Measurement of strain distribution in metals for tensile test using digital image correlation method and consideration of stress-strain relation

Displacement distribution in plate specimen of aluminum during tensile test was measured with digital image correlation method and strain distribution was derived based on the displacement distribution. Change of strain distribution was obtained during whole tensile test. Local concentrated strain starts to appear before proof stress and the local strain becomes larger as the deformation proceeds and strain at other area in the specimen almost does not change. In consideration of stress-strain relation, true stress derived from the conventional average stress in uniform area of the specimen decreases after proof stress with increase of true strain. True stress derived from the maximum strain in specimen increases with increase of true strain and it is found that work hardening exists. Stress-strain relation for aluminum was compared with the result for steel which the author already published. It was found that we have to measure strain distribution and consider local maximum strain to evaluate stress-strain relation accurately.