THE EFFECT OF A YIELD DROP ON CHIP FORMATION OF SOFT CARBON STEELS

It is a common experience, confirmed in a recent co-operative action, that finite element simulations of the machining of soft carbon steels (when continuous chips are formed) do not correctly predict all three together of cutting force, thrust force and chip thickness ratio. This paper investigates the possibility that the source of error is in modelling the work materials' strain hardening behaviour, in ignoring the yield delay phenomenon that also shows itself as an upper yield point. First, the capability of a particular software to include an upper yield point is demonstrated. Then, the general effects of inclusion are shown before finally comparing simulations and experiments for a BS 970 070M20 steel (similar to AISI 1021). For agreement with experiment, not only an upper yield point, but also a correct dependence of flow stress on strain rate and temperature, is needed.