Martínez-Donaire Andrés Jesús, Morales-Palma Domingo, Vallellano Carpóforo
Department of Mechanical and Manufacturing Engineering, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain.
Materials (Basel). 2020 Aug 19;13(17):3660. doi: 10.3390/ma13173660.
The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process windows in industrial stretch-bend forming processes. In this paper, the use of critical distance rule and two path-independent stress-based metrics are explored to numerically predict failure of AA7075-O stretch-bend sheets with 1.6 mm thickness. Formability limits of the material were experimentally obtained by means of a series of Nakazima and stretch-bending tests at different thickness-over-radius ratios for inducing controlled non-uniform strain distributions across the sheet thickness. By using a 3D calibrated finite element model, the strain-based forming limit curve was numerically transformed into the path-independent stress and equivalent plastic strain polar spaces. The numerical predictions of necking strains in the stretch-bending simulations using the above approaches were successfully compared and critically discussed with the experimental results for different values of the critical distance. It was found that failure was triggered by a critical material volume of around the half thickness, measured from the inner surface, for the both path-independent metrics analyzed.
基于应变的成形极限曲线是评估金属板材成形性的传统工具。然而，其应用应限于均匀应变条件下的比例加载过程。一些研究致力于克服这一限制，以表征工业拉伸弯曲成形过程中的安全工艺窗口。本文探讨了使用临界距离规则和两个与路径无关的基于应力的度量，对厚度为1.6mm的AA7075-O拉伸弯曲板材的失效进行数值预测。通过一系列不同厚度与半径比的 Nakazima 试验和拉伸弯曲试验，在板材厚度方向上诱导可控的非均匀应变分布，从而实验获得材料的成形极限。通过使用三维校准有限元模型，基于应变的成形极限曲线被数值转换为与路径无关的应力和等效塑性应变极坐标空间。使用上述方法对拉伸弯曲模拟中的颈缩应变进行数值预测，并与不同临界距离值的实验结果进行了成功比较和批判性讨论。结果发现，对于所分析的两个与路径无关的度量，失效是由从内表面测量的大约一半厚度处的临界材料体积触发的。
