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Tensile fatigue properties of 3D composites with through-thickness reinforcement
Composites Science and Technology 68 (2008) 2503–2510
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Tensile fatigue properties of 3D composites with through-thickness reinforcement A.P. Mouritz * School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476V, Melbourne, Australia
a r t i c l e
i n f o
Article history: Received 16 January 2008 Received in revised form 22 April 2008 Accepted 2 May 2008 Available online 8 May 2008 Keywords: A. Polymer-matrix composites B. Fatigue, through-thickness reinforcement
a b s t r a c t The tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites with through-thickness reinforcement are compared in this paper. Tensile tests under monotonic and cyclic loading were performed on the 3D composite materials to determine the influence of the z-reinforcement type – woven z-binder, stitch or z-pin – on the tensile modulus, strength and fatigue life. The in-plane Young’s modulus of the composites was not affected by the type or volume content of the z-reinforcement. The tensile strength of the 3D woven and stitched composites was also not affected by the z-reinforcement, however the strength of the z-pinned composite dropped steadily with increasing volume content of z-reinforcement. The fatigue life of the 3D composites was reduced by the z-reinforcement, regardless of whether they were woven z-binders, stitches or z-pins. The fatigue lives of the 3D composites decreased with increasing volume content of z-reinforcement. The tensile fatigue properties are degraded by the z-reinforcement causing damage to the microstructure of the 3D composites. The fatigue damage mechanisms caused by the different types of z-reinforcement are described. The results indicate that through-the-thickness reinforcement is detrimental to the tensile fatigue life, although the study was restricted to specific types of materials and further research into a wider variety of 3D woven, stitched and z-pinned composites is required for a general assessment of their fatigue performance. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A long-standing problem with fibre-polymer laminates is poor delamination resistance and impact damage tolerance due to their low through-thickness strength and fracture toughness. Various methods have been developed to increase the delamination toughness and impact properties of composites, that include well-established methods such as rubber toughening of the matrix and fibre adhesion treatment as well as contemporary methods such as nanoparticle toughening of the matrix and carbon nanotube treatment of the fibres, e.g. [1–4]. Another important method is the reinforcement of the composite in the through-thickness direction using fibrous yarns, rods or pins, which collectively is called z-reinforcement. The z-reinforcement of composites can be achieved using various techniques, with the most common being threedimensional (3D) weaving, stitching and z-pinning, e.g. [5–7]. zReinforcement is a remarkably effective method for improving the delamination toughness and impact damage tolerance of composites. Large improvements to these properties are achieved with a relatively small amount of z-reinforcement in the composite. zReinforcement contents of 0.5–5.0% by volume are usually sufficient to greatly enhance the damage resistance.
* Tel.: +61 3 9925 6269; fax: +61 3 9925 6108. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.003
A variety of damage tolerant structures have been fabricated using z-reinforced composites. Examples include stiffened aircraft panels, rocket nose cones and beams made using 3D woven composites; aircraft wing sections and stiffened panels using stitched composites; and stiffened panels and structural joints using z-pinned composites [5,7,8]. While many types of structural components have been made using 3D composites, the acceptance of these materials by the aerospace industry has been a problem. A concern with using z-reinforced composites in damage tolerant aircraft structures is their load-bearing properties. Many studies have reported that the z-reinforcement degrades the in-plane mechanical properties of composites, e.g. [5,9–20]. The in-plane stiffness and strength properties can be reduced by microstructural damage to the composite caused by the z-reinforcement, which includes crimping, waviness and damage to the fibres, formation of polymer-rich zones and, in some materials, swelling that reduces the average fibre volume content. The in-plane fatigue properties of composites are also reduced by through-thickness weaving [21– 23], stitching [24–31] and z-pinning [18–20]. However, a comparison between the fatigue performance of 3D woven, stitched and pinned composites have not yet been performed. It is not known whether the fatigue properties of these materials are the same or whether the type of z-reinforcement (i.e. woven z-binder, stitch or z-pin) has a major influence on the fatigue life of 3D composites. This study compares the tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites. The
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composites were made using fibreglass/vinyl ester containing different volume contents of z-reinforcement. The influence of the type and volume content of the z-reinforcement on the tensile modulus, strength and fatigue properties are compared for the different types of 3D composites. It is important to note that this study is comparing specific types of composites with throughthe-thickness reinforcement. 3D woven composites are available in many forms (e.g. orthogonal, layer interlock), although this study examines only one type. Likewise, there are various types of stitched and pinned composites, but this study only examines one type of each material. 2. Materials and experimental methods
ite without z-reinforcement (called the 2D woven composite) was produced as the control specimen. This composite contained plain woven [0/90]s E-glass fabric (600 g/m2) supplied by ACI Pty Ltd. The 2D woven composite was made using the same VARTM process and curing conditions as the 3D composites. Table 1 gives the volume contents of the glass fibres and polymer matrix to the woven composite specimens. The total fibre content of the 2D woven composite (Vf = 0.39) is lower than for the 3D composites (Vf 0.73), although the volume content of load-bearing warp yarns is about the same (Vwarp 0.2). Because the tensile properties of woven composites are controlled predominantly by the volume content of load-bearing yarns, the 2D composite is suitable as a control material to evaluate the effect of woven z-reinforcement on the tensile fatigue properties.
2.1. 3D woven composite specimens 2.2. Stitched composite specimens Fabric preforms for the 3D woven composites were made using E-glass yarns woven on a computer controlled Jacquard loom. The weaving process used to manufacture the fabrics is described by Lee et al. [32]. The fabrics were woven with an orthogonal fibre structure that consisted of in-plane warp (0°) and weft (90°) glass yarns interlaced in the through-thickness direction with z-binder glass yarns. The fabric contained a total of 13 ply layers: six layers of 900 tex warp yarns and seven layers of 1200 tex weft yarns. The z-reinforcement was woven in an orthogonal pattern by passing over two weft yarns at one surface, passing through the fabric, passing over two weft yarns at the opposite surface, and then passing back through the fabric. This pattern is repeated through the fabric to create a fully interlocked orthogonal fibre structure. The z-reinforcement was woven in parallel lines in the warp fibre direction. The fabrics were interlocked with 68 tex, 204 tex or 408 tex E-glass yarns to produce 3D woven composites with volume contents of z-reinforcement of 0.3%, 0.5% or 1.1%, respectively. The profile of the z-reinforcement in the 3D woven composites is shown schematically in Fig. 1. The 3D woven fabrics were infused with vinyl ester resin (DerakaneÒ 411 from Dow Chemicals) using the vacuum assisted resin transfer moulding (VARTM) process. After infusion, the composites were cured in the mould at 110 °C for 1 h. A woven fabric compos-
The stitched composite specimens were made of E-glass fibres and vinyl ester resin. Two types of glass reinforcement were used in the composite: (i) plain woven fabric (600 g/cm2) and (ii) chopped strand mat (300 g/cm2), which were supplied by ACI Pty. Ltd. Preforms to the composite were made by stacking the woven fabric and chopped mat in an alternating sequence to a total of 14 plies. The preforms were then stitched using an industrial sewing machine with 40 tex Kevlar yarn in a modified lock stitch pattern. The preforms were stitched with an areal density of 3 or 6 stitches/cm2, which is equivalent to a stitch volume content of about 0.4% or 0.8%, respectively. The stitches were inserted in straight rows spaced evenly apart in the warp fibre direction. This z-reinforcement pattern in the stitched specimens is similar to the pattern in the 3D woven specimens. Fig. 2 shows a schematic diagram of the z-reinforcement in the stitched composites. The stitched performs were infused with vinyl ester resin (DerakaneÒ 411) using the VARTM process. The composites were cured under ambient conditions for several weeks. Control specimens without stitches were also made with the same fibre lay-up and processing conditions as the stitched composites. The volume contents of fibres and polymer matrix to the specimens are given in Table 2.
warp (0o) yarns
weft (90o) yarns
woven z-reinforcement Fig. 1. Schematic of the z-reinforcement in the 3D woven composites.
Table 1 Fibre and resin contents for the 3D woven composites Composite
2D 3D 3D 3D
control woven woven woven
z-Binder content (%)
0 0.3 0.5 1.1
Polymer volume fraction Vm
0.61 0.48 0.47 0.46
Fibre volume fraction
Thickness (mm)
Warp fibres (Vwarp)
Total fibre content (Vf)
0.196 0.213 0.209 0.211
0.392 0.524 0.527 0.541
3.8 3.8 3.8 3.8
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needle thread
woven fabric
chopped mat
stitches
bobbin thread
Fig. 2. Schematic of the z-reinforcement in the stitched composites.
In addition, control specimens without z-pins were manufactured. The fibre volume contents of the specimens are given in Table 3.
Table 2 Fibre and resin contents for the 3D stitched composites Composite
z-Binder content (%)
Polymer volume fraction Vm
Fibre volume fraction
Thickness (mm)
2D control 3D stitched 3D stitched
0 0.4 0.8
0.42 0.35 0.33
0.58 0.65 0.67
5.87 5.21 5.11
2.3. z-Pinned composite specimens Composite specimens containing z-pins were made using plain woven E-glass fabric (600 g/m2) and vinyl ester resin (DerakaneÒ 411). The composite was fabricated using the wet hand lay-up process, which basically involved brushing the woven glass fabric with liquid resin and then stacking the fabric/resin layers to the required thickness.1 The fabric layers were stacked with a [0/90]s lay-up pattern. Before the resin gelled, which took about 45 min at 20 °C, the composite stack was reinforced in the through-thickness direction using z-pins. The z-pins were supplied by Axtex Inc. (Waltham, MA), and sold commercially under the brand-name Z-FiberTM. The pinning process involved several steps, which are described in detail by Chang et al. [18]. Briefly, the process begins by placing a foam preform containing the z-pins over the top surface of the uncured composite stack. The z-pins are arranged in a square pattern inside the foam. The foam is used to ensure an even spacing between the z-pins and to provide lateral support to the pins during insertion. The z-pins are forced from the foam into the composite using a hand-held ultrasonically actuated horn. The ultrasonic horn generates high frequency compressive stress waves that travel down to the tool end where they are transmitted into the foam preform. Pressure applied on the horn by the operator together with the compressive waves drove the z-pins into the uncured composite. After the z-pins were inserted the composite was cured under ambient conditions. The foam and any excess length of pin protruding from the upper and lower surfaces of the composite was then carefully polished away to ensure a smooth finish. A schematic of z-pins in the composite is presented in Fig. 3. Composite specimens were made containing 0.5%, 2.0% or 4.0% by volume of z-pins. The z-pins used for the lowest (0.5%) and highest (4.0%) contents were thin (0.28 mm) diameter pultruded rods of unidirectional T300 carbon/bismaleimide. The z-pins used for the intermediate volume content (2.0%) were slightly thicker (0.51 mm) diameter T300 carbon/bismaleimide pultruded rods. 1 The VARTM process could not used to manufacture the z-pinned composites because the z-pins are displaced when the resin flows through the fabric. This problem did not occur with the wet hand lay-up process.
2.4. Tensile and fatigue testing Tensile tests on the 3D woven, stitched and z-pinned composites were performed under monotonic or cyclic loading. The dimensions of the tensile specimens are given in Fig. 4. The width of the stitched specimens is greater than the other composite specimens because their z-reinforcement content is relatively low (only 0.4% and 0.8% by volume). The wider specimen ensured a sufficient number of stitches were present in the gauge region to represent the bulk material properties of the stitched composite. Fatigue tests were performed under cyclic tension–tension loading over a range of peak fatigue stress levels from 20% to 90% of the ultimate tensile strength. The fatigue tests were performed until the composite failed, at which point the number of load cycles-to-failure was recorded. The composites did not fail at the lower fatigue stresses, and in these cases the tests were stopped after one million cycles. The monotonic and fatigue tests were performed with the load direction parallel to the warp fibre direction, which is also the direction of the woven z-binder, stitch and z-pin rows (as indicated in Fig. 4).
3. Results and discussion The tensile fatigue properties of composites are influenced by their in-plane tensile modulus; a composite with a high modulus will often have a longer fatigue life than a material with lower modulus under the same cyclic load condition. For this reason, the tensile modulus of the 3D composite materials was measured as part of the study into their fatigue properties. The effect of zreinforcement content on the tensile modulus of the 3D woven, stitched and z-pinned composites is shown in Fig. 5. The Young’s modulus values for the 3D composites (E3D) are normalised to the value of their control composite material without z-reinforcement (E2D). The normalisation of the modulus values allows a direct comparison between the different composite materials. The data does not show any strong relationship between the tensile modulus of the 3D composites and their z-reinforcement content. The 3D woven composite appears to show a gradual increase in modulus with z-reinforcement content, although the large amount of scatter makes it difficult to conclude that the trend is significant. The modulus of the stitched composites appears to decline with increasing z-reinforcement content, but again the trend is not significant based on the large degree of scatter and the limit amount of data.
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woven fabric
z-pin Fig. 3. Schematic of the z-reinforcement in the z-pinned composites.
Table 3 Fibre and resin contents for the z-pinned composites Composite
z-Binder content (%)
Polymer volume fraction Vm
Fibre volume fraction
Thickness (mm)
2D control z-Pinned z-Pinned z-Pinned
0 0.5 2.0 4.0
0.51 0.46 0.54 0.50
0.49 0.54 0.46 0.50
4.27 3.90 4.53 4.23
Other studies, however, have shown that the z-reinforcement can reduce the tensile modulus up to 20%, e.g. [9]. The loss in modulus is caused by increased fibre waviness by the z-reinforcement, as shown schematically in Fig. 6. The fibres are forced to bend around the z-reinforcement which causes local ply waviness in the 3D composites. The deflection angle is dependent on several factors, including the diameter of the z-reinforcement and the fibre packing density, although the angles are usually in the range of 4–10° [5,18]. The deflection of the fibres away from the tensile load causes the modulus to decrease. However, those studies that report a relatively large reduction in modulus are performed on composite materials containing straight fibres (usually made of unidirectional tape) that are sensitive to small changes in fibre angle
Previous studies on 3D woven [12], stitched [9,17,33–36] and zpinned composites [18] have already shown that the z-reinforcement may not change significantly the tensile modulus, and the data presented in Fig. 5 appear to support these earlier studies.
a
Tensile Load Direction woven z-binders
width = 25 mm
gauge length = 200 mm thickness ~ 4 mm
b
Tensile Load Direction gauge radius = 156 mm stitches
width = 70 mm
gauge length = 110 mm thickness ~ 6 mm
c
Tensile Load Direction z-pins
width = 25 mm
gauge length = 100 mm thickness ~ 4 mm Fig. 4. Dimensions of the (a) 3D woven, (b) stitched and (c) z-pinned specimens. Note that the rows of woven z-binders, stitches and z-pins are parallel with the load direction.
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1.4
1.4
1.2
1.2
Normalised Tensile Strength (σ3D/σ2D)
Normalised Tensile Modulus (E3D/E2D)
A.P. Mouritz / Composites Science and Technology 68 (2008) 2503–2510
1.0 0.8 0.6 0.4
3D woven composite (Ε2D = 20.9 GPa) Stitched composite (Ε2D = 16.2 Pa) Z-pinned composite (Ε2D = 23.5 GPa)
0.2 0.0
1.0 0.8 0.6 0.4
3D woven composite (σ2D = 345 MPa) Stitched composite (σ2D = 264 MPa)
0.2
Z-pinned composite (σ2D = 429 MPa)
0.0
0
1
2
3
4
Volume Percent of z-Reinforcement Fig. 5. Plot of volume content of z-reinforcement vs. normalised tensile modulus for the 3D woven, stitched and z-pinned composites. The error bars represent one standard deviation in the measured modulus values.
caused by the z-reinforcement. The 3D composites studied here contain woven fabric that has an inherent amount of fibre waviness. The fibres in the fabric have a waviness angle of 4–8°, which is close to the waviness caused by the z-reinforcement. Therefore, the z-reinforcement does not increase significantly the fibre waviness in the 3D composites, and therefore the tensile modulus is not changed appreciably. Fig. 7 shows the effect of z-reinforcement content on the tensile failure strength of the 3D composites measured under monotonic loading. The strength values for the 3D composites (r3D) have been normalised to the value of their control composite material without z-reinforcement (r2D). The strength of the z-pinned composite falls steadily with increasing volume content of z-pins. The strength decreases at a rate of 7.5% for every 1% of z-pins, with the strength dropping by 30% at the highest pin content (4%). Chang et al. [18] found the tensile strength of carbon/epoxy tape composites also decrease at a linear rate with increasing z-pin content. They attributed the reduction to microstructural damage caused to the composite during the z-pinning process. When z-pins are forced into the composite material during the manufacturing stage the fibres are crimped, distorted and broken. The fibres are broken and damaged under the tip of the z-pins when forced into the composite. This is because the uncured resin was viscous, and many of the fibres were not easily pushed aside during z-pinning but instead were caught under the pin tip which caused them to break. The damage was localised to a small volume of material surrounding each z-pin. Chang et al. [18] believe that there are a sufficient number of damaged fibres at each z-pin to form a critical flaw. Ibnabdeljalil and Curtin [37] estimate that the number of broken fibres within a cluster that is required to create a critical flaw is only 5–50 filaments. When the z-pinned composite specimens are
resin-rich region
0
1
2
3
4
Volume Percent of z-Reinforcement Fig. 7. Plot of volume content of z-reinforcement vs. normalised tensile strength for the 3D woven, stitched and z-pinned composites. The dashed line shows the reduction in strength for the z-pinned composite only. The error bars represent one standard deviation in the measured strength values.
loaded in tension the unbroken fibres surrounding the critical flaws are believed to rupture at lower stress due to the reduced effectiveness of load transfer across the damaged region via the shear lag mechanism. The number density of critical flaws within the z-pinned composites increases with the z-reinforcement content, resulting in the measured reduction to the tensile strength. Chang et al. [18] also attribute the reduction to the tensile strength of their pinned carbon/epoxy specimens to volumetric swelling and fibre waviness caused by the z-pins. The swelling reduced the average fibre volume content of their z-pinned composite and thereby reduced the tensile strength. However, swelling of the z-pinned composites studied here was not significant, as shown by the thickness and fibre content values in Table 3. Fibre waviness in the z-pinned composites is caused by the fibres bending around the pins, as shown in Fig. 6. The fibre waviness caused by z-pins is typically only 4–6° [7,18], which is about the same value as the waviness of the woven fabrics used in the composite specimens. For this reason, the fibre waviness due to z-pins will not reduce the strength. Therefore, the reduction to the tensile strength of the z-pinned composites shown in Fig. 7 is attributed solely to fibre breakage caused by the pinning process. Fig. 7 shows that the tensile strengths of the 3D woven and stitched composites did not change significantly with the z-reinforcement content. The average strength values of the 3D woven composites are slightly higher and lower than their control material, although due to the large amount of scatter the changes in strength are not statistically significant. The volume content of zreinforcement in both the 3D woven and stitched materials is relatively low (61%), and this may account for the constant strength values. Tong et al. [5] and Mouritz and Cox [9] examined the tensile
z-reinforcement
Fig. 6. Schematic of in-plane fibre waviness caused by the z-reinforcement.
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at the z-reinforcement content of 1%. The fatigue life of the stitched composite was reduced by nearly one order of magnitude at a stitch content of only 0.8%. The fatigue life of the z-pinned composite dropped by the same amount at the z-reinforcement content of 2–4%. The large reduction to the fatigue performance is attributed to changes to the composite microstructure caused by the z-reinforcement which promote the early formation of fatigue-induced damage. Rudov-Clark and Mouritz [23] and Mouritz [30] examined the fatigue damage in the 3D woven and stitched composites, respectively, and similar fatigue damage mechanisms occur in both materials. (The reader is encouraged to examine these papers for photographic evidence of the fatigue damage mechanisms.) The large reduction to the fatigue lives of the 3D woven and stitched materials is due to several types of damage caused by the z-reinforcement under cyclic tensile loading. The z-reinforcement in both the 3D woven and stitched composites debonds from the surrounding material during fatigue loading. Debonding of the z-reinforcement occurs because of the generation of shear stress along the interface with the surrounding material as a result of the mismatch of their elastic moduli. Cracks initiate and spread along the interface under cyclic loading until eventually large segments of the z-reinforcement have detached. This cracking changes the stress transfer mechanics in the composite material because higher
strength values for 3D woven and stitched composites with much higher z-reinforcement contents (up to 18%), and found no statistically significant trend between the tensile strength and z-reinforcement. The woven z-binders and stitches were inserted in dry fabric, unlike the z-pins that were inserted into an uncured composite. The insertion of z-reinforcement into dry fabric allows the fibres to be easily pushed aside which minimises the amount of damage and breakage. In the case of z-pinning, the z-reinforcement is inserted into the fibre preform that has already been preimpregnated with the polymer resin. The resin provides some resistance against the displacement of the fibres during insertion of the z-pins. For this reason, the 3D woven and stitched composites may experience less fibre damage than the z-pinned composites, and therefore their tensile strengths did not decrease as rapidly with increasing z-reinforcement content. Fatigue-life (S–N) curves for the 3D woven, stitched and z-pinned composites are presented in Fig. 8. The curves represent lines-of-best fit through the fatigue life data. The results show that the fatigue lives of the 3D composites are always lower than the fatigue life of the control material. Furthermore, their fatigue life drops with increasing volume content of z-reinforcement, with a large loss in fatigue performance caused by a relatively small amount of through-thickness reinforcement. For example, the fatigue life of the 3D woven composite fell by one order of magnitude
a
b
400
300
300 0.3% z-binders
250 200
0.5% z-binders
150 1.1% z-binders
100 50 0 10
0
1
10
2
3
10
4
10
10
10
5
Tensile Fatigue Stress (MPa)
250 0% z-binders
0% stitches
200
150 0.4% stitches 0.8% stitches
100
50
0
6
10
0
10
1
2
10
10
Load Cycles to Failure
c
500
400
0% z-pins
350 0.5% z-pins
300 250 200 2% z-pins
150 4% z-pins
100 50 0 10
0
10
3
10
1
2
10
3
10
10
4
10
5
6
10
Load Cycles to Failure Fig. 8. S–N curves for the (a) 3D woven, (b) stitched and (c) z-pinned composites.
4
10
Load Cycles to Failure
450
Tensile Fatigue Stress (MPa)
Tensile Fatigue Stress (MPa)
350
10
5
6
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A.P. Mouritz / Composites Science and Technology 68 (2008) 2503–2510
loads are exerted onto the undamaged regions. The number of debonding cracks increases with the volume content of z-reinforcement, which accounts for the corresponding reduction to the fatigue life. It is also possible that pre-existing damage within the z-reinforcement itself may reduce the fatigue life of the 3D woven and stitched composites. The tensile strength of the z-binders in the 3D woven composite is reduced by about 50% due to fibre damage and breakage during the weaving process [38]. Likewise, the tensile strength of the Kevlar stitches is reduced by 20% due to fibre damage incurred by the stitching process [39]. The bending and twisting of the stitches when they are sewn into the material are believed to damage the fibres within the stitches. The damaged fibres in the woven z-binders or stitches are believed to break early in the fatigue life of the composite and develop into flaws. These flaws within the z-reinforcement are believed the generate cracks within the 3D woven and stitched composite that reduce the fatigue life. The number of these flaws must increase with the z-reinforcement content, which would account for the corresponding reduction to the fatigue life. The fatigue damage mechanisms of the z-pinned composites are different to the 3D woven and stitched materials. Debonding of the z-pins was not observed in the fatigued specimens, unlike the 3D woven and stitched materials. Also, the z-pins were not damaged during the z-pinning process, again unlike the woven z-binders and stitches that were damaged and weaken. Instead, the reduction to the fatigue life of the z-pinned composites is attributed mostly to the damage and breakage of the load-bearing fibres, which reduced the tensile modulus and strength. As mentioned, the fibres were not easily pushed aside when the z-pins were inserted by the z-pinning process because the uncured resin matrix is viscous. Fibres that do not move aside can break under the force needed to insert the z-pins. The broken fibres are clustered near the pins, which can develop into flaws under cyclic loading that leads to fatigue-induced failure. The number of fibres broken by the z-pinning process rises with the z-pin content. For this reason, the fatigue life of the z-pinned composites decreases with increasing z-reinforcement content. Chang et al. [18] measured a large reduction of the tensile fatigue life of carbon/epoxy tape composites with increasing z-pin content, and also attribute this to damage to the loading-bearing fibres during the pinning process. As mentioned, the 3D woven and stitched composites do not experience significant fibre damage during insertion of the z-reinforcement, and therefore is not an important cause of the reduced fatigue life of these materials.
4. Conclusions The impact damage resistance and damage tolerance of composite materials can be improved by through-the-thickness reinforcement using woven z-binders, stitches or z-pins. These properties increase with the volume content of the z-reinforcement, and therefore it is common practice to use a relatively large amount of through-thickness reinforcement in composite structures requiring high damage tolerance. This study has shown, however, that increasing the z-reinforcement content reduces the tensile fatigue life of 3D composites. The fatigue life of 3D woven, stitched and z-pinned composites decreased with increasing zreinforcement content. The reduction to the fatigue performance is due to microstructural damage to the composite caused by the insertion of the z-reinforcement. Debonding of the z-reinforcement and the development of cracks within the z-reinforcement itself are believed responsible for the large reduction to the fatigue lives of the 3D woven and stitched composites. Breakage and damage to the load-bearing fibres by the z-pins are responsible for the reduction to the fatigue life of the z-pinned composites. The research
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presented in this paper shows that the deterioration to the tensile fatigue properties of 3D composites must be a consideration when using these materials in impact damage tolerance structures. However, as noted earlier, this study was confined to one type of 3D woven, stitched and pinned material, and the tensile fatigue properties may be somewhat different with other types of composites with through-the-thickness reinforcement. This uncertainty provides the rationale for a larger study into the fatigue properties of a diverse variety of 3D woven, stitched and pinned composites. Acknowledgements The author thanks Dr Rudov-Clark for manufacturing and testing the 3D woven composite specimens, Dr Shah Khan for testing the stitched specimens, and Mr Cull for manufacturing and testing the z-pinned specimens. The assistance of Mr Tkatchyk in the tensile and fatigue testing is also gratefully acknowledged. References [1] Collyer AA, editor. Rubber toughening engineering plastics. Springer; 1994. [2] Kim J-K, Mai Y-W. Engineered interfaces in fibre reinforced composites. Oxford: Elsevier Science; 1998. [3] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44:1624–52. [4] Veeda VP, Cao A, Li X, Man K, Soldano C, Kar S, et al. Multifunctional composites using reinforced laminae with carbon-nanotube forests. Nature Mater 2006;5:457–61. [5] Tong L, Mouritz AP, Bannister MK. 3D fibre reinforced polymer composites. London: Elsevier Science Ltd.; 2002. [6] Dransfield KA, Baillie C, Mai Y-W. Improving the delamination resistance of CFRP by stitching – a review. Compos Sci Technol 1994;50:305–17. [7] Mouritz AP. Review of z-pinned composite laminates. Composites 2007;38A:2383–97. [8] Mouritz AP, Bannister MK, Falzon PJ, Leong KH. Review of applications for advanced three-dimensional fibre textile composites. Composites 1999;30A:1445–61. [9] Mouritz AP, Cox BN. A mechanistic approach to the properties of stitched laminates. Composites 2000;31A:1–27. [10] Dickinson LC, Farley GL, Hinders MK. Prediction of effective three-dimensional elastic constants of translaminar reinforced composites. J Compos Mater 1999;33:1002–29. [11] Guess TR, Reedy EDJ. Comparison of interlocked fabric and laminated fabric Kevlar 49/Epoxy composites. J Compos Technol Res 1985;7:136–42. [12] Arendts F-J, Dreschler K, Brandt J. Manufacturing and mechanical performance of composites with 3-D woven fiber reinforcement. In: Proceedings of the 4th textile structural composites symposium. Philadelphia; 1989. [13] Kang TJ, Lee SH. Effect of stitching on the mechanical and impact properties of woven laminate composite. J Compos Mater 1994;28:1574–87. [14] Steeves C, Fleck NA. In-plane properties of composite laminates with throughthickness pin reinforcement. Int J Solids Struct 2006;43:3197–212. [15] Larsson F. Damage tolerance of a stitched carbon/epoxy composite. Composites 1997;28A:923–34. [16] Farley GL. A mechanism responsible for reducing compression strength of through-the-thickness reinforced composite material. J Compos Mater 1992;26:1784–95. [17] Harris H, Schinske N, Kruger R, Swanson B. Multiaxial stitched preform reinforcement. In: Proceedings of the 6th annual ASM.ESD advanced composites conference; 8–11 October 1991. p. 433–4. [18] Chang P, Mouritz AP, Cox BN. Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension. Compos A 2006;37: 1501–13. [19] Mouritz AP. Compression properties of z-pinned sandwich composites. J Mater Sci 2006;41:5771–4. [20] Chang P, Mouritz AP, Cox BN. Flexural properties of z-pinned laminates. Compos A 2007;38:224–51. [21] Dadkhah MS, Morris WL, Cox BN. Compression–compression fatigue in 3D woven composites. Acta Metall Mater 1995;43:4235–45. [22] Ding YQ, Yan Y, McIlhagger R, Brown D. Comparison of the fatigue behaviour of 2-D and 3-D woven fabric reinforced composites. J Mater Process Technol 1995;55:171–7. [23] Rudov-Clark S, Mouritz AP. Tensile fatigue properties of a 3D orthogonal woven composites. Compos A 2008;39:1018–24. [24] Portanova MA, Poe CC, Whitcomb JD. Open hole and postimpact compressive fatigue of stitched and unstitched carbon–epoxy composites. In: Glenn GC, editor. Composite materials: testing and design (10th volume). ASTM STP 1120. Philadelphia: American Society for Testing and Materials; 1992. p. 37–53.
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[25] Furrow KW, Loos AC, Cano RJ. Environmental effects on stitched RTM textile composites. J Reinf Plast Compos 1996;15:378–419. [26] Lubowinski SJ, Poe CC. Fatigue characterization of stitched carbon/epoxy composites. In: Proceedings of fiber-tex 1987 conference; 1987. p. 253–71. [27] Vandermey NE, Morris DH, Masters JE. Damage development under compression-compression fatigue loading in stitched uniwoven graphite/ epoxy composite material. NASA Report PB91-236026; 1991. [28] Aymerich F, Priolo P, Sanna R, Sun CT. Fatigue behaviour of stitched composite laminates. In: Proceedings of the 13th international conference on composite materials, 25–29 June 2001, Beijing, Paper No. 1461. [29] Aymerich F, Priolo P, Sun CT. Static and fatigue behaviour of stitched graphite/ epoxy composite laminates. Compos Sci Technol 2003;63:907–17. [30] Mouritz AP. Fracture and tensile fatigue properties of stitched fibreglass composites. Proc Institut Mech Eng. Part L: Mater: Design Appl 2004;218:87–93. [31] Mouritz AP. A simple fatigue model for 3D fibre–polymer composites. J Compos Mater 2006;40:455–69. [32] Lee L, Rudov-Clark S, Mouritz AP, Bannister MK, Herszberg I. Effect of weaving damage on the tensile properties of three-dimensional woven composites. Compos Struct 2002;57:405–13.
[33] Wu E, Wang J. Behaviour of stitched laminates under in-plane tensile and transverse impact loading. J Compos Mater 1995;29:2254–79. [34] Thuis HGSJ, Bron E. The effect of stitching density and laminate lay-up on the mechanical properties of stitched carbon fabrics. National Lucht-en Ruimtevaartlaboratorium, Report NCL CR 96126L. [35] Jain LK, Mai Y-W. Recent work on stitching of laminated composites – theoretical analysis and experiments. In: Proceedings of the 11th international conference on composite materials; 14–18 July 1997. p. I-25–51. [36] Dexter HB, Funk JG. Impact resistance and interlaminar fracture toughness of through-the-thickness reinforced graphite/epoxy. AIAA Paper 86-CP; 1986. p. 700–9. [37] Ibnabdeljalil I, Curtin WA. Strength and reliability of fiber-reinforced composites: Localized load-sharing and associated size effects. Int J Solids Struct 1997;34:2649–68. [38] Mouritz AP, Lee L, Rudov-Clark S, Bannister MK. Fibre damage in the manufacture of advanced three-dimensional woven composites. Composites 2003;34A:963–70. [39] Morales A. Structural stitching of textile preforms. In: Proceedings of the 22nd international SAMPE technical conference; 6–8 November 1990. p. 1217–30.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Tensile fatigue properties of 3D composites with through-thickness reinforcement A.P. Mouritz * School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476V, Melbourne, Australia
a r t i c l e
i n f o
Article history: Received 16 January 2008 Received in revised form 22 April 2008 Accepted 2 May 2008 Available online 8 May 2008 Keywords: A. Polymer-matrix composites B. Fatigue, through-thickness reinforcement
a b s t r a c t The tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites with through-thickness reinforcement are compared in this paper. Tensile tests under monotonic and cyclic loading were performed on the 3D composite materials to determine the influence of the z-reinforcement type – woven z-binder, stitch or z-pin – on the tensile modulus, strength and fatigue life. The in-plane Young’s modulus of the composites was not affected by the type or volume content of the z-reinforcement. The tensile strength of the 3D woven and stitched composites was also not affected by the z-reinforcement, however the strength of the z-pinned composite dropped steadily with increasing volume content of z-reinforcement. The fatigue life of the 3D composites was reduced by the z-reinforcement, regardless of whether they were woven z-binders, stitches or z-pins. The fatigue lives of the 3D composites decreased with increasing volume content of z-reinforcement. The tensile fatigue properties are degraded by the z-reinforcement causing damage to the microstructure of the 3D composites. The fatigue damage mechanisms caused by the different types of z-reinforcement are described. The results indicate that through-the-thickness reinforcement is detrimental to the tensile fatigue life, although the study was restricted to specific types of materials and further research into a wider variety of 3D woven, stitched and z-pinned composites is required for a general assessment of their fatigue performance. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A long-standing problem with fibre-polymer laminates is poor delamination resistance and impact damage tolerance due to their low through-thickness strength and fracture toughness. Various methods have been developed to increase the delamination toughness and impact properties of composites, that include well-established methods such as rubber toughening of the matrix and fibre adhesion treatment as well as contemporary methods such as nanoparticle toughening of the matrix and carbon nanotube treatment of the fibres, e.g. [1–4]. Another important method is the reinforcement of the composite in the through-thickness direction using fibrous yarns, rods or pins, which collectively is called z-reinforcement. The z-reinforcement of composites can be achieved using various techniques, with the most common being threedimensional (3D) weaving, stitching and z-pinning, e.g. [5–7]. zReinforcement is a remarkably effective method for improving the delamination toughness and impact damage tolerance of composites. Large improvements to these properties are achieved with a relatively small amount of z-reinforcement in the composite. zReinforcement contents of 0.5–5.0% by volume are usually sufficient to greatly enhance the damage resistance.
* Tel.: +61 3 9925 6269; fax: +61 3 9925 6108. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.003
A variety of damage tolerant structures have been fabricated using z-reinforced composites. Examples include stiffened aircraft panels, rocket nose cones and beams made using 3D woven composites; aircraft wing sections and stiffened panels using stitched composites; and stiffened panels and structural joints using z-pinned composites [5,7,8]. While many types of structural components have been made using 3D composites, the acceptance of these materials by the aerospace industry has been a problem. A concern with using z-reinforced composites in damage tolerant aircraft structures is their load-bearing properties. Many studies have reported that the z-reinforcement degrades the in-plane mechanical properties of composites, e.g. [5,9–20]. The in-plane stiffness and strength properties can be reduced by microstructural damage to the composite caused by the z-reinforcement, which includes crimping, waviness and damage to the fibres, formation of polymer-rich zones and, in some materials, swelling that reduces the average fibre volume content. The in-plane fatigue properties of composites are also reduced by through-thickness weaving [21– 23], stitching [24–31] and z-pinning [18–20]. However, a comparison between the fatigue performance of 3D woven, stitched and pinned composites have not yet been performed. It is not known whether the fatigue properties of these materials are the same or whether the type of z-reinforcement (i.e. woven z-binder, stitch or z-pin) has a major influence on the fatigue life of 3D composites. This study compares the tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites. The
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composites were made using fibreglass/vinyl ester containing different volume contents of z-reinforcement. The influence of the type and volume content of the z-reinforcement on the tensile modulus, strength and fatigue properties are compared for the different types of 3D composites. It is important to note that this study is comparing specific types of composites with throughthe-thickness reinforcement. 3D woven composites are available in many forms (e.g. orthogonal, layer interlock), although this study examines only one type. Likewise, there are various types of stitched and pinned composites, but this study only examines one type of each material. 2. Materials and experimental methods
ite without z-reinforcement (called the 2D woven composite) was produced as the control specimen. This composite contained plain woven [0/90]s E-glass fabric (600 g/m2) supplied by ACI Pty Ltd. The 2D woven composite was made using the same VARTM process and curing conditions as the 3D composites. Table 1 gives the volume contents of the glass fibres and polymer matrix to the woven composite specimens. The total fibre content of the 2D woven composite (Vf = 0.39) is lower than for the 3D composites (Vf 0.73), although the volume content of load-bearing warp yarns is about the same (Vwarp 0.2). Because the tensile properties of woven composites are controlled predominantly by the volume content of load-bearing yarns, the 2D composite is suitable as a control material to evaluate the effect of woven z-reinforcement on the tensile fatigue properties.
2.1. 3D woven composite specimens 2.2. Stitched composite specimens Fabric preforms for the 3D woven composites were made using E-glass yarns woven on a computer controlled Jacquard loom. The weaving process used to manufacture the fabrics is described by Lee et al. [32]. The fabrics were woven with an orthogonal fibre structure that consisted of in-plane warp (0°) and weft (90°) glass yarns interlaced in the through-thickness direction with z-binder glass yarns. The fabric contained a total of 13 ply layers: six layers of 900 tex warp yarns and seven layers of 1200 tex weft yarns. The z-reinforcement was woven in an orthogonal pattern by passing over two weft yarns at one surface, passing through the fabric, passing over two weft yarns at the opposite surface, and then passing back through the fabric. This pattern is repeated through the fabric to create a fully interlocked orthogonal fibre structure. The z-reinforcement was woven in parallel lines in the warp fibre direction. The fabrics were interlocked with 68 tex, 204 tex or 408 tex E-glass yarns to produce 3D woven composites with volume contents of z-reinforcement of 0.3%, 0.5% or 1.1%, respectively. The profile of the z-reinforcement in the 3D woven composites is shown schematically in Fig. 1. The 3D woven fabrics were infused with vinyl ester resin (DerakaneÒ 411 from Dow Chemicals) using the vacuum assisted resin transfer moulding (VARTM) process. After infusion, the composites were cured in the mould at 110 °C for 1 h. A woven fabric compos-
The stitched composite specimens were made of E-glass fibres and vinyl ester resin. Two types of glass reinforcement were used in the composite: (i) plain woven fabric (600 g/cm2) and (ii) chopped strand mat (300 g/cm2), which were supplied by ACI Pty. Ltd. Preforms to the composite were made by stacking the woven fabric and chopped mat in an alternating sequence to a total of 14 plies. The preforms were then stitched using an industrial sewing machine with 40 tex Kevlar yarn in a modified lock stitch pattern. The preforms were stitched with an areal density of 3 or 6 stitches/cm2, which is equivalent to a stitch volume content of about 0.4% or 0.8%, respectively. The stitches were inserted in straight rows spaced evenly apart in the warp fibre direction. This z-reinforcement pattern in the stitched specimens is similar to the pattern in the 3D woven specimens. Fig. 2 shows a schematic diagram of the z-reinforcement in the stitched composites. The stitched performs were infused with vinyl ester resin (DerakaneÒ 411) using the VARTM process. The composites were cured under ambient conditions for several weeks. Control specimens without stitches were also made with the same fibre lay-up and processing conditions as the stitched composites. The volume contents of fibres and polymer matrix to the specimens are given in Table 2.
warp (0o) yarns
weft (90o) yarns
woven z-reinforcement Fig. 1. Schematic of the z-reinforcement in the 3D woven composites.
Table 1 Fibre and resin contents for the 3D woven composites Composite
2D 3D 3D 3D
control woven woven woven
z-Binder content (%)
0 0.3 0.5 1.1
Polymer volume fraction Vm
0.61 0.48 0.47 0.46
Fibre volume fraction
Thickness (mm)
Warp fibres (Vwarp)
Total fibre content (Vf)
0.196 0.213 0.209 0.211
0.392 0.524 0.527 0.541
3.8 3.8 3.8 3.8
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needle thread
woven fabric
chopped mat
stitches
bobbin thread
Fig. 2. Schematic of the z-reinforcement in the stitched composites.
In addition, control specimens without z-pins were manufactured. The fibre volume contents of the specimens are given in Table 3.
Table 2 Fibre and resin contents for the 3D stitched composites Composite
z-Binder content (%)
Polymer volume fraction Vm
Fibre volume fraction
Thickness (mm)
2D control 3D stitched 3D stitched
0 0.4 0.8
0.42 0.35 0.33
0.58 0.65 0.67
5.87 5.21 5.11
2.3. z-Pinned composite specimens Composite specimens containing z-pins were made using plain woven E-glass fabric (600 g/m2) and vinyl ester resin (DerakaneÒ 411). The composite was fabricated using the wet hand lay-up process, which basically involved brushing the woven glass fabric with liquid resin and then stacking the fabric/resin layers to the required thickness.1 The fabric layers were stacked with a [0/90]s lay-up pattern. Before the resin gelled, which took about 45 min at 20 °C, the composite stack was reinforced in the through-thickness direction using z-pins. The z-pins were supplied by Axtex Inc. (Waltham, MA), and sold commercially under the brand-name Z-FiberTM. The pinning process involved several steps, which are described in detail by Chang et al. [18]. Briefly, the process begins by placing a foam preform containing the z-pins over the top surface of the uncured composite stack. The z-pins are arranged in a square pattern inside the foam. The foam is used to ensure an even spacing between the z-pins and to provide lateral support to the pins during insertion. The z-pins are forced from the foam into the composite using a hand-held ultrasonically actuated horn. The ultrasonic horn generates high frequency compressive stress waves that travel down to the tool end where they are transmitted into the foam preform. Pressure applied on the horn by the operator together with the compressive waves drove the z-pins into the uncured composite. After the z-pins were inserted the composite was cured under ambient conditions. The foam and any excess length of pin protruding from the upper and lower surfaces of the composite was then carefully polished away to ensure a smooth finish. A schematic of z-pins in the composite is presented in Fig. 3. Composite specimens were made containing 0.5%, 2.0% or 4.0% by volume of z-pins. The z-pins used for the lowest (0.5%) and highest (4.0%) contents were thin (0.28 mm) diameter pultruded rods of unidirectional T300 carbon/bismaleimide. The z-pins used for the intermediate volume content (2.0%) were slightly thicker (0.51 mm) diameter T300 carbon/bismaleimide pultruded rods. 1 The VARTM process could not used to manufacture the z-pinned composites because the z-pins are displaced when the resin flows through the fabric. This problem did not occur with the wet hand lay-up process.
2.4. Tensile and fatigue testing Tensile tests on the 3D woven, stitched and z-pinned composites were performed under monotonic or cyclic loading. The dimensions of the tensile specimens are given in Fig. 4. The width of the stitched specimens is greater than the other composite specimens because their z-reinforcement content is relatively low (only 0.4% and 0.8% by volume). The wider specimen ensured a sufficient number of stitches were present in the gauge region to represent the bulk material properties of the stitched composite. Fatigue tests were performed under cyclic tension–tension loading over a range of peak fatigue stress levels from 20% to 90% of the ultimate tensile strength. The fatigue tests were performed until the composite failed, at which point the number of load cycles-to-failure was recorded. The composites did not fail at the lower fatigue stresses, and in these cases the tests were stopped after one million cycles. The monotonic and fatigue tests were performed with the load direction parallel to the warp fibre direction, which is also the direction of the woven z-binder, stitch and z-pin rows (as indicated in Fig. 4).
3. Results and discussion The tensile fatigue properties of composites are influenced by their in-plane tensile modulus; a composite with a high modulus will often have a longer fatigue life than a material with lower modulus under the same cyclic load condition. For this reason, the tensile modulus of the 3D composite materials was measured as part of the study into their fatigue properties. The effect of zreinforcement content on the tensile modulus of the 3D woven, stitched and z-pinned composites is shown in Fig. 5. The Young’s modulus values for the 3D composites (E3D) are normalised to the value of their control composite material without z-reinforcement (E2D). The normalisation of the modulus values allows a direct comparison between the different composite materials. The data does not show any strong relationship between the tensile modulus of the 3D composites and their z-reinforcement content. The 3D woven composite appears to show a gradual increase in modulus with z-reinforcement content, although the large amount of scatter makes it difficult to conclude that the trend is significant. The modulus of the stitched composites appears to decline with increasing z-reinforcement content, but again the trend is not significant based on the large degree of scatter and the limit amount of data.
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woven fabric
z-pin Fig. 3. Schematic of the z-reinforcement in the z-pinned composites.
Table 3 Fibre and resin contents for the z-pinned composites Composite
z-Binder content (%)
Polymer volume fraction Vm
Fibre volume fraction
Thickness (mm)
2D control z-Pinned z-Pinned z-Pinned
0 0.5 2.0 4.0
0.51 0.46 0.54 0.50
0.49 0.54 0.46 0.50
4.27 3.90 4.53 4.23
Other studies, however, have shown that the z-reinforcement can reduce the tensile modulus up to 20%, e.g. [9]. The loss in modulus is caused by increased fibre waviness by the z-reinforcement, as shown schematically in Fig. 6. The fibres are forced to bend around the z-reinforcement which causes local ply waviness in the 3D composites. The deflection angle is dependent on several factors, including the diameter of the z-reinforcement and the fibre packing density, although the angles are usually in the range of 4–10° [5,18]. The deflection of the fibres away from the tensile load causes the modulus to decrease. However, those studies that report a relatively large reduction in modulus are performed on composite materials containing straight fibres (usually made of unidirectional tape) that are sensitive to small changes in fibre angle
Previous studies on 3D woven [12], stitched [9,17,33–36] and zpinned composites [18] have already shown that the z-reinforcement may not change significantly the tensile modulus, and the data presented in Fig. 5 appear to support these earlier studies.
a
Tensile Load Direction woven z-binders
width = 25 mm
gauge length = 200 mm thickness ~ 4 mm
b
Tensile Load Direction gauge radius = 156 mm stitches
width = 70 mm
gauge length = 110 mm thickness ~ 6 mm
c
Tensile Load Direction z-pins
width = 25 mm
gauge length = 100 mm thickness ~ 4 mm Fig. 4. Dimensions of the (a) 3D woven, (b) stitched and (c) z-pinned specimens. Note that the rows of woven z-binders, stitches and z-pins are parallel with the load direction.
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1.4
1.4
1.2
1.2
Normalised Tensile Strength (σ3D/σ2D)
Normalised Tensile Modulus (E3D/E2D)
A.P. Mouritz / Composites Science and Technology 68 (2008) 2503–2510
1.0 0.8 0.6 0.4
3D woven composite (Ε2D = 20.9 GPa) Stitched composite (Ε2D = 16.2 Pa) Z-pinned composite (Ε2D = 23.5 GPa)
0.2 0.0
1.0 0.8 0.6 0.4
3D woven composite (σ2D = 345 MPa) Stitched composite (σ2D = 264 MPa)
0.2
Z-pinned composite (σ2D = 429 MPa)
0.0
0
1
2
3
4
Volume Percent of z-Reinforcement Fig. 5. Plot of volume content of z-reinforcement vs. normalised tensile modulus for the 3D woven, stitched and z-pinned composites. The error bars represent one standard deviation in the measured modulus values.
caused by the z-reinforcement. The 3D composites studied here contain woven fabric that has an inherent amount of fibre waviness. The fibres in the fabric have a waviness angle of 4–8°, which is close to the waviness caused by the z-reinforcement. Therefore, the z-reinforcement does not increase significantly the fibre waviness in the 3D composites, and therefore the tensile modulus is not changed appreciably. Fig. 7 shows the effect of z-reinforcement content on the tensile failure strength of the 3D composites measured under monotonic loading. The strength values for the 3D composites (r3D) have been normalised to the value of their control composite material without z-reinforcement (r2D). The strength of the z-pinned composite falls steadily with increasing volume content of z-pins. The strength decreases at a rate of 7.5% for every 1% of z-pins, with the strength dropping by 30% at the highest pin content (4%). Chang et al. [18] found the tensile strength of carbon/epoxy tape composites also decrease at a linear rate with increasing z-pin content. They attributed the reduction to microstructural damage caused to the composite during the z-pinning process. When z-pins are forced into the composite material during the manufacturing stage the fibres are crimped, distorted and broken. The fibres are broken and damaged under the tip of the z-pins when forced into the composite. This is because the uncured resin was viscous, and many of the fibres were not easily pushed aside during z-pinning but instead were caught under the pin tip which caused them to break. The damage was localised to a small volume of material surrounding each z-pin. Chang et al. [18] believe that there are a sufficient number of damaged fibres at each z-pin to form a critical flaw. Ibnabdeljalil and Curtin [37] estimate that the number of broken fibres within a cluster that is required to create a critical flaw is only 5–50 filaments. When the z-pinned composite specimens are
resin-rich region
0
1
2
3
4
Volume Percent of z-Reinforcement Fig. 7. Plot of volume content of z-reinforcement vs. normalised tensile strength for the 3D woven, stitched and z-pinned composites. The dashed line shows the reduction in strength for the z-pinned composite only. The error bars represent one standard deviation in the measured strength values.
loaded in tension the unbroken fibres surrounding the critical flaws are believed to rupture at lower stress due to the reduced effectiveness of load transfer across the damaged region via the shear lag mechanism. The number density of critical flaws within the z-pinned composites increases with the z-reinforcement content, resulting in the measured reduction to the tensile strength. Chang et al. [18] also attribute the reduction to the tensile strength of their pinned carbon/epoxy specimens to volumetric swelling and fibre waviness caused by the z-pins. The swelling reduced the average fibre volume content of their z-pinned composite and thereby reduced the tensile strength. However, swelling of the z-pinned composites studied here was not significant, as shown by the thickness and fibre content values in Table 3. Fibre waviness in the z-pinned composites is caused by the fibres bending around the pins, as shown in Fig. 6. The fibre waviness caused by z-pins is typically only 4–6° [7,18], which is about the same value as the waviness of the woven fabrics used in the composite specimens. For this reason, the fibre waviness due to z-pins will not reduce the strength. Therefore, the reduction to the tensile strength of the z-pinned composites shown in Fig. 7 is attributed solely to fibre breakage caused by the pinning process. Fig. 7 shows that the tensile strengths of the 3D woven and stitched composites did not change significantly with the z-reinforcement content. The average strength values of the 3D woven composites are slightly higher and lower than their control material, although due to the large amount of scatter the changes in strength are not statistically significant. The volume content of zreinforcement in both the 3D woven and stitched materials is relatively low (61%), and this may account for the constant strength values. Tong et al. [5] and Mouritz and Cox [9] examined the tensile
z-reinforcement
Fig. 6. Schematic of in-plane fibre waviness caused by the z-reinforcement.
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at the z-reinforcement content of 1%. The fatigue life of the stitched composite was reduced by nearly one order of magnitude at a stitch content of only 0.8%. The fatigue life of the z-pinned composite dropped by the same amount at the z-reinforcement content of 2–4%. The large reduction to the fatigue performance is attributed to changes to the composite microstructure caused by the z-reinforcement which promote the early formation of fatigue-induced damage. Rudov-Clark and Mouritz [23] and Mouritz [30] examined the fatigue damage in the 3D woven and stitched composites, respectively, and similar fatigue damage mechanisms occur in both materials. (The reader is encouraged to examine these papers for photographic evidence of the fatigue damage mechanisms.) The large reduction to the fatigue lives of the 3D woven and stitched materials is due to several types of damage caused by the z-reinforcement under cyclic tensile loading. The z-reinforcement in both the 3D woven and stitched composites debonds from the surrounding material during fatigue loading. Debonding of the z-reinforcement occurs because of the generation of shear stress along the interface with the surrounding material as a result of the mismatch of their elastic moduli. Cracks initiate and spread along the interface under cyclic loading until eventually large segments of the z-reinforcement have detached. This cracking changes the stress transfer mechanics in the composite material because higher
strength values for 3D woven and stitched composites with much higher z-reinforcement contents (up to 18%), and found no statistically significant trend between the tensile strength and z-reinforcement. The woven z-binders and stitches were inserted in dry fabric, unlike the z-pins that were inserted into an uncured composite. The insertion of z-reinforcement into dry fabric allows the fibres to be easily pushed aside which minimises the amount of damage and breakage. In the case of z-pinning, the z-reinforcement is inserted into the fibre preform that has already been preimpregnated with the polymer resin. The resin provides some resistance against the displacement of the fibres during insertion of the z-pins. For this reason, the 3D woven and stitched composites may experience less fibre damage than the z-pinned composites, and therefore their tensile strengths did not decrease as rapidly with increasing z-reinforcement content. Fatigue-life (S–N) curves for the 3D woven, stitched and z-pinned composites are presented in Fig. 8. The curves represent lines-of-best fit through the fatigue life data. The results show that the fatigue lives of the 3D composites are always lower than the fatigue life of the control material. Furthermore, their fatigue life drops with increasing volume content of z-reinforcement, with a large loss in fatigue performance caused by a relatively small amount of through-thickness reinforcement. For example, the fatigue life of the 3D woven composite fell by one order of magnitude
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loads are exerted onto the undamaged regions. The number of debonding cracks increases with the volume content of z-reinforcement, which accounts for the corresponding reduction to the fatigue life. It is also possible that pre-existing damage within the z-reinforcement itself may reduce the fatigue life of the 3D woven and stitched composites. The tensile strength of the z-binders in the 3D woven composite is reduced by about 50% due to fibre damage and breakage during the weaving process [38]. Likewise, the tensile strength of the Kevlar stitches is reduced by 20% due to fibre damage incurred by the stitching process [39]. The bending and twisting of the stitches when they are sewn into the material are believed to damage the fibres within the stitches. The damaged fibres in the woven z-binders or stitches are believed to break early in the fatigue life of the composite and develop into flaws. These flaws within the z-reinforcement are believed the generate cracks within the 3D woven and stitched composite that reduce the fatigue life. The number of these flaws must increase with the z-reinforcement content, which would account for the corresponding reduction to the fatigue life. The fatigue damage mechanisms of the z-pinned composites are different to the 3D woven and stitched materials. Debonding of the z-pins was not observed in the fatigued specimens, unlike the 3D woven and stitched materials. Also, the z-pins were not damaged during the z-pinning process, again unlike the woven z-binders and stitches that were damaged and weaken. Instead, the reduction to the fatigue life of the z-pinned composites is attributed mostly to the damage and breakage of the load-bearing fibres, which reduced the tensile modulus and strength. As mentioned, the fibres were not easily pushed aside when the z-pins were inserted by the z-pinning process because the uncured resin matrix is viscous. Fibres that do not move aside can break under the force needed to insert the z-pins. The broken fibres are clustered near the pins, which can develop into flaws under cyclic loading that leads to fatigue-induced failure. The number of fibres broken by the z-pinning process rises with the z-pin content. For this reason, the fatigue life of the z-pinned composites decreases with increasing z-reinforcement content. Chang et al. [18] measured a large reduction of the tensile fatigue life of carbon/epoxy tape composites with increasing z-pin content, and also attribute this to damage to the loading-bearing fibres during the pinning process. As mentioned, the 3D woven and stitched composites do not experience significant fibre damage during insertion of the z-reinforcement, and therefore is not an important cause of the reduced fatigue life of these materials.
4. Conclusions The impact damage resistance and damage tolerance of composite materials can be improved by through-the-thickness reinforcement using woven z-binders, stitches or z-pins. These properties increase with the volume content of the z-reinforcement, and therefore it is common practice to use a relatively large amount of through-thickness reinforcement in composite structures requiring high damage tolerance. This study has shown, however, that increasing the z-reinforcement content reduces the tensile fatigue life of 3D composites. The fatigue life of 3D woven, stitched and z-pinned composites decreased with increasing zreinforcement content. The reduction to the fatigue performance is due to microstructural damage to the composite caused by the insertion of the z-reinforcement. Debonding of the z-reinforcement and the development of cracks within the z-reinforcement itself are believed responsible for the large reduction to the fatigue lives of the 3D woven and stitched composites. Breakage and damage to the load-bearing fibres by the z-pins are responsible for the reduction to the fatigue life of the z-pinned composites. The research
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presented in this paper shows that the deterioration to the tensile fatigue properties of 3D composites must be a consideration when using these materials in impact damage tolerance structures. However, as noted earlier, this study was confined to one type of 3D woven, stitched and pinned material, and the tensile fatigue properties may be somewhat different with other types of composites with through-the-thickness reinforcement. This uncertainty provides the rationale for a larger study into the fatigue properties of a diverse variety of 3D woven, stitched and pinned composites. Acknowledgements The author thanks Dr Rudov-Clark for manufacturing and testing the 3D woven composite specimens, Dr Shah Khan for testing the stitched specimens, and Mr Cull for manufacturing and testing the z-pinned specimens. The assistance of Mr Tkatchyk in the tensile and fatigue testing is also gratefully acknowledged. References [1] Collyer AA, editor. Rubber toughening engineering plastics. Springer; 1994. [2] Kim J-K, Mai Y-W. Engineered interfaces in fibre reinforced composites. Oxford: Elsevier Science; 1998. [3] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44:1624–52. [4] Veeda VP, Cao A, Li X, Man K, Soldano C, Kar S, et al. Multifunctional composites using reinforced laminae with carbon-nanotube forests. Nature Mater 2006;5:457–61. [5] Tong L, Mouritz AP, Bannister MK. 3D fibre reinforced polymer composites. London: Elsevier Science Ltd.; 2002. [6] Dransfield KA, Baillie C, Mai Y-W. Improving the delamination resistance of CFRP by stitching – a review. Compos Sci Technol 1994;50:305–17. [7] Mouritz AP. Review of z-pinned composite laminates. Composites 2007;38A:2383–97. [8] Mouritz AP, Bannister MK, Falzon PJ, Leong KH. Review of applications for advanced three-dimensional fibre textile composites. Composites 1999;30A:1445–61. [9] Mouritz AP, Cox BN. A mechanistic approach to the properties of stitched laminates. Composites 2000;31A:1–27. [10] Dickinson LC, Farley GL, Hinders MK. Prediction of effective three-dimensional elastic constants of translaminar reinforced composites. J Compos Mater 1999;33:1002–29. [11] Guess TR, Reedy EDJ. Comparison of interlocked fabric and laminated fabric Kevlar 49/Epoxy composites. J Compos Technol Res 1985;7:136–42. [12] Arendts F-J, Dreschler K, Brandt J. Manufacturing and mechanical performance of composites with 3-D woven fiber reinforcement. In: Proceedings of the 4th textile structural composites symposium. Philadelphia; 1989. [13] Kang TJ, Lee SH. Effect of stitching on the mechanical and impact properties of woven laminate composite. J Compos Mater 1994;28:1574–87. [14] Steeves C, Fleck NA. In-plane properties of composite laminates with throughthickness pin reinforcement. Int J Solids Struct 2006;43:3197–212. [15] Larsson F. Damage tolerance of a stitched carbon/epoxy composite. Composites 1997;28A:923–34. [16] Farley GL. A mechanism responsible for reducing compression strength of through-the-thickness reinforced composite material. J Compos Mater 1992;26:1784–95. [17] Harris H, Schinske N, Kruger R, Swanson B. Multiaxial stitched preform reinforcement. In: Proceedings of the 6th annual ASM.ESD advanced composites conference; 8–11 October 1991. p. 433–4. [18] Chang P, Mouritz AP, Cox BN. Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension. Compos A 2006;37: 1501–13. [19] Mouritz AP. Compression properties of z-pinned sandwich composites. J Mater Sci 2006;41:5771–4. [20] Chang P, Mouritz AP, Cox BN. Flexural properties of z-pinned laminates. Compos A 2007;38:224–51. [21] Dadkhah MS, Morris WL, Cox BN. Compression–compression fatigue in 3D woven composites. Acta Metall Mater 1995;43:4235–45. [22] Ding YQ, Yan Y, McIlhagger R, Brown D. Comparison of the fatigue behaviour of 2-D and 3-D woven fabric reinforced composites. J Mater Process Technol 1995;55:171–7. [23] Rudov-Clark S, Mouritz AP. Tensile fatigue properties of a 3D orthogonal woven composites. Compos A 2008;39:1018–24. [24] Portanova MA, Poe CC, Whitcomb JD. Open hole and postimpact compressive fatigue of stitched and unstitched carbon–epoxy composites. In: Glenn GC, editor. Composite materials: testing and design (10th volume). ASTM STP 1120. Philadelphia: American Society for Testing and Materials; 1992. p. 37–53.
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