Abstract
It has been suggested recently [1–4] that an increase in the hardness value often observed when the indentation size is reduced (indentation size effect) in metals is a consequence of the dependence of the flow stress of the metal on the strain gradient. Here, we show based on an analysis of the strain gradient in machining, that a similar size effect should be observable when machining at small values of the undeformed chip thickness. Such a size effect will manifest itself as a continuous increase in the specific cutting force as the chip thickness is reduced. Furthermore, the size effect in machining is likely to be much more pronounced than in indentation, because of the more intensive strain gradient prevailing in the deformation field in machining. The dependence of the flow stress on strain gradient is not an artificial construct but has a well-established basis in the dislocation theory of hardening [5].
Our analysis suggests that an effective test of plasticity constitutive laws that incorporate the strain gradient as a parameter can be achieved using a simple 2-D cutting experiment with a sharp tool carried out at very low speeds. Such a test may be carried out with ductile pure metals as workpieces wherein this size effect is expected to be most significant.
