缝合参数对复合材料T型接头拉脱承载能力的影响

利用二维平面应变模型对缝合增强试验件进行失效分析,采用内聚力模型模拟界面的破坏情况,通过在分层的上下界面加入非线性弹簧元来模拟缝线的增强作用,非线性弹簧元的力学性能(桥联律)由细观力学方法获得。有限元分析结果与试验值吻合较好。在此基础上,对缘条区的缝合增强进行缝线的材料、直径和缝合密度的参数化分析,研究各参数对T型接头拉脱承载能力的影响。结果表明:缝合可显著提高T型接头的拉脱承载能力,同时能使其在较大的加载位移下仍保持较高的承载性能。T型接头的拉脱承载能力随缝线直径和缝合密度的增大而增大,且直径和密度的影响显著。缝线的拉伸强度是影响缝线性能最主要的因素, T型接头的拉脱强度随缝线拉伸强度的升高而升高。T型接头的拉脱强度随缝线拉伸模量的降低而升高,但拉伸模量的影响较拉伸强度的影响小。      

Z-pin/缝合对复合材料T型接头剪切承载能力的影响

利用三维有限元模型对Z-pin/缝合增强试验件进行有限元分析,采用内聚力模型模拟界面的破坏情况,通过在分层的上下界面加入非线性弹簧元来模拟Z-pin/缝合的增强机制,非线性弹簧元的力学性能（桥联律）由细观力学方法获得。通过与试验结果的对比发现,由于未进行界面增强的T型结构的剪切承载能力已较高,Z-pin/缝合增强较难提高T型接头的剪切承载能力。从少数几种可提高T型接头的剪切承载能力的增强方案中可看到,应选择拉伸强度较高而拉伸模量较低的缝线来进行T型接头剪切界面增强设计。     

缝合连接三维编织复合材料拉伸性能试验研究

研究了多种缝合方向、 搭接长度和缝合密度对缝合连接三维编织复合材料拉伸性能的影响 , 并与三维整体编织复合材料的拉伸性能进行对比。结果表明: 缝合连接三维编织复合材料与三维整体编织复合材料相比 ,拉伸强度下降了 25 %～50 % , 初始模量下降了 35 %～55 %; 搭接长度对拉伸强度和模量的影响较大 , 拉伸性能随搭接长度的增加呈现先增大后减小的特征 , 当搭接长度与试件宽度的比值在 2. 5 左右时 , 试件的拉伸性能较好 ,与未缝合的三维整体编织试件相比 , 仅下降了 25 %; 相对而言 , 中密度缝合试件的拉伸性能最优 ; 缝合方向对试件拉伸性能的影响不明显。      

缝合复合材料T型加筋壁板的抗剪与抗弯性能

为了防止复合材料T型加筋壁板在服役过程中因剪切和弯矩作用而发生筋条与蒙皮的脱粘失效,引入了缝合技术来提高筋条-蒙皮界面的结合性能.采用自主研发的单线弯针缝合设备来缝合干纤维,通过真空辅助树脂灌注技术(VARI)固化成型,脱模后制成缝合T型加筋壁板试样.通过对试样进行剪切和弯曲试验,研究缝合的增强机理以及缝线细度对T型接头性能的影响规律.结果表明:在剪切应力作用下,缝合试样的峰值载荷比未缝合试样有明显提高,随着缝线细度增大,T型接头的峰值载荷升高,缝线细度增大到1 500 D时,峰值载荷提高55. 0% .在弯曲应力作用下,随着缝线细度的增大,T型接头的峰值载荷先升高后降低.缝合对T型接头在两种不同应力下的初始损伤载荷均无明显影响.     

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。      

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(G_IC)的基础上,分析和阐明了缝合工艺参数(缝合密度)与G_IC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.     

复合材料缝合结构强度试验研究

本文对复合材料缝合结构的基本力学性能、单针连接性及多钉拉伸和剪切强度进行了试验研究,结果表明,虽然缝合工艺本身可使层间性能成倍增加,特别是使冲击后压缩后的强度值增加了一倍,但同时也使缝合／ＲＦＩ板的面内拉伸、压缩和剪切性能有所降低。缝合／ＲＦＩ单钉连接的钉孔位置对缝合／ＲＦＩ板挤压强度的影响可以忽略。文中还给出了缝合／ＲＦＩ板和层压板的两排两列四钉伸连接和多钉剪发连接强度试验结构。     

缝合参数对复合材料板面内刚度和强度影响研究

通过数值计算和对37组18个试件的实验测试与分析,研究了四种缝合参数对缝合复合材料板面内刚度和强度的影响.结果表明:1) 缝合密度越大,缝合所产生的损伤就越大,对面内刚度的影响也就越大.几种常用缝合密度下的面内刚度差最大为纵向6%,横向11%,剪切9%;2) 缝合密度和缝合工艺水平对面内拉伸强度和剪切强度有很大影响,缝合引起的拉伸强度和剪切强度降低分别达14%和17%,而对面内压缩强度的影响很小,最大强度降不到4%.3) 当缝线方向与测试方向垂直时,可以得到较高的面内刚度值和较低的面内强度值.反之,当缝线方向与测试方向平行时,可以得到较低的面内刚度值和较高的面内强度值.最大变化幅值分别为21%和18%.     

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%。     

Evaluation of durability and strength of stitched foam-cored sandwich structures

The static and fatigue characteristics of polyurethane foam-cored sandwich structures are investigated. Three types of specimens with glass fabric faces and polyurethane foam core are used; non-stitched, stitched and stiffened sandwich specimens. The bending strength of the stitched specimen is improved by 50% compared with the non-stitched specimen, and the stiffened specimen is over 10 times stronger than the non-stitched specimen. After fatigue loading of 10⁶ cycles, the static bending strengths of all specimens decrease compared with those of the static test. To verify the aging effect of polyurethane foam, ultrasonic C-scanning equipment is used to detect damage of the skin laminate alone after the fatigue test for non-stitched specimens. From the results of UT C-scanning images, no damage is found to have occurred during the fatigue test. Results indicate that the decrease in bending strength of foam-cored sandwich structures is caused by the degradation of stiffness due to the aging of the polyurethane foam core during fatigue cycles. To investigate the effect of distance and diameter of stitching thread, four types of stitched specimens are used. The strength of stitched specimen is improved by increasing the stitching thread diameter and decreasing the stitching thread distance. But fatigue characteristics are not predominantly affected by the variation of stitching thread diameter and distance.     

Effect of natural stitched composites on the crashworthiness of box structures

In the present paper the effects of stitching on the energy absorption and crashworthy behaviour of composite box structures will be studied. The combination of unidirectional carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) composite materials are used to laminate the composite boxes. Delamination study in Mode-I with the same lay-up was carried out to investigate the effect of stitching on delamination crack growth on energy absorption of stitched and non-stitched composite box structures. The double cantilever beam (DCB) standard test method was chosen for delamination studies. For non-stitched and stitched composite boxes the lamina bending and brittle fracture crushing modes were observed. It was found that the stitched composite boxes which show higher fracture toughness in Mode-I delamination tests, are not necessarily able to absorb more crushing energy in comparison with non-stitched composite boxes. It was also observed that the position of stitched area can affect the crushing mode and consequently energy absorption capability of composite box structures. The main reason can be related to other mechanisms such as bending, friction and bundle fracture which significantly contribute to energy absorption. The analytical model based on energy balance approach is proposed to estimate the mean crushing force, F_m, in axial crushing of square composite box.     

The damage to stitched GRP laminates by underwater explosion shock loading

This paper examines the effect of impulse loading from an underwater explosive shock wave on the damage to glass-reinforced polymer (GRP) laminates stitched through-the-thickness with Kevlar thread. Glass-fibre preforms were stitched in the parallel or transverse directions with a density of 3 or 6 stitches/cm², and then fabricated by resin-transfer moulding. The response of the stitched GRP laminates to explosion loading at two shock levels was compared to a non-stitched laminate. The damage to the non-stitched and stitched laminates appeared to be similar, with both the polymer matrix and glass fibres experiencing cracking while some of the polymer/glass interfaces were delaminated. The damage occurred by the bending of the laminates under the pressure exerted by the shock wave. At a relatively low shock level, no significant damage to the Kevlar threads occurred, but at a higher shock level many of the threads were broken. The stitching was effective in reducing the spread of delamination damage, particularly when the stitch orientation was in the parallel direction and/or the stitch density was high. However, the stitches acted as stress concentrators under shock loading, resulting in a large amount of damage localised around the stitches. Measurements of the residual tensile strengths of the damaged laminates are presented.     

全厚度缝合复合材料泡沫芯夹层结构力学性能研究与损伤容限评定

探索了全厚度缝合复合材料闭孔泡沫芯夹层结构低成本制造的工艺可行性及其潜在的结构效益。选用3 种夹层结构形式, 即相同材料和工艺制造的未缝合泡沫芯夹层和缝合泡沫芯夹层结构及密度相近的Nomex 蜂窝夹层结构, 完成了密度测定、三点弯曲、平面拉伸和压缩、夹层剪切、结构侧压和损伤阻抗/ 损伤容限等7 项实验研究。结果表明, 泡沫芯夹层结构缝合后, 显著提高了弯曲强度/ 质量比、弯曲刚度/ 质量比、面外拉伸和压缩强度、剪切强度和模量、侧压强度和模量、冲击后压缩(CAI) 强度和破坏应变。这种新型结构形式承载能力强、结构效率高、制造维护成本低, 可以在飞机轻质机体结构设计中采用。      

缝合复合材料可用性--环境条件下层合板的冲击后压缩性能

为了解决缝合复合材料在航空航天结构中的应用问题,对国内生产的缝合/树脂膜渗透成型工艺复合材料层合板在三种不同环境条件下的低速冲击后压缩性能进行试验研究.结果表明,缝合改变了含冲击损伤层合板的压缩破坏机理,可以大幅度提高层合板在干态常温下的冲击后剩余压缩强度,但是对湿态高温下层合板的冲击后剩余压缩强度影响不大;缝合方向对冲击损伤面积和剩余压缩强度的影响较小,其中以0°缝合较为有利.     

The analysis of skin-to-stiffener debonding in composite aerospace structures

A comprehensive finite element (FE) analytical tool to predict the effect of defects and damage in composite structures was developed for rapid and accurate damage assessment. The structures under consideration were curved, T-stiffened, multi-rib, composite panels representative of those widely used in aerospace primary structures. The damage assessment focussed on skin-to-stiffener debonding, a common defect that can critically reduce the performance of composite structures with integral or secondary bonded stiffeners. The analytical tool was validated using experimental data obtained from the structural test of a large stiffened panel that contained an artificial skin-to-stiffener debond. Excellent agreement between FE analysis and test results was obtained. The onset of crack growth predictions also compared well with the test observation. Since the general damage tolerance philosophy in composite structures follows the “no-growth” principle, the critical parameters were established based the onset of crack growth determined using fracture mechanics calculations. Parametric studies were conducted using the analytical tool in order to understand the structural behaviour in the postbuckling range and to determine the critical parameters. Parameters considered included debond size, debond location, debond type, multiple debonds and laminate lay-up.     

薄层化碳布缝合碳/碳复合材料制备与性能

为获得高性能、低成本碳/碳复合材料,以商用级T700大丝束薄层化碳纤维展宽平纹布和航空航天级T300小丝束碳纤维缎纹布为原材料制备缝合预制体,采用化学气相沉积工艺方法制备了一系列缝合碳/碳复合材料,对材料的气相致密化特征、微观结构特征和力学性能进行了测试与分析.研究结果表明,碳布规格和缝合间距对材料气相致密化效果和力学...     

缝合复合材料面内刚度和强度的神经网络预测

基于神经网络的BP算法,建立了预测缝合复合材料刚度和强度性能的模型。根据试验所得的缝合复合材料的性能参数,训练人工神经网络,拟合出输入参数 (各种缝纫参数与等效未缝纫层合板性能参数)与输出参数(缝合层合板性能参数) 之间的非线性关系,设计完成了缝合复合材料弹性性能与强度分析软件,并以此软件分析计算在新的缝纫参数和等效未缝纫层合板性能参数情况下的缝合复合材料性能。与实验结果对比,两者符合较好。为缝合复合材料刚度强度预测开辟了一条新的有效途径。      

Effect of stitch density on tensile properties and damage mechanisms of stitched carbon/epoxy composites

Experimental investigation is performed to study tensile properties, damage initiation and development in stitched carbon/epoxy composites subjected to tensile loading. T800SC-24kf dry preforms with tow orientation of [+45/90/−45/0₂/+45/90₂/−45/0]s are stitched using 200 denier Vectran® thread. Modified-lock stitch pattern is adopted, and stitch density is varied, viz. moderate density (stitched 6 × 6: stitch density = 2.8 cm⁻²) and high density (stitched 3 × 3: stitch density = 11.1 cm⁻²). The stitched preforms are then infiltrated by epoxy XNR/H6813 using resin transfer molding process. Tensile test is conducted to obtain in-plane mechanical properties (tensile strength, failure strain, tensile modulus and Poisson’s ratio). Effect of stitch density on the mechanical properties is assessed, and it is found that stitched 3 × 3 modestly improves the tensile strength by 10.4%, while stitched 6 × 6 reduces the strength by only 1.4%. In stitched 3 × 3 cases, the strength increase is mainly due to an effective impediment of edge-delamination. Tensile stiffness and Poisson’s ratio of carbon/epoxy are slightly reduced by stitching. Fiber misalignment in in-plane and out-of-plane directions is responsible for stiffness reduction, whilst reduction of Poisson’s ratio is probably caused by the orthogonal binding effect of modified-lock stitch architecture. Damage mechanisms in stitched and unstitched composites are studied using acoustic emission testing and interrupted test coupled with X-ray radiography and optical microscopy. The detailed damage observation reveals that stitch thread promotes early formation of transverse and oblique cracks. These cracks rapidly develop, and higher density of cracks ensues in stitched composites. Although this behavior triggers early formation of delamination, stitched 3 × 3 effectively impedes the growth the delamination. In contrast, stitched 6 × 6 is ineffective in suppressing the delamination yet the cracks are vast in this specimen. One of the plausible reasons of the rapid development of cracks in stitched composites is fiber compaction effect whereby fibers are compacted and the gap among fibers is reduced. The verification of compaction effect is done experimentally by performing burn-off test to measure the local fiber volume fraction. It is confirmed that fiber compaction indeed occurs as indicated by higher local fiber volume fraction between stitch lines.     

Evaluation of impact damage mechanism of multi-axial stitched CFRP laminate

This study focuses on multi-axial stitched fabric, which is a thick, high performance reinforcement for large-scale composite structures. The effects of impact damage on multi-axial stitched CFRP laminates molded by vacuum-assisted resin transfer molding (VARTM) method were evaluated. Impact damage within material was evaluated by ultrasonic scanning device and optical cross-sectional observations. Probed images obtained by both non-destructive and destructive methods were compared, and internal damage distributions of multi-axial stitched CFRP laminates were clarified. In addition, residual compressive strength and fatigue property of impact-damaged CFRP laminates were evaluated by in situ damage growth monitoring using the thermo-elastic stress analyzer (TESA). Three-dimensional damage distribution of impacted CFRP laminate was obtained from ultrasonic C-scan images and cross-sectional photographs. Damage progress behavior was observed on a destructive and non-destructive basis by post-impact fatigue (PIF) test.