22. Direct Measurement of R Value for Aluminum Alloy Sheet Metal Using Digital Image Correlation

Plastic Strain Ratio, also called the R value, is a key parameter to evaluate the formability of sheet metal which is widely used in the yield functions and constitutive equations. To measure the R value during a tensile test, the strain along the width direction and thinning direction have to be measured simultaneously. Conventionally, due to the difficulties of measuring the thinning strain, the R value is usually calculated from the principle strain 1 and 2 under zero plastic volume change assumption. This paper shows a novel testing setup using a 3D digital image correlation which can measure the strain in width and thinning strain simultaneously, thus the R value can be measured directly without volume constant assumption. The test plan, experiment setup and results are shown in detail in this article.