缝合层合板的I型层间断裂韧性研究

通过缝合的方法改善织物增强复合材料层合板的层间断裂韧性.采用双悬臂梁(DCB)试验测试和研究了缝合层合板的层间断裂韧性与断裂行为.为了评价缝合工艺参数(缝合密度)对层间断裂韧性的影响, 用改进的插入型夹具在实测不同缝合工艺层合板的I型层间断裂韧性值(GIC)的基础上, 分析和阐明了缝合工艺参数(缝合密度)与GIC间的关系; 以提高层合板的平均层间断裂韧性值为目标, 以拉伸和弯曲强度为约束条件优化了缝合工艺; 采用摄影显微镜对分层断裂面进行了观察, 分析和考察了缝合对其它性能的影响.结果表明 改进的插入型夹具可方便地完成缝合层合板的I型层间断裂韧性测试; 缝合后裂纹不连续扩展, 缝合密度对裂纹扩展行为有较大影响; 随着缝合密度的增大, 层间断裂韧性值增大, 但拉伸和弯曲强度降低, 缝合密度存在最佳值.     

缝合复合材料II型层间断裂特性研究

分别采用测量ENF试样加载点位移与测量其端部剪切位移CSD(Crack Shear Displacement)的试验方法,研究了缝合复合材料层合板的II型层间断裂韧性以及缝合密度,缝合线的直径等缝合参数对于缝合复合材料层合板II型层间断裂韧性和分层模式的影响。结果表明,缝合降低了层合板初始分层韧性G_IIi,但对于分层的扩展有良好的抑制作用。缝合参数对此有较大影响。      

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。      

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。      

缝合/RTM复合材料层合板的力学性能研究

分别采用高强玻璃纱、Kevlar-29纤维和T300 3K纤维为缝合线对玻璃纤维方格布进行缝合,研究了缝合/RTM复合材料层合板的面内拉伸性能和层间剪切性能.研究结果表明,与未缝合复合材料层合板相比,缝合复合材料层合板面内拉伸性能有所降低(碳纤维缝合复合材料除外),在给定缝合密度下最大降幅为14%;缝合复合材料层合板的层间剪切性能较未缝合层合板都有不同程度的提高,在给定缝合密度下最大达到了40%,缝合可显著提高复合材料层合板的层间性能.     

超细纤维增强复合材料 Ⅰ 型层间断裂韧性分析

采用静电纺丝技术制备了厚度约0.1mm的超细纤维无纺布薄膜, 并入层合板中间界面, 固化成型后加工为双悬臂梁(DCB)试样。根据ASTM D5528标准测试了 Ⅰ 型层间断裂韧性。实验结果表明, 增强试样比空白试样的 Ⅰ 型临界应变能释放率(G_{Ⅰ C})提高了约35％。同时采用有限元分析方法研究了含无纺布薄膜试样和空白试样的裂纹扩展过程, 数值结果与实验结果吻合较好, 更好地解释了含无纺布薄膜层合板的层间断裂机理。      

低温环境下测试复合材料Ⅰ型层间断裂韧性的简易方法

目前ASTM复合材料Ⅰ型层间断裂韧性测试标准需不断观测裂纹扩展长度。然而在低温环境下,裂纹扩展长度不易测量且过程繁琐。为克服这一缺陷,本文采用双柔度法测试复合材料低温环境下Ⅰ型层间断裂韧性,该方法的步骤与ASTM标准基本相同,但不需观测裂纹扩展长度便能获得低温下Ⅰ型层间断裂韧性。为了验证该方法的可靠性和精度,采用5件碳纤维增强环氧树脂基复合材料双悬臂梁（DCB）试样在-10℃环境下进行Ⅰ型层间裂纹扩展实验,应用ASTM标准所推荐的三种方法及本文的双柔度法进行数据处理获得复合材料Ⅰ型层间断裂韧性。结果表明:ASTM标准的三种方法与双柔度法得到的Ⅰ型层间断裂韧性结果一致,相对差别小于5%,而本文的双柔度法不需测量裂纹扩展长度,因此更简单,为测试低温环境下Ⅰ型层间断裂韧性提供了一种准确、简单的新方法。      

Z-pin增强CMC层间Ⅰ+Ⅱ混合型断裂韧性

提出手工预缝纫方法将3K丝束的T300碳纤维引入预成型体,采用CVI工艺在预成型体和缝线处同时渗透SiC基体,制备了Z-pin增强平纹编织C/SiC陶瓷基复合材料。通过三点弯曲试验测定了Ⅰ+Ⅱ混合型应变能释放率,分析了材料的裂纹扩展行为和Z-pin增强机理。结果表明:随着裂纹扩展长度的增大,Ⅰ+Ⅱ型裂纹扩展阻力不断增大,相同裂纹扩展长度,增加Z-pin植入密度可以提高粘结强度,增大止裂作用。Z-pin增强平纹编织C/SiC陶瓷基复合材料裂纹扩展的耗能途径主要是层间界面剥离、Z-pin弹性剪切和拉伸变形。     

缝合密度对缝合/VARI成型复合材料力学性能的影响EI北大核心CSCD

采用改进锁式缝合和真空辅助树脂注射(VARI)成型工艺制备不同缝合密度的碳纤维/环氧树脂复合材料,研究缝合行距和缝合针距对复合材料力学性能的影响,得出最佳缝合密度。结果表明:随着缝合行距的增大,拉伸性能和弯曲性能均有所提升,层间剪切强度先增大后减小;随着缝合针距的增大,拉伸性能和弯曲性能均有提高的趋势;当缝合密度为5 mm×8 mm时,缝合复合材料具有最佳的综合力学性能,与未缝合复合材料相比,拉伸强度和模量分别下降了13.3%和12.7%,弯曲强度和模量分别下降了23.0%和25.2%,层间剪切强度提高了11.3%。     

缝合复合材料可用性——一般层合板的基本性能

通过试验研究了缝合T300帘子布/QY9512常用层合板的拉伸和压缩性能,考察了缝合方向、铺层顺序和环境因素对层合板拉、压性能的影响,得到了3种常用层合板及其孔板的拉伸、压缩强度与模量。研究结构表明:缝合与缝合方向对常用层合板的拉伸强度与模量的影响不大;不同铺层顺序层合板的拉伸强度和模量受缝合与缝合方向的影响程度不同。缝合方向与铺层形式对孔板的缝合效果均有影响,层合板的最佳缝合方向随铺层形式不同而发生变化。缝合对层合板湿热状态下压缩性能的影响与铺层顺序有很大关系。缝合使含孔层合板的干态常温压缩强度明显提高,使湿热时的强度明显降低。      

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.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

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%).     

Effect of mendable polymer stitch density on the toughening and healing of delamination cracks in carbon–epoxy laminates

This paper presents an investigation into the effect of stitch density on the delamination toughening and self-healing properties of carbon–epoxy laminates. The stitches provide the laminate with the synergistic combination of high mode I interlaminar fracture toughness to resist delamination cracking and healing properties to repair delamination damage. The results show that the fracture toughness of the laminate increased with stitch density, due to higher traction (crack closure) loads exerted by the stitches bridging the delamination. During the healing process these bridging stitches first melt and then flow into the delamination, leading to self-healing with full restoration of the mode I fracture toughness. Furthermore, the stitches were capable of repairing delamination cracks many times larger than the original size of the stitches. The effect of stitch density on the healing process of delamination cracks and restoration of fracture toughness was found to remain approximately the same under multiple repair operations.     

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.     

Experimental investigation of bridging law for single stitch fibre using Interlaminar tension test

In this study, a novel Interlaminar tension test (ITT) method was performed to experimentally investigate the bridging and fracture process of a single stitch fibre used to improve the delamination strength of composite laminates. Kevlar-29, of various thread thicknesses (44, 66, 88 and 132 tex), was used as the through-thickness stitch fibre in the ITT experiments. Key empirical force and displacement parameters, which governed the stitch fibre bridging law, were characterised and identified. Relationships of such parameters with thread thicknesses were determined. Fibre fracture load and fibre fracture energy are found to increase with increasing thread thickness. Frictional pull-out force greatly depends on the type of stitch fracture modes, which can be grouped into three categories. This paper aims to provide better physical understanding of the mechanics and mechanisms of stitch fibre fracture. By correlating critical stitch fracture parameters with stitch fibre thicknesses, the results expect to provide useful reference, which is essential and important for accurate stitch computational modeling and strength prediction of composites using stitching as the interlaminar reinforcement technique.     

Traction law for inclined through-thickness reinforcement using a geometrical approach

Stitching of laminated composites is a proven way to improve damage tolerance and increase interlaminar fracture toughness. However, the size and shape of various composite parts manufactured across many industries has limited possible applications of stitching. Innovative one-sided stitching techniques incorporating inclined stitches have emerged to overcome these limitations.Models for determining traction laws for individual stitches including inclined stitches have progress over the years but with limitations. A model for analysing a stitch, pin or other through-thickness reinforcement in a composite laminate has been developed and validated with finite element analysis (FEA). This model is formulated based on treating the stitch as a rope supported by a plastic foundation, with pull-out resisted by frictional stresses. A new approach was taken to determine the displacement by integrating the function describing the shape of the stitch. The model accurately predicts the traction law of a stitch and is most accurate for cases where μ and θ₀ are small. This model can be incorporated with FEA to simulate the delamination of laminates. This reduces the need for expensive experimental testing and allows for the most effective stitching parameters to be determined, resulting in optimal design.     

Improved fracture toughness in carbon fibre epoxy composite through novel pre-preg coating method using Epoxy Terminated Butadiene Nitrile rubber

A novel pre-preg coating method was used to improve the interlaminar fracture toughness in carbon fibre epoxy composite laminates, using reactive liquid rubber. The Epoxy Terminated Butadiene Nitrile (ETBN) liquid rubber incorporated between pre-pregs using automatic draw bar coating technique. Experimental test results reveal that by adding ETBN in small quantities in the range of 15.55–22.66 g/m², inter laminar critical energy release rates (G_IC and G_IIC) can be improved up to 140% in mode-I loadings and 32% in mode-II loadings respectively. It was confirmed that the effect of ETBN rubber concentration in carbon epoxy pre-preg system on interlaminar fracture toughness under mode-I and mode-II loadings, was discussed by on the bases of fractographic observations and mechanism considerations using SEM.