Understanding and optimising the mechanical effects of textured indentation in cold roll metal forming

Textures are commonly applied to steel sheet for the mechanical benefits produced however, the mechanisms responsible for these benefits are not well understood, causing undesirable variation in the results. This thesis aims to investigate these mechanisms and their impact on the mechanical behaviour of dimpled steel. Investigating UltraSTEEL®, developed by Hadley Industries, the possibility of grain boundary strengthening was explored; microstructural examination of dimpled sheet revealed possible grain size reductions resulting from the dimpling process. It was noted that this should result in a correlation between Lüders elongation in the plain steel and the strengthening due to dimpling. To test this and understand how the mechanical response of dimpled steel differs from that of the plain steel source material, numerous tensile tests were conducted. This enabled statistical analysis to identify correlations such as the aforementioned. Confirming the Lüders elongation – strengthening link, an expression predicting strengthening based on Lüders elongation in the plain steel was established. Investigating the stiffness implications of dimpling, the geometric effects were assessed using FEA of plain and dimpled cross-sections. This illustrated that dimpled geometry necessitates a reduction in stiffness. However, the material property changes due to dimpling counteract this reduction. Observing that standard measures for yield strength are not representative of the behaviour of dimpled steel, alternatives were explored. The inapplicability of these measures was attributed to macroscopic geometric non-uniformity, leading to the proposal of the novel derivative yield criterion. This novel criterion was applied successfully to both plain and dimpled steel (discontinuous and continuous yield, respectively) and the implications for the predictive expression developed here have been addressed. FEA was used to apply new understanding to optimise the dimple tooth profile. Existing and novel tooth geometries and configurations were assessed based on plastic strain distributions and thinning. Consequently, the novel star profile was proposed for experimental evaluation.