Detecting the beginning of the shear band formation in uniaxial tensile tests by out-of-plane displacement measurements

We have used both electronic speckle-pattern interferometry (ESPI) and whole-field Subtractive Moiré (WSM) to follow the thickness reduction of sheet metal specimens subjected to uniaxial tensile tests. By analyzing the out-of-plane displacements induced between close load stages, we evaluated nonlinear effects linked to the progression of the thickness necking or transverse reduction in area of the sample. We observed that, during the transition zone of the test, long before reaching instability, the samples were thinned mainly along a relatively broad band. Due to its remarkable degree of localization, we identified this band as the diffuse necking. This diffuse necking preceded the development of the local neck and indicated the beginning of shear band formation.