Fracture mechanisms in chip formation processes

Abstract

Metal cutting is accompanied by extensive plastic deformation and fracture. Quick stop sectional specimens of the chip formation of medium carbon and resulphurised free cutting steel have been studied in order to determine the nucleation and growth mechanism of cracks during machining and their relation to the fracture process. During machining two main types of cracks were observed: internal crack formation in the primary shear zone and cracks associated with the formation of the built up edge (BUE). Internal crack formation is due to void formation either at MnS inclusions or on the ferrite/pearlite interface, while cracks associated with the BUE were formed below the flank face and ahead of the rake face of the tool. The type of chip formation was classified according to the mechanism of crack formation. In both of the steels used, quasicontinuous chips and chips with BUEs were obtained. Although, the quasicontinuous chips contained cracks and voids, they did not propagate right through the shear band, so that the chip remained continuous. Discontinuous chip formation occurred only in resulphurised free cutting steel as crack propagation and void coalescence progressed right through the shear band.