Failure behavior of steel under various combinations of biaxial stress

Abstract

Understanding the failure behavior of thin-walled steel structures under large-deformation plasticity and fracture is crucial for assessment of accidental loading conditions. This thesis will experimentally determine the crack propagation direction in a thin steel sheet under different combinations of in-plane strain, which is essential for improving simulations. Necking and crack angles in a commercially available ductile steel sheet (DC01) are determined under various stress conditions, ranging from uniaxial to equi-biaxial tension. A state-of-the-art formability testing method (Marciniak) is adapted to meet the specific needs of this project. Through tensile testing, the Lankford Parameters of DC01 steel are determined, with which a theoretical assumption of the crack angle can be made using Hills theory (1952). By varying the specimen and carrier dimensions, the strain ratio range of −1/2 to near 1 is tested. A Digital Image Correlation (DIC) system is used to measure deformations and strains during the whole deforming process. Pictures taken from the DIC system and post-mortem section cuts allow for the determination of the crack angle with respect to the in-plane and through-thickness directions. The resulting crack angles agree with the theoretical angles determined by Hill (1952) when the frame of reference of determining the crack angles are rotated.