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Characterisation of model asperity flattening under longitudinal subsurface elongation is presented with a variety of normal pressure, flank angle and strain hardening behaviour. A systematic investigation outlines an optimal specimen design to ensure a homogenous deformation field in the workpiece under stress-strain conditions, which are typical in sheet metal forming. Asperity flattening increases with normal pressure, and the longitudinal subsurface strain reduces the necessary yield pressure and promotes asperity flattening. When the flank angle is increased, the longitudinal stress component due to elongation gets smaller in the asperity, reducing the effect of longitudinal straining. Strain hardening decreases the asperity flattening rate as strain hardening extends the deformation field further into the underlying material, which results in less asperity flattening.