Measurement of Plastic Strain Ratio Using Digital Image Correlation

Abstract Plastic strain ratio, or r-value is used extensively as an indicator of anisotropy of a material. The r-value is usually measured using the “line method” as defined in ASTM E517-00 using an automatic or manual line method. In this contribution, digital image correlation is introduced as an alternative measuring technique to determine r-value using the “line method” or “point method” that utilize a small gauge length of 0.5 mm. These measuring methods have been used to measure the r-values of six different sheet alloys (i.e., AA5754-O, AA6063-T6, AA7055-T751, IF-steel, AZ31, and ZEK100) with different starting textures. The results show that the r-values calculated using the “line method” are very sensitive to specimen geometry and gauge length, and the measurements are also limited to maximum uniform strain corresponding to the maximum load. However, the newly proposed “point method” using a local gauge length of 0.5 mm produces consistent r-value results up to large true axial strains and is also not sensitive to the specimen geometry being used. The measured r-values from the ASTM specimen and a new test specimen with a 25-mm gauge length using both methods are consistent up to maximum uniform strain. This indicates that the new test specimen with a 25-mm (1-in.) gauge length is sufficient to measure r-value accurately using digital image correlation.