含面内波纹缺陷的复合材料层合板刚度性能

基于经典层合板理论（CLT）,并考虑面内波纹引起的拉伸-弯曲耦合作用,提出了含面内波纹缺陷的复合材料层合板刚度预测模型,定量研究了波纹比、纤维偏转角和波纹位置等面内波纹参数对其三维刚度性能的影响。结果表明：理论模型预测值与文献中的结果吻合较好;面内波纹对纵向弹性模量、横向弹性模量、面内剪切模量、主泊松比和面内弯曲刚度均产生了显著影响。该建模方法为研究波纹缺陷对复合材料力学性能影响提供了参考。     

含纤维波纹缺陷复合材料层合板的损伤分析

纤维波纹是复合材料层合板制备过程中的一种常见缺陷,会导致其刚度和强度显著下降,有效地预测含波纹缺陷复合材料层合板的失效强度具有显著的意义。基于此,本文采用解析的方式分别构造了纤维波纹呈正弦起伏与余弦起伏状的复合材料层合板模型。利用该模型,以Tsai-Wu准则作为失效判据,研究了一种含纤维波纹的碳纤维/环氧树脂复合材料层合板在受压情况下的损伤演化过程,得到了碳纤维/环氧树脂复合材料层合板的初始损伤强度。与有限元方法计算得到的损伤位置和损伤强度非常吻合,验证了本文算法的正确性。另外,相比于有限元方法,本文所述计算方法具有模型构造简单、计算效率高等优点,便于快速分析和确定含纤维波纹缺陷复合材料层合板的损伤位置与损伤强度。      

复合材料性能反演单向板弹性常数的方法

通过对复合材料层合板等效弹性模量的反演推算,得到单向板的弹性常数。当层合板中0°和90°铺层比例不等时,通过直接反推法得到唯一解,即单向板的弹性常数;但是当层合板中0°和90°铺层比例相等时,运用反推法得到无穷多组解,此时添加额外已知条件可确定唯一解。由算例分析得出:对于均衡层合板该方法预测结果较准确;但对于非均衡层合板预测结果误差偏大。复合材料弹性模量的反演推算充分考虑了材料因加工缺陷引起的随机性,所得结果能更真实地反映单向板的弹性常数。     

基于细观力学的大开孔层合板缝合补强力学性能计算的新方法基于细观力学的大开孔层合板缝合补强力学性能计算的新方法

从细观力学的角度出发,考虑了面内纤维弯曲及富树脂缺陷,建立了大开孔层合板缝合补强孔边针脚损伤的单胞模型。建立了纤维弯曲函数,推导了纤维弯曲区域的纤维体积分数及纤维弯曲角度。基于复合材料力学分析方法,计算得出了单胞的材料弹性常数。研究表明:缝合导致单胞面内纤维最大弯曲角不超过20°,单层板纵向杨氏模量减小,横向杨氏模量、剪切模量及泊松比均增大,变化幅度均在-8%~20%之间;且对于大开孔层合板缝合补强而言,针距变化引起的材料性能变化相对边距大许多。由上述计算结果,建立了一种缝合补强大开孔层合板力学性能计算的新方法,同时引入针孔模拟针脚处的应力集中现象,结果表明:缝合会造成层合板面内力学性能降低,并且对面内的压缩性能影响大于对面内拉伸性能的影响。      

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.     

平面编织复合材料层板弹性性能的预测

本文在平面编织纤维增强树脂复合材料的单方向直波纹模型的基础上,分别采用经典层合板理论和有限元应变能等效方法,预测了平面编织复合材料层合板迭层对其弹性性能的影响。根据经典层合板理论,由对平面织物复合材料的单向直纹模型的上、下表面施加不同的约束条件,得出层板弹性常数变化范围。用有限元能量法,则预测出不同铺层数的编织复合材料层板的弹性性能。与实验结果比较,表明纵向模量的预测是可靠的。     

基于多尺度模型的复合材料层合板性能预测

运用均匀化理论与有限元相结合的方法,预测了风机叶片复合材料层合板的性能。将复合材料层合板内部结构分为宏观、细观和纳观3个层次,建立了复合材料层合板的多尺度模型。通过3次均匀化方法,并编写APDL程序输入商业软件ANSYS,预测材料各参数(碳纳米管体积分数、长径比、弹性模量,纳米薄层体积分数、弹性模量)对复合材料层合板性能的影响。结果表明,当分别增大碳纳米管体积分数、长径比、弹性模量以及纳米薄层体积分数、弹性模量时,风机叶片复合材料层合板的性能均能得到提高。同时表明加入一定量的碳纳米管可以适当提高复合材料层合板的性能。实验结果对风机叶片复合材料的制备有一定的指导作用。     

考虑纤维面外起伏的变刚度层合板优化策略研究

针对变刚度层合板在自动铺放制造过程中因间隙/重叠缺陷产生大量纤维面外起伏缺陷的问题,提出采用铺层偏移法与断送纱策略两种铺层优化策略来进行变刚度层合板的铺层设计,在研究过程中同时引入考虑间隙/重叠缺陷建模的方法。根据变刚度层合板铺层的特点提出缺陷重复单元的概念,通过对缺陷重复单元的分析来反映纤维面外起伏的影响,并提出通过纤维面外系数来表征变刚度层合板的纤维面外起伏尺度,最后对不同优化策略的变刚度层合板的屈曲性能进行分析。研究表明:基准设计方案、铺层偏移法与断送纱策略所对应的纤维面外起伏系数为0.83、0.95、0.93,所提出的优化策略对变刚度层合板的纤维面外起伏尺度有着明显的抑制作用。铺层偏移法优化后的[±<50/65>]_6s变刚度层合板最大厚度超差为33%,所对应的屈曲载荷为9117.1 N,屈曲载荷提升17.6%;断送纱策略优化后的[±<50/65>]_6s变刚度层合板最大厚度超差为50%,所对应的屈曲载荷为9716.3N,屈曲载荷提升25.3%。      

考虑面内载荷的复合材料层合板冲击性能

考虑面内初始载荷作用,开展复合材料层合板高速冲击响应与损伤特性研究。设计一种面内拉伸/压缩/剪切初始载荷施加装置,结合气炮试验装置,提出一种初始载荷复合材料高速冲击试验方法,针对X850/IM+和M21C/IMA两种牌号复合材料层合板开展高速冲击试验。结果表明:面内初始载荷对复合材料层合板高速冲击响应和损伤特性有显著影响;相比无初始载荷冲击情况,面内拉伸载荷作用提高了结构抗弯刚度,使冲击剩余速度提高,穿透速度降低,分层损伤面积减小;而面内压缩载荷则反之。      

分层缺陷深度对复合材料层合板力学性能的影响

通过对含分层缺陷的复合材料层合板进行压缩、弯曲以及拉伸试验,研究了分层缺陷深度对层合板力学性能的影响;对影响层合板力学性能的因素进行定性分析,得到了层合板力学性能随分层缺陷深度的变化规律。结果表明：分层缺陷深度对层合板的抗压、抗弯强度有直接影响,而对抗拉强度的影响十分有限。     

Analytical modeling of effect of interlayer on effective moduli of layered graphene-polymer nanocomposites

Nanocomposites enhanced with two-dimensional, layered graphene fillers are a new class of engineering materials that exhibit superior properties and characteristics to composites with conventional fillers.However, the roles of "interlayers" in layered graphene fillers have yet to be fully explored. This paper examines the effect of interlayers on mechanical properties of layered graphene polymer composites.As an effective filler, the fundamental properties(in-plane Young's modulus E_(L1), out-of-plane Young's modulus E_(L2); shear modulus G_(L12), major Poisson's ratio V_(L12)) of the layered graphene were computed by using the Arridge's lamellar model. The effects of interlayers on effective moduli of layered graphene epoxy composites were examined through the Tandon-Weng model. The properties of the interlayer show noticeable impact on elastic properties of the composites, particular the out-of-plane properties(Young's modulus E_2 and shear modulus G_(12)). The interlayer spacing is seen to have much great influence on properties of the composites. As the interlayer spacing increases from 0.34 nm to 2 nm, all elastic properties of the composites have been greatly decreased.     

An Experimental Study of the Influence of in-Plane Fiber Waviness on Unidirectional Laminates Tensile Properties

As one of the most common process induced defects of automated fiber placement, in-plane fiber waviness and its influences on mechanical properties of fiber reinforced composite lack experimental studies. In this paper, a new approach to prepare the test specimen with in-plane fiber waviness is proposed in consideration of the mismatch between the current test standard and actual fiber trajectory. Based on the generation mechanism of in-plane fiber waviness during automated fiber placement, the magnitude of in-plane fiber waviness is characterized by axial compressive strain of prepreg tow. The elastic constants and tensile strength of unidirectional laminates with in-plane fiber waviness are calculated by off-axis and maximum stress theory. Experimental results show that the tensile properties infade dramatically with increasing magnitude of the waviness, in good agreement with theoretical analyses. When prepreg tow compressive strain reaches 1.2%, the longitudinal tensile modulus and strength of unidirectional laminate decreased by 25.5% and 57.7%, respectively.     

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.     

Compression properties of z-pinned composite laminates

The effect of z-pinning on the in-plane compression properties and failure mechanisms of polymer laminates is experimentally studied in this paper. The reduction to the compression modulus, strength and fatigue performance of carbon/epoxy laminates with increasing volume content and diameter of pins is determined. The elastic modulus decreases at a quasi-linear rate with increasing pin content and pin diameter. Softening is caused by fiber waviness around the pins and reduced fiber volume content due to volumetric swelling of the laminate from the pins. A simple model is presented for calculating the compression modulus of pinned laminates that considers the softening effects of fiber waviness and fiber dilution. The compression strength and fatigue life also decrease with increasing volume content and diameter of the pins. The strength and fatigue properties are reduced by fiber kinking caused by fiber waviness around the pins and the reduced fiber content caused by swelling. The deterioration to the compression properties is also dependent on the fiber lay-up pattern of the laminate, with the magnitude of the loss in properties increasing with the percentage of 0° (load bearing) fibers in the laminate. The paper gives suggestions for minimizing the loss to the compression properties to laminates due to pinning.     

Effect of on-axis tensile loading on shear properties of an orthogonal 3D woven SiC/SiC composite

The present study examines in-plane and out-of-plane shear properties of an orthogonal 3D woven SiC fiber/SiC matrix composite. A composite beam with rectangular cross-section was subjected to a small torsional moment, and the torsional rigidities were measured using an optical lever. Based on the Lekhnitskii’s equation (Saint–Venant torsion theory) for a orthotropic material, the in-plane and out-of-plane shear moduli were simultaneously calculated. The estimated in-plane shear modulus agreed with the modulus measured from ±45° off-axis tensile testing. The effect of on-axis (0°/90°) tensile stress on the shear stiffness properties was also investigated by the repeated torsional tests after step-wise tensile loading. Both in-plane and out-of-plane shear moduli decreased by about 50% with increasing the on-axis tensile stress, and it is mainly due to the transverse crack propagation in 90° fiber bundles and matrix cracking in 0° fiber bundles. It was demonstrated that the torsional test is an effective method to estimate out-of-plane shear modulus of ceramic matrix composites, because a thick specimen is not required.     

Experimental determination of elastic properties of impact damage in carbon fibre/epoxy laminates

This paper describes an investigation of in-plane elastic properties of impact damaged regions in composite laminates. Quasi-isotropic carbon fibre/epoxy laminates were impacted and the impact damage examined by ultrasonic C-scanning, optical microscopy and thermal deplying. After impact damage observations, specimens were cut from the laminates and tested in tension and compression. The elastic modulus of the impact damage was, in both tension and compression, mainly controlled by the amount of fibre breakage. Interestingly, layers with broken fibres could sustain some load in compression, which led to higher elastic modulus in compression than in tension. The effect of delaminations on the elastic modulus was quite small in both tension and compression. The through-the-thickness variation of in-plane stiffness was studied by successively removing plies. The variation in stiffness was negligible, probably as a result of the very uniform distribution of delaminations and fibre breakage through the thickness of the laminates.     

On the transverse shear modulus of negative Poisson's ratio honeycomb structures

The transverse shear modulus of in-plane negative Poisson's ratio honeycombs is determined using numerical simulations. General honeycombs have theoretical upper and lower bounds of transverse shear moduli. The actual transverse shear modulus is an intermediate value, depending on the relative density and the gauge thickness ratio. The difference between upper and lower bounds is particularly significant for negative Poisson's ratio honeycombs. Finite element simulations confirm the theoretical trend, and show gauge ratio dependence different from that envisaged in the literature for conventional honeycombs. A modified approximate formula is thus proposed to predict the actual transverse shear modulus for honeycombs having in-plane negative Poisson's ratio values.     

Effect of fiber,matrix and interface properties on the in-plane shear deformation of carbon-fiber reinforced composites

The effect of fiber, matrix and interface properties on the in-plane shear response of carbon-fiber reinforced epoxy laminates was studied by means of a combination of experiments and numerical simulations. Two cross-ply laminates with the same epoxy matrix and different carbon fibers (high-strength and high-modulus) were tested in shear until failure according to ASTM standard D7078, and the progressive development of damage was assessed by optical microscopy in samples tested up to different strains. The composite behavior was also simulated through computational micromechanics, which was able to account for the effect of the constituent properties (fiber, matrix and interface) on the macroscopic shear response. The influence of matrix, fiber and interface properties on each region and on the overall composite behavior was assessed from the experimental results and the numerical simulations. After the initial elastic region, the shear behavior presented two different regions, the first one controlled by matrix yielding and the second one by the elastic deformation of the fibers. It was found that in-plane shear behavior of cross-ply laminates was controlled by the matrix yield strength and the interface strength and was independent of the fiber properties.