缝合复合材料层合板拉伸疲劳损伤及其机理

对有、无缝合复合材料层合板的拉伸疲劳性能进行了试验研究,考察了0^\circ缝合对复合材料光滑板拉伸疲劳损伤扩展规律的影响. 通过有限元素法分析了有、无缝合复合材料层合板的应力状态分布情况,对缝合复合材料层合板的拉伸疲劳损伤及其扩展机理进行了分析. 研究表明,缝合改变了复合材料层合板拉伸疲劳损伤起始与扩展的机理,针脚附近的面内正应力\sigma_{x}与层间剪应力的集中对层合板拉伸疲劳损伤的发生与扩展有着重要的作用,自由边界处的层间集中应力对缝合板的疲劳性能也有影响. 自由边界处的层间集中应力是导致无缝合层合板疲劳损伤及其扩展的主要原因.      

变形对非饱和土渗透系数影响规律模拟研究

对有、无缝合复合材料层合板的拉伸疲劳性能进行了试验研究,考察了0^\circ 缝合对复合材料光滑板拉伸疲劳损伤扩展规律的影响. 通过有限元素法分析了有、无缝合复 合材料层合板的应力状态分布情况,对缝合复合材料层合板的拉伸疲劳损伤及其扩展机理进 行了分析. 研究表明,缝合改变了复合材料层合板拉伸疲劳损伤起始与扩展的机理,针脚 附近的面内正应力\sigma_{x}与层间剪应力的集中对层合板拉伸疲劳损伤的 发生与扩展有着重要的作用,自由边界处的层间集中应力对缝合板的疲劳性能也有影响. 自 由边界处的层间集中应力是导致无缝合层合板疲劳损伤及其扩展的主要原因.     

含表面缺口玻纤／环氧正交层压板的疲劳特性

本文探讨含表面缺口玻纤／环氧树脂正交层压板的疲劳力学特性，测量和分析了疲劳寿命特征及脱层扩散规律，监测了缺口张开位移，试样刚度降低随循环历史的变化特怔，结果表明，疲劳脱层扩展速率与应变能率的关系服从Ｐａｒｉｓ方程，监视缺口张开位移或刚度降低的变化可能是监视和预测含表面缺口复合材料构件疲劳破坏的有效手段之一。     

复合材料层板基于剩余刚度比的剩余强度模型

本文从唯裂 学的观点提出了一个复合材料疲劳剩余刚度衰退模型,根据该模型导出了基于疲劳乘余刚度比的复合材料层合板疲劳剩余强度概率分布模型,给出了确定该模型中各参数的计算方法,根据该模型可预报复合材料层合板在某一应力水平的疲劳载荷作用下疲劳剩余刚度比为某一确定值时的疲劳剩余强度概率分布。为验证该模型的合理性,用典型复合材料层合板做了几组疲劳实验,并用实验数据对模型中的参数进行了估计,计算结果证明,该模型预报的疲劳剩余强度概率理论分布与实验结果符合较好。     

Q235钢缺口试样拉压低循环疲劳的实验研究

以Q235钢制U型缺口板试样为研究对象,用有限元方法计算其缺口根部等效应变幅对应的试样标距段位移,以此控制试验机进行拉压循环疲劳试验。然后用局部应力应变法对试验测得的寿命结果进行分析。结果表明：无论用有限元还是修正Neuber公式计算缺口根部的应力应变,局部应力应变法的疲劳寿命评估只适用于缺口半径较大的试样;对缺口半径较小试样的估计寿命明显低于实测值,且有限元法比修正Neuber法更保守。进而又对试样缺口区域应变梯度的影响进行了探讨：参照有限元计算的应变梯度,利用Taylor模型估算了缺口根部的屈服应力和流动应力;在此基础上重新计算应变分布并估计试样的疲劳寿命,结果证实考虑应变梯度影响可改善缺口试样的疲劳寿命估计。     

复合材料加筋板连接剪切性能试验模拟

建立了带泡沫加筋的复合材料层合板连接件及试验夹具模型,用MSC.Nastran进行非线性有限元分析,模拟加筋层合板剪切性能试验,分析试验件过渡段和夹具刚度参数对剪切试验结果的影响,结果表明,考核区的应力随着夹具刚度及过渡段铺层的增加而减小,当夹具钢片厚度增大到15mm,过渡段铺层增加到考核区铺层的7倍时,考核区应力值较为稳定,可以作为工程试验夹具及过渡段设计的参考.     

镍基单晶合金非对称循环载荷低周疲劳实验研究与寿命预测

对三种不同晶体取向的DD3单晶合金试样在680℃温度下进行非对称循低周疲劳试验,表明晶体取向对疲劳寿命有显著影响.用取向函数修正总应变范围可在很大程度上消除晶体取向对疲劳寿命的影响,引入参量k表示非对称循环载荷对疲劳寿命的影响,它与循环寿命之间呈幂函数关系.根据影响单晶合金低周疲劳寿命的主要因素,提出由总应变范围、取向函数和参量k构成循环塑性应变能的计算方法。用塑性应变能作为损伤参量导出单晶合金低周疲劳寿命预测模型,利用DD3单晶合金低周疲劳试验数据进行验证,光滑试样和缺口试样试验数据分别落在2.6倍和2倍的偏差分布带内.     

错位铺层拼接层合板力学性能分析与模拟方法探索

针对特殊使用条件,需使用单向带逐层拼接或对接方法制备复合材料层合板,层合板内存在大量的,有意设计的间断点.论文首先通过多种方法测定试件不同位置应变数据来计算不同参数下刚度值变化情况,然后基于可反映错位铺层拼接层合板结构的细观尺度,使用ABAQUS有限元,将层合板中存在间断的增强纤维视为rebar-layer插入树脂基体,建立表征结构、纤维体积百分含量相符的等效模型进行模拟.经对比模拟与试验结果可见,该方法模拟结果准确,贴近真实情况;有效反映出参数不同所引起试样刚度变化特征.     

含分层损伤的变角度纤维层合板后屈曲力学行为研究

变角度（Variable angle tow,简称VAT）纤维复合材料层合板的纤维方向能够连续变化．相较于传统的直线纤维层合板,此类层合板通过刚度变化,整体的屈曲性能可以得到很大的提升．本文利用ABAQUS 自带的粘结单元(Cohesive Element)对预制圆形分层的变角度纤维复合材料层合板进行了后屈曲力学行为研究,得出载荷位移曲线,以及分层裂纹萌生和扩展的情况．然后本文分析了预制分层尺寸对板的刚度、前后屈曲阶段和裂纹萌生及扩展的影响．最后通过变角度纤维层合板和直线纤维层合板的后屈曲力学行为进行对比,深入探索了变角度复合材料层合板在抵抗分层裂纹萌生和扩展方面的优势．     

含缺口复合材料层合板的三维有限元失效分析

本文研究了复合材料层合板最终强度计算的有限元理论,讨论了修正的Newton-Raphson迭代方法在层合板失效过程中应力场计算的迭代过程.同时本文建立了带缺口的复合材料层合板三维有限元模型,充分考虑层合板的纤维断裂、基体开裂和分层三种失效模式,采用修正的三维Hashin准则作为失效判断的依据,计算了三种不同铺层的层合板最终失效载荷值,与试验值吻合得很好.鉴于层压板材料常数ν23的数值难于测定的特点,讨论其对最终失效载荷的影响.在三维有限元模型的基础上,实现了失效扩展仿真分析.     

层板复合材料的疲劳剩余刚度统计分布模型

在分析现有关于层板复合材料疲劳剩余刚度衰退模型的基础上，根据层板复合材料在疲劳载荷作用下刚度衰退变化的现象，研究了疲劳载荷对层板复合材料刚度衰退的影响，建立了一个新的、更为合理的用于描述层板复合材料在常幅疲劳载荷作用下的刚度衰退模型，导出了剩余刚度统计分布的表达式，给出了确定模型参数的方法。为验证该模型，设计了几组测试实验，并利用试验结果对模型参数进行了估计。利用该统计分布模型，可以预报层板复合材料在给定应力水平的疲劳载荷作用下循环指定周次时剩余刚度的统计分布。实验数据表明，理论预报和实验结果符合得是很好的。     

缺口件疲劳寿命分布预测的有效应力法

本文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法。该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的。在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布。该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响。进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的。     

缺口件表象疲劳极限的定量化分析

测定了淬火、高温回火35CrMo钢光滑试样和三种缺口试样的三点弯曲疲劳极限,并利用ANSYS有限元软件计算了缺口试样加载时缺口截面上的应力分布.用“疲劳源形成的微细观过程理论”对试验结果进行了分析,认为,疲劳源的形成虽然发生在个别薄弱晶粒内部,是其中位错往返运动及交互作用的结果,但还必须满足一定的形变协调条件和概率条件,因而必须形成由相当多晶粒组成的“细观屈服区”.对试验结果的分析表明,如取“细观屈服区”临界尺寸(xW)等于11个奥氏体晶粒平均直径,则计算得到的缺口试样的表象疲劳极限,与实测值相比,其综合误差最小.据此计算的三种缺口试样的表象疲劳极限,其误差都小于5％.该xW可认为是材料的特征参量.     

Fatigue damage in centrally notched GR/EP laminates

The fatigue properties of graphite/epoxy (Gr/Ep) T300/5208 composite laminates of 16 plies with a central circular hole subjected to tension-tension (T-T) constant-stress amplitudes at room temperature and low humidity have been fully investigated. Studied are four types of notched laminates which are classified as unidirectional, off-axis, orthotropic shear and quasi-isotropic. Some of them were precracked to initiate and guide the crack growth transversely. Our work is experimental and the analysis is based on a semiempirical approach. We have experimentally measured S-N curves, failure surfaces, crack lengths and their corresponding growth directions, delamination areas and transverse delamination lengths for the above series of composites. The fatigue failure mechanism was observed and expressed schematically. To analyze the experimental results, we have categorized the S-N curves by three common equations. The effective transverse crack length of quasi-isotropic laminates was found to be independent of the applied stress. For simplicity, it was modeled by a power law of applied cycles. It was also found that the delamination area could be expressed by a power law of applied cycles. Hence, the so-called modified Paris law, i.e., the power law of cycles, proposed here has been verified as satisfactorily acceptable.     

Strength and stiffness of sandwich beams in bending

This investigation is concerned with the experimental versus analytical correlation of the mechanical properties of sandwich-beam specimens. Such sandwich structures are commonly employed in the aircraft industry. Four-point and three-point load tests were conducted on a large number of sandwich-beam specimens, fabricated by using fiber-glass reinforced plastics (both unidirectional and woven-glass cloth) and DTD 685 aluminum alloy for the facings with aluminum honeycomb core and polyurethane foam cores and the indigenously available Araldite as the bonding medium between the core and the facings.The flexural stiffness of the composite sandwich specimens used in this investigation compared favorably with theoretical predictions. The shear stiffness was found to be about 55 percent and 45 percent of the theoretically predicted values for FRP (fiberglass-reinforced-plastic) cloth and FRP unidirectional laminates with aluminum honeycomb core sandwich, respectively. The failure load as determined by experiments was less than the theoretically predicted safe load. There was a loss of strength as well as a steep decrease in the failure load in the case of low density foam core.It was concluded that FRP facing plates with aluminum honeycomb core sandwich structure may be preferred to similar aluminum-alloy facing sandwich construction if high flexural stiffness and shear stiffness properties are required at less cost and weight. Indigenously available Araldite was quite satisfactory for bonding the core to the facings.This investigation has confirmed the importance of experiments in the field of sandwich structures which can effectively replace other conventional uneconomical structural or machine members which are currently in use.     

Effects of joining techniques on impact perforation resistance of assembled composite plates

A previous study on impact response of composite laminates concluded that impact perforation was the most important damage stage in composite laminates subjected to impact loading, since impact characteristics (peak force, contact duration and absorbed energy) and mechanical properties degradation of composite laminates reached critical points once perforation took place. It was also found that thickness had a greater influence on impact perforation resistance than did in-plane dimensions. However, as the composite laminates became very thick, the manufacturing cost for obtaining high-quality composite laminates increased. In an effort to meet design requirements and reduce manufacturing costs, assembled composite plates, which were organized by assembling multiple thin composite laminates, were considered as alternatives for thick single-laminate composite plates. Various joining techniques including mechanical riveting, adhesive bonding and stitch joining, and their combinations, were used in assembling two- and three-laminate plates. Experimental results revealed that adhesive bonding outperformed other joining techniques. Although good bonding resulted in higher joining (bending) stiffness and subsequently higher perforation thresholds, increasing the laminate thickness or the number of laminates was found to be more efficient in raising perforation threshold than in improving the joining stiffness. The assembled three-laminate plates were found to have higher perforation thresholds than their thick single-laminate counterpart.     

缝纫复合材料层合板面内弹性模量分析

基于对缝纫孔附近局部细观结构的分析，提出了一种预测缝纫复合材料层合板面内力学性能的理论模型．从分析缝孔单胞的纤维弯曲几何特征入手，最终得到单向板及层合板的弹性常数．通过有限元分析研究了缝纫参数对复合材料层合板面内等效模量的影响．研究结果表明，缝纫造成单向板及层合板面内材料性质的不均匀，随着缝纫密度和缝纫线直径的增加，层合板的等效模量逐渐降低．     

缺口试样疲劳短裂纹行为研究

采用复型方法对中碳结构钢单边钝缺口试样的非穿透和穿透疲劳短裂纹的演化过程进行了研究。结果表明：试样的疲劳破坏主要是由形成于缺口根部的表面短裂纹扩展引起的；形成穿透裂纹以前的短裂纹形成和扩展过程占试样疲劳寿命的７０％以上；线弹性断裂力学不适合于描述表面裂纹和穿透裂纹早期阶段的扩展特性；某些条件下，裂纹初始几何形状的影响是导致穿透裂纹早期扩展出现奇异特性的主要原因。