预估在非比例加载下薄金属板成型极限的损伤基力学模型

探讨了用损伤基力学模型研究应变路径对薄金属板塑性失稳的影响,这种力学模型考虑了材料损伤作用．基于这种模型,在等效应变空问建立了考虑损伤的塑性失稳判据,并用以预估在比例或非比例加载下薄金属板成型极限曲线(FLC)．借助这种理论模型和方法,预估薄金属板的理论成型极限曲线与Graf和Hosford的实验结果一致。     

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.      

考虑损伤和界面脱粘的复合材料加筋板稳定性试验与模拟研究

对复合材料帽型加筋板的压缩稳定性进行了试验研究,并在包含复合材料多种损伤模式和胶层损伤的条件下,对加筋板失稳及损伤演化特征开展了有限元分析.试验与模拟结果表明:在加筋壁板失稳前,基体、纤维、加强筋和壁板之间的胶层均未产生明显损伤,加筋板的失稳会导致快速的损伤演化;壁板近表面损伤较内部严重,胶层界面脱粘降低了壁板的极限承载能力;考虑损伤的模拟结果与试验相符性好,证明了该方法的可行性和有效性.     

板材成型中的分叉和断裂及其空洞扩展效应

用平面应力有限元方法分析空洞模型以模拟一种双相钢板材在成型过程中所遇到的微空洞损伤,经试算可使模型的总体和局部的响应与已有的实验相一致,由此可提供描述该材料的损伤本构参数并研究局部剪切带和扩散型颈缩等分叉现象,临界应变值的分布形成了成型极限图中的下限曲线,当空洞模型的总体应力急剧下降或微裂纹开始出现,其相应的总体应变值提供了上限曲线。     

全局性能水准与构件局部损伤状态

传统的最大层间位移作为一个全局性的性能指标,超出给定的性能极限状态阈值就不能映射构件局部损伤状态。本文基于能量平衡原理,将塑性能量耗散和塑性铰的等效能相比得到累积塑性应变,将累积塑性应变这一反映局部损伤状态的损伤指标作为工程需求参数,假定一系列累积塑性应变阈值建立局部构件的易损性曲线族。以传统的最大层间位移作为反映全局性能水准的工程需求参数,建立结构全局性能极限状态下的易损性曲线。对比全局和局部易损性曲线,发现全局性能水准和局部构件损伤状态之间存在良好的相关性,通过易损性曲线的吻合程度可获得给定全局性能水准相对应的累积塑性应变阈值。进而也建立起全局性能指标与局部损伤状态的映射关系。基于累积塑性应变的机构易损性分析是一种更精确的易损性分析方法。     

层状基岩上坝体抗滑稳定性的塑性理论极限平衡法

关于坝体抗滑稳定性计算,现在通用的极限平衡法指的是双斜滑动法.它把滑动体和阻滑体看作刚体来分析.对于坝体沿倾向下游的夹泥层产生深层滑动的情形,石膏模型试验表明失稳前坝趾处出现塑性区.本文用塑性力学原理分析失稳前的塑性区,得出了夹泥层上均质岩基及层状岩基安全系数与极限抗力的计算公式.文末给出大坝典型剖面抗滑稳定安全系数计算实例及与刚体分析法的比较.     

30CrMnSiA钢板表面塑性损伤的激光检测

本文根据激光衍射谱谱面光强分布随塑性损伤的变化规律，用３０ＣｒＭｎＳｉＡ材料进行了实测，给出了应变与损伤变量Ｄ（Ｋ）的关系曲线，并对加载和卸载条件下的Ｄ（Ｋ）～ε曲线进行了比较。结果再次表明，用衍射光强比来定义损伤变量是可行的；该方法检测方便，计算简单，具有较高的精度，可望成为塑性损伤检测的一种有效方法；３０ＣｒＭｎＳｉＡ材料沿垂直于轧制方向拉伸的力学性能及损伤状态优于平行轧制方向，且用空穴面积百分数Ｆ＿Ａ得到验证；弹性变形对塑性损伤的影响可以忽略不计。     

用损伤理论方法预测铝合金薄板成型极限

应用各向异性损伤理论研究2024-T3铝合金薄板的成形极限,通过构造有限元单胞模型预测薄板结构的极限应变.单胞模型由两相材料组成:铝合金基体和金属强化物.基体采用全耦合弹塑性-损伤本构方程描述,而金属强化物则视为弹脆性材料.采用所提出的缩颈准则,得到了双轴拉伸状态下铝合金薄板的极限应变,和实验结果比较两者吻合较好.研究结果揭示有限元单胞模型可以提供铝合金的细观损伤机理信息,当忽略材料的损伤影响,采用金属薄板成型理论的研究结果将过高估计薄板的极限应变.     

纵向磁场对聚能射流极限拉伸系数的影响

在分析纵向磁场能够增强聚能射流稳定性的基础上,根据聚能射流的运动方程以及聚能射流的塑性失稳条件,推导得到了聚能射流在纵向磁场中的极限拉伸系数计算公式,并计算了有、无磁场情况下极限拉伸系数的比值。通过两种炸高下的实验研究对理论模型进行了验证。结果表明:由于磁场的存在而引起的电磁力抑制了聚能射流颈缩的发展,进而延长了射流成型的惯性拉伸阶段,最终使聚能射流在磁场中的极限拉伸系数在一定程度上得到了增加;理论和实验所得结果吻合较好。运用所建立模型可以较准确地反映磁场对聚能射流极限拉伸系数的影响。     

基于突变理论深埋硬岩隧道的失稳分析

在分析隧道开挖引起围岩扰动和损伤的基础上,运用突变理论探讨了深埋硬岩隧道失稳的机理;基于势能原理,建立了深埋硬岩隧道失稳的尖点突变模型,导出了失稳的力学判据条件。结果表明:深埋硬岩隧道的失稳不仅受岩体特性及所受载荷的影响,而且与围岩中弹性区刚度和塑性区刚度的比值有关;开挖过程中,围岩受扰动和损伤程度越大,岩体完整性系数越小,力学参数指标弱化程度越高,塑性区范围越大,刚度比越小,围岩系统发生失稳的可能性就越大。工程实例分析表明,分析结果与实际经验基本一致;研究成果可为预防隧道失稳、采取合理的施工工艺和支护措施提供参考依据。     

Stress wave experiments in plasticity

Plastic wave experiments are reviewed, beginning with the earliest experiments of Bell on the propagation of incremental waves in prestressed bars. Attention is directed to experiments in which the plastic wave profile at different distances of propagation is used to infer information on the dynamic plastic response of the material in which the wave is propagating. Plastic waves in bars, tubes, and plates are considered. Principal results are reviewed on such primary physical features as the velocity of propagation of incremental waves, the dynamic elastic limit, the wave profiles of finite amplitude waves, and the effects of nonproportional loading. Objectives for future research are suggested.     

金属成形过程中的塑性失稳

本文评述了金属成形过程中塑性失稳现象(剪切带、颈缩、皱曲)研究的最新进展和仍存在的问题,着重讨论了塑性失稳预报和本构关系选择的密切关系,指出了塑性失稳预报结果取决于所选择的本构关系。还讨论了材料各向异性、应变率敏感性、不均匀性及其幅度、惯性等因素对塑性失稳的影响。除此之外,对作为分析塑性失稳的基本理论工具——Hill分叉准则亦作了简要讨论。      

Suppression of necking in incremental sheet forming

Incremental sheet forming enables sheet metal to deform above a conventional strain-based forming limit. The mechanics reason has not been clearly explained yet. In this work, the stress-based forming limit was utilized for through-thickness necking analysis to explain this uncovered question. Stress-based forming limit which has path-independency shows that the stress states in top, middle and bottom surfaces did not exceed the forming limit curve at the same time and each layer has different stress state in terms of their deformation history to suppress necking. It has been found that it is important to consider the gradient stress profile following the deformation history for the proper forming limit analysis of incremental sheet forming.     

金属塑性变形极限判据

研究出金属材料塑性变形极限判据，它为断裂力学和失效分析提供理论依据。利用这判据，裂纹尖端的临界应变和相应的临界应力可以确定。在板料成形中，拉应力下颈缩失稳的临界应变值可以定出。节省大量试验工作。     

Crystal plasticity-based forming limit prediction for non-cubic metals: Application to Mg alloy AZ31B

A viscoplastic crystal plasticity model is incorporated within the Marciniak–Kuczynski (M–K) approach for forming limit curve prediction. The approach allows for the incorporation of crystallographic texture-induced anisotropy and the evolution of the same. The effects of mechanical twinning on the plastic response and texture evolution are also incorporated. Grain-level constitutive parameters describing the temperature dependent behavior of hexagonal close packed Mg alloy, AZ31B, sheets at discrete temperatures are used as a first application of the model. A trade-off between significant strain hardening behavior at lower temperatures (∼150 °C), and significant strain rate hardening at higher temperatures (∼200 °C) lead to similarities in the predicted forming limits. The actual formability of this alloy depends strongly on temperature within this range, and this distinction with the current modeling is related to more localized instability-based failure mechanisms at the lower temperatures than is assumed in the M–K approach. It is shown that the strain path dependence in the strain hardening response is significant and that it influences the forming limits in a predictable way. For broader applicability, a means of incorporating dynamic recrystallization into the crystal plasticity model is required.     

Analytical and numerical analysis of sheet metal instability using a stress based criterion

Strain based Keeler–Goodwin diagrams are widely used in forming processes to predict onset of local necking. Plastic instability is determined once the forming limit strain is exceeded. Use of these diagrams requires proportional strain paths, which is not necessarily the case in sheet metal forming operations. In many forming processes, the strain path changes during deformation. This may change the forming limit curve significantly. In the paper, a stress based forming limit criterion is adopted to deal with strain path non-linearities. Comparisons with earlier published work on forming limits are made through analytical considerations. Furthermore, the criterion is implemented into the finite element code LS-DYNA and verified numerically against results from large scale bulge tests.     

非关联流动的安定性理论及在土力学中的应用

将Melan静力型安定定理加以推广，使其适用于非关联流动法则下的土工结构的安定性分析．应用Palmer的方法构造塑性势面，根据其性质，证明了静力型安定定理在非关联流动法则下同样成立．在非关联的Mok—Coulomb准则下，求解浅黏土层地基极限载荷，得到了比较合理的解答．最后，计算了非关联流动法则下的条形基础地基的安定裁荷．     

Comparison of bifurcation and imperfection analyses of localized necking in rate-independent polycrystalline sheets

Plastic flow localization into a narrow band is investigated for a rate-independent polycrystalline sheet. Strain localization is considered to initiate at a point of bifurcation from the homogeneous deformation state or to result from the growth of a preexisting imperfection. Bifurcation analysis based on the rate-independent crystal plasticity model is found to give limit strains at a realistic strain level in the biaxial-stretching range. Limit strains predicted by the imperfection analysis are lower than those for bifurcation analysis and tend to approach those strain levels with decreasing amount of initial imperfection. Then, the bifurcation analysis is applied to the simulation of the forming limit for strongly textured sheets. The bifurcation analysis reveals the same trend of a texture dependence of limit strains as that for the imperfection analysis employing a rate-dependent model reported in the literature.