复杂加载路径下各向异性薄板流动特性及成形极限理论

Research on the formability theory of sheet metal has been focused and limited on single forming process. As the development of integral and light-weight components, preformed specimen and multi-step forming process are usually required, which can be characterized as non-continuous and non-proportional loading. As compared with single forming process, both the flow behavior and the formability of material will be quite different, which tends to result in complex forming process or even failure. Therefore, it is vital to develop the formability theory for complex loading conditions, in order to establish the basis for manufacturing of complex components and development of new techniques. Based on the newly proposed step-die bulging method, so-called dynamic hardening model based on stress state parameter will be developed to give precise description of the hardening of anisotropic sheet metal under complex stress state. Qualitative evaluation of the performing process on subsequent hardening behavior, and new coupled forming limit criterion for non-continuous loading will also be given. Combining the dynamic hardening rule and coupled forming limit criterion, a new forming limit predicting theory and FLC for anisotropic sheet metal under complex loading condition will be established, and will be used as theoretical basis for process determination and optimization for manufacturing of complex-shaped components.

金属板材成形理论方面的研究始终针对并一直局限于单一成形工序，但随着构件向整体化和轻量化发展，复杂构件成形时多需采用中间预制坯或多个成形工序。相比简单加载和单一成形工序，呈现出加载历程非连续和加载比例非线性，成形过程中材料的变形流动特性及成形性能将发生显著变化，导致成形工序异常复杂甚至无法实现整体成形。因此，迫切需要开展复杂加载路径成形极限理论研究，以指导复杂构件成形并促进新技术的发展。本项目基于发明的“阶梯凹模胀形”新方法，提出基于应力状态系数的硬化模型，实现复杂应力条件下各向异性薄板硬化特性的准确描述，解决经典“单一硬化曲线假设”用于各向异性材料时的困难；定量表征预变形对后继硬化行为的综合影响，建立从多角度判断材料极限状态的耦合判断准则；基于所建立的薄板流动特性演变模型及极限判断准则，构建适用于复杂加载条件的薄壁板材成形极限理论及FLC，为复杂构件制造时的成形工艺选择及优化奠定理论基础。

随着制造技术向整体化、轻量化发展，传统金属板材成形性能的理论已不能满足实际生产的需要，而复杂加载实验技术的缺乏严重限制了相关理论的发展。本项目提出薄板“阶梯凹模胀形”实验法，建立了阶梯凹模胀形专用测试系统；基于薄管复合加载原理，建立了薄管可控双向加载专用测试系统；实现了薄壳材料复杂加载条件下的性能测试，为研究复杂加载路径下各向异性材料的流动特性和成形极限提供了基础。提出了专用于金属薄壳的HAH-2d各向异性硬化模型，描述复杂加载路径下材料的硬化特性。HAH-2d模型很好地描述了ST16钢板的永久软化和潜在硬化现象。基于Swift失稳准则和改进的最大载荷准则（MMFC）准则提出广义的最大载荷准则（eMFC），建立适用于一般连续非线性加载的成形极限理论预测模型。基于挤压管初始壁厚不均匀的特点提出了改进的M-K模型。各向异性硬化模型和成形极限理论预测模型的提出推进了塑性成形理论的发展。新提出用于管内高压成形的成形极限图，通过引入管胀形区几何参数以反映复杂加载路径的影响，为薄壁管内高压成形工艺的制定和优化提供依据。本项目的研究具有重要的科学意义和应用价值。