【作者】 黄婧；

【导师】 沈成武；

【作者基本信息】 武汉理工大学 ， 船舶与海洋结构物设计制造， 2006， 博士

【摘要】 薄板成形是众多金属加工工艺中一类非常重要的加工方式。近年来，随着越来越多新型材料的投入应用，对这些材料的成形性能进行详细研究具有重要意义。本文以此为背景，利用实验研究和理论建模分析相结合的方法，对板料成形极限及其损伤过程进行了研究。建立了各向异性塑性的损伤模型，对损伤和极限应变分别进行了预测，并和实验结果作了比较分析。首先，介绍了经典塑性理论，和几种常用的各向异性塑性屈服函数。并建立了各向异性的塑性本构模型。并将各向异性塑性理论和各向同性流动理论进行了比较。其次，对连续损伤力学的基本概念和理论进行了综合阐述。同时对Lemaitre损伤模型和基于热力学理论框架的一般损伤理论这两者进行了公式推导和综合的比较与分析。论文的实验部分，先阐述了成形极限图的实验测定原理和方法，对实验中关键的环节——应变测量进行详细介绍，包括网格测量分析方法及网格印制技术。然后以板材密度的变化作为损伤变量，针对近年来工业生产中常用的新型冲压材料IF钢板，开展了损伤实验研究。根据实验结果，得出了单向拉伸和胀形两种成形条件下的密度损伤值与厚向应变之间的关系，对IF钢板料成形机理从宏观尺度进行了一定的讨论。同时，借助金相仪分析，以及宏细观相结合的方法，就金属晶粒的尺寸变化对损伤过程机理做了定性分析。分析表明，不同应力状态下晶粒的尺寸变化规律不一样。相比较来说，平面应变情况下板材的塑性变形能力较差。从纵、横向晶粒长短轴比的变化情况来看，等双拉的应力状态下，其变化幅度明显超过其他两种方式，尤其是单向拉伸的情况。此外，在热力学基本原理的基础上，推导了一般各向异性损伤情况下的塑性流动关系与损伤演化方程；并简化后得到含损伤的各向同性塑性本构方程的具体形式。将Lemaitre各向同性损伤模型应用于本文的试验材料——薄板冲压中常用的某IF和03A1两种型号钢板，对预测的成形极限图和试验值进行了比较，结果两者相差较大。本文采用Lemaitre的损伤耗散势形式框架，认为损伤是各向同性的，同时考虑材料是各向异性塑性的情况下，提出了含损伤的塑性模型用于薄板成形加工过程的损伤和极限应变分析。在假设比例加载情况下，演算得到损伤的演化方程，并通过数值计算得到三种简单应力状态下损伤变量随累积塑性应变变化曲线。将该曲线同试验值进行了比较分析，结果表明，理论预测曲线和试验值趋势均一致，而在单向拉伸情况下两者相当吻合。根据损伤判则和含损伤的塑性模型，考虑比例加载的情况，得到了板材成形的极限应变关系式。应用于两种典型钢板的成形极限预测，计算得到两种板材的FLD。并将理论预测结果和试验值进行了比较分析，结果表明，在FLD的左半区域，两者吻合较好；在FLD右半区域的等双拉应力状态部分，两者有较大差异。而对于板材成形性能的重要指标——平面应变点，理论预测值和实验值相当接近。最后，通过数值计算讨论分析了面内应变比、材料各向异性系数对应力状态函数的影响，以及材料参数对应力三轴因子和成形极限曲线的影响。理论分析和实验验证的研究结果说明，本文建立的各向异性塑性的损伤模型是有效的。更多还原

【Abstract】 Sheet metal forming processes are among the most important metal-working operations. In recent years, more and more advanced materials have been used in metal industry, so it is indispensable to study their formability thoroughly. In this dissertation, theoretical method was applied to analyzed sheet metal forming limit and its damage process, as well as testing investigation. The damage-coupled anisotropic plasticity models were presented, to estimate the damage and limit strains of sheet metal during stamping process. A comparision of theoretical results with testing measures was carried out.First of all, classic theory of plasticity was summarized, along with several anisotrpical yielding functions. A general anisotropic constitutive model was founded. Moreover, comparision of anisotropic plasticity with isotropic flow theory was carried out. Then, the foundational concepts and theory of Continuum Damage Mechanics were introduced, including the formulas deduction and comprehensiveanalysis on two kinds of theories--Lemaitre damage model and a general damagetheory in the framework of continuum thermodynamics.As to the experiment research in this dissertation, the principle and methods of measure of FLD (or FLDs, short for Forming Limit Diagrams) were introduced,especially for the key part -- strain measurement, including analysis method and silk screening technique. Then by regarding the change of sheet metal’s density as the damage variable, testing study of damage was carried out, on one kind of advanced sheet metal -IF steel which has been broadly used in forming industry production recently. According to the results obtained in the experiments in two cases of forming path - uniaxial tension and equivalent bi-axial tension, the forming mechanism of IF sheet metal has been discussed.Furthermore, basing on the thermodynamics fundamentals, anisotropic plasticity flow theory and damage evolution functions were deduced, together with a simplified damage-coupled isotropic plastic constitutive model. For the experimental materials in this study - some IF and 03A1 sheet metal, Lemaitre and Chaboche isotropic damage model was applied to predict their forming limits. The result has shown a great difference of the FLD predicted from those obtained by experiment. ’For the case of isotropic damage assumed in this dissertation, and by using the damage dissipation potential function presented by Lemaitre, a damage coupled anisotropic plasticity model was founded. Then for giving proportional loading condition, damage evolutional functions were obtained, as well as the curves of the damage variable changing with acumulation plastic strains by numerical analysis. Comparision of the curves predicted with those tested shows quite accordance in tendency, especially for the case of uniaxial tension.Assuming the propotional loading condition, with the damage coupled anisotropic plasticity model , the limit strains expressions of sheet metal were obtained. Then the limit strain curves of two types of sheet metal were calculated. Comparision of the FLD predicted with forming limit experiments measured has been carried out, which shows a good accordence in the left half region of FLD.At last, through numerical calculation the influence of plane strain ratio, material anisotropic parameters on stress function was discussed, as well as that of material parameters on stress triaxial factor and forming limit curves.更多还原