Analysis of plastic strain localization by a combination of the speckle interferometry with the bulge test

Abstract : The process of localization of strains, diffuse and localized necking, up to fracture in equi-biaxial loading was analyzed through the images obtained by electronic speckle pattern interferometry (ESPI). The problem of localization is important in the sheet metal forming processes. The ESPI technique is used to have a better resolution on the measured strains (10−5) than other technique such as the image correlation (10−2, 10−3). The bulge test is currently used to determine the mechanical properties of materials by measuring the deformation that occurs in response to the application of a controlled pressure. This test is used to determine the mechanical properties of sheet metals submitted to an equi-biaxial loading path, the strains at failure are used as data to determine forming limit diagrams (FLDs). The aim of this study is to detect the localization of plastic strain (diffuse and localized necking) during the bulge test combined with a common speckle interferometer. This paper describes an original technique to detect the localization using the strain rate at different positions on the sample. The main originality of this work is the application of an optical interferometer to determine the field of plastic strain increments and strain rates in the region of the top of the specimen, at various stages of global strains.