On cropping and related processes

Two distinct aspects of cropping and related processes are considered, (i) the maximum in punch force caused by plastic instability and (ii) the initiation and propagation of cracks after the onset of plastic flow.An expression for punch travel at the peak load is derived in terms of the work hardening index of the workpiece, its thickness and state of prestrain. Good agreement is found with experimental results from a variety of sources. The expression is also modified to include cases where cracks appear before the peak in load.A re-examination of experimental autographic punch load-punch penetration traces shows that it is possible, within the limitations of a single shear plane model, to partition the total work of deformation into flow and fracture components. It would seem that crack propagation in cropping or blanking starts when the incremental energy consumed by cracking plus flow is smaller than the alternative process of carrying on flowing over a larger plastic volume with no cracking. Estimates for fracture toughness are possible by this method, e.g. 500–600 lbf-in/in² for brass and aluminium and 170 lbf-in/in² for lead from the results of Chang and Swift1]. A criterion for the occurrence of multiple cracking on the cropped faces is derived in terms of toughness, flow stress and workpiece/punch geometry and is shown to agree with experimental observations.An appendix highlights some more general aspects of the area of combined flow and fracture, and discusses how cracks in plastic flow fields may be identified and dealt with analytically.