Electronic speckle pattern interferometry analysis of tensile tests of semihard copper sheets

Uniaxial tension tests of semihard copper sheets were studied by means of e lectronic speckle pattern interterometry (ESPI). The setup allowed the auth ors to analyze in detail the transitions from elastic to plastic behavior a nd from homogeneous to inhomogeneous plastic deformation. In agreement with the conventional definition of the yield point for copper, fully plastic b ehavior started at permanent strains close to 0.005. The strain-hardening c oefficient was very low at the early stage of plastic flow ("easy glide"), increasing progressively until values on the order of 0.13 to 0.14 were rea ched at maximum load. At this point, the appearance of unequally spaced fri nges signaled the beginning of inhomogeneous deformation. With ESPI, this o ccurrence may thus serve as a criterion to establish the forming limit of t he material.