Fracture criterion based on the higher-order asymptotic fields

A complete development for the higher-order asymptotic solutions of the crack tip fields and finite element calculations for mode I loading of hardening materials in plane strain are performed. The results show that in the higher-order asymptotic solution (to the twentieth order), only three coefficients are independent. These coefficients are determined by matching with the finite element solutions carried out in the present paper (our attention is focused on the first five terms of the higher-order asymptotic solution). We obtain an analytic characterization of crack tip fields, which conform very well to the finite element solutions over wide range. A modified two parameter criterion based on the asymptotic solution of five terms is presented. The upper bound and lower bound fracture toughness curves predicted by modified two parameter criterion are given. These two curves agree with most of the experimental data and fully capture the proper trend.

Numerical simulation of evolution-induced catastrophe

A numerical simulation of damage evolution in a two-dimensional system of micocracks is presented. It reveals that the failure is induced by a cascade of coalescences of microcracks, and the fracture surface appears fractal. A model of evolution-induced catastrophe is introduced. The fractal dimension is found to be a function of evolution rule only. This result could qualitatively explain the correlation of fractal dimension and fracture toughness discovered in experiments.

Fatigue fracture-behavior of carbon-fiber-reinforced modified bismaleimide composites

The fracture toughness and fatigue fracture behaviour of carbon-fiber-reinforced modified bismaleimide (BMI) composites have been studied. These composites were found to have higher fracture toughnes, better damage tolerance and longer fatigue life than carbon-fiber composites with epoxy matrices. Delamination is the major mode of failure in fatigue and it is controlled by the properties of the matrix and interface. The improved performance is dire to the presence of thermoplastic particles in the modified BMI matrix which gives rise to enhanced fiber/matrix adhesion and more extensive plastic deformation. The fatigue behaviour also depends on the stacking sequence, with the multidirectional [45/90/-45/0] fiber-reinforced modified BMI composite having a lower crack propagation rate and longer fatigue life than the unidirectional laminate. This arises because of the constraint on the damage processes due to the different fiber orientation in the plies.

Kinking of an interface crack between 2 dissimilar anisotropic elastic solids

A detailed analysis of kinking of an interface crack between two dissimilar anisotropic elastic solids is presented in this paper. The branched crack is considered as a distributed dislocation. A set of the singular integral equations for the distribution function of the dislocation density is developed. Explicit formulas of the stress intensity factors and the energy release rates for the branched crack are given for orthotropic bimaterials and misoriented orthotropic bicrystals. The role of the stress parallel to the interface, sigma0 is taken into account in these formulas. The interface crack can advance either by continued extension along the interface or by kinking out of the interface into one of the adjoining materials. This competition depends on the ratio of the energy release rates for interface cracking and for kinking out of the interface and the ratio of interface toughness to substrate toughness. Throughout the paper, the influences of the inplane stress sigma0 on the stress intensity factors and the energy release rates for the branched crack, which can significantly alter the conditions for interface cracking, are emphasized.

Higher-order analysis of crack tip fields in elastic power-law hardening materials

A HIGHER-ORDER asymptotic analysis of a stationary crack in an elastic power-law hardening material has been carried out for plane strain, Mode 1. The extent to which elasticity affects the near-tip fields is determined by the strain hardening exponent n. Five terms in the asymptotic series for the stresses have been derived for n = 3. However, only three amplitudes can be independently prescribed. These are K1, K2 and K5 corresponding to amplitudes of the first-, second- and fifth-order terms. Four terms in the asymptotic series have been obtained for n = 5, 7 and 10; in these cases, the independent amplitudes are K1, K2 and K4. It is found that appropriate choices of K2 and K4 can reproduce near-tip fields representative of a broad range of crack tip constraints in moderate and low hardening materials. Indeed, fields characterized by distinctly different stress triaxiality levels (established by finite element analysis) have been matched by the asymptotic series. The zone of dominance of the asymptotic series extends over distances of about 10 crack openings ahead of the crack tip encompassing length scales that are microstructurally significant. Furthermore, the higher-order terms collectively describe a spatially uniform hydrostatic stress field (of adjustable magnitude) ahead of the crack. Our results lend support to a suggestion that J and a measure of near-tip stress triaxiality can describe the full range of near-tip states.

Crack extension and kinking in laminates and bicrystals

Singular fields at the tip of an interface crack in anisotropic solids are reviewed with emphasis on establishing a framework to quantify fracture resistance under mixed mode conditions. The concepts of mode mixity and surface toughness are unified by using generalized interface traction components. The similarity between the anisotropic theory and existing isotropic theory is shown. Explicit formulae are given for misoriented orthotropic bimaterials with potential applications envisioned including composite laminates and semiconductor crystals. Competition between crack extension along the interface and kinking into the substrate is investigated using a boundary layer formulation. Several case studies reveal the role of anisotropy. An explicit complex variable representation for orthotropic materials and a solution to a dislocation interacting with a crack are presented in two self-contained Appendices.

Orthotropy rescaling and implications for fracture in composites

For most practically important plane elasticity problems of orthotropic materials, stresses depend on elastic constants through two nondimensional combinations. A spatial rescaling has been found to reduce the orthotropic problems to equivalent problems in materials with cubic symmetry. The latter, under favorable conditions, may be approximated by isotropic materials. Consequently, solutions for orthotropic materials can be constructed approximately from isotropic material solutions or rigorously from cubic ones. The concept is developed to gain insight into the interplay between anisotropy and finite geometry. The inherent simplicity of the solutions allows a variety of technical problems to be addressed efficiently. Included are stress concentration related cracking, effective contraction of orthotropic material specimens, crack deflection onto easy fracture planes, and surface flaw induced delamination.

Ballistic impact of ceramics

The impact behaviour of a range of glass and ceramic materials has been studied using high-speed photography. A gas gun was used to project hardened spheres at plate specimens in the velocity range 30 to 1000m s-1. The target materials included soda-lime glass, boron carbide and various glass ceramics and aluminas. The performance of a particular ceramic was found to depend on a combination of parameters but of key importance was the relative hardness of the projectile and target materials. The fracture toughness, K(IC), had only a secondary effect.

Analysis of Kic and its temperature dependence of metals by a simplified dislocation model

The relative Kic values of metals are calculated with a simplified dislocation model. It is found that the ratio of KIIc to KIc and the temperature dependence of fracture toughness of some metals estimated with this model are consistent with the experimental results.

测量镀铬层界面韧性的激光屈曲法

为了测量强界面电镀铬层的界面韧性，利用连续CO_2激光器对钢基体上的电镀铬层表面进行循环扫描实验。结果表明：该种加热方式能够诱发铬层沿激光扫描方向呈周期性分布的屈曲变形。在此基础上，结合涂层屈曲变形理论，提出测量镀铬层界面韧性的激光屈曲法。该方法只需对一个屈曲单元的最大屈曲高度和屈曲半长进行测量，就可给出界面韧性。作为应用举例，利用该方法对上述镀铬层／钢基体结构界面韧性进行了测量。

激光淬火基体对镀铬层断裂韧性的影响

为提高30CrNi2MoVA钢镀铬身管的寿命,采用YAG激光器对钢基体进行激光搭接淬火再镀铬的复合工艺。用多裂纹拉伸技术对经激光搭接处理的30CrNi2MoVA钢再镀铬后的铬层和普通铬层的断裂韧性进行比较。结果表明,激光淬火处理基体可以提高镀铬层的断裂韧性。力学模型分析表明,基体残余压应力以抵抗基体拉伸变形的方式提高了铬层的断裂韧性;化学去基体法研究铬层界面表明,激光处理基体以细化基体影响层的晶粒形式提高了铬层的强度和韧性。离子刻蚀横截面对拉伸结果作了进一步验证。

涂层断裂韧性的声发射辅助拉伸测量方法

涂层的断裂韧性与界面结合强度是表征涂层/基体材料体系力学性能的重要指标.但如何准确地测量涂层的断裂韧性和强界面结合的涂层/基体材料体系的界面结合强度至今仍存在困难.以铬涂层/钢基体材料为对象,采用声发射和显微镜实时动态检测技术与拉伸实验相结合的方法,探索了铬涂层的断裂韧性以及铬涂层/钢基体的界面剪切强度.根据相关力学模型和实验测量结果,得到铬涂层在室温下的断裂韧性为27.41J/m~2. 同时,发现在铬涂层裂纹饱和后界面开裂都未发生,获得了该种材料体系界面剪切强度的一个下限值.

El rol de los contratos en viticulturas

[ES] A pesar de que los economistas han dedicado enorme esfuerzo a examinar la racionalidad de los contratos en agricultura, pocos estudios se han llevado a cabo en viticultura. Sin embargo, se observan diferencias contractuales en la producción de uva y vino. En el presente trabajo se analizan las estructuras contractuales actualmente utilizadas en varias regiones vitivinícolas. Independientemente de las características inherentes en los contratos, se obtiene que estos mecanismos permiten a las bodegas articular sus requisitos con respecto a la calidad de las uvas producidas por los agricultores.

Capacidad informativa de los resultados contables en la empresa familiar

[ES] El presente trabajo analiza la relación entre el control familiar y la credibilidad de los resultados contables. Utilizando un panel de datos de empresas españolas cotizadas no financieras para el período 1997-2003, los resultados alcanzados muestran que la credibilidad de la información contable de la empresa familiar es inferior a la de la no familiar. Asimismo, el incremento de los derechos de voto en manos del último propietario familiar incide negativamente en la credibilidad de sus resultados divulgados. Las únicas empresas en las que la naturaleza familiar incide positivamente en la credibilidad de la información contable son aquellas en las que el presidente del consejo de administración no pertenece a la familia controladora.

Determinantes de la intención del consumidor de ir a un destino turístico. Aplicación al caso de "Mundo Maya-México"

[ES] Cada destino turístico tiene una marca que puede elegir el consumidor a la hora de viajar. Este trabajo examina, a través de un modelo causal que contrastamos empíricamente para el caso de Mundo Maya-México, el papel que desempeña la reputación, las emociones y la confianza en la intención de compra de los consumidores de un destino turístico.