提出一种创新的试验方法——覆层板椭圆胀形试验，利用该试验方程可以实现厚度法向应力的加载和研究。建立覆层板椭圆胀形的理论解析模型，通过试验确定不同椭圆度下板材的等效应力?应变曲线，并研究覆板材料性能和厚度对目标板材流动性能的影响。结果表明，相比无覆板液压胀形，覆层板液压胀形得到的等效应力?应变曲线缩颈前的应变值更大。并且椭圆凹模椭圆度越小，所得的等效应力?应变曲线离圆形凹模(b/a=1)胀形曲线越远。同时，成形过程中选择强度系数K较高、加工硬化指数n较大及适当厚度的覆板有利于目标板材成形性能的提高。
A novel experimental approach was presented, namely the overlapping elliptical bulge test, which can load and research thickness normal stress. Theoretical analysis model of the overlapping elliptical bulging was described, the equivalent stress?strain curves of target sheets with different ellipticity ratios were determined experimentally, and influences of the material performance and thickness of overlapping sheets on the flow property of the target sheet were also researched. The results show that, in the overlapping hydraulic bulge test, the equivalent stress?strain curve can be determined up to larger strains before necking than in the no overlapping hydraulic bulge test. And as the die ellipticity ratio decreases, the flow stress curves tend to move away from the curve obtained by circular (b/a=1) bulging test. Meanwhile, the flow property of the target sheet can be improved by choosing higher strength coefficient K, larger work hardening exponent n and proper thickness of the overlapping sheet.
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