Prediction of mixed-mode cracking direction in random, short-fibre composite-materials

The prediction of cracking direction in composite materials is of significance to the design of composite structures. This paper presents several methods for predicting the cracking direction in the double grooved tension-shear specimen which gives mixed-mode cracking. Five different criteria are used in this analysis: two of them have been used by other investigators and the others are proposed by the present authors. The strain energy density criterion proposed by G.C. Sih is modified to take account of the influence of the anisotropy of the strength on the direction of crack. The two failure criteria of Tsai-Hill and Norris are extended to predict the crack orientation. The stress distributions in the near-notch zone are calculated by using the 8-node quadrilateral isoparametric finite element method. The predictions of all the criteria except one are in good agreement with the experimental measurement. In addition, on the basis of the FEM results, the size of the zone in which the singular term is dominant is estimated.

Study on cracked plates, shells and 3 dimensional bodies

This paper presents a summary of the authors' recent work in following areas: (1) The stress-strain fields at crack tip in Reissner's plate. (2) The calculations of the stress intensity factors in finite size plates. (3) The stress-strain fields at crack tip in Reissner's shell. (4) The calculations of the stress intensity factors and bulging coefficients in finite size spherical shells. (5) The stress-strain fields along crack tip in three dimensional body with surface crack. (6) The calculation of stress intensity factors in a plate with surface crack.

Study on cracked spherical shells

The local-global anatysis method is systematically extended to the fracture analysis of spherical shells. On the basis of the shallow shell theory, which takes into account transverse shear deformations, governing equations for cracked spherical shells expressed in displacement and stress functions f, F and φ are proposed, and then a general solution including Modes, Ⅰ, Ⅱ, Ⅲ for stress-strain fields at crack tip in a spherical shell is obtained, which plays the same role as Williams's expansion in plane elasticity. The numerical results for finite-size spherical shells under different boundary conditions have been obtained. Furthermore, the bulging factors are analyzed with regard to shearing stiffness and an approximate formula is given.

Experimental relaxation behavior of chopped-strand mat glass-fiber reinforced polyester beams in 3 point bending

A dimensionless relation of the form for collating fatigue crack starting growth data is proposed in which Δkth represents the stress intensity factor range at the threshold. Based on experimental results, this relation attains the value of 0.6 for a fatigue crack to start growth in the Austenitic stainless steel investigated in this work. Metallurgical examinations were also carried out to show a transgranular shear mode of cyclic cleavage and plastic shear.

Static and fatigue failure behavior of carbon-fiber reinforced epoxy composite-materials with edge notch

A study of carbon fiber reinforced epoxy composite material with 0° ply or ±45°ply(unnotched or with edge notch) was carried out under static tensile and tension-tensioncyclic loading testing. Static and fatigue behaviour and damage failure modes in unnotched/notched specimens plied in different manners were analysed and compared with each other.A variety of techniques (acoustic emission, two types of strain extensometer, high speed pho-tography, optical microscopy, scanning electron microscope, etc.) were used to examine thedamage of the laminates. Experimental results show that when these carbon/epoxy laminateswith edge notch normal to the direction of the load are axially loaded in static or fatiguetension, the crack does not propagate along the length of notch but is in the interface (fiberdirection). The notch has no substantial effect on the stresses at the unnotched portion. Thedamage failure mechanism is discussed.

Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of ΔK_I/ΔK_II the shear mode growth shows a self-arrest character and it would only start again when ΔK_I and ΔK_II are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.

Elastic-plastic finite element analyses of short cracks

Stress and strain distributions and crack opening displacement characteristics of short cracks have been studied in single edge notch bend and centre cracked panel specimens using elastic–plastic finite element analyses incorporating both a non strain hardening and a power law hardening behaviour. J contour integral solutions to describe stress strain conditions at crack tips for short cracks differ from those for long cracks. The analyses show that (i) short cracks can propagate at stress levels lower than those required for long cracks and (ii) a two-parameter description of crack tip fields is necessary for crack propagation.

不确定边界条件下悬臂梁裂纹参数识别方法研究

基于Bernoulli-Euler梁振动理论,以等效弹簧来模拟裂纹引起的局部软化效应和由非完全固支边界条件引起的转角效应.推导了悬臂梁在不确定边界条件下确定其振动频率的特征方程,直接利用该特征方程,提出一种有效估计裂纹参数的优化方法,通过计算测量频率和理论频率之间的误差目标函数最小化即可识别裂纹参数-裂纹位置和深度.最后,应用两个实例-理想固支边界条件下和非完全固支边界条件下的悬臂梁实验来说明本文方法的有效性.实验结果表明:只需梁结构前三阶频率即可识别裂纹位置和深度.对于理想边界条件下的裂纹参数识别,在测量频率存在小误差情况下,该方法仍能给出比较满意的结果,对于非完全固支边界条件下的裂纹参数识别,利用本文方法能得到比Narkis的方法更精确的裂纹位置识别结果.同时本文方法还能给出比较满意的裂纹深度识别结果.

冲击载荷下纤维复合材料裂纹起裂的实验研究

采用SHTB技术对纤维增强复合材料裂纹动态起裂行为进行了实验研究。使用应变片方法确定了裂纹的起裂时间，结合有限元数值模拟得到了裂纹的起裂韧性；同时观察了裂纹在冲击载荷作用下的裂纹起裂和扩展方式，分析了纤维的铺层角度对裂纹起裂和扩增的影响。

抽油杆裂纹扩展的实验研究和剩余疲劳寿命的预测

应用超声波探测抽油杆疲劳裂纹扩展时的深度a, 通过室内实验找出D级抽油杆裂纹深度a和裂纹宽度c之间的比趋向于0.78, 简化后可直接研究超声波信号V和裂纹深度a之间的关系, 从而用V-a关系估算有疲劳裂纹D级抽油杆的剩余寿命, 避免杆断带来的经济损失.

黏弹性材料界面裂纹应力场奇性分析

研究两半无限大黏弹性体间Griffith界面裂纹在简谐载荷作用下裂纹尖端动应力场的奇异特性。通过引入裂纹张开位移和裂纹位错密度函数，相应的混合边值问题归结为一组耦合的奇异积分方程。渐近分析表明裂尖动应力场的奇异特征完全包含在奇异积分方程的基本解中。通过对基本解的深入分析发现黏弹性材料界面裂纹尖动应力场具有与材料参数和外载荷频率相关的振荡奇异特性。以标准线性固体黏弹材料为例讨论了材料参数和载荷频率对奇性指数和振荡指数的影响。

氢对锆基块体非晶合金形变和开裂的影响

通过充氢和未充氢缺口拉伸试样和三点弯曲试样在SEM下的原位加载,研究了氢对Zr65Al7.5Ni10Cu17．5块体非晶合金形变和开裂过程的影响．结果表明,无论是否有氢,块体非晶的剪切带发展到临界尺寸,剪切裂纹就沿剪切带形核、扩展,它一旦张开就导致快速的断裂．断口边缘观察到的无特征区是剪切带,而不是剪切裂纹断口;剪切断口形貌和拉伸断口形貌没有本质区别．只有当长时间充氢才能形成氢鼓泡,如鼓泡很小或尚未形成,则氢对剪切带以及裂纹的形核、扩展没有明显影响;如存在较大的氢鼓泡,则当剪切带尚未充分发展时微裂纹就形核,导致低应力脆断．

梯度涂层材料中裂纹问题的非均匀元分析

本文采用非均匀等参有限元的方法研究了薄膜梯度涂层/均匀基材中的界面裂纹问题，并与双材料界面裂纹情况进行了对比计算。研究表明：在均匀基材上采用梯度涂层，与双材料相比可以有效地降低裂尖场应力强度因子；同时还分析了涂层厚度与梯度参数对界面应力强度因子的影响。结果表明：当薄膜厚度大于或等于裂纹长度时，应力强度因子（K_I、K_(II)）对其尺度的变化显得不敏感；对梯度参数的影响而言，当材料性能曲线的幂指数m大于1时，裂尖场的应力强度因子K_(II)相对K_I很小且基本不随m变化，因此裂尖场与均匀材料情况类似；当m小于1时，应用强度因子K_(II)随m减小而急剧增大，裂尖场由K_I及K_(II)控制，断裂趋于混合型。

机械载荷作用下单边裂纹载流薄板的应力场

采用坐标变换的方式,将单边裂纹载流薄板通电瞬间由温度产生的应力场表达式中的各应力分量分离,并用极坐标进行表示.给出了Ⅰ型穿透裂纹尖端附近的应力场的表达式.最后将温度产生的应力场与单向拉伸载荷作用产生的应力场相叠加,推导出用极坐标表示的机械载荷作用下单边裂纹载流薄板的应力场的表达式,并给出算例.

折线裂纹和两相材料界面的相互作用

利用螺位错基本解建立了和界面相交的折线裂纹的Cauchy型积分方程．根据奇异积分方程理论，得出了确定折线裂纹和界面交点处的奇性应力指数的特征方程，以及交点处各角形域内的奇性应力．利用所得的交点处的奇性应力定义了折线裂纹和界面交点处的应力强度因子．对所得积分方程进行数值求解，可得裂纹端点以及裂纹和界面交点处的应力强度因子．