Thermomechanical design in the automotive industry

In this paper thermomechanical fatigue assessment in the automotive industry is discussed. The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.     

Vibration Analysis of Tb-Dy-Fe Magnetostriction Actuator and Transducer

In this paper, theoretical analysis of longitudinal free vibration was carried out for Tb-Dy-Fe series magnetostrictive actuator and transducer. The formulations considered two constitutive laws; in one we employ the standard square nonlinear constitutive equation of magnetostriction and in the other we employ the linear piezomagnetic equation. The results obtained from the nonlinear equation can be reduced to the linear piezomagnetic equation when the amplitude of the excitation magnetic field provided by the coil is very small compared to the bias magnetic field and its frequency does not induce resonance of the system. For the case of a relatively large excitation magnetic field, which usually exists for an actuator, the nonlinear constitutive equation should be adopted in order to provide an accurate prediction for the design and analysis of actuator and transducer. Another important aspect is the resonance of the Tb-Dy-Fe series transducers that was revealed in the analysis using the nonlinear constitutive equation. The resonance not only appears at the natural frequencies of the system, but also arises when the frequency of excitation current in the coil happens to be half of one of the natural frequencies of the system. This conclusion cannot be reached using the linear piezomagnetic formulation.     

锈蚀钢筋与混凝土间黏结性能试验

为研究锈蚀对钢筋与混凝土间黏结性能的影响,制作20个中心拉拔试件.通过电化学加速锈蚀得到理论锈蚀率为0%、0.5%、1%、2%、5%的拉拔试件,完成了不同锈蚀率下钢筋与混凝土的拉拔试验.从钢筋几何外形特征与混凝土强度在锈蚀过程中变化的角度分析了锈蚀率对极限黏结强度的影响,以割线刚度的方法定量研究了锈蚀率对黏结刚度的影响.阐述了锈胀裂纹、黏结刚度及试件破坏模式随锈蚀率变化的规律.试验结果表明:随着锈蚀率增大,钢筋与混凝土间的黏结强度、黏结刚度呈现先增大后减小趋势.黏结刚度的退化与锈胀裂纹的出现具有一定相关性.通过整理国内外学者黏结强度退化数据,得到了两段式黏结强度退化试验模型及一定程度上可供工程参考的黏结强度退化保守模型.最后,基于两段式黏结-滑移模型得到了不同锈蚀率下钢筋与混凝土间黏结-滑移本构模型.     

钢材的低周疲劳特性及双曲面本构模型的改进

为合理评估钢结构的地震损伤,研究结构钢的塑性变形与低周疲劳之间的耦合关系以及循环软化特性.通过对Q345qC钢材进行高应变反复加载试验,获得了相应的低周疲劳破坏和循环软化特性.通过研究承载力下降与塑性耗能密度之间的关系及对边界面进行缩放和移动,将循环软化特征引入材料的本构关系模型.在此基础上,采用改进后的本构关系模型对钢桥墩进行了反复荷载作用下的力学行为分析,讨论了循环软化对构件承载能力的影响.结果表明:Q345钢材具有较好的延性和较强的抗低周疲劳性能,试验未发现钢材塑性变形与低周疲劳之间存在明显的相关性;循环软化特征在较大的塑性应变状态下表现得更明显,修正后的双曲面本构关系模型可以较好地模拟钢材循环软化行为;考虑材料循环软化效应后,钢桥墩在反复荷载或地震作用下的承载力略有降低.     

From experimental data,the mechanics relationships describing the behaviour of four different low alloyed steel powders during uniaxial cold compaction

This work aims at determining the constitutive model of four commercial water atomised low alloyed steel powders during cold compaction. Single-action experiments were performed, obtaining cylindrical specimens with different H/D ratios. The distribution of axial and radial stresses was investigated, and the relationships describing both the radial stress transmission coefficient and the flow stress as functions of the relative density were determined. The radial stress transmission coefficient also confirmed the hypothesised value of Poisson’s coefficient. The friction coefficient between the powder column and the die wall was determined, also highlighting the influence of the H/D ratio. Measuring the axial and radial strains due to spring-back, the axial and radial elastic moduli were determined, as functions of the relative density. The results obtained for the four materials were compared, also highlighting both differences and similarities.     

饱和土体固结压缩和蠕变的热力学本构理论及模型分析

岩土热力学模型（thermodynamic soil model,TSM）是基于颗粒固体的非平衡态热力学理论,建立的一种崭新的描述岩土力学问题的统一理论模型。该模型引入“颗粒熵运动”和“弹性弛豫”,对土体颗粒层次的耗散机制进行了合理地考虑,这些使得模型能够更深入描述土体的变形和能量耗散机理,从而能够在统一理论框架中描述岩土体复杂多变的物理力学行为。基于该理论模型,研究了饱和土体的固结压缩和蠕变问题,分析了加载速率、应力/应变路径和非单调荷载等因素的影响规律。模拟结果表明：模型具有描述复杂条件下的饱和土体的固结压缩和蠕变特性的能力,具有较高的理论和工程应用价值。     

A constitutive model for saturated compacted soils under undrained shear condition

Abstract

In this paper, elasto‐plastic constitutive equations extended from theory of plasticity and based on laws of thermo‐dynamics were derived to predict the mechanical behavior of saturated geological media. Besides, these equations can also be adapted to form the stiffness matrix for finite element analysis on deformation and stress distribution in geotechnical engineering practice. The test data were obtained from compacted crushed mudstone under consolidated un‐drained triaxial tests with pore water pressure measurement. The preparation of the specimen was different from those which had usually been adopted. Hence, it is hoped that this study will throw some light on the constitutive equations applied on compacted geological media.     

Constitutive model and failure criterion for orthotropic ceramic matrix composites under macroscopic plane stress

To predict the nonlinear stress-strain behavior and the rupture strength of orthotropic ceramic matrix composites (CMCs) under macroscopic plane stress, a concise damage-based mechanical theory including a new constitutive model and two kinds of failure criteria was developed in the framework of continuum damage mechanics (CDM). The damage constitutive model was established using strain partitioning and damage decoupling methods. Meanwhile, the failure criteria were formulated in terms of damage energy release rate (DERR) in order to correlate the failure property of CMCs with damage driving forces, and the maximum DERR criterion and the interactive DERR criterion were suggested simultaneously. For the sake of model evaluation, the theory was applied to a typical CMC with damageable and nonlinear behavior, that is, 2D-C/SiC. The damage evolution law, strain response and rupture strength under incremental cyclic tension along both on-axis and off-axis directions were completely investigated. Comparison between theoretical predictions and experimental data illustrates that the newly developed mechanical theory is potential to give reasonable and accurate results of both stress-strain response and failure property for orthotropic CMCs.     

Experimental investigation and numerical simulation analysis of sintered micro-fluidic devices

ABSTRACT

This study investigates the use of numerical simulations to describe the solid-state diffusion of a sintering stage during a metal injection moulding process for micro-fluidic components with 316L stainless steel powders. Finite element (FE) analysis based on a thermo-elasto-viscoplastic model was conducted to describe the densification process of a stainless steel porous component during solid-state sintering. The numerical analyses, which were performed on a 3D micro-structured component with various powder volume loadings to take into account the thermal debinding effect to propose a full debinding sintering simulation, demonstrated that the FE simulation results are in agreement with the experimental ones.