Atomistic Modeling of Fatigue Crack Growth in Magnesium Single Crystals Under Cyclic Loading

The fatigue crack propagation behavior of magnesium single crystal was analyzed using molecular dynamics simulation. The inter-atomic potential used in this investigation is Embedded Atom Method (EAM) potentials. The studies of the influences of crystal orientation and strain rate were perfomred using CC (center crack) and EC (edge crack) specimen. For CC specimen, the periodic boundary conditions were assigned in the x and z direction, while for EC specimen, only z direction was allowed periodic boundary conditions. In order to study the orientation dependence of fatigue crack growth mechanism, ten crystal orientations of initial crack, namely, orientation A-(12¯10) [101¯0], orientation B-(101¯0)[12¯10], orientation C-(101¯0)[0001], orientation D-(12¯10)[0001], orientation E-(0001)[101¯0], orientation F-(0001)[12¯10], orientation G (101¯1)[1¯012], orientation H (101¯1)[12¯10], orientation I (101¯2)[101¯1], and orientaton J (101¯2)[12¯10] were analyzed and the simulation results reveal that the fatigue crack growth rate and the crack path vary significantly with the crystallographic orientations of initial crack. The growth rate of orientaton G is the highest and the resistance of fatigue crack growth of orientation B is the highest. A CC specimen was employed to demonstrate the fatigue damage caused by pyramidal slip band under increased maximum strain cyclic loading in the specimen of orientation E. The analysis of the influences of strain rate was carried out on the orientation C, D, F, and G and the results revealed that the growth rate of fatigue crack decreasing with increasig strain rate. Fatigue growth rate can be expressed by da/dN = cΔCTOD, where the constant c was determined by the present atomistic simulations. The values of the constant c are large and vary widely from on orientation to another.