. Tech. 1995,55, 365-374 Mouritz, A.P. ‘Proc. ICCM-10, Vol. 5’(eds A. Poursartip and K. Street), Woodhead Publishing Ltd, pp. 695-701 Mouritz, A.P., Gallagher, J. and Goodwin, A.A. Comp. Sci. Tech. (submitted) Fuller, R., Gallagher, J., Goodwin, A.A. and Mouritz, A.P. ‘Materials Research 96’, 9-12 July 1996 (in press) Shah Khan, Z. and Mouritz, A.P. J. Comp. Sci. Tech. (in press) ASTM D790M-84, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials (Metric), ASTM Standards and Literature References for Composite Materials, American Society for Testing and Materials, Philadelphia, 1984 Boukhili, R., Hubert, P. and Gauvin, R. Composites 1991,22,39
Compositr.c Part A 27A (1996) 525-530 Copyright r(“’1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359~83SX!96/$15.00
S1359-835X(96)00010-3
I.I.SEVIER
Flexural properties laminates
of stitched
GRP
A. P. Mouritz Department of Defence, DSTO, Aeronautical P.O. Box 4331, Melbourne, 3001 Australia (Received 70 April 1995; revised 2 January
and Maritime
Research Laboratory,
1996)
The flexural properties (elastic moduli, yield strength and maximum strength) and flexural failure processes of GRP laminates reinforced through-the-thickness with Kevlar ’-49 stitches were studied under four-point bending at strain rates between about 10e6 and 3 x 10e2 s-‘. The flexural properties of the non-stitched laminate were larger than the stitched laminates over the range of strain rates studied. Scanning electron microscopy revealed that some of the failure processes in the non-stitched laminate differed from those of the stitched laminates, which occurred at lower flexural stresses. It appears that the lower flexural properties of the stitched laminates result from damage caused by stitching as well as by the stitches being stress concentration sites during bending. Copyright cl 1996 Elsevier Science Limited (Keywords: glass/polymer
laminates; stitching; four-point bending; flexural properties; failure processes)
INTRODUCTION A problem for many fibre reinforced polymer laminates used in aircraft and maritime structures is their relatively low mechanical strength in the through-thickness (translaminar) direction, which is determined mainly by the strength of the polymer matrix’. This can result in the relatively easy growth of delamination cracks between the fibres plies under impact loading, which usually reduces the strength and stiffness. However, it has been shown that the impact delamination resistance of carbon fibre reinforced polymer (CFRP) laminates2-‘, glass reinforced polymer (GRP) laminates8-‘0 and Kevlar ” reinforced epoxy laminates”.” can be improved by stitching through-the-thickness using a high tensile strength thread made from carbon, glass or Kevlar ’ “. Stitching improves the delamination resistance by raising the Mode I interlaminar fracture resistance of the laminate, which makes it more difficult for a delamination crack to propagate between the fibre plies4.‘4-‘6. The improved delamination resistance often results in stitched laminates having higher mechanical strength after impact loading compared with the non-stitched material. While stitching improves the mechanical properties in the through-thickness direction, it can degrade the in-plane properties of the laminate. In the stitching process a needle and thread are forced through the laminate, and this can damage the microstructure by breaking. spreading and crimping the fibres around the
stitch holes and by forming polymer-rich regions within the holes7.‘3.‘4. This damage has been found to reduce the in-plane tensile5”0”6.‘8 and compressive33’~~7~‘6~~‘9 strengths. The flexural properties also appear to be affected by stitching, but in some cases stitching appears to improve the flexural strength’.’ ’while in other cases it is reduced5,‘0,‘2. The cause for this discrepancy remains unclear, and therefore the aim of this paper is to study changes to the flexural properties and micro-fracture mechanisms during the bending of GRP laminates stitched with KevlarR thread. The changes to the mechanical properties of stitched laminates at high strain rates has not yet been studied, and therefore another aim of this paper is to measure the strain rate sensitivity of the flexural properties of stitched GRP. This work forms part of an evaluation study’0-‘4 into the possible use of stitching to improve the damage resistance of GRP used in composite naval ships, particularly small patrol boats and mine countermeasures vessels.
EXPERIMENTAL
DETAILS
The GRP laminates were fabricated by resin transfer moulding (RTM) using E-glass fibres and a vinyl ester polymer. and were stitched through-the-thickness with Kevlar R thread. The RTM procedure and materials have been described by Mouritz”. The glass preforms were made from an alternating stacking sequence of
525
Flexural properties
of stitched
5mm
GRP laminates: A. P. Mouritz
Figure 2 shows the dimensions
STITCH
SPACING
KNOT Figure 1 A schematic diagram of a modified lock stitch. The knots were at the back surface (tensile side) of the stitched laminates, and the stitches were spaced 5 mm apart
woven roving (WR) and chopped strand mat (CSM) plies to a total of 14 layers. This sequence was used because many GRP ships are made from an alternating stacking sequence of WR and CSM fibreglass. The preforms were stitched with Kevlar@-49 yarn using an industrial sewing machine by the Cooperative Research Centre-Aerospace Structures (Victoria, Australia). The Kevlar@ was sewn into the preform using a modified lock stitch with a stitch spacing of 5mm. Figure I presents a schematic diagram of the stitch geometry, and the stitch knots (or loops) were located along the bottom surface of the preform. The stitching was performed in parallel rows along the length of the preform, which was 1 m long and 0.5 m wide. The separation distance between the stitch rows was about 7 mm or 3.5 mm, and this produced laminates with average stitch densities of 3 and 6 stitches per cm2, respectively. The Kevlar@ was sewn into the glass preform with a slight but variable residual tensile stress. The magnitude of this stitch tension could not be accurately controlled, and therefore some variability exists between stitches. Examination of the stitched preforms revealed that where a stitch appeared to have high residual stress, the fibre greatest disturbance, the showed architecture particularly around the stitch knots. The non-stitched and stitched glass preforms were impregnated with the vinyl ester resin (known commercially as Derakane@ 41 l-45) using vacuum-assisted RTM, and then cured under ambient conditions. Panels of the cured laminates (1 m x 0.5 m) were about 6.1 mm thick, and the non-stitched GRP had a polymer content of 42% (by weight) while the stitched GRPs had polymer contents of about 38%. The polymer content was reduced slightly because of compaction to the preform caused by stitching. The flexural properties of the laminates were determined using the four-point bend method according to ASTM D790M-8425. Five types of laminate were tested:
1) non-stitched GRP; GRP stitched along the length of the specimen -7-Y (parallel direction) with 3 stitches per cm2; 3) GRP stitched along the length of the specimen with 6 stitches per cm2; 4) GRP stitched across the specimen (transverse direction) with 3 stitches per cm2; 5) GRP stitched across the specimen with 6 stitches per cm2.
526
of the flexural test specimens, and the stitching pattern for laminates with a parallel and transverse stitch direction. The specimens with a parallel stitch direction were cut from the resin transfer moulded panels along the same direction as the stitch rows, whereas those specimens with a transverse stitch direction were cut normal to the direction of the stitch rows. The four-point loading fixture used in the flexural tests had a load span and support span of 70 and 140 mm, respectively. The specimens were placed on the loading fixture with the top surface, being woven roving glass, on the compressive side and the bottom surface, being chopped strand mat, on the tensile side. Consequently, the stitched specimens were placed with the knots on the bottom (tensile) surface. Testing was performed using an Instron 1121 tensile machine driven at cross-head speeds between 0.05 and 1000 mm mini, which generated strain rates between about 10m6 and 3 x lop2 s-l. For each loading rate, the flexural tests were repeated five times on each type of stitched and non-stitched GRP to determine an average value and standard deviation for bending modulus, yield strength and maximum flexural strength. The development and growth of microstructural damage resulting from bending the non-stitched and stitched laminates was studied using scanning electron microscopy (SEM). The laminates were bent at a strain rate of lop4 s-’ under a range of flexural stresses which resulted in increasing amounts of damage, and then unloaded. A separate specimen was used for the different flexural stress levels. To observe the damage in crosssection, the four-point bend specimens were carefully sectioned, polished and etched in dilute hydrofluoric acid. The SEM examination was performed in the backscattered image mode to distinguish between the glass, polymer and Kevlar% phases in the GRP microstructures.
EXPERIMENTAL
RESULTS
Figure 3 shows examples
of flexural stress against displacement curves for the non-stitched and a stitched laminate measured in four-point bending at a strain rate of 1O-4s-i. In this case the laminate was stitched in the parallel direction with a high density (6 stitches per cm2), however the stress-displacement curves for the other stitched laminates were similar to this. The curves show a linear increase up to a stress (gv), when plastic yielding became noticeable. Beyond the yield stress, the curves increase up to a maximum stress (c,,,), before decreasing incrementally due to the progressive failure of the glass plies. Table 1 lists the elastic moduli, flexural yield strengths and flexural breaking strengths of the laminates tested at a strain rate of 10e4 s-l. The flexural properties of the stitched laminates are significantly lower (between 15 and 30%) than those of the non-stitched laminate.
Flexural
k34
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I-lT
I I I I I
I II dtitch
I I I I d
Rows
Non-Stitched
A. P. Mouritz
I I I I I
I II II IIII
I I I I I
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I I I I I
I Ill1 IIII
l l I I I
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I I l I I
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I I I I I
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I I I I I
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I I I I I
Laminate
I II
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1
II II
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-4l3.5 mm
7 mm
GRP laminates:
I ll
I I I I 1
--II-
of stitched
400
I I I I I-
ll
properties
(b)
(21)
Stitched
Laminate
100
50
0
5
I
I
1
10
15
20
Maximum Displacement
(mm)
Figure 3 Flexural stress-displacement curves for the non-stitched laminate and the laminate stitched in the longitudinal direction with a density of 6 stitches per cm-. mY and omax are the yield and flexural strengths. respectively. The open circles on the curves represent the stresses to which the laminates were loaded before being unloaded and examined for microstructural damage
-a-
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C--
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(c) Figure flexural stitches stitches
Cd)
2 Diagrams showing the dimensions (in millimetres) of the test specimens stitched in the parallel direction to (a) 3 and (b) 6 per cm- and stitched in the transverse direction to(c) 3 and (b) 6 per cm’
Although there is no statistical difference between the flexural properties of the different stitched laminates, the average values of elastic modulus and yield strength of the laminates stitched in the parallel direction are slightly
lower than those stitched in the transverse direction. Furthermore, the average elastic modulus values were about 20% higher in the two laminates with the low stitch density compared with the high stitch density. Figure 4 shows the variation in the flexural strengths of the non-stitched laminate and the laminate stitched in the parallel direction with a high density when the strain rate was increased from ~10~~ to 3 x lo-’ s ’. Both materials show the same rate of increase in strength with strain rate, however, the strength of the non-stitched laminate remained about 15-30% higher than the stitched laminate. The elastic modulus of the nonstitched and stitched laminates was not affected by changes in the strain rate. The failure mechanisms involved during four-point bending were studied by loading the laminates up to the flexural stresses shown in Figure 3 (represented by the open circles), and then unloading and examining by SEM. No damage to the non-stitched GRP was observed until a flexural stress of about 300MPa, when the first sign of damage was the growth of short cracks (less than 20pm in length) between the glass fibres, as shown in Figure Sa. As shown, these cracks only developed along the tensile side of the four-point flexural specimens. Raising the flexural stress above 300 MPa caused these short cracks to grow, and by about 350MPa delaminations developed between the plies, as shown in Figure 5h. At the maximum flexural stress, the nonstitched laminate failed by the rapid growth of delaminations combined with the compressive buckling of the
527
Flexural properties Table 1
of stitched
GRP laminates: A. P. Mouritz
Flexural properties of the non-stitched and stitched laminates; the values given are based on five measurements Flexural yield
Flexural
Elastic
Laminate
strength (MPa)
strength (MPa)
modulus (GPa)
Non-stitched GRP
253 (f32)
386 (f15)
20.9
Parallel high density stitched GRP
180 (f29)
324 (k37)
12.3
Parallel low density stitched GRP
189 (f31)
327 (f12)
15.5
Transverse high density stitched GRP
214 (f16)
334 (f28)
15.7
Transverse low density stitched GRP
220 (133)
330 (It15)
19.4
500
C 4 S 400 $ iz Td S ; u
Stitched Laminate
g 300 E ‘ii ;”
200 10’7
IO.’
1o-3 Strain Rate
I 0-l W’ )
The effect of strain rate on the flexural strengths of the nonstitched laminate and the laminate stitched in the longitudinal direction to a high stitch density
Figure 4
glass plies. When the laminate was bent beyond this load, the delaminations spread along the specimen and extended through the thickness while the plies continued to buckle on the compressive side and fracture on the tensile side until complete failure. In comparison, the first sign of damage in the stitched laminates was observed at a bending stress of about 250 MPa, when cracks developed in the polymer matrix and debonding occurred along the fibre/polymer interfaces leading to small delaminations as shown in Figure 6a. It was interesting to find that this damage was only observed in the GRP microstructure immediately surrounding some of the stitch knots along the tensile side, as illustrated in Figure 6. However, not all of the knots were surrounded by this damage; but rather the damage only occurred around the knots which had caused considerable disturbance to the fibre architecture due to the higher stitching stress. When the flexural stress was increased above 250 MPa, the damage surrounding the knots spread into the microstructure, as shown in Figure 6b, and some of the glass fibres near the knots began to break. In addition, the plies on the compressive side began to buckle and delaminations developed, particularly along the stitch lines. At the maximum
528
W
cr= 350 MPa
Figure 5 A schematic representation of a non-stitched flexural specimen showing the location of bending damage, and scanning electron micrographs of (a) fine-scale cracking near the tensile surface when loaded to a flexural stress of 306 MPa and (b) delamination cracking between glass plies when loaded to 360 MPa
flexural stress, the stitched laminates failed by similar processes to the non-stitched material, with the Kevlar@ threads breaking across the widest delaminations. When bent beyond the maximum stress, the stitched laminates
Flexural properties
(a)
(r =
250MPa
Figure 6 A schematic representation of a stitched flexural specimen showing the location of bending damage, and scanning electron micrographs showing cracking and delaminations surrounding a stitch knot near the tensile surface of a laminate stitched in the transverse direction with 3 stitches per cm’ loaded to (a) 261 MPa and (b) 323 MPa
usually failed along one of the stitching rows across the specimen. DISCUSSION The flexural properties of the GRP laminate were reduced considerably when stitched through-thethickness with Kevlar@ thread. This finding is consistent with three-point bending studies by Harris et al.’ and Cholakara rt al.” who found the flexural strength and stiffness of graphite/epoxy and Kevlar’%/epoxy laminates were reduced by stitching. The results in Table I also show that the yield and flexural strengths of the different stitched laminates were statistically similar, indicating that they were not affected by stitch direction (parallel or transverse) or stitch density (3 or 6 stitches per cm2). This observation differs from flexural studies by Kang and Lee” on a glass/polyester laminate which was stitched with Kevlar ’ using a plain or chain stitch. With the plain stitch, the flexural strength initially increased with stitch density but decreased at high stitch densities. In contrast, with the chain stitch the flexural strength decreased with increasing stitch density. Kang and Lee concluded that the variation in flexural strength with stitch density is dependent on the type of stitching. The GRP laminates studied here were stitched with a modified lock stitch.
of stitched
GRP laminates: A. P. Mouritz
which has a different geometry to the plain and chain stitches used by Kang and Lee”, and this may account for the flexural strength of the GRP being similar for the two stitch densities. The reduction in the flexural properties may be caused, in part, by the damage introduced into the glass preform during stitching. Examination of the microstructure of the stitched GRP laminates revealed breakage of glass fibres surrounding the stitch holes, and polymer-rich regions within the stitch holes. This type of damage is typical of stitching, and has been proposed to cause the reduction in the tensile and compressive strengths of stitched laminates”. Examination of the GRP microstructures following flexural loading revealed some differences in the damage processes between the non-stitched and stitched laminates. The first signs of damage in the stitched laminates occurred in the vicinity of those stitch knots which had disturbed the fibre architecture. It appears that during bending these knots are stress concentration sites, which promote the formation of defects at a lower stress (about 250MPd) than in the non-stitched laminate (about 300MPa). When the non-stitched and stitched laminates were subjected to higher loads up to their maximum strengths, they experienced similar failure mechanisms, including delaminations, and compressive buckling and tensile fracture of the fibres. However, complete failure of the stitched laminates was localised to along the stitch rows while the non-stitched laminates failed in a more wide-spread region by delamination and fibre failure through the thickness. Kang and Lee” and Cholakara et al.” suggest that stitch holes can be stress concentration sites during three-point bending. and thereby reduce the flexural strengths of laminates. The study presented here, however, shows that damage is nucleated near the stitch knots and then spreads around the stitch holes, which ultimately leads to lower flexural properties. It is expected that laminates which are stitched in ‘1 way to produce stress concentration sites, such as the knots in lock and chain stitches, will exhibit a similar failure sequence to that observed here for the modified lock stitch. Figure 4 shows that the relationship between llexural strength and strain rate for the non-stitched and stitched laminates was the same, indicating that the strain rate sensitivity of GRP is not affected by stitching for strain rates below 3 x 10 -’ ss’. No work has been reported on strain rate sensitivity of stitched laminates, although Boukhili et 01.~~measured an increase in the flexural strength with strain rate of non-stitched polyester- and epoxy-based GRP laminates which was similar to the non-stitched GRP studied here. The data presented in Figure 4 shows that the Kevlar’ stitches and the damage arising from stitching does not affect the strain rate sensitivity of GRP.
CONCLUSIONS The flexural properties of GRP laminates are reduced
529
Flexural properties
of stitched
GRP laminates: A. P. Mouritz
when stitched through-the-thickness with Kevlar@ at strain rates between low6 and 3 x 10m2s-’. This reduction was partly caused by the damage introduced during the stitching process, including breakage of glass fibres and formation of polymer-rich regions at the stitch holes. The reduction was also caused by the stitch knots and holes, which are stress concentration sites in the GRP microstructure, and thereby promote the growth of damage at relatively low flexural stresses. The yield and flexural strengths of the stitched laminates were not affected by either the orientation or density of the stitches, however, the elastic modulus was higher for laminates stitched in the transverse direction and/or with a low stitch density.
8 9
10 11 12 I3
50,305
14
Hull, D. ‘Introduction to Composite Materials’, University of Cambridge Press, Cambridge, 1981 Mignery, L.A., Tan, T.M. and Sun, C.T. in ‘Delamination and Debondina. ASTM STP 876’ (ed. W.S. Johnson). American Society f& Testing and Materials, Philadelphia;’ 1985, pp. 371-385 Dow, M.B. and Smith, D.L. in ‘21st Int. SAMPE Tech. Conf.‘, 25-28 September, 1989, pp. 595-605 Pelstring, R.M. and Madan, R.C. in ‘34th Int. SAMPE Symp.‘, 8-11 May 1989, pp. 1519-1528 Harris, H., Schinske, N., Krueger, R. and Swanson, B. in ‘36th Int. SAMPE Symp.‘, 15-18 April 1991, pp. 521-535 Farley, G.L., Smith, B.T. and Maiden, J. J. Rein. Plusf. Comp. 1992, 11, 787 Portanova, M.A., Poe, C.C. and Whitcomb, J.D. in ‘Composite Materials: Testing and Design (Tenth Volume), ASTM STP 1120’ (ed. G.C. Grimes), American Society for Testing and Materials, Philadelphia, 1992, pp. 37-53
530
Su, K.B. in ‘Advances in Thermoplastic Matrix Composite Materials, ASTM STP 1044’ (ed. G.M. Newas), American Society for Testing and Materials, Philadelphia, 1989, pp. 279-300
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REFERENCES
Liu, D. J. Rein. Plast. Comp. 1990, 9, 59 Adanur, S. and Tsao, Y.P. in ‘26th Int. SAMPE Tech. Conf.‘, 17-20 October 1994, pp. 25-34 Kang, T.J. and Lee, S.H. J. Comp. Mat. 1994, 28, 1574 Chang, W.C., Jang, B.Z., Chang, T.C., Hwang, L.R. and Wilcox, R.C. Mat. Sci. Eng. 1989, A112, I57 Cholakara, M.T., Jang, B.Z. and Wang, C.Z., in ‘34th Int. SAMPE Conf.‘, 8-l 1 May 1989, pp. 2153-2160 Dransfield, K., Baillie, C. and Mai, Y. W. Comp. Sci. Tech. 1994,
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Morales, A. in ‘22nd Int. SAMPE Tech. Conf.‘, 6-8 November 1990, pp. 1217-1230 Dexter, H.B. and Funk, J.G. in ‘27th Conference on Structures, Structural Dynamics and Materials’, 19-21 May 1986, pp. 700709 Hertzberg, I. and Bannister, M.K. in ‘Proc. 5th Aust. Aero. Conf.‘, 13-15 September 1993, pp. 213-218 Du, X., Xue, F. and Gu, Z. in ‘Proc. Int. Symp. Comp. Mat. and Struct.‘, Technomic, Lancaster, PA, 1986, pp. 912-918 Ogo, Y. MSc thesis, University of Delaware, Newark, 1987 Mouritz, A.P. Comp. Sci. Tech. 1995,55, 365-374 Mouritz, A.P. ‘Proc. ICCM-10, Vol. 5’(eds A. Poursartip and K. Street), Woodhead Publishing Ltd, pp. 695-701 Mouritz, A.P., Gallagher, J. and Goodwin, A.A. Comp. Sci. Tech. (submitted) Fuller, R., Gallagher, J., Goodwin, A.A. and Mouritz, A.P. ‘Materials Research 96’, 9-12 July 1996 (in press) Shah Khan, Z. and Mouritz, A.P. J. Comp. Sci. Tech. (in press) ASTM D790M-84, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials (Metric), ASTM Standards and Literature References for Composite Materials, American Society for Testing and Materials, Philadelphia, 1984 Boukhili, R., Hubert, P. and Gauvin, R. Composites 1991,22,39