高强度钢板热成形数值模拟-静力显式

在已建立的高强度钢热成形热、力、相变耦合本构方程的基础上,基于虚功率方程及持续平衡方程建立了热成形静力显式多场耦合有限元列武。将热成形过程中的相变潜热引入温度场,并进行了有限元分析;在自主开发的商业化金属成形CAE软件KMAS(King-Mesh Analysis System)基础上,开发了考虑多场耦合的非线性、大变...     

高强度钢板热成形技术及力学问题研究进展

高强度钢板热成形技术是将传统的热处理技术(淬火)与冷冲压技术相结合的最新制造工艺,可以成形强度超过1 600 MPa的超高强度钢板.本文首先介绍了热成形技术的原理、一次及多次成形工艺和热成形过程中的核心制造技术;随后分析了热成形过程中材料的微观组织及力学性能变化,包括成形前、成形中及成形后材料性能的演变,介绍了与温度、马氏体转变速率相关的高温奥氏体材料模型,指出了其在热成形过程中的应用方法;研究了高强度钢板热成形材料的成形性能:分析了硬化指数n值对材料高温成形性的影响,并给出了最佳n$值温度区间;介绍了材料初始轧制各向异性对材料性能的影响,实验结果说明在热成形温度下晶格的各向异性将消失.对热成形过程中材料的本构关系进行了介绍:重点介绍了热、力及相变耦合本构模型;对热成形过程中的多相材料热力学参数和力学性能进行等效分析的混合定律进行了说明;介绍了相变体积应力及相变塑性应力等新概念;随后对热成形数值模拟技术进行了介绍:重点介绍了自主开发的KMAS(King-Meshanalysis sytem)热成形软件的动力显示、静力显示算法及温度场模拟技术;说明了将材料的高温性能引入接触与摩擦模型的接触控制参数的概念;随后基于车身碰撞力学的分析,介绍了热成形钢板在车身中应用的设计方法.最后对热成形技术今后的研究作出了展望.      

基于连续体损伤理论的硼钢高温成形极限确定法

为了应对节能减排及汽车安全性的双重要求,先进高强钢的开发和应用受到愈来愈高的重视. 超高强度钢板热冲压成形技术是减轻车身重量、提高汽车抗冲击和防撞性能的重要途径,目前已经成为世界汽车制造行业的热门技术. 成形极限是硼钢热冲压成形过程中一个非常重要的表征参数,但由于高温半球形凸模胀形试验(Nakazima) 的复杂性造成实验测试面临诸多困难. 针对硼钢热冲压成形过程特点,提出将韧性损伤模型与有限元数值模拟技术结合起来预测硼钢高温成形极限的方法. 在连续体损伤力学基础上,建立了考虑有效应力和等效塑性应变对微孔损伤影响的勒迈特(Lemaitre) 韧性损伤演化方程,在损伤耗散势函数中引入了等效塑性应变因子,从而寻找到能够准确反映硼钢在奥氏体状态下的成形特性的勒迈特耗散势函数. 选取基于遗传算法的基于非支配排序的多目标优化算法(NSGA-II) 与有限元分析软件“FORGETM” 相结合的方法优化韧性损伤模型中材料的损伤因子,并利用优化后的损伤因子准确预测到板料成形过程的拉伸失稳和断裂现象. 将建立的韧性损伤模型引入到硼钢高温半球形凸模胀形有限元仿真模型中,通过预测的成形极限与实验测定结果的比较,验证了所提韧性损伤模型预测硼钢高温成形极限的可靠性.      

聚合物注塑残余应力研究进展

综述了聚合物注塑残余应力的研究进展,详细介绍计算残余应力所用的理论模型及其基本概念,注塑制品残余应力由两部分组成,并且有不同的测试方法,这些应力是造成制品曲变形的原因,对于残余流动应力,重点分析本构模型的发展和注塑控制方程的建立;对于残余热应力,则讨论了冷却过程中的相变问题,以及热粘弹性模型在应力分析中的应用;最后介绍测量残余应力的几种实验方法。     

形状记忆合金热力学行为的模拟

基于塑性流动法则和马氏体相变动力学 ,引入马氏体体积分数和相变应力间的关系 ,对形状记忆合金的热力学行为进行了模拟 ,算例表明本文提出的形状记忆合金本构模型与实验结果比较吻合 ,且实施起来简单易行 ;最后还用该本构模型进行了有限元分析     

单轴拉伸下液晶高弹体力学行为的热力序耦合特性

本文利用液晶高弹体的应力－应变本构方程和力－序耦合方程,研究了沿指向矢单轴拉伸下液晶高弹体力学行为的热力序耦合特性。由于力－序耦合,有序度在拉伸作用下变大,从而影响了液晶高弹体的应力－应变关系,使得应力在相同伸长下变小。不同温度下应力受到有序度变化影响的程度不同.由于有序度随温度升高而减小,当工程应力不变时,伸长随着温度升高而减小;当伸长不变时,应力随着温度升高而增大;伸长随温度的变化关系和应力随温度的变化关系都呈非线性。     

汽车玻璃高温蠕变特性的实验与理论研究

通过对汽车用挡风玻璃进行不同温度、不同应力水平下的单轴拉伸蠕变实验,揭示了其非线性蠕变变形特性.实验发现,不同温度和载荷下的蠕变曲线形状类似,高温(550℃～590℃)下该材料的减速蠕变变形不很明显,主要表现为稳态和加速蠕变变形,且其断裂数据符合Monkman-Grant关系,应力和温度对其影响不显著.基于实验结果,本文建立了非线性高温蠕变本构方程.理论预测与实验数据进行了比较,结果表明所提出的本构方程能较好地描述汽车玻璃高温蠕变变形的全过程.     

高温下混凝土化学塑性-损伤耦合本构模拟及破坏分析

提出了一个用于模拟高温下混凝土中化学-热-湿-力学耦合行为的化学塑性-损伤耦合本构模型.发展了用于积分率形式的耦合本构方程的三步算子分裂算法.导出了化学-热-湿-力学(CTHM)耦合本构模型的一致性切线模量矩阵,以保证对于全局耦合控制方程Newton迭代过程的二阶收敛率.算例数值结果显示了本文发展的化学塑性-损伤耦合本构模型在重现火灾和热辐射条件下的混凝土中化学-热-湿-力学耦合行为的有效性.     

用于板料成形反向模拟的特殊应力更新算法

提出了一种改进的反向模拟法,以最终构型为研究对象,采用Euler坐标系,基于虚功原理获得有限元列式. 改进的反向模拟法采用了一种基于塑性流动理论的本构方程,可以充分考虑应变历史对塑性变形的影响. 为了避免流动理论应力更新算法过程中关于未知量\Delta\lambda 的非线性方程的求解,引入等效应力思想,无需Newton-Raphson迭代直接计算未知量\Delta \lambda . 盒形件的拉深实例中,传统的基于塑性形变本构方程的反向模拟法和改进的基于塑性流动本构方程的反向模拟法计算结果,分别与基于增量有限元法的正向数值模拟求解器LS-DYNA计算结果进行对比. 通过获得的坯料轮廓、成形极限图、等效应变分布、计算效率等的比较,验证了所提出的基于塑性流动理论本构模型的应力更新算法的有效性.      

有限长厚壁管在过冷沸腾状态下的热应力响应

在求得有限长厚壁管水淬时瞬态温度分布的基础上［１］，引入了包含相变的热弹塑性本构方程的增量形式．用有限元法求得了瞬态热应力和残余应力．对影响热应力和残余应力的各种因素进行了分析和讨论．     

Plasticity and fracture of martensitic boron steel under plane stress conditions

Two series of multiaxial experiments are performed to characterize the mechanical behavior of a hot formed martensitic 22MnB5 boron steel. In the first series, flat specimens of uniform cross-section are subjected to various combinations of tensile and shear loading to characterize the elasto-plastic response. Butterfly-shaped specimens of non-uniform cross-section are used for the second series to study the onset of fracture in the martensitic steel. It is found from the analysis of the experimental results that the planar isotropic Hill’48 yield function along with an associated flow rule provides good estimates of the stress–strain response over a wide range of loading paths. The fracture experiments demonstrate that the crack initiation depends strongly on the loading state. A simple stress triaxiality dependent phenomenological fracture model is calibrated to describe the onset of fracture. Using the proposed plasticity and fracture model, numerical simulations of the fracture of tensile specimens of different notch radii are performed and compared with experiments.     

Artificial neural network prediction to the hot compressive deformation behavior of Al–Cu–Mg–Ag heat-resistant aluminum alloy

The behavior of the flow stress of Al-Cu-Mg-Ag heat-resistant aluminum alloys during hot compression deformation was studied by thermal simulation test. The temperature and the strain rate during hot compression were 340-500 °C, 0.001 s⁻¹ to 10 s⁻¹, respectively. Constitutive equations and an artificial neural network (ANN) model were developed for the analysis and simulation of the flow behavior of the Al-Cu-Mg-Ag alloys. The inputs of the model are temperature, strain rate and strain. The output of the model is the flow stress. Comparison between constitutive equations and ANN results shows that ANN model has a better prediction power than the constitutive equations.     

Evaluation of constitutive models for voided solids

In this work four constitutive models for voided solids are reviewed and evaluated; namely, Gurson's, Green's, Liao's et al. and Shima-Oyane's models. Two modified forms of Gurson's model and a non-quadratic form of Green's model are developed for normal anisotropy prevailing in the analysis of sheet metal forming. A simplified version — more amenable to analytical derivations — of Gurson's yield function and its modified forms are proposed for plane stress conditions. The associated flow rules are presented and the laws governing void growth with accumulated strain are derived using the above models. Their predictions are compared with experiments. As an application, the flow curves of voided materials of known initial porosities are predicted and compared with experiments.     

内时理论在板料成形失稳中的应用

应用Valanis提出的内时本构方程，研究了板料成形的拉伸失稳问题，推导出单向和双向拉伸应力状态下的内时本构方程，据此分析了分散性失稳和集中性失稳。该文推导出应用于拉伸失稳分析时内时理论的近似表达式，它对应于经典塑性理论解，同时给出了内时理论的完整迭代数值解。结果表明内时理论具有很好的适用性。     

钨合金长杆弹侵彻陶瓷层合板的数值模拟

用自编计算程序给出了长杆弹侵彻陶瓷层合板的数值模拟。其中层合板的面板和背板均为装甲钢,中间为3层氧化铝陶瓷。装甲钢和长杆弹材料的本构关系采用Johnson-Cook模型,将Mohr-Coulomb盖帽模型与Bodner-Partom模型相结合来描述氧化铝陶瓷的失效和流动过程。为了提高有限元的计算精度,使用了一致质量矩阵迭代法,破坏网格的处理采用了网格侵蚀法。数值模拟的最终穿深计算结果与实验一致。     

A viscoplastic constitutive model with effect of aging

It is recognized experimentally that differences in plastic flow stress due to the change in strain rate of SUS304 stainless steel are found to decrease after cyclic preloadings, but minimal change is observed in relaxation properties. Therefore, viscosity function based on tensile stress–strain properties differs from that obtained from relaxation behavior. For such case, the existing visco-plastic constitutive concept, such as the so-called overstress model where only viscosity is taken into consideration, has a poor capability in predicting the time-dependent mechanical properties systematically. A new viscoplastic constitutive concept is presented to analyze the phenomenon. In the constitutive concept, the dynamic strain aging even at room temperature, as well as viscosity, are introduced as the dominant factors of the time-dependent plastic deformation. An experimental technique is proposed and some experimental results are presented to estimate the effects of aging and viscosity separately on the time-dependency of a SCM435 low alloyed steel under tensile loading. The proposed constitutive model with aging is verified for the systematical predictions of both plastic flow properties and relaxation behavior of the SCM435 low alloyed steel.     

A plasticity and anisotropic damage model for plain concrete

A plastic-damage constitutive model for plain concrete is developed in this work. Anisotropic damage with a plasticity yield criterion and a damage criterion are introduced to be able to adequately describe the plastic and damage behavior of concrete. Moreover, in order to account for different effects under tensile and compressive loadings, two damage criteria are used: one for compression and a second for tension such that the total stress is decomposed into tensile and compressive components. Stiffness recovery caused by crack opening/closing is also incorporated. The strain equivalence hypothesis is used in deriving the constitutive equations such that the strains in the effective (undamaged) and damaged configurations are set equal. This leads to a decoupled algorithm for the effective stress computation and the damage evolution. It is also shown that the proposed constitutive relations comply with the laws of thermodynamics. A detailed numerical algorithm is coded using the user subroutine UMAT and then implemented in the advanced finite element program ABAQUS. The numerical simulations are shown for uniaxial and biaxial tension and compression. The results show very good correlation with the experimental data.