Effect of fibre content on the interlaminar fracture toughness of unidirectional glass-fibre/polyamide composite

In this paper, the effects of fibre content on the interlaminar fracture in continuous glass-fibre/polyamide 12 composite have been investigated under model I (DCB) loading condition. The specimens were fabricated with different fibre volume contents (21%, 26%, 34% and 39%) by using a powder impregnation method. It was observed that the values of G_IC(NL) and G_IC(PROP) of this material have a dropping tendency with increasing fibre volume content in the range of 21%–39%, while no general trends in G_IC(5%) and G_IC(VIS). Results show that the glass-fibre/polyamide 12 composites possess high mode I fracture toughness, which is mainly attributed to the high ductility of the polyamide 12 matrix, and the increased fibre bridging caused by the increasing of the fibre volume content can not change the decrease tendency of G_IC(PROP). The fracture surfaces of the specimens were observed by scanning electron microscopy, and the fracture mechanism was analysed.     

Effect of the loading rate on mode I interlaminar fracture toughness of laminated composites

The objective of the present study is to determine the influence of the loading rate on the critical energy release rate G_Ic of fibre-reinforced epoxy laminates. In order to perform pure mode I loading at higher opening velocities, a new test device is developed. The approach is based on a symmetrical opening displacement applied to a DCB specimen. In the data reduction, the influence of the kinetic energy has to be taken into account. The results obtained on the unidirectional carbon-epoxy laminate T300/914 at crack opening rates up to 1.6 m/s show a slight effect of the loading rate on G_Ic.     

Prestandardization study on mode I interlaminar fracture toughness test for CFRP in Japan

This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.     

Effect of ionomer thickness on mode I interlaminar fracture toughness for ionomer toughened CFRP

Effect of ionomer thickness on mode I interlaminar fracture toughness was investigated for the ionomer-interleaved CFRP. Laminates were fabricated with Toho UT500/111 prepregs. Ethylene-based ionomer, which has high ductility and good adhesion to epoxy resin, was used as an interleaf material in this study. Thickness of the ionomer film selected was 12, 25, 100 and 200 μm. EPMA analysis showed the existence of the interphase region between the interleaf film and the base prepreg lamina where ionomer and epoxy were mixed. Mode I fracture toughness tests were carried out using DCB specimens. Precracks were introduced into all of the specimens. Fracture toughness values were much improved by interleaving the ionomer films. The fracture toughness value increased sharply by inserting thin ionomer film; however, the additional increase with the increase of the ionomer thickness was smaller. The thickness effect of the ionomer interleaf differs from that of the other kinds of thermoplastic-interleaf. Microscopic observation revealed that the crack path depended on the thickness of the ionomer region. Crack propagated in the interphase/ionomer interfaces for thinner-ionomer-interleaved CFRP, and in the interphase region, at the interphase/base lamina interface and interphase/ionomer interface for thicker-ionomer-interleaved CFRP. Ionomer resin deformed largely only near the crack surfaces, and this fact is responsible for the nonlinear increase of the fracture toughness with the increase of the ionomer thickness.     

Nesting effect on the mode I fracture toughness of woven laminates

The fracture toughness of two woven laminates is evaluated for different nesting/shifting values between advanced layers. The analysed woven composites are manufactured using the same resin-reinforcement and same architecture, but have a different tow size (3K/12K). Three different nesting/shifting configurations are applied to the plies at the fracture surface: zero shifting, middle shifting and maximum shifting. Before being tested, the internal geometry of the material is evaluated and any shifting error is measured. For all these configurations mode I fracture tests are carried out. The differences obtained between 3K and 12K cases can be explained by fibre bridging, but not the differences between the nesting configurations. Depending on the nesting/shifting value the delaminated surface waviness is different, and consequently the fracture toughness is also influenced.     

PBO纤维增强环氧树脂复合材料层间Ⅰ型断裂韧性的DIC技术测量

Ⅰ型双悬臂梁(DCB)试验通常用于单向复合材料的层间抗拉性能研究,目标是测量Ⅰ型层间断裂韧性,其可作为复合材料分层扩展及失效机制研究的重要输入参数。在DCB试验中必须经常暂停试验以实现多次测量裂纹长度,这不仅会对裂纹传播产生潜在影响,造成测量误差且多次反复试验的时效性较差。数字图像相关(DIC)测试技术应用于裂纹扩展长度测量具有实时跟踪、精确定位的优点,可有效提高Ⅰ型断裂韧性试验的测量效率,但应用于非连续变形行为仍存在局限性,且易受到图像噪声的干扰,产生测量误差。本文发展了一种基于DIC测试技术的实时获取裂纹长度的检测方法,通过图像匹配算法获取试件的非连续变形位移场,并提出一种根据全局横向位移离散程度的辨别方法,实现了裂纹尖端的实时捕捉。再通过DCB试验,与传统测量方式对比,裂纹长度的测量误差平均不超过2.76%,验证了该方法的准确性和高效性,同时也克服了聚对苯撑苯并双噁唑(PBO)/环氧树脂复合材料侧表面毛糙、散斑质量较差及纤维桥接对测量结果的干扰,最终获取了有效的Ⅰ型层间断裂韧性初始值及稳态扩展值。     

Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross‐ply laminates

One of the major difficulties in interlaminar fracture tests of multidirectional laminates is the high tendency for intralaminar cracking and the resulting wavy crack propagation. Experimental work showed that this occurred in double cantilever beam (DCB) tests of cross‐ply laminates having a starter crack on a 0°/90° interface. Moreover, under steady‐state propagation conditions, the apparent values of the critical strain energy release rate G_Ic were two times higher than those of 0°/0° specimens. In this paper, a finite‐element‐based progressive damage model was used to simulate crack propagation in cross‐ply specimens. The results showed that transverse cracking alone cannot be responsible for the above difference of G_Ic values. Therefore, the higher propagation G_Ic values for cross‐plies must be attributed to the more extensive fibre bridging observed and to plastic deformations of the 90° interfacial ply.     

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

Prestandardization study on mode II interlaminar fracture toughness test for cfrp in japan

A three year long cooperative research programme has been carried out to establish a Japanese standard for an interlaminar fracture toughness test method for CFRPs. For mode II fracture toughness tests, the end-notched flexure test was employed. Two series of round robin tests (RRTs) were carried out using conventional brittle epoxy CFRPs, a toughened epoxy CFRP and APC-2. The following points became clear in the investigation: 1) crack shear displacement measurement gives precise information on crack initiation; 2) the fracture toughness values at the start of non-linear behaviour showed a considerably large scatter; 3) the toughness at the 5% offset points has a lower scatter and is still conservative; and 4) it is necessary to take account of the increase in crack length for the toughness calculation after the 5% offset point. Considering this information, together with other important observations obtained in the RRTs, a proposal for the mode II fracture toughness test was made and it was established as Japanese Industrial Standard K 7086 in March 1993.     

Analysis of stitched laminated ENF specimens for interlaminar mode II fracture toughness

Two simple micromechanics based models are proposed to predict the effect of through-thickness reinforcement (stitching) on the improvement of delamination crack growth resistance in end-notched flexure (ENF) specimens. In the first model, it is assumed that stitches stretch elastically and then rupture when the load carried approaches the failure load. In the second model, it is assumed that stitches are discontinuous and that the stitch thread-matrix interface is completely frictional. Approximate closed form solutions for energy release rates are obtained, and the effects of stitch density, matrix-stitch thread interfacial shear stress, stitch thread diameter, volume fraction of stitches, critical energy release rate and Young's modulus are then examined. A simple design study for sizing the ENF specimen to minimise geometric nonlinear response is presented. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.     

Environmental and time dependence of fracture toughness and crack growth in glass-fibre reinforced polyester resins

The validity of simple (maximum load) fracture toughness testing of glass-fibre reinforced polyester resin laminates has been examined for SEN specimens using slow strain rate test techniques developed for metallic materials. Laminates were tested both in air and whilst immersed in an acidic environment. In general, valid test conditions were only established at test speeds <10^–2 mm min^–1 which are slower than those generally reported in the past. Fracture toughness was found to be greatly reduced by the presence of the chemical environment, as would be expected from the known sensitivity of these materials to environmental stress cracking. The slow strain rate test technique indicated a threshold level of critical stress intensity for environmental stress cracking and hence could be useful in ranking materials.Environmental effects were also found to determine crack growth rates as measured on DCB specimens. Crack growth data was analysed in terms of initial and/or inherent flaws and its use in predicting creep-rupture life examined. In principle it was concluded that use of crack growth data could provide an alternative to the present qualitative approach to design and a more predictable alternative to long term creep-rupture testing.     

Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites

The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness (G_Ic) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G_Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.     

Prediction of Mode I interlaminar fracture toughness of stitched flax fiber composites

This paper aims to propose a simulation procedure to predict the interlaminar fracture toughness of stitched flax fiber composites through a virtual double cantilever beam test. The proposed procedure is constituted of two steps. First, the interlaminar failure of unstitched flax fiber laminate, as the parent laminate, is modeled using cohesive elements with a nonlinear softening law in order to model the large-scale fiber bridging occurred during delamination. The experimental results are used to calibrate the parameters of the cohesive law. Second, two-node beam elements are superposed onto the cohesive interface of the parent laminate at a prescribed stitch density and distribution to model the bridging stitches present in the validation samples. The stitch material behavior and properties are obtained from the tensile test of impregnated stitch fibers. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves was achieved.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.