Measuring fracture toughness from machining tests

Abstract

An analysis of the forces involved in orthogonal cutting or machining is presented in which yielding on a shear plane is assumed. The fracture toughness G_c is included and it is observed that G_c may be determined by measuring the cutting and transverse forces together with the chip thickness for a range of cutting depths. This latter measurement enabled the shear plane angle ϕ to be determined experimentally. A simplified version of the analysis is also given in which ϕ is predicted by a cutting force minimization scheme. Neither scheme requires any details of the friction condition to be known since the transverse force is sufficient information for any type to be included in the analysis. A friction model including a coefficient of friction and an adhesion toughness is also utilized. Data for both polymer and metal cutting are taken from the literature and G_c is determined. In some datasets the tool rake angle α is also varied and the values of G_c and the yield stress σ_Y are found to be independent of α. The force minimization method gives a good estimate of ϕ for most polymers. For metals (aluminium alloy, steel, and brass) the method worked well. For aluminium alloy G_c was independent of α and the predicted and measured ϕ values agreed. For steel and brass this was not so. G_c was mostly independent of α except at low values where high values of G_c were observed. A constant value of the coefficient of friction was observed for each α value but values for both the coefficient of friction and the adhesion toughness varied significantly with increasing rake angle.

Keywords

fracture toughness measurements cutting machining polymers metals

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