Metallic friction under near-seizure conditions

Abstract In metal-working theory extreme frictional (perfectly rough) conditions are assumed to apply when the shear stress opposing motion at the tool-work interface is equal to the shear flow stress of the work material. This is normally likened to seizure (sticking) with zero velocity at the actual interface and with plastic flow occurring in the adjacent work material. If the flow stress is assumed constant as in ideal plasticity theory, then this condition can be readily defined. However, if the flow stress is allowed to vary with, for example, strain rate and temperature, it is no longer obvious at what value of flow stress the perfectly rough condition becomes applicable. With the assumption that the velocity changes from zero at the interface to the full rigid body velocity over a narrow plastic zone of intense shear, an analysis is presented in which it is proposed that the thickness of this zone (which must be finite in a strain-rate-sensitive material) and hence the associated strain rate, temperature and flow stress are determined by a minimum work criterion. Recent results in support of this are presented from machining experiments where the flow at the tool-chip interface approximates to perfectly rough conditions. It is suggested that for steady state conditions true seizure cannot occur and a possible mechanism is given for near-seizure conditions in which the velocity at the interface, although very low, is not zero.