Study On Fatigue Crack Propagation Rate Of Lamellar Low Carbon Steel With Ulti-Fine Grain

In this paper, martensite low carbon steel sheet was multi-pass cold rolled to 60%, 70% and 80% deformation rate (DR). Tensile-tensile test and nanoindentation test were carried out on specimens with the three DR. the length of fatigue cracks in specimen with 60% and 80% DR were measured by combined ways of photography and replica method, and a-N curves and da/dN-â–³K curves were drawed. The morphology of fatigue fracture of specimens with 60% and 80% DR were observed by scanning electron microscopy (SEM). We also used Transmission electron microscopy (TEM) to reveal the microstructure features of dislocation and interface, the mechanism of fatigue crack propagation in low carbon steel with nano-lamellar microstructure was also analyzed. The results show that:Under low cyclic stress, there is no big difference of fatigue life among specimens with 60%, 70% and 80% DR; under high cyclic stress, fatigue life increased as the DR increasing, and the fatigue life of 80% DR is the longest.The curves of fatigue crack propagation and its rate of specimen with 60% and 80% DR are consistent with conventional steel materials. In the first and second stage of the curves, fatigue crack of specimen with 60% DR shows an evident lower speed comparing with that of specimen with 80% DR. In the third stage, crack propagation rate of the two type specimens are almost the same. Theâ–³Kth, threshold of fatigue crack propagation, of specimen with 80% DR is higher than 60% DR.Average thickness of pan-shape grain in specimen with 60% and 80% DR are 187nm and 78nm respectively. Compare with 80% DR, specimen with 60% DR has a lower microhardness, density of weak interface, and more plastic deformation on fracture.There are three mechanisms for mother material to inhibit the propagation of fatigue crack: develop delamination crack along weak interface, deformation of weak interface and plastic deformation of matrix. These three mechanisms act on alternately as crack growing. Crack propagation is inhibited mainly by weak interface and delamination crack in specimen with 80% DR. While in specimen with 60% DR, crack propagation is inhibited mainly by plastic deformation of matrix and crack tip passivation, this difference improves the threshold of fatigue crack propagation to a level higher than that of specimen with 80% DR.Both photography and replica method could be used to measure fatigue crack length. Photography method is easy, but has a low resolution, while replica method is complicated but its revolution is high, and fit for measuring crack length in the first stage.