Analyzing the properties promoting shear bands and damage initiation in 3-point bending of ultra-high strength steel

Abstract

Ultra-high strength steels (UHSS) have been developed to reduce the weight and increase strength capability of high value products such as crane structures. The bendability of UHSS steels has limited its application in production as their yield strength has increased, the bendability has been found to reduce with the reduction of tensile ductility. In this paper, the material properties affecting bendability in UHSS have been investigated and factors have been identified by analysing specimens pre and post bend testing [1-3] using a combined approach of in- and ex-situ small mechanical testing, Digital Image Correlation (DIC), modelling and characterisation. The propagation of shear bands in bending has been identified as the mechanism promoting damage in the tensile face to occur in the bending of steels. Identifying these factors combined to promote failure is of great importance while also developing a practical approach to identifying procedures to improve bendability.A small-scale bend test with new tooling and specimen geometry has been conducted inside the chamber of a Camscan SEM to observe the propagation of shear bands and damage initiation at the scale of the microstructure. Micrographs produced during the test were processed using DIC to study shear band formation in relation to strain distributions.Using these techniques shear bands and damage are observed at the tensile surface of the bend test. The strain localises in shear bands in a bifurcating pattern from the tensile surface in bending. Damage initiates at the surface where these shear bands intersect promoting a high strain at the surface. This damage promotes the shear bands to move through the sub-surface promoting further deformation then promoting more damage when the plasticity is exhausted. In this test a large 5µm inclusion is observed to interact with shear bands and seen to affect the damage propagation of the specimen and localise high strains at a region local to it. This region is where the failure originates from. Further analysis of the interaction between subsurface properties and the localisation of strain and shear bands will be studied.