Mechanisms of extrusion and intrusion formation in fatigued crystalline materials

Experimental data on the formation of persistent slip markings (PSMs) at the egressing persistent slip bands (PSBs) and physically based models of surface relief formation and fatigue crack initiation are surveyed. The original Polák's model of fatigue crack initiation based on the formation, migration and annihilation of vacancy-type defects in PSBs is further refined and extended by taking into consideration the annihilation of vacancies in the matrix and the formation of internal stresses. The proposed mechanisms of surface relief formation are based on the plastic relaxation of internal compression stresses in PSBs and internal tensile stresses in the matrix. By solving the migration of vacancies from the PSB and their annihilation in the matrix, the shapes of extrusion and parallel intrusions are derived analytically under some simplifying assumptions.