基于有限元辅助试验技术的板材成形极限精确研究

The accurate analysis of forming limit in sheet metals is dependent on accurate characterization of flow localization during sheet forming, but the measurement of flow localization is an impassable obstacle for the current approaches of forming limit. The innovation of this project lies in ascribing flow localization in sheet metals to unknown factor of an inverse problem which is to be solved by finite element analysis aided tests (FAT) in term of the difference between experimental loading curve in sheet forming test and simulative loading curve in ideal sheet simulation. The flow localization of real sheet, as a basis for accurately measuring forming limit of sheet metals, is inversely solved through introducing a parametric necking model, instead of the measurement or characterization of flow localization. From this, the strain-based and stress-based forming limit diagrams related to diffused necking, localized necking and fracture are accurately measured. Additionally, the flow localization factor, as an empirical index which characterizing the level of flow localization in real sheet metal, is introduced into ideal sheet model, and then an empirical approach for evaluating the formability of real sheet metals through finite element analysis of ideal sheet forming is presented from the view of engineering application. In this project, forming limit analysis only requires the experimental loading curve to be accurately measured, and involves no strain measurement which is indispensable in traditional forming limit tests, and combines the advantages of full information in optical measurement and accurate calculation in finite element method (FEM). The results in the project would be added into the current theoretical system of forming limit in sheet metals, make the measurement of forming limit more accurate, and be of high industrial interest in practical application.

板材成形极限精确研究的前提是准确掌握材料成形不均匀流动，而其相应的测量与表征是现有研究方法无法克服的难题。本项目创新思想在于将板材成形不均匀流动归结为反问题的未知因素进行求解，应用有限元辅助试验技术作为反问题求解工具，并引入参数化颈缩模型，依据实际板材成形试验加载曲线与理想板材模拟加载曲线的差异，反求实际板材成形不均匀流动，从而避免了板材不均匀的测量与表征难题。基于此，精确测量分散性失稳、集中性失稳和断裂的成形极限图与成形极限应力图，另外在理想板材模型中引入不均匀流动因子，作为衡量实际板材成形不均匀流动程度的经验指标，从工程应用角度提出基于理想板材有限元分析评估实际板材成形性的经验方法。本项目成形极限研究仅需测量试验加载曲线，无需传统测量的应变测量环节，兼具光学测量信息充分和有限元分析计算准确的优势。其成果将丰富板材成形极限理论体系，推动成形极限研究精确化发展，并具有重要的工程价值。

随着现代塑性精密成形技术的发展，对板材成形极限曲线（FLC）的精确性提出更高的要求，而成形极限曲线测量难点在于板材断裂临界状态的判定。本项目首次提出利用载荷位移曲线，通过有限元辅助试验技术反求板材不均匀变形规律，从而判定断裂临界状态的研究思路。首先，项目针对板材本构模型进行精确测量，作为有限元精确分析的基础，主要包括各向异性屈服准则的参数识别和大应变范围硬化曲线的测量。其中，基于DIC技术的板材颈缩阶段硬化曲线的测量方法为本项目首次提出，显示出优异的测量精度和显著的应变范围。第二，开展板材不均匀变形反求研究，首先考察单向拉伸情况颈缩扩展的不均匀发展规律，获得了切实可行的有限元辅助试验反求技术实施方案，并逐步向板材其他应力状态过渡，同时开发了可精确采集载荷位移曲线的板材成形试验机，奠定反求研究的试验基础。研究结果显示基于载荷位移曲线的有限元辅助试验反求技术和三维动态应变测量DIC技术直接测量均适于板材不均匀变形的定量分析。第三，通过板材不均匀变形规律分析，首次提出板材不均匀程度的定量表征指标，基于此提出相应的断裂临界判据。最后，开展半球凸模胀形试验（Nakazima试验），应用DIC技术分析板材不均匀变化规律，根据提出的断裂临界判据，测量板材的成形极限曲线，并与目前的横截面拟合法和时间相关法进行比较，证明本项目所提断裂临界状态判据的精确性和可靠性。本项目研究对于丰富当前板材成形性能测量手段和提升板材成形极限测量精度具有重要的借鉴意义和应用价值。