In-Situ Examination of the Development of Shear Localizations that Occur in High Strain Rate Compressive Loading in Steel

Abstract

The objective of this thesis is to investigate material failure phenomena, specifically related to localizations with applications to naval and rail steels. In order to accomplish this objective, steels commonly used in naval and rail applications were examined in quasi-static and dynamic compression using a universal testing machine and a split-Hopkinson pressure bar, respectively, with in-situ imaging. The work was performed on two commonly used rail weights of 115 lb/yd and 136 lb/yd rail steel and a commonly used naval steel, HSLA 65. In this thesis, state-of-the-art ultra-high-speed imaging was used with testing procedures to visualize the deformation at high strain rates to capture phenomena that occur over microseconds to provide process information not readily available in the literature for the compressive stress-state. The quasi-static and dynamic tests were performed in compression to gain material behaviour in more representative strain rates and stress states of the steels in blast and impact loading. The in-situ imaging of the deformation process provided velocities, orientation and propagation of shear bands not widely reported in the literature with an appropriate experimental setup. The localizations that occur in the context of rail applications, is the white etching layer, which is a hardened layer that forms on top of the rail from repeated plastic deformation, heat and follows the development of shear banding. Process information was gained in regards to the formation of the white etching layer on the surface of the rail by displaying grain texturing not known to occur at low strains. The new information found in this thesis was further discussed in context to the current understanding of shear banding and the formation of the white etching layer. Altogether, this work provides fundamental information for the validation of material models in studying high rate applications and use in industrial applications. It provides information into the current understanding of the shear banding and the white etching layer phenomena, improves understanding of material behaviour for design, and gives insight for the performance of the current materials commonly used in the naval and rail industries.