开裂是由于材料的强度或塑性不足,当拉应力超过临界值时便会发生。开裂发生的位置主要在:凸模端部、侧壁、凸模圆角部位、法兰部分和拉伸筋部分[29]。由于开裂的影响因素也很多,因此到目前为止还没有十分精确的判断开裂的准则。对于开裂的判断现在用的比较多的主要还是成形极限图[30]:包括FLCN(forming limit curves at neck)以及FLCF(forming limit curves at fracture);最大拉深率也是主要判据之一,Min Wan等人[31]结合了圆柱和圆锥杯形的内在联系,从理论上给出了圆锥杯形的极限拉深系数的判断公式。
在有限元模拟上,人们一方面在探索新的模型,另一方面将现有的判断准则与有限元模拟结合起来进行研究。K.Komori[32]提出了一种节点分离的模型:将服从开裂准则的单元体和与它邻近的单元体共有的节点分离开来,也就是假设开裂沿着单元体的边界扩展;而一般的有限元模型中常常将服从开裂准则的单元直接删除,在一定程度上改变了材料本来的特性,导致模拟结果误差相对较大。Z.H.Chen等人[33]应用了混合有限元法(Mixed finite element method),混合了位移和压力的有限元法,可以对板料成形中的不可压缩的塑性变形进行比较精确的模拟。而Ridha Hambli等人[34]用了一种反向技术的模型,可以识别开裂准则的临界值,从而将其应用于开裂模拟中,对研究开裂准则和开裂模拟都有很大的帮助。
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Present research on the numerical simulation of metal sheet forming
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