先进高强钢板成形失效的弯曲效应与拉弯成形极限预测

Advanced high strength steel sheets show severe failure issue in stretch-bending forming process, which increases the manufacturing cost with lower forming quality. The forming necking failure under stretch-bending shows strong bending effect, which is due to the induced out-of-plane bending moment and nonuniform normal stresses from small radius bending deformation. The traditional forming limits based on in-plane deformation and homogeneous hardening through thickness assumptions cannot predict the impact of bending effect on forming necking failure. This project proposes coupled in-plane and out-of-plane deformation effect as well as inhomogeneous hardening distribution along thickness direction and constructs new forming limits analysis method under bending effect. This project will reveal the physical nature of necking failure of advanced high strength steel sheets under bending effect and extends traditional in-plane forming limits prediction method. To develop new small radius stretch-bending process failure criteria and propose stretch-bending forming limits based on the top surface strain with the capability of accurately predict different layer material’s strain distribution are also included. This project also aims to develop experimental test methods for forming necking failure under typical stretch-bending loading conditions under bending effect and provide accurate foundation for theoretical prediction and engineering application.

先进高强钢板小圆角拉弯成形开裂问题突出，导致车身安全构件成形质量下降、制造成本上升。小圆角拉弯成形中板材受到面外弯矩与非均匀厚向应力作用，造成面内外加载耦合、分层硬化显著，使得成形颈缩失效行为表现出强烈的弯曲效应。传统成形极限理论基于面内拉伸变形与厚向均匀硬化假设，无法准确描述颈缩失效的弯曲效应。本项目拟引入面外弯曲与面内拉伸变形的耦合作用以及沿板材厚度方向的非均匀硬化特性描述，建立弯曲效应下板材成形颈缩失效分析新方法，揭示先进高强钢板成形颈缩失效弯曲效应问题的机理，扩展基于面内变形模式的传统成形极限理论。建立适用于高强钢板小圆角拉弯成形的颈缩失效判据，提出基于顶层极限应变的拉弯成形极限图（BFLD），能精确计算沿板厚方向顶层、中性层和底层材料的极限应变，提高先进高强钢构件成形失效的预测精度。设计并建立符合典型小圆角拉弯加载条件的拉弯成形极限实验测试方法，为理论预测和工程应用提供准确依据。

针对高强钢拉弯成形条件下小圆角失效严重，废品率高，传统失效准则无法准确预测的问题，开展了高强钢失效行为的弯曲效应研究，设计并开发了高强钢拉压循环加载下的实验夹具和测试方法，获得了非对称加载下的力学性能数据；借助新夹具实现了不同变形模式、不同应变率和不同变形温度条件下相变高强钢的微观组织测试，并揭示了微观演化和宏观变形之间的内在关系，建立了可靠的预测模型；以此为物理依据，建立了各向异性的硬化模型，准确描述拉弯过程的应力应变，并开发材料子程序构建拉弯有限元仿真模型，进一步研究弯曲效应下的板材颈缩失效演化规律；设计开发考虑厚向应力的面外剪切断裂强度测试试样，建立各向异性断裂准则，提出高强钢小圆角拉弯条件下剪切断裂的新机制，提供了准确预测的理论方法和实验手段；相关研究在车身冲压制造中进行了推广应用，获得了一定的工程效果。