基于双平面镜辅助多视角三维数字图像相关方法的板材Lankford系数直接测量

The direct measurement of the Lankford coefficient of sheet metal based on the mirror-assisted multi-view digital image correlation

Lankford系数是评价板材成形性能的关键参数,准确测量Lankford系数对于经典的各向异性屈服准则的应用有重要意义。本文提出了一种基于双平面镜辅助多视角数字图像相关方法的板材Lankford系数测量方法,在双平面镜的辅助下,仅使用一套基于双目立体视觉的三维数字图像相关系统即可同时测量试件前后表面的形貌与变形场,再通过反射变换,即可直接测量板材全场厚度及厚度应变,进而实现对板材Lankford系数的直接测量,避免了现有测量方法对体积守恒假设的依赖。通过45号钢和DP780双相钢板状试样的单轴拉伸实验,比较了本文提出的直接测量方法与已有间接测量方法的测量差异,结果显示,本文方法与基于体积守恒假设的Lankford系数测量偏差对于颈缩区分别为2.62%和0.41%,对于非颈缩区分别为7.07%和1.21%,证明了本文方法的测量准确性。本文方法提供了一种低成本且简便有效的板状试样双表面及厚度变形场测量技术,在Lankford系数、硬化指数、杨氏模量等金属材料本构参数的准确表征中具有实用前景。

The Lankford coefficient is a key parameter for evaluating the formability of sheet metal.Accurate determination of the Lankford coefficient is of significance to the application of classical yield criteria for anisotropic materials.This paper proposes a novel method for measuring the Lankford coefficient of sheet metal based on the mirror-assisted multi-view digital image correlation(mirror-assisted MV-DIC)technique.With the assistance of dual planar mirrors,the surface profile and deformation field of both the front and rear surfaces of the specimen can be simultaneously measured by a single set of three-dimensional digital image correlation(3D-DIC)system based on binocular stereovision.Then,by applying reflection transformation,the full-field thickness and thickness strain of the sheet metal can be directly measured,enabling the direct measurement of the Lankford coefficient without relying on the volume conservation assumption used in existing measurement methods.A series of uniaxial tensile experiments were respectively conducted on 45 steel and DP780 dual-phase steel sheet specimens to compare the measurement accuracy of the proposed direct measurement method with existing indirect measurement methods.The results showed that the deviations of the proposed method and the volume conservation-based Lankford coefficient measurement method were 2.62%and 0.41%for the necking region,respectively,and 7.07%and 1.21%for the non-necking region,respectively.The proposed method provides a low-cost and convenient technique for measuring the dual-surface deformation fields and the thickness of sheet specimens,which has practical prospects for accurately characterizing the Lankford coefficient,hardening exponent,Young's modulus,and other constitutive parameters of metal materials.