A New and Direct R-Value Measurement Method of Sheet Metal Based on Multi-Camera DIC System

Siyuan Fang,Lianxiang Yang,Bicheng Guo,Gang Zheng,Xiaowan Zheng,Boyang Zhang

doi:10.3390/met11091401

Siyuan Fang, Lianxiang Yang + Show 4 more

Open Access

https://doi.org/10.3390/met11091401

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Abstract

More and more attention has been given in the field of mechanical engineering to a material’s R-value, a parameter that characterizes the ability of sheet metal to resist thickness strain. Conventional methods used to determine R-value are based on experiments and an assumption of constant volume. Because the thickness strain cannot be directly measured, the R-value is currently determined using experimentally measured strains in the width, and loading directions in combination with the constant volume assumption, to determine the thickness strain indirectly. This paper provides an alternative method for determining the R-value without any assumptions. This method is based on the use of a multi-camera DIC system to measure strains in three directions simultaneously. Two sets of stereo-vision DIC measurement systems, each comprised of two GigE cameras, are placed on the front and back sides of the sample. Use of the double-sided calibration strategy unifies the world coordinate system of the front and back DIC measurement systems to one coordinate system, allowing for the measurement of thickness strain and direct calculation of R-value. The Random Sample Consensus (RANSAC) algorithm is used to eliminate noise in the thickness strain data, resulting in a more accurate R-value measurement.

Highlights

The plastic strain ratio r is a parameter that indicates the ability of a sheet metal to resist thinning or thickening when subjected to either tensile or compressive forces in the plane of the sheet [1,2]
The 3D contour and strain distribution on the surfaces of the DP980 was obtained through Digital Image Correlation (DIC) measurement
R-valuemeasurement measurement method method based based on onthe theuse useof oftwo twostereo-DIC

Summary

Introduction

The plastic strain ratio r is a parameter that indicates the ability of a sheet metal to resist thinning or thickening when subjected to either tensile or compressive forces in the plane of the sheet [1,2]. It has been used as an important parameter to evaluate the formability of automotive sheet metal. It is typically advantageous for the material to experience a minimal reduction in the area when subjected to a force; this means that it is a good drawing material with a high R-value. Since the crystal structure of polycrystalline materials is often difficult to model accurately, the accuracy of the current theoretical estimates is not high [5,6]

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