织物顶出测试中纱线应力传递模型的拉曼研究

弹头侵彻织物过程中的载荷传递是冲击能量耗散的重要途径,有必要从细观尺度上研究纱线上的应力传递行为。本文使用微拉曼光谱法测量了织物离面顶出测试中的纱线应力分布,建立了相应的纱线应力传递模型。针对周边固支的Kevlar 49芳纶纤维平纹织物受中心加载实验,拉曼测试显示出织物存在十字形的高应力分布,主纱是载荷传递的主要路径,辅纱是传递主纱载荷的次要路径,理论模型预测的纱线应力分布符合实验测量趋势。上述工作有利于进一步探讨柔性防弹织物的能量吸收机制。     

单纤维细丝微力学性能实验研究

使用数字标记点识别方法分别对T300碳纤维和Kevlar29芳纶纤维单丝进行了拉伸实验测试,给出了单纤维细丝的拉伸性能参数。其中T300碳纤维单丝的直径在 7μm左右,比Kev lar29芳纶纤维的直径 (约 12μm左右 )要小;T300碳纤维的拉伸模量为 230GPa左右,远大于Kevlar29芳纶纤维的拉伸模量(约 80GPa);它们的拉伸强度相当,都在 2. 5GPa左右;而T300碳纤维的断裂伸长率在 1. 0%左右,小于Kevlar29芳纶纤维的 3. 0%。使用扫描电子显微镜(SEM)对纤维单丝断裂前后的结构进行了观察,这些不同的微观结构决定了纤维单丝拉伸性能的差异。     

微拉曼光谱研究M55JB碳纤维/微滴的拉伸变形行为

使用评价纤维/基体界面力学性能的新方法纤维微滴拉伸测试,来研究M55JB碳纤维/环氧树脂基体之间的界面应力传递性能。使用自制的微加载装置对碳纤维/环氧树脂微滴试样进行对称式拉伸测试,用微拉曼光谱仪记录下不同应变下的嵌入微滴内纤维上的拉曼频移信号,经过应力/频移关系转换成纤维轴向应力。实验结果显示,微滴内纤维轴向应力随载荷而明显增加。根据界面力平衡模型得到相应的界面剪切应力呈反对称式分布,在纤维嵌入端存在剪应力集中。新测试方法能保证嵌入微滴内纤维上的应力呈对称式分布,而且能降低纤维嵌入端附近的应力奇异性。     

Kevlar纤维增强复合材料动态压缩力学性能实验研究

通过实验较系统地研究了Kevlar纤维增强复合材料的动态压缩力学性能,实验结果表明,在冲击压缩载荷作用下Kevlar纤维增强复合材料有明显的损伤软化现象和应变率效应,针对Kevlar纤维增强复合材料动态应力应变实验曲线,提出了含损伤的率相关动态本构方程,由于所引入的损伤最反映了Kevlar纤维增强复合材料内部基体开裂、脱层、纤维断裂等多种破坏模式的总体效果,因此所提出的本构方程形式相对说来比较简便并易于嵌入目前有关冲击力学的有限元或有限差分程序,有一定的工程应用价值。     

超高速撞击玄武岩及Kevlar纤维布填充防护结构研究

利用二级轻气炮对玄武岩及Kevlar纤维布填充Whipple防护结构进行了超高速撞击实验研究.以Nextel/Kevlar撞击极限曲线为参照,分析了双层未涂胶玄武岩及Kevlar纤维布填充防护结构和双层涂环氧树脂胶玄武岩及Kevlar纤维布填充防护结构的防护性能、填充材料及舱壁的损伤情况.实验表明:双层未涂胶玄武岩及Kevlar纤维布填充防护结构和双层涂环氧树脂胶玄武岩及Kevlar纤维布填充防护结构在高速区都具有优良的防护性能.此外.通过涂环氧树脂胶改善了玄武岩及Kevlar纤维布填充防护结构在高速区的防护性能.Kevlar纤维丝在低速区主要依靠塑性变形及断裂吸收弹丸的动能,玄武岩纤维丝在高速区主要依靠脆性断裂及高温碳化将弹丸破碎或融化为尺寸更小的碎片或熔球,减轻了对舱壁的损伤.玄武岩及Kevlar纤维丝在高速区存在高温熔化及碳化现象.     

聚四氟蜡和二硫化钼填充凯夫拉纤维织物复合材料的摩擦磨损性能研究

采用玄武三号栓-盘式摩擦磨损试验机研究凯夫拉(Kevlar)纤维织物材料及聚四氟蜡(PFW)和MoS2填充Kevlar纤维织物复合材料的摩擦磨损性能,采用扫描电子显微镜观察其磨损表面形貌.结果表明,PFW和MoS2均可以改善Kevlar纤维织物材料的摩擦磨损性能,其中PFW的改善效果尤为显著.当PFW质量分数为20%时,Kevlar纤维织物复合材料的摩擦系数减小75%、磨损率降低82%,Kevlar纤维织物复合材料的减摩抗磨性能最佳.     

基于弹性-塑性内聚力模型的纤维拔出界面宏观行为分析

用解析法分析了单纤维从聚合物基体中的拔出过程,采用弹性—塑性内聚力模型模拟裂纹的扩展和界面失效,确定了临界纤维埋入长度,该值区分两种不同长度的纤维拔出过程. 在纤维拔出过程,界面经历不同的阶段. 纤维埋长小于临界长度时,界面的脱粘载荷与纤维的埋长成正比;超过临界长度后,界面的脱粘载荷近似为常数. 分析了界面参数对脱粘载荷的影响:增加界面的剪切强度和界面的断裂韧性,或减小界面裂纹萌生位移,均能提高界面的脱粘载荷;界面脱粘后无界面摩擦应力时,拔出载荷—位移曲线的峰值载荷等于界面的脱粘载荷;界面摩擦应力存在时,使峰值载荷大于脱粘载荷,需要较长的纤维埋入长度和较大的界面摩擦应力.      

单向纤维增强复合材料中纤维断裂及其发展

纤维增强复合材料中某根纤维断裂后，断口作为裂纹向何处发展？它可以向纤维和基体的界面发展形成界面脱粘，也可向基体发展，造成基体开展，从而殃及邻近纤维。另外，一根纤维的断裂会在其邻近纤维中造成应力集中。本文采取轴对称边界元法对这些问题进行仔细研究。本文假定纤维在基体中成六角形分布，即每根纤维周围有六根纤维，均匀地分布在以该纤维为中心的圆周上。     

单向KFRP的应变率相关的本构方程

在旋转盘式杆杆型冲击拉伸试验装置上 ,对单向Kevlar 49纤维增强酚醛树脂复合材料(KFRP)进行了冲击拉伸试验 ,得到了应变率为 15 0 ,40 0和 15 0 0s-1下的单向KFRP的完整拉伸应力应变曲线 ;结果表明 ,单向KFRP的力学性能是应变率相关的。通过改进复合丝束模型 ,建立了计及应变率效应的单向KFRP的一维损伤宏观本构方程。     

单向复合材料纤维断裂对局部应力集中的影响

本文采用剪切滞后理论，对单向辅设的增强复合材料中，纤维根数有限时纤维断裂所引起的局部应力集中的进行研究，确证了以往研究中假定纤维无限根的可行性，并定量展示了在断裂纤维和邻近纤维中的应力重分布。     

Measurement and modeling of stress concentration around a circular notch

When designing composite materials, the presence of stress concentration at locations such as circular nothces is unavoidable. Such locations in structural elements arise from joints required to form a structure. The stress concentration, observed around the notch, is quantified by the stress concentration factorK. This quantity is normally calculated analytically and/or numerically and is an important design parameter. In this work, the experimental technique of remote laser Raman microscopy is used for the in situ measurement ofK in Kevlar 49 fiber/epoxy composite plates containing a circular hole. The results obtained by this technique are compared with those calculated analytically and by finite element analysis. Both analytical and numerical methods underestimate the experimental results for maximumK by approximately 10 percent, which is considered reasonable within experimental error. In addition, very good agreement between analytical and experimental data is obtained for the decay of the stress concentration factor as a function of distance from the edge of the hole. The numerical results, however, overestimate the decayK with distance from the notch boundary and only converge at relatively large distances.B. P. Arjyal has obtained his PhD from the Materials Department, Queen Mary & Westfield College, University of London, London, E1 4NS, United Kingdom.     

Influence on fiber inclination and interfacial conditions on fracture in composite materials

Dynamic photoelasticity has been used to study the effect of the fiber-matrix interface and fiber orientation on dynamic crack growth in fiber composites. Two types of fiber-matrix interfaces are considered: well bonded and partly debonded. The fiber-matrix interface is characterized by conducting fiber pullout tests. Partly debonded fibers aligned with the loading direction, result in higher fiber debonded lengths, lower dynamic stress-intensity factorK _ID and lower fracture surface roughness compared to well bonded fibers. Orientation of brittle fibers, with respect to the loading direction, impairs their ability to lowerK _ID , while oriented ductile fibers produce no significant change inK _ID . Misalignment of fibers from the loading direction reduces the fiber debonded length due to kinding of the fiber at the crack face.     

Dynamic Crack Growth Past a Stiff Inclusion: Optical Investigation of Inclusion Eccentricity and Inclusion-matrix Adhesion Strength

Interactions between a dynamically growing matrix crack and a stationary stiff cylindrical inclusion are studied optically. Test specimens with two different bond strengths (weak and strong) and three crack-inclusion eccentricities (e = 0, d/2 and 3d/4, d being inclusion diameter) are studied using reflection mode Coherent Gradient Sensing (CGS) and high-speed photography. These variants produce distinct dynamic crack trajectories and failure behaviors. A weaker inclusion-matrix interface attracts a propagating crack while a stronger one deflects the crack away. The former results in a propagating crack lodging (‘key-hole’) into the inclusion-matrix interface whereas in the latter the crack tends to circumvent the inclusion. When the inclusion is in the prospective crack path, the maximum attained crack speed is much higher in the weakly bonded inclusion cases relative to the strongly bonded counterparts. For a crack propagating towards a weakly bonded inclusion, the effective stress intensity factor (K _e) value remains constant for each inclusion eccentricity considered. But these constant K _e values increase with increasing eccentricity. A distinct drop in K _e occurs when the crack is near the inclusion. In strongly bonded inclusion cases, on the other hand, monotonically increasing K _e before the crack reaches the inclusion is observed. A drop in K _e is seen just before the crack reaches the inclusion. The mode-mixity estimates are of opposite signs for weakly and strongly bonded inclusions in case of the largest eccentricity studied, confirming the observed crack attraction and deflection mechanisms.

Edge-cracked stiffened panels analyzed by caustics

Externally bonded composite patches have been proven to be an effective method for repairing damaged aircraft structural components. They are ease in application and provide excellent in-service performance. The major function of a repair is to reduce the stress intensity factor at the crack tip. Calculation of stress intensity factor of a repaired crack has been performed by analytical and numerical methods. However, these methods are based on simplifying assumptions regarding material behavior and repair conditions. In the present paper an experimental determination of mode-I stress intensity factor (SIF), K_I, at the tip of an edge-crack or a V-notch reinforced with double bonded strips or with compression pre-stresses applied along the crack surfaces is undertaken by using the optical method of caustics. This method is simple in its application and has successfully been used for the solution of a host of crack problems of engineering importance.     

A photoelastic study of crack repair

Birefringent coatings have been employed to study the effectiveness of an adhesively bonded repair of a center-cracked tension panel. The repair was one sided, with photoelastic coatings applied to the opposite side. Photoelastic coatings were also applied over the patch. Analysis methods are presented to permit the stress intensity factor to be determined from the isochromatic fringe patterns recorded from both continuous andX- andY-edged coatings. The results showed that the one-sided adhesively bonded patch reduced the stress intensity factor; however, the repair did not markedly change the character of stress distributions. Fringe loops formed near the crack tips for both the cracked and repaired tension panels. The primary difference was in the size of the loops. The reduction inK _I due to repair was smaller than anticipated, but even small improvements in ΔK _I markedly enhance the life of a repaired panel. The Paris power law is used to show the relation between the reduction in ΔK _I and the improvement in the crack growth rateda/dN. Fringe patterns from the birefringent coatings applied to the patch provided a means not only to investigate the stresses in the patch but also to detect the initiation of the local debonding of the adhesive in the neighborhood of the crack. The birefringent coating on the patch is an approach for producing an optically “smart” repair.     

Influence of debonding on the efficiency of crack patching

A simplified analysis of crack patching is presented in which the restraining effect of the external bonded reinforcement on the crack opening is simulated by distributed springs acting between the crack faces. It is suggested that elastic-perfectly plastic springs provide an adequate idealization for the case where debonding can occur. The analysis is thereby reduced to the solution of a one-dimensional integral equation involving only two material parameters: a spring constant k and a limit stress σ_L, both of which can be determined from the behaviour of a suitable overlap joint. The efficiency of the reinforcement is shown to be determined by the normalized crack length ka and the normalized stress σ/σ_L, so that a comprehensive characterization can be obtained from relatively little computation or experimental study.     

钢轨表面剥离掉块路径预测研究

轮轨滚动接触下,钢轨表面会产生典型的鱼钩形剥离掉块,其形成机理目前暂未明确.为了探究轮轨滚动接触下钢轨表面裂纹扩展机理,基于最大周向拉应力准则,建立轮轨滚动接触疲劳计算模型,提出裂尖扩展路径预测方法,并对不同初始角度裂纹的扩展路径进行预测.结果表明,钢轨表面微裂纹为Ⅰ-Ⅱ复合型裂纹,随着裂纹长度增加,KⅠ先增加后减小,...     

The interaction between crack and electric dipole of piezoelectricity

Discrete dipoles located near the crack tip play an important role in nonlinear electric field induced fracture of piezoelectric ceramics. A physico-mathematical model of dipole is constructed of two generalized concentrated piezoelectric forces with equal density and opposite sign. The interaction between crack and electric dipole in piezoelectricity is analyzed. The closed form solutions, including those for stress and electric displacement, crack opening displacement and electric potential, are obtained. The function of piezoelectric anisotropic direction,p _a (θ)=cosθ+p _a sinθ, can be used to express the influence of a dipole's direction. In the case that a dipole locates near crack tip, the piezoelectric stress intensity factor is a power function with −3/2 index of the distance between dipole and crack tip. Supported by National Natural Science Foundation of China(No. 10072033)     

Deflection criterion of a crack approaching a bi-material interface obliquely

Deflection criterion for oblique cracks terminating at a bi-material interface was established numerically based on a remote loading condition where the crack deflection event took place well within the K-dominant stress field. The criterion was described in terms of the ratio of the energy release rate of a deflected crack (G _d) to the maximum energy release rate of a penetrated crack (). The criterion was markedly more conservative than the existing solution based on wedge loading which did not converge with respect to limited number of a/L ratios in the literature (a is the size of the putative crack; L is the loading distance). Further, the criterion established herein for the cracks slightly oblique from the normal direction to the interface was more conservative than the crack normal to the interface.     

Bifurcation condition of crack pattern in the periodic rectangular array

Bifurcation condition of crack pattern in the periodic rectangular array plays an important role in determining the final failure pattern of rock material. An approximation for the critical crack size/spacing ratio is established for a uniformly growing periodic rectangular array yields to a non-uniform growing pattern of crack growth. Numerical results show that the critical crack size/spacing ratio λ_cr depends on the number of cracks, the crack spacing, the perpendicular distance between two adjacent rows, as well as the loading conditions. In general, λ_cr increases with the number of lines. It is observed that the critical crack size/spacing ratio λ_cr for the periodic rectangular array decreases with an increase in the perpendicular distance between two adjacent rows. It is clear that the critical crack size/spacing ratio λ_cr for the periodic rectangular array under shear stress increases with increasing the crack spacing.