A new Thermo-viscoplastic Material Model for Finite-Element-Analysis of the Chip Formation Process

A new material model for describing the thermo-viscoplastic flow behavior of workpiece material in metal cutting is presented. In order to express the complex flow behavior which depends on the local strain, strain rate and temperature, a new methodology for sequential formulation is proposed. The material parameters which are achieved by using the flow stress data available at low strain rates are enhanced by matching the results of the experimental investigations and finite element simulations of the orthogonal cutting process. As a result, a material model which has a wide validity range of strain, strain rate and temperature is established.     

A mesomodel for the numerical simulation of the multiphasic behavior of materials under anisothermal loading (application to two low-carbon steels)

The determination of residual stresses induced by welding or heat treatment operations requires the use of complex models taking into account thermal, metallurgical and mechanical phenomena. In this paper, we propose a mechanical model in which each phase can follow its own constitutive law. This model also takes into account phase transformation plasticity, which is treated independently of the behavior of each phase. This model has been implemented into the French FEM code Castem 2000. The interest of the proposed method is that it allows one to mix any type of nonlinear behavior using Taylor homogenization hypothesis. There is no need to develop a theory to get the equations of the homogenized material law. Two numerical examples demonstrate the efficiency and the flexibility of this approach. The results obtained are compared to experimental values for a typical welding situation and a high-temperature response. This comparison seems to indicate that viscous effects in the materials have a significative influence on the residual stresses produced by welding.     

Numerical simulation model of vibration responses of rectangular plates embedded with piezoelectric actuators

A numerical simulation model for random large amplitude vibration control of composite plate using piezoelectric material is presented. The H_∞ control design is employed to suppress the large amplitude vibrations of composites plates under random loading. The numerical simulation model is developed and based on the finite element method. The finite element governing equation includes fully coupled structural and electrical nodal degrees of freedom, and consider the von Karman large amplitude vibration. The modal reduction method using the structural modes is adopted to reduce the finite element equations into a set of modal equations with fewer degrees of freedom. The modal equations are then employed for controller design and time domain simulation. In the simulations without control, the value of the linear mode to the nonlinear deflection is quantified; and the minimum number of linear modes needed for accurate model is obtained. In the simulations with control, it is shown that the truncated modes, which are neglected in the control design, deteriorate the controller performance. Generally, the vibration reduction level is not monotonically increasing with the size of the piezoelectric actuator. The optimal piezoelectric actuator size depends on the excitation level. For higher excitation level, optimal actuator size is larger. The H_∞ controller based on the linear finite element formulation gives better vibration reduction for small amplitude vibration, but it still gives reasonable performance for large amplitude vibration provided that the piezoelectric actuator is big and powerful enough.     

Thermal shock and thermal fatigue study of ceramic materials on a newly developed ascending thermal shock test equipment

A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples.     

有限元法应用于微波和光波器件的问题和发展

本文就有限元法应用于微波和光波器件的原理、问题和最新进展作了综合评述。文中讨论了适用于不同问题的变分公式的意义；着重讨论了避免和消除虚模的方法和处理开放边界的技术；还对代表该领域发展趋势的三个方面，即边棱元法。非线性结构及有限元程序的前后处理问题进行了评论和展望。     

A finite element parametric study on the mechanical properties of J-shaped steel hysteresis devices

The present paper discusses the mechanical properties of J-shaped steel hysteresis devices, abbreviated to “J-dampers”, interposed between the upper structure of a spatial structure and its substructure. The mechanical properties of a J-damper, which are characterized by its key geometrical and material parameters, are evaluated through parametric analyses using an in-house Finite Element program. The hysteresis behavior of a J-damper is also modeled by a bi-linear elasto-plastic hysteresis model with the strain-hardening effect. Its elastic stiffness, yield strength and dimensionless hardening ratio, which are expressed as functions of the parameters, are formulated on the basis of mathematical models and the results of parametric studies. Finally, some remarks relevant to the application of the proposed hysteresis model are made for the design of the J-damper.     

超声变幅杆解析法设计的可靠性研究

本文提出了用有限元法验证变幅杆解析法设计的可靠性，对三种类型的变幅杆先作解析法设计；然后利用有限元分析软件ANSYS进行模态分析，确定了变幅杆的固有频率，计算了固有频率与工作频率之间的相对误差，以此来评价解析法设计的可靠性；并分析了影响解析法设计可靠性的两个基本因素。     

Neutron Stress Measurement of W‐Fiber Reinforced Cu Composite

Stress measurement methods using neutron and X‐ray diffraction were examined by comparing the surface stresses with internal stresses in the continuous tungsten‐fiber reinforced copper‐matrix composite. Surface stresses were measured by X‐ray stress measurement with the sin²ψ method. Furthermore, the sin²ψ method and the most common triaxal measurement method using Hooke's equation were employed for internal stress measurement by neutron diffraction. On the other hand, microstress distributions developed by the difference in the thermal expansion coefficients between these two phases were calculated by FEM. The weighted average strains and stresses were compared with the experimental results. The FEM results agreed with the experimental results qualitatively and confirmed the importance of the triaxial stress analysis in the neutron stress measurement.     

桥丝式电火工品静电发火过程的数值模拟

根据灼热桥丝式电火工品的电热起爆机理，考虑桥丝的轴向和径向散热以及药剂自身的分解放热，建立了灼热桥丝式电火工品静电发火的数理模型。用有限差分法对该模型方程进行求解，得出了桥丝式电火工品的三维温度场分布，并对结果进行了分析。     

Analysis of crack tip plasticity for microstructurally small cracks using crystal plasticity theory

Crack tip plastic zone sizes and crack tip opening displacements (CTOD) for stationary microstructurally small cracks are calculated using the finite element method. To simulate the plastic deformation occurring at the crack tip, a two-dimensional small strain constitutive relationship from single crystal plasticity theory is implemented in the finite element code ANSYS as a user-defined plasticity subroutine. Small cracks are modeled in both single grains and multiple grains, and different crystallographic conditions are considered. The computed plastic zone sizes and CTOD are compared with those found using conventional isotropic plasticity theory, and significant differences are observed.