缝合参数对泡沫夹层结构复合材料力学性能的影响

采用真空辅助树脂注射(VARI)成型工艺制备不同缝合方式和缝合密度的缝合泡沫夹层复合材料, 研究缝合参数对平面拉伸、三点弯曲、芯子剪切以及滚筒剥离性能的影响。结果表明: 缝合使泡沫夹层复合材料的平面拉伸强度和芯子剪切强度明显降低, 可以改善弯曲性能并大幅提高滚筒剥离性能, 改进锁式缝合方式优于临缝式缝合方式; 适当地增加缝合行距对力学性能有一定的积极作用, 但不利于滚筒剥离性能的提高; 与未缝合泡沫夹层复合材料相比, 当缝合密度为30 mm×10 mm时, 改进锁式缝合泡沫夹层复合材料的平拉强度和芯子剪切强度分别降低了14.75%和24.79%, 弯曲强度和平均剥离强度分别提高了7.96%和80.78%。     

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.     

Adhesive interface element for bonding of laminated plates

An isoparametric adhesive interface element is used in the stress analysis of adhesive-bonded structures. The model assumes that transverse shear and peel stresses prevail in the adhesive layer. The analyses articulate separate responses of the plate, overlays and the adhesive. The stress distribution in the adhesive layer, obtained for lap joints, is found to be in agreement with those obtained by previous authors. To extend the use of bonded joints, the deflection of patched plates under transverse loading and the stress concentration in a plate having a center hole reinforced by a ring patch are also analyzed. The present element, together with the eight-node isoparametric plate element based on first order shear deformation theory used to model the plate and overlay patch, is found to have an advantage in solving problems of adhesive-bonded structures.     

缝纫层合板的本构关系研究(I)——缝纫单层板有效弹性常数分析

建立了缝纫单层板的有效弹性常数分析模型。通过考虑缝纫引起的铺层纤维的面内弯曲和非均匀分布,分析了缝纫参数(如缝纫行距、针距和缝线半径)对等效弹性常数的影响。结果表明,当缝纫方向垂直于铺层纤维方向时,随缝纫行距的减小或缝线半径的增大,纵向模量逐渐下降,而横向模量和剪切模量则逐渐提高;缝纫针距对纵向模量影响很小。     

An analytical solution for the analysis of symmetric composite adhesively bonded joints

Analytical solutions for adhesively bonded balanced composite and metallic joints are presented in this paper. The classical laminate plate theory and adhesive interface constitutive model are employed for this deduction. Both theoretical and numerical (finite element analysis) studies of the balanced joints are conducted to reveal the adhesive peel and shear stresses. The methodology can be extended to the application of various joint configurations, such as single-lap and single-strap joints to name a few. The methodology was used to evaluate stresses in several balanced adhesively bonded metallic and composite joints subjected to the tensile, moment and transverse shear loadings. The results showed good agreements with those obtained through FEM.     

A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites

This paper reviews over fifty studies into the effect of through-the-thickness stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Reviewed are the in-plane tensile, compressive, flexure, interlaminar shear, creep, fracture and fatigue properties, although little work has been undertaken on the last three properties. When comparing studies it is apparent that many contradictions exist: some studies reveal that stitching does not affect or may improve slightly the in-plane properties while others find that the properties are degraded. In reviewing these studies it is demonstrated that predicting the influence of stitching on the in-plane properties is difficult because it is governed by a variety of factors, including the type of composite (eg. type of fibre, resin, lay-up configuration), the stitching conditions (eg. type of thread, stitch pattern, stitch density, stitch tension, thread diameter), and the loading condition. The implications of these findings for the use of stitching in lightweight engineering structures are discussed.     

Nonlinear finite element analysis of stress and strain distributions across the adhesive thickness in composite single-lap joints

A geometrically nonlinear, two-dimensional (2D) finite element analysis has been performed to determine the stress and strain distributions across the adhesive bond thickness of composite single-lap joints. The results of simulations for 0.13 and 0.26 mm bond thickness are presented. Using 2-element and 6-element mesh schemes to analyze the thinner bond layer, good agreement is found with the experimental results of Tsai and Morton. Further mesh refinement using a 10-element analysis for the thicker bond has shown that both the tensile peel and shear stresses at the bond free edges change significantly across the adhesive thickness. Both stresses became increasingly higher with distance from the centerline and peak near but not along the adherend–adhesive interface. Moreover, the maximum shear and peel stresses occur near the overlap joint corner ends, suggesting that cohesive crack initiation is most likely to occur at the corners. The dependence of stress and corresponding strain distributions on bond thickness and adhesive elastic modulus are also presented. It is observed that the peak shear and peel stresses increase with the bond thickness and elastic modulus.     

Structurally stitched NCF CFRP laminates. Part 1: Experimental characterization of in-plane and out-of-plane properties

The insertion of local through-thickness reinforcements into dry fiber preforms by stitching provides a possibility to improve the mechanical performance of polymer-matrix composites perpendicular to the laminate plane (out-of-plane). Three-dimensional stress states can be sustained by stitching yarns, leading to increased out-of-plane properties, such as impact resistance and damage tolerance. On the other hand, 3D reinforcements induce dislocations of the in-plane fibers causing fiber waviness and the formation of resin pockets in the stitch vicinity after resin infusion which may reduce the in-plane stiffness and strength properties of the laminate.In the present paper an experimental study on the influence of varying stitching parameters on in-plane and out-of-plane properties of non-crimp fabric (NCF) carbon fiber/epoxy laminates is presented, namely, shear modulus and strength as well as compression after impact (CAI) strength and mode I energy release rate. The direction of stitching, thread diameter, spacing and pitch length as well as the direction of loading (which is to be interpreted as the direction of the three rail shear loading or the direction of crack propagation in case of mode 1 energy release rate testing) were varied, and their effect on the mechanical properties was evaluated statistically.The stitching parameters were found to have ambivalent effect on the mechanical properties. Larger thread diameters and increased stitch densities result in enhanced CAI strengths and energy release rates but deteriorate the in-plane properties of the laminate. On the other hand, a good compromise between both effects can be found with a proper selection of the stitching configurations.     

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.     

金属裂纹板复合材料单面胶接修补结构应力分析

考虑金属裂纹板复合材料单面胶接修补结构的几何非线性和边界条件,建立了考虑弯曲变形单面修补结构力学分析模型,计算出承受面内载荷时修补结构的弯矩和挠度,将补片自由端和金属板裂纹处的弯矩作为胶层应力控制微分方程的边界条件,推导出剪应力和剥离应力的解析解,及裂纹张开位移的表达式,并与有限元数值结果进行对比。分析结果表明,胶接修补结构应力分析理论模型和相关简化假设合理、正确。利用所建立的解析模型研究了金属裂纹复合材料单面胶接修补结构的应力分布特点及胶层主导破坏模式的失效机制,为胶接修补结构的承载能力分析以及结构改进设计提供了一定的理论依据。     

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.     

Analysis of thick orthotropic laminated composite plates based on higher order shear deformation theory

A two-dimensional finite element model is presented to perform the linear static analysis of laminated orthotropic composite plates based on a refined higher order shear deformation theory. The theory accounts for parabolic distributions of transverse shear stresses and requires no shear correction factors. A finite element program is developed using serendipity element with seven degrees of freedom per node. The present solutions are compared with those obtained using three-dimensional elasticity theory and those obtained by other researchers. The theory accurately predicts displacements and transverse shear stresses compared to previously developed theories for thick plates and are very close to three-dimensional elasticity solutions. The effects of transverse shear deformation, material anisotropy, aspect ratio, fiber orientation and lamination sequence on transverse shear stresses are investigated. The error in values of transverse shear stresses decreases as the number of lamina increases, for a plate of same thickness. An increase in degree of anisotropy results in lower values of deflection in the plate. For cross-ply plate an increase in anisotropy results in an increase in effective stress whereas for angle-ply plate the effect is almost negligible. Through thickness variation of transverse shear stresses are independent of anisotropy. The maximum effective stress increases exponentially at lower values of anisotropy and reaches to an asymptotic value at higher values. The stacking sequence has a significant effect on the transverse deflections and shear stress. Rectangular plates experience less effective, in-plane and transverse shear stresses compared to square plates.     

Analytical investigation of thermal and mechanical load effects on stress distribution in adhesive layer of double-lap metal-composite bonded joints

Significant thermal stresses are induced in the adhesive layers of a metal-composite bonded joint owing to the large temperature change associated with the difference in the coefficients of thermal expansions of metals and composite adherends. In this study, a theoretical analysis of shear and peel stresses in adhesive layers of a double-lap metal-composite bonded joint is carried out to evaluate the effects of thermal and mechanical loads on the stress distribution in the adhesive layer. The effects of temperature change and adhesive thickness on the shear and peel stresses in the adhesive layer of the bonded joint, with and without external forces, are examined based on the theoretical analysis. The results calculated for the condition involving a mechanical load application to the bonded joint and a decrease in temperature indicate that the absolute value of the shear and peel stresses peak at both ends of the adhesive layer, and that the absolute value of the peak stresses increases in the case of a thinner adhesive layer. When mechanical and thermal loads are simultaneously applied to a double-lap joint, shear and peel stresses synergistically increase at one end of the adhesive layer and decrease with an offset at the other end.     

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。      

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%。     

Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

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.     

The J-integral for single-lap joint using the stress field from the mixed variational principle

The closed-form solution for the J-integral of a single-lap joint is presented based on the stress field derived from Reissner’s mixed variational principle. In an adhesive-bonded joint, loads are carried by the surface of the adherends in shear through an adhesive layer, and thus, the shear effect is important. To improve the accuracy of shear response in fracture analysis, all transverse effects of the shear and peel stresses are considered, and then the constitutive equations and the equilibrium equations are derived from the variational principle. The obtained J-integral gives additional terms on the transverse shear part of the total integral compared with the results from previous conventional analysis, and illustrative examples are provided to show the effects of the current approach. Also, the formulation proposed in this paper can deal with non-identical adherends and laminates easily.     

复合材料正交加筋层合圆柱壳结构阶梯式挖补修理的参数化研究

利用三维参数化有限元分析模型,对复合材料正交加筋层合圆柱壳结构的阶梯式挖补修理进行了参数分析.主要考虑了载荷情况、阶梯数、铺层顺序以及补片铺层错误等修理参数对阶梯式挖补修理结构胶层剥离应力和剪切应力分布情况的影响规律,并利用绝对值的平均值和均方差来描述胶层剪切应力剥离应力的分布情况,以确定最优的阶梯式挖补修理参数.基于该文建立的三维有限元模型可以深入地了解复合材料正交加筋层合圆柱壳结构阶梯式挖补修理区域胶层应力分布的详细情况,而且参数化分析结论也可为复合材料正交加筋层合板圆柱壳结构的阶梯式挖补修理方案的设计和分析提供参考.     

复合材料夹芯管胶接连接结构力学特性分析

针对在航空结构中广泛应用的复合材料蜂窝夹芯圆管中的接头这一最脆弱的部分,发展了一种分析复合材料蜂窝夹芯圆管胶粘接头力学特性的解析模型.该模型根据Gibson修正公式得到了蜂窝芯子的等效弹性参数,再运用经典的复合材料壳理论和线弹性理论得到管接头的控制方程,并通过状态空间法进行求解.运用本文模型,计算了管接头在扭矩和弯矩作用下胶层内的剪应力和剥离应力;同时采用有限元法对模型进行了数值模拟,并将模拟结果与模型计算结果进行了对比,最后分析了搭接长度对胶层内应力的影响.