The direct measurement of the Lankford coefficient of sheet metal based on the mirror-assisted multi-view digital image correlation
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.
Lankford coefficientmulti-view digital image correlationthickness strainplanar mirrors
NETL
NSTL
万方数据
