Effect of coherent twin boundary in nanotwinned materials on fatigue crack growth based on dislocation emission

A theoretical model to describe the dislocations emitted from the crack tip and penetrating the coherent twin boundary (CTB) in nanotwinned materials under cyclic far field shear stress was proposed in this paper. The dislocations emitted from the tip are subjected to four stresses: the shear stress on the primary slip plane generated by applied stress, the image stress which drives the dislocation glide to the surface of the crack, the back stress from the other dislocations and the lattice friction stress. Combining the theory of continuously distributed dislocation, we developed a method to analyze the dislocation movement from the crack tip under cyclic far field shear stress and derived the calculation method of fatigue crack growth rate in nanotwinned materials. It can be figured out that the fatigue crack growth rate is dependent on the applied stress, the lamellae thickness and the angle between slip system and CTB. When the critical conditions are satisfied, the dislocation piling up around the CTB will penetrate the CTB, the critical stress can be calculated as a function of the misorientation angle and the lamellae thickness.