缝合增强复合材料帽型加筋壁板界面拉脱性能

为了防止复合材料帽型加筋壁板结构在服役过程中发生筋条与蒙皮的脱粘失效,引入了缝合技术来提高筋条-蒙皮界面拉脱性能.采用单线弯针缝合设备缝合纤维编织布,通过真空辅助树脂灌注技术(VARl)固化成型,制备缝合帽型加筋壁板试样.通过对试样进行筋条拉脱试验和有限元数值模拟,研究界面的失效机制及缝合参数对帽型加筋壁板界面结合性能的影响规律.结果表明:在拉脱载荷作用下,缝合试样的峰值载荷比未缝合试样明显增大.帽型试样的拉脱承载力随缝合密度的增大先增加后减小,在缝合密度(注:缝合密度表示缝合针距(单位mm)×行距(单位mm),下文同)为5×10时,相比未缝合试样,最高增加了26.7％;帽型接头的拉脱承载力随缝线细度的增加而增加,在缝线细度为1500 D时增加了39.7％.蒙皮/筋条厚度比为2时,结构拉脱承载力增加了27.35％,缝合的增强效果最明显.     

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158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%。     

三维机织复合材料/钛合金混杂板缝合连接剪切失效机理EI北大核心CSCD

为研究缝合密度、缝线纤维束规格以及钛合金板上预制的缝合孔直径对经过缝合的三维机织复合材料/钛合金混杂板缝合连接结构抗剪切能力的影响,对7组缝合参数各不相同的单搭接实验件进行剪切实验。通过加载条件下的原位细观实验观察,获得不同缝合参数下接头的失效模式,给出对应载荷-位移曲线上特征点的损伤形貌。结果表明:增加碳纤维缝线的丝束规格以及增加缝合密度均能提高混杂接头的失效载荷,且增加缝合密度比增加碳纤维缝线的丝束规格对提高失效载荷的效果更明显;缝合孔直径为2 mm或4 mm对结构承载能力无明显影响,当缝合孔直径达到6 mm时,承载能力明显降低;通过细观原位力学实验观察了三维机织复合材料/钛合金混杂板缝合结构剪切破坏过程;实验结果表明,搭接区的失效模式有缝线纯剪断、缝线抽出与剪断混合以及缝线挤出/剪断混合3种。缝合密度的变化是接头失效模式改变的主要因素。     

缝合夹层结构复合材料树脂传递模塑成型工艺充模仿真

对复合材料与金属经缝合连接形成的夹层结构板的树脂传递模塑成型（RTM）工艺进行了充模模拟研究。首先通过实验和数值计算的方法,分别获得缝合夹层结构织物和芯层孔洞的渗透率;随后,建立能够反映缝孔内流动情况的二维和三维简化模型,进行RTM充模仿真,讨论不同工艺参数对成型流动的影响;最后通过成型实验验证工艺的可行性。缝线与孔洞直径之比为0.3~0.8时,孔洞渗透率随缝线直径的增大而减小,预制体织物渗透率与孔洞渗透率相差两个数量级;缝孔内容易产生缺陷,没有缺陷的区域随着注射压力的增加、孔洞密度和芯层厚度的减小而增大,在芯层表面沿每排孔洞单向开槽能够改善树脂在孔洞内的浸润;线注射时,树脂整体流动情况优于点注射,而点注射时,将进胶口设置在一角,能够减少表面干斑。     

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。      

曲面复合材料缝合机器人自动路径规划技术

为提高复合材料预制件缝合机器人的灵活性与加工柔性,针对缝合机器人自动路径规划问题,研究了基于三维光栅扫描技术的曲面复合材料缝合机器人视觉接缝提取技术.针对三维扫描摄像机视场固定的特点,对摄像机进行标定实验,改进传统的手眼标定方法,提出一种用于求解手眼关系矩阵的四点标定新方法,建立了机器人坐标系与摄像机坐标系的位姿映射关系.经过手眼关系标定,三维光栅扫描仪采样得到的缝合预制件点云数据可转化为机器人坐标系下的点云数据坐标.进一步对采集的预制件点云数据进行接缝中心线提取,提出了一种缝合轨迹及姿态规划算法,采用三次多项式对接缝中心线进行空间曲线拟合,采用最小二乘法对缝合微切平面进行平面拟合,完成机器人操作空间前进矢量及接近矢量的计算.最后,将规划结果应用于缝合机器人的离线编程仿真与实验中.实验结果表明,该系统精度高,线迹成型良好,可以满足机器人缝合系统的要求.     

缝合参数对缝合泡沫夹芯结构复合材料VARTM工艺树脂充模的影响

对不同缝合参数的缝合泡沫夹芯结构复合材料真空辅助树脂传递模塑成型(VARTM)工艺进行数值模拟,研究了针距、行距、缝针直径、芯板厚度及纤维面板厚度等缝合参数对缝合泡沫夹芯结构复合材料VARTM工艺树脂流动充填的影响。结果表明,改变缝合行距对树脂的流动充填速度影响不大,缝合行距越大,树脂在下层纤维面板流动的同步性越差,制品出现空隙及干斑的可能性越大;缝合针距越小,树脂完成充填的时间越长;分别增加缝针直径和泡沫芯板的厚度,树脂完成充填时间呈线性增长,缝针直径越大,下层纤维面板树脂浸润效果越好;纤维面板厚度增加,树脂完成充填的时间变长,且相对于其他缝合参数,纤维面板厚度对树脂流动充填时间影响最大;缝合针距、泡沫芯板的厚度及纤维面板的厚度都不影响树脂在下层纤维面板的浸润效果。     

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。      

基于“离位”增韧技术Z向注射RTM成型的浸润研究

针对"离位"增韧技术和Z-RTM成型技术,引入饱和度参数修正Darcy定律,建立描述树脂在纤维预制件中非稳态流动的偏微分方程,研究恒流注射过程中体积流量、树脂黏度和纤维预制件渗透率等工艺参数对非稳态浸润过程注入压力的影响,模拟树脂在层间未增韧和增韧纤维预制件束内和束间的流动。结果表明:数值模拟结果具有可靠性;随着注射时间的增加,纤维预制件内部各点的压力增加;随着体积流量、树脂黏度的增加,注入压力线性增加,而随着纤维渗透率的增加,注入压力减少,符合Darcy定律;实现了树脂在纤维预制件细微观层次浸润的可视化,这种可视化结果为预测树脂在预制件中的宏观流动提供了重要补充,并为实际工艺提供了一定指导作用。     

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.     

Stitched glass/PP composite. Part I: Tensile and impact properties

Stitching has proven to be an effective way to increase the through-the-thickness mechanical properties of fibre-reinforced polymer composites. However, there are rare investigations which concentrate on the stitching effect on fibre-reinforced thermoplastic polymer composite, particularly under different temperature environments. Here, we investigate the tensile and impact behaviours of stitched glass/polypropylene woven composites. The effect of various sewing threads, stitch row orientations, and spacing are evaluated. Our data indicate that the stitching in through-the-thickness direction considerably increases the impact damage tolerance especially at low temperature. In addition, glass sewing threads does not deteriorate the tensile performance of the stitched composite. The study of ductile ratio (D.R.) shows that suitable sewing thread can reduce the sensitivity of ductile behaviour of composite to the variation of temperature. A strong correlation of energy absorption with respect to sewing thread fracture work in relation to its fibre volume fraction was found.     

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.     

Shock loading response of sandwich panels with 3-D woven E-glass composite skins and stitched foam core

Sandwich composite are used in numerous structural applications, with demonstrated weight savings over conventional metals and solid composite materials. The increasing use of sandwich composites in defense structures, particularly those which may be exposed to shock loading, demands for a thorough understanding of their response to suc highly transient loadings. In order to fully utilize their potential in such extreme conditions, design optimization of the skin and core materials are desirable. The present study is performed for a novel type of sandwich material, TRANSONITE^® made by pultrusion of 3-D woven 3WEAVE^® E-glass fiber composites skin preforms integrally stitched to polyisocyanurate TRYMER^TM 200L foam core. The effect of core stitching density on the transient response of three simply supported sandwich panels loaded in a shock tube is experimentally studied in this work. The experimental program is focused on recording dynamic transient response by high-speed camera and post-mortem evaluation of imparted damage. The obtained experimental results reveal new important features of the transient deformation, damage initiation and progression and final failure of sandwich composites with unstitched and stitched foam cores. The theoretical study includes full 3-D dynamic transient analysis of displacement, strain and stress fields under experimentally recorded surface shock pressure, performed with the use of 3-D MOSAIC analysis approach. The obtained theoretical and experimental results for the transient central deflections in unstitched and two stitched foam core sandwiches are mutually compared. The comparison results reveal large discrepancies in the case of unstitched sandwich, much smaller discrepancies in the case of intermediate stitching density, and excellent agreement between theoretical and experimental results for the sandwich with the highest stitching density. The general conclusion is that further comprehensive experimental and theoretical studies are required in order to get a thorough understanding of a very complex behavior of composite sandwiches under shock wave loading.     

缝合复合材料层板低速冲击及冲击后压缩实验研究

通过对缝合复合材料层板进行低速冲击和冲击后压缩实验, 研究了不同类型的缝合复合材料层板的冲击损伤特性及冲击后压缩的剩余强度。实验研究表明: 基体损伤和分层是缝合层板与未缝合层板低速冲击的主要损伤模式, 缝合层板具有更好的抗冲击性能, 更高的冲击后压缩强度。缝合密度越大的层板其抗冲击性能越好, 冲击后压缩强度越高。缝合方向为0°的缝合层板较缝合方向为90°的缝合层板具有更好的抗冲击性能和更高的冲击后压缩强度。增加0°方向铺层, 减少45°、-45°方向铺层, 可以提高缝合层板的抗冲击性能和冲击后压缩强度。     

Effect of Through-Thickness Stitching on the Fatigue Life of Composite Single-Lap Joints

An exploratory experimental evaluation has been undertaken to investigate the effect of through-thickness stitching on the fatigue life of composite single lap joints. Balanced single lap joints were considered, and the lay-up for the adherends was (0/±45/90)_s Specimens were stitched transversely and through-the-thickness with Kevlar^® thread in a zigzag pattern and were manufactured using the resin transfer moulding (RTM) technique. Preliminary experimental results have indicated that stitches remain intact when the crack starts growing from either side of the overlap. This consequently imparts a significant improvement in the fatigue life of stitched versus unstitched counterparts.     

The effect of stitching on FRP cylindrical shells under axial compression

FRP composites have usually poor through-thickness mechanical properties and, therefore, are able to sustain more loads in tension than in compression and fail as a consequence of buckling. The through-thickness reinforcement is carried out by stitching to improve the delamination strength and to reduce the in-plane crack growth rate.Experiments were performed on both stitched and unstitched laminated plates which were prepared by using woven roving glass fibre mat and chopped strand glass fibre mat with polyester resin, and the effect of stitching was studied. It is observed that stitching increases delamination strength to a great extent. There are losses in the in-plane mechanical properties due to in-plane fibre damage and creation of resin-rich pockets.Various energy-absorbing modes observed during progressive crushing of both stitched and unstitched FRP cylindrical shells under axial compression were studied both experimentally and theoretically. Analytical expressions for the calculation of energy absorption in various modes and average crush stress were derived, and the results thus obtained were compared with the experiments.     

Development of Through-Thickness Reinforcement in Advanced Composites Incorporating Rigid Cellular Foams

This paper presents a method of joining carbon-fibre plies and rigid cellular foam core with stitching for producing light-weight composite structures. After resin infusion and consolidation, the stitched sandwich panel exhibits superior damage tolerance as well as improved transverse properties due to the presence of through-thickness fibre reinforcement. First part of the paper deals with the conceptual development of a multi-needle stitching machine for rigid foams. A needle penetration model for computing the penetration forces has been reported—there is a good agreement between the experimental and theoretical penetration force-displacement curves. A number of sandwich panels with orthogonal and bias stitch orientations have been developed and examined for stitch quality with the aid of X-ray tomography. The paper also presents results from quasi-static indentation, three-point bending and transverse compression tests, on both the stitched and unstitched sandwich panels.     

Effects of Stitching on Delamination of Satin Weave Carbon-Epoxy Laminates Under Mode I,Mode II and Mixed-Mode I/II Loadings

The objective of the present study is to characterize the effect of modified chain stitching on the delamination growth under mixed-mode I/II loading conditions. Delamination toughness under mode I is experimentally determined, for unstitched and stitched laminates, by using untabbed and tabbed double cantilever beam (TDCB) tests. The effect of the reinforcing tabs on mode I toughness is investigated. Stitching improves the energy release rate (ERR) up to 4 times in mode I. Mode II delamination toughness is evaluated in end-notched flexure (ENF) tests. Different geometries of stitched specimens are tested. Crack propagation occurs without any failure of stitching yarns. The final crack length attains the mid-span or it stops before and the specimen breaks in bending. The ERR is initially low and gradually increases with crack length to very high values. The mixed-mode delamination behaviour is investigated using a mixed-mode bending (MMB) test. For unstitched specimens, a simple mixed-mode criterion is identified. For stitched specimens, stitching yarns do not break during 25% of mode I ratio tests and the ERR increase is relatively small compared to unstitched values. For 70% and 50% of mode I ratios, failures of yarns are observed during crack propagation and tests are able to capture correctly the effect of the stitching: it clearly improves the ERR for these two mixed modes, as much as threefold.     

Ballistic impact and explosive blast resistance of stitched composites

The effectiveness of stitching in increasing the damage resistance of polymer composites against ballistic projectiles and explosive blasts is determined. Glass-reinforced vinyl ester composites stitched in the through-thickness direction with thin Kevlar®-49 yarn were impacted with a bullet travelling at 0.9 km s⁻¹ or an underwater explosive shock wave moving at 1.5 km s⁻¹. The amount of delamination damage to the composite caused by a ballistic projectile was reduced slightly with stitching. Stitching was highly effective in increasing the damage resistance against explosive blast loading. The increased damage resistance was due to the stitching raising the Mode I interlaminar fracture toughness of the composite. While the stitched composites experienced slightly less damage, their flexural modulus and strength was similar to the properties of the unstitched composite after ballistic impact testing. The post-blast flexural properties of the stitched composites, on the other hand, were degraded less than the properties of the unstitched material.