考虑冲剪诱发初始损伤的金属板料成形边缘断裂预测研究

The sheared edge is prone to fracture much earlier than the parent material because the shearing process introduces pre-damage into its shear affected zone. Hence edge fracture cannot be accurately predicted by the conventional fracture model without considering the pre-damage from shearing process. The present work will take the aluminum alloy sheet as an example and introduce its pre-damage from previous shearing into the subsequent forming process to predict its edge fracture. Firstly, macro fracture modes of different stress states will be related their micro fracture mechanism and therefore the variables which can describe the evolution modes of micro-defects will be determined to construct the fracture criterion with fracture strain. The experimental shearing processes will be conducted and parallel numerical models will also be established using the proposed fracture criterion. The dimensions of macro-defects as well as damage distribution in shear affected zone can be extracted from simulative results. Besides, the micro-damage featured by micro-defects in fracture band will be estimated by the damage indicator. As a result, the three forms of pre-damage including dimensional defects, micro-damage and macro-damage will be quantitated in a macroscopic way. Then, the pre-damage will be inherited from the shearing process into the subsequent forming process and therefore the fracture model will be updated by considering the pre-damage of edges. The new fracture model will be adopted to predict the edge fracture of typical hole expansion and stretchflanging processes and then to sum up the effect of pre-damage on edge fracture. An improved fracture model will be proposed to enrich the theories for edge fracture prediction in this research and will be utilized to provide a theoretical guidance for avoiding fracture of forming process as well as optimizing fracture profiles of shearing process.

冲剪工序诱发的金属板料边缘损伤是导致后续成形出现边缘断裂的重要因素之一，而常规不考虑初始损伤的断裂模型在预测边缘断裂时存在局限性。项目提出建立考虑冲剪初始损伤的断裂模型以提高边缘断裂的预测精度。以铝合金板为研究对象，研究不同应力状态的微观断裂形貌及宏观断裂行为并建立反映微观断裂机理的唯象型断裂准则。开展板料冲剪实验及断裂预测研究，提出边缘损伤的宏观几何缺陷、微观损伤和宏观损伤的统一表征模型。引入板料边缘损伤属性而建立考虑初始损伤的断裂模型，应用于揭示初始损伤对边缘断裂的影响规律和预测典型成形工艺的边缘断裂。项目的研究意义在于完善金属板料断裂预测的理论模型，为避免板料成形过程的断裂缺陷和控制分离过程的断面质量提供指导。

板料边缘切割过程诱发的初始损伤是导致后续拉伸翻边类成形边缘断裂的主要诱因之一，急需开展初始损伤表征及考虑其影响的断裂预测研究，以更好地控制初始损伤影响和提高成形极限。本项目揭示了金属板料微观断裂形貌和宏观断裂形式的关联规律并构建反映微观断裂机理的MMC断裂准则，确定冲剪影响区的断面形貌及宏观损伤区的表征方法，建立了考虑初始损伤的断裂预测模型，揭示了晶粒尺寸对冲剪影响区的损伤影响规律，并结合多级冲裁工艺降低冲剪损伤区的影响，进一步提高了板料的拉伸翻边成形极限。取得的主要进展为：(1)板料在负应力三轴度区间的断面形貌以微剪切带缺陷为主，与正应力三轴度区间的微空洞主导型断裂机理存在明显差异。建立两种微缺陷演变与正应力和剪应力的关联关系，确定考虑微观机理的MMC断裂准则。(2)建立考虑/不考虑初始冲剪损伤的边裂预测模型，发现前者能准确地预测扩孔实验的裂纹位置、裂纹数量和扩孔率，而后者预测的扩孔率误差超过100%，慢走丝切割可近似认为是不造成初始边缘损伤的理想切割方式。(3)晶粒尺寸对冲剪断面形貌及损伤影响区宽度的影响显著，增大晶粒尺寸可在一定程度上提高断面质量和降低损伤影响区宽度，但会牺牲材料的承载性能，因此要平衡承载性能和断面质量来确定适合板料冲压的晶粒尺寸。(4)提出改变晶粒尺寸和多级冲裁工艺相结合的方法提高边缘质量和降低冲剪损伤区影响。材料经二级和三级冲裁后，断面质量提高且损伤影响区范围变窄，但随着晶粒尺寸增大改善效果逐步减弱，且三级冲裁的改善效果不如二级冲裁。原始基体材料的扩孔率由12.67%提高至25.49%，对应的拉伸翻边成形极限提高两倍以上。项目研究不仅为提高板料成形边缘断裂预测精度提供理论及方法支撑，而且负应力三轴度区间成形极限高的相关研究为发展增强压剪应力状态的塑性成形新工艺提供理论依据，晶粒尺寸对断面质量影响的相关研究对于微成形/极薄板成形等对晶粒尺寸极敏感的研究领域具有重要的借鉴意义。