Strain rate measurements by speckle interferometry for necking investigation in stainless steel

Localization that occurs at the end of the tensile test of a ductile 316L stainless steel has been followed in detail by in-plane Electronic Speckle Pattern Interferometry (ESPI). A global description of the whole strain-rate field with an analytical function and physical descriptors such as band width, band inclination and maximum strain rate is proposed. The description with two straight bands of constant amplitude along the width of the specimens is valid from the beginning of the diffuse necking to the fracture of the specimens. It allows distinguishing between two localization scenarii which occur for specimens with a different width to thickness ratio, one with a fracture inclined along the width and the other with a fracture inclined in the thickness. For the former, the two bands keep a constant angle while for the latter, the two bands rotate progressively until they become perpendicular to the tensile direction. The bandwidth can be defined and monitored during the whole necking evolution.