  We investigate the formation dynamics of plastic creases in thin
elasto-plastic sheets. In contrast to the commonly accepted description of
crumpled thin sheets, which asserts that creases form only by elastic
interaction between two d-cones, the creases we study in this letter are
created by plastic deformations left in the wake of a single propagating
d-cone. Upon application of load, a d-cone initially remains stationary and
responds by deforming globally. However, above a critical load, the d-cone
undergoes a sharpening transition that focuses the stresses at its tip,
allowing it to propagate along the sheet, leaving a furrow-like scar in its
wake. Our results show that the dynamics of plastic defect creation are
important for predicting the final geometry and statistics of a defect network
in a crumpled thin sheet.
