DCB test simulation of stitched CFRP laminates using interlaminar tension test results

A simulation model for the delamination extension of stitched CFRP laminates and 3-D orthogonal interlocked fabric composites (3-D OIFC) has been developed using a 2-D finite element method incorporating interlaminar tension test results to simulate the experimental results of their DCB tests. The mechanical properties of through-the-thickness fiber were determined from the results of interlaminar tension tests in which the specimen included only one through-the-thickness yarn. The fracture phenomena around the through-the-thickness thread, such as debonding from the in-plane layer, slack absorption, fiber bridging, and the pull-out of broken threads from the in-plane layers, are also introduced into the FEM model. The present FEM simulation results were compared to DCB test results for certain stitched laminates and a 3-D OIFC, and the simulation results showed good agreement with the experimental results of DCB tests, including the load–displacement curve and Mode I strain energy release rate (GI). While it was difficult to estimate GI accurately when the DCB test specimen included different types of z-fiber fracture modes, the present model of FEM analysis can simulate the experimental results of DCB tests of stitched laminates and 3-D OIFC. It is suggested that the GI of CFRP with arbitrary z-fiber densities can be predicted by using this FEM analysis model together with interlaminar tension test results.     

Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Multifunctional composites using reinforced laminae with carbon-nanotube forests

Traditional fibre-reinforced composite materials with excellent in-plane properties fare poorly when out-of-plane through-thickness properties are important. Composite architectures with fibres designed orthogonal to the two-dimensional (2D) layout in traditional composites could alleviate this weakness in the transverse direction, but all of the efforts so far have only produced limited success. Here, we unveil an approach to the 3D composite challenge, without altering the 2D stack design, on the basis of the concept of interlaminar carbon-nanotube forests that would provide enhanced multifunctional properties along the thickness direction. The carbon-nanotube forests allow the fastening of adjacent plies in the 3D composite. We grow multiwalled carbon nanotubes on the surface of micro-fibre fabric cloth layouts, normal to the fibre lengths, resulting in a 3D effect between plies under loading. These nanotube-coated fabric cloths serve as building blocks for the multilayered 3D composites, with the nanotube forests providing much-needed interlaminar strength and toughness under various loading conditions. For the fabricated 3D composites with nanotube forests, we demonstrate remarkable improvements in the interlaminar fracture toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behaviour, and thermal and electrical conductivities making these structures truly multifunctional.     

Influence of adsorption behaviour of a silane coupling agent on interlaminar fracture in glass fibre fabric-reinforced unsaturated polyester laminates

The relationship between the interphase consisting of physisorbed and chemisorbed silane on glass fibres and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate, was studied. The Mode I interlaminar fracture toughness of the laminate specimen was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of the laminate specimen finished with two silane concentrations and washed in methanol solvent, is discussed on the basis of the interlaminar fracture toughness. In order to determine the amount of physisorbed and chemisorbed silane on the glass fibre, the amount of total carbon was determined using an analysis instrument. The physisorbed silane migrated into the resin matrix and influenced the mechanical properties and interlaminar fracture of the laminate specimen. The amount of unsaturated polyester resin blended with a silane coupling agent was measured using dynamic mechanical spectroscopy, and a DCB specimen for mechanical properties and fracture toughness.     

Influence of silane coupling agents on interlaminar fracture in glass fibre fabric reinforced unsaturated polyester laminates

The relationship between the adhesive properties of the interphase of glass fibre/resin and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate (GFFL) was studied. The Mode I interlaminar fracture toughness of GFFL was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of GFFLs which have two silane coupling agents and three concentration finishes is discussed on the basis of interlaminar fracture toughness. The crack propagation behaviour of DCB testing was mainly divided into stable and unstable manners. The fracture toughness and the crack propagation behaviour were dependent on the types and concentration of silane coupling agents.     

铺层方向和裂纹混合比对复合材料层压板混合型断裂韧性影响的试验研究

试验研究了复合材料层压板的铺层方向以及裂纹混合比对层间裂纹分层扩展的影响规律。试验结果显示: 在非0°单向板的 Ⅰ 型层间裂纹分层扩展过程中, 会出现层间裂纹穿过分层开裂面的铺层而偏离到相邻铺层间扩展的现象, 而0°铺层具有阻止该裂纹偏离扩展的作用; 在不同裂纹混合比的层压板分层开裂试验中, 相应的0°单向板的断裂韧性均可以作为下限值而偏安全; 混合断裂韧性( Ⅰ 型断裂韧性+ Ⅱ 型断裂韧性)随着裂纹混合比的变化呈现类似正弦曲线的变化规律。      

Electrospun nanofibre toughened carbon/epoxy composites: Effects of polyetherketone cardo (PEK-C) nanofibre diameter and interlayer thickness

Polyetherketone cardo (PEK-C) nanofibres were produced by an electrospinning technique and directly deposited on carbon fabric to improve the interlaminar fracture toughness of carbon/epoxy composites. The influences of nanofibre diameter and interlayer thickness on the Mode I delamination fracture toughness, flexure property and thermal mechanical properties of the resultant composites were examined. Considerably enhanced interlaminar fracture toughness has been achieved by interleaving PEK-C nanofibres with the weight loading as low as 0.4% (based on weight of the composite). Finer nanofibres result in more stable crack propagation and better mechanical performance under flexure loading. Composites modified by finer nanofibres maintained the glass transition temperature (T_g) of the cured resin. Increasing nanofibre interlayer thickness improved the fracture toughness but compromised the flexure performance. The T_g of the cured resin deteriorated after the thickness increased to a certain extent.     

Toughening and self-healing of epoxy matrix laminates using mendable polymer stitching

This paper presents an experimental study into a new type of stitched fibre–polymer laminate that combines high interlaminar toughness with self-healing repair of delamination damage. Poly(ethylene-co-methacrylic acid) (EMAA) filaments were stitched into carbon fibre/epoxy laminate to create a three-dimensional self-healing fibre system that also provides high fracture toughness. Double cantilever beam testing revealed that the stitched EMAA fibres increased the mode I interlaminar fracture toughness (by ∼120%) of the laminate, and this reduced the amount of delamination damage that must subsequently be repaired by the self-healing stitches. The 3D stitched network was effective in delivering self-healing EMAA material extracted from the stitches into the damaged region, and this resulted in high recovery in the delamination fracture toughness (∼150% compared to the original material). The new self-healing stitching method provides high toughness which resists delamination growth while also having the functionality to repeatedly repair multiple layers of damage in epoxy matrix laminates.     

A modification of the mixed-mode bending test apparatus

In this article, the mixed-mode bending test appartaus was modified in order to reduce the influence of the weight of the lever on the mixed-mode fracture toughness during the measurement. A unidirectional glass fibre reinforced polyamide 12 composite laminate was measured at G_I/G_II ratios of 3/1 and 1/1, by using the original and the modified MMB apparatus respectively, and the results of the mixed-mode interlaminar fracture toughness data were compared. It has been found that the modified MMB apparatus can be used to avoid the preloading on the specimens caused by the weight of the lever, and the fracture toughness can directly be calculated by applying the beam theory or the modified beam theory equations without the lever-weight correction.     

Effect of delamination failure in crashworthiness analysis of hybrid composite box structures

In the present paper the effects of delamination failure of hybrid composite box structures on their crashworthy behaviour will be studied and also their performance will be compared with non-hybrid ones. The combination of twill-weave and unidirectional CFRP composite materials are used to laminate the composite boxes. Delamination study in Mode-I and Mode-II with the same lay-ups was carried out to investigate the effect of delamination crack growth on energy absorption of hybrid composite box structures. The end-loaded split (ELS) and double-cantilever beam (DCB) standard test methods were chosen for delamination studies. In all hybrid composite boxes the lamina bending crushing mode was observed. Regarding the delamination study of hybrid DCB and ELS the variation of the specific energy absorption (SEA) versus summation of G_IC and G_IIC were plotted to combine the effect of Mode-I and Mode-II interlaminar fracture toughness on the SEA. From this relationship it was found the hybrid laminate designs which showed higher fracture toughness in Mode-I and Mode-II delamination tests, will absorb more energy as a hybrid composite box in crushing process. The crushing process of hybrid composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental result.     

Z-pin增强陶瓷基复合材料拉伸和层间剪切性能

研究了Z-pin横向增强平纹编织陶瓷基复合材料的拉伸和层间剪切性能。炭纤维平纹编织物和炭纤维Z-pin制备的预成型体, 通过化学气相渗透(CVI)工艺制成Z-pin增强平纹编织陶瓷基复合材料。通过单轴拉伸试验及加-卸载试验研究材料拉伸力学性能参数及破坏规律。采用双切口压缩试验测试材料的层间剪切强度。结果表明, Z-pin增强平纹编织陶瓷基复合材料拉伸应力-应变曲线具有非线性特性; Z-pin嵌入降低了平纹编织陶瓷基复合材料的拉伸强度, 显著提高了陶瓷基复合材料层间剪切强度, 使原来单纯层间基体与织物表面的脱离转变为Z-pin的剪切破坏和层间基体与织物的脱离双重破坏机理。     

Characterization of transverse tensile, interlaminar shear and interlaminate fracture in CF/EP laminates with 10 wt% and 20 wt% silica nanoparticles in matrix resins

The transverse tensile properties, interlaminar shear strength (ILSS) and mode I and mode II interlaminar fracture toughness of carbon fibre/epoxy (CF/EP) laminates with 10 wt% and 20 wt% silica nanoparticles in matrix were investigated, and the influences of silica nanoparticle on those properties of CF/EP laminates were characterized. The transverse tensile properties and mode I interlaminar fracture toughness (G_IC) increased with an increase in nanosilica concentration in the matrix resins. However, ILSS and the mode II interlaminar fracture toughness (G_IIC) decreased with increasing nanosilica concentration, especially for the higher nanosilica concentration (20 wt%). The reduced G_IIC value is attributed to two main competing mechanisms; one is the formation of zipper-like pattern associated with matrix microcracks aligned 45° ahead of the crack tip, while the other is the shear failure of matrix. The ratio of G_IIC/G_IC decreased with the concentration of silica nanoparticles, comparable with similar CF/EP laminates with dispersed CNTs in matrix. Fractographic studies showed that interfacial failure between carbon fibre and epoxy resin occurred in the neat epoxy laminate, whereas a combination of interfacial failure and matrix failure occurred in the nanosilica-modified epoxy laminates, especially those with a higher nanosilica concentration (20 wt%).     

平纹织物结构对织物增强复合材料层间断裂韧性影响的研究

本文采用粘贴片式双悬臂梁(DCB)试件和端部切口弯曲(ENF)试件研究了平纹织物的经纬纱密度对玻璃平纹织物/环氧树脂复合材料的Ⅰ型和Ⅱ型层间断裂韧性的影响。实验结果表明织物的密度对层间断裂韧性有显著的影响。提出了在织物增强复合材料层合板中,基体在织物孔洞中形成层间铆接,并且就其与层间G_IC和G_IC的关系进行了研究。      

Assessment of transverse impact damage in GF/EP laminates of conductive nanoparticles using electrical resistivity tomography

GF/EP composite laminates with an epoxy matrix modified by carbon black (CB) of 2.0 wt.% and copper chloride (CC) were manufactured by the vacuum assisted resin infusion (VARI) technique. The effects of CB nanoparticles and CC on improvement in Modes I and II interlaminar fracture toughness and impact damage resistance and on the electrical conductivity of GF/EP laminate composites were investigated. Delamination growth was calibrated by in situ electrical resistance changes during interlaminar fracture tests. The relationship between growth of delamination and change in electrical resistance was characterised. A damage index based on the change in electrical resistance was introduced, and a new method of electrical resistivity tomography was developed to access transverse impact damage in GF/EP laminates based on a matrix of conductive points in both in-plane and through-thickness directions. The damage images from in-plane and through-thickness electrical resistivity tomography were finally estimated with the corresponding C-scan.     

Fracture Toughness (Mode-I) of Para-Aramid/Phenolic Nano Preform Composites

The mode-I interlaminar toughness properties of nanostitched para-aramid/phenolic multiwall carbon nanotube composites were studied. The toughness strength of the stitched and stitched/nano composites demonstrated 40 fold and 38 fold (beam theory) increases compared to the base composites, respectively. It was found that stitching yarn type, especially prepreg para-aramid stitching yarn, was effective. On the other hand, the initiation and propagation of the G_IC values for stitched and stitched/nano composites were considerably deviated due to strengthening mechanism of the para-aramid stitch yarn in the transverse direction of the composite. The fracture toughness resistance to arrest crack propagation in the stitched/nano composite was mainly due to through-the-thickness stitching fiber bridging and pull-out, and was also due to warp and weft directional fiber bridging and multiwall carbon nanotubes. The results demonstrated that mainly stitching and some extent the nanotubes arrested the crack growth. Therefore, the stitched/nano and especially stitched para-aramid/phenolic composites showed a better damage resistance performance.     

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.     

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.     

Interlaminar and intralaminar fracture modes in 0/90 cross-ply glass/epoxy laminate

Delamination of a cross-ply 0/90 glass fibre-reinforced composite laminate with an epoxy-phenol matrix was studied using a double cantilever beam test. Fracture toughness was determined by measurement of bend angle of the cantilever beams. Results obtained with this method were in agreement with those from conventional compliance and area methods. Two different fracture modes were observed: interlaminar and intralaminar. In the interlaminar fracture mode, crack jumps in the space between two neighbouring 0° and 90° plies were observed. With the interlaminar fracture mode, during crack initiation G_Ic decreased with crack length. Intralaminar fracture mode consisted of the gradual growth of a crack through a 0° ply. Fibres bridging the opposite sides of the crack were observed in this case, and fracture toughness G_Ic did not change with crack length. G_Ic (420 J m⁻²) at intralaminar fracture mode was approximately twice that at interlaminar fracture mode (220 J m⁻²). The difference in fracture toughness was explained by the dissipation of energy by fibres bridging the opposite sides of the crack at intralaminar fracture mode.     

Interlaminar fracture properties of weft-knitted/woven fabric interply hybrid composite materials

An experimental study has been undertaken to characterize the delamination behavior and tensile properties of interply hybrid laminated composites reinforced by interlock weft-knitted and woven glass fiber preform fabrics. The hybrid composites, comprising the alternate layers of interlock and uniweave fabrics, were compared to interlock knitted (only) and uniweave (only) composites with respect to delamination and tensile performances. Mode-I double cantilever beam and mode-II end-notched flexure tests were carried out to assess the interlaminar fracture toughness using aluminum-strip stiffened specimens. The mode-I and mode-II interlaminar fracture toughness values, G _IC and G _IIC, for the hybrid composite were about three and two times higher than that for the uniweave composite, respectively. The tensile strength and modulus of the hybrid composite were 315 MPa and 12.8 GPa in the wale direction, respectively, demonstrating that the strength and modulus were found to be slightly lower than those of the uniweave composite, and significantly improved in comparison with the interlock knitted composites.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.