Fatigue and fracture paths in cold drawn pearlitic steel

This paper analyses the influence of microstructural anisotropy of a progressively drawn pearlitic steel (orientation of pearlitic lamellae in the drawing direction) on the microscopic and macroscopic evolution of cracking paths produced by fatigue and fracture. The fatigue crack path is always contained in the transverse section of the wires, i.e., the subcritical propagation develops under a global mode I, so that the main crack path is associated with mode I and some very local deflections take place to produce a roughness in the fatigue crack path depending on the drawing level. The fracture crack path evolves from a global mode I propagation following the transverse plane in slightly drawn steels (including the hot rolled bar that is not cold drawn at all) to a global mixed-mode propagation associated with crack deflection in intermediate and heavily drawn steels (the latter with a strong mode II component), the deviation angle being an increasing function of the drawing degree in the steel.     

高地应力条件下围岩破损区岩体力学特性研究

利用细观损伤力学的基本原理,建立了高地应力区围岩稳定分析的细观损伤力学模型,对围岩破坏全过程进行了分析模拟。引入等效模型的概念,对破损区域内围岩在破坏前后分别建立等效模型,进行数值力学试验,根据试验得到的力学特性曲线,对围岩开挖破碎带岩体主要力学参数（强度、变形模量等）随施工过程的变化规律进行了深入的定量研究,得到了岩体主要力学参数的变化数值,为岩体稳定分析和结构设计提供了可靠的依据,同时也进一步明确了开挖后围岩扰动区域的划分标准。     

金属磁记忆检测法研究进展

金属磁记忆法不仅可以实现传统无损检测技术对宏观裂纹的测定,而且可以确定由于应力集中造成的损伤,进而实现对结构剩余寿命的评估。该方法具有检测设备轻便、操作过程简单以及缺陷识别判据直观等优点,易于工程应用。但作为一种新型无损检测技术,其测试原理的理论物理模型、测试结果的可靠性和定量化、测试信息的有效利用性等方面还存在许多争议和不确定之处。在简单介绍其测试原理基础上,列举了一些典型工程应用实例,并就该方法存在的问题和未来发展方向进行了探讨。     

Hypervelocity impact damage to Ti–6Al–4V meshes reinforced Al–6Mg alloy matrix composites

An Al–6Mg alloy matrix composite reinforced with Ti–6Al–4V meshes was fabricated by pressure infiltration method; its damage behaviors impacted by hypervelocity aluminum projectiles were investigated. Results showed that the thin Ti_f/Al–6Mg composite target exhibits better protection efficiency and energy absorption ability than Al–6Mg alloy target. With projectile sizes increasing, bulge and spallation were observed on the back of the composite target. The Ti–6Al–4V meshes were tensed and deformed drastically in the spallation region, where micro-damages such as interfacial debonding and cracks were dominant. Shear localization was the primary failure characteristic for thin Al–6Mg alloy target. The adiabatic shear bands were observed near the crater of Al–6Mg alloy, not in Ti_f/Al–6Mg composite target. It was ascribed to the Ti–Al interfacial bonding strength and the high temperature strength for Ti–6Al–4V alloy.     

A micro-damage healing model that improves prediction of fatigue life in asphalt mixes

The focus of the current paper is on the development and validation of a micro-damage healing model that improves the ability of an integrated nonlinear viscoelastic, viscoplastic, and viscodamage constitutive model based on continuum damage mechanics for predicting the fatigue life of asphalt paving mixtures. The model parameters of the continuum-based healing model are related to fundamental material properties. Recursive-iterative and radial return algorithms are used for the numerical implementation of viscoelasticity and viscoplasticity models respectively, whereas the viscodamage and micro-damage healing models are implemented using the concept of the effective undamaged-healed natural configuration. Numerical algorithms are implemented into the well-known finite element code Abaqus via the user material subroutine UMAT. Finally, the model is validated by comparing its predictions with experimental data on an asphalt mix that include repeated creep-recovery tests for different loading times and rest periods in both tension and compression. The significant enhancement of the ability of the constitutive model to predict fatigue life due to inclusion of the micro-damage healing is clearly demonstrated.     

Predicting Young's modulus of glass/ceramic sealant for solid oxide fuel cell considering the combined effects of aging,micro-voids and self-healing

We study the temperature dependent Young's modulus for the glass/ceramic seal material used in solid oxide fuel cells (SOFCs). With longer heat treatment or aging time during operation, further devitrification may reduce the residual glass content in the seal material while boosting the ceramic crystalline content. In the meantime, micro-voids induced by the cooling process from the high operating temperature to room temperature can potentially degrade the mechanical properties of the glass/ceramic sealant. Upon reheating to the SOFC operating temperature, possible self-healing phenomenon may occur in the glass/ceramic sealant which can potentially restore some of its mechanical properties. A phenomenological model is developed to model the temperature dependent Young's modulus of glass/ceramic seal considering the combined effects of aging, micro-voids, and possible self-healing. An aging time-dependent crystalline content model is first developed to describe the increase of the crystalline content due to the continuing devitrification under high operating temperature. A continuum damage mechanics (CDM) model is then adapted to model the effects of both cooling induced micro-voids and reheating induced self-healing. This model is applied to model the glass–ceramic G18, a candidate SOFC seal material previously developed at PNNL. Experimentally determined temperature-dependent Young's modulus is used to validate the model predictions.