Atomistic modeling of crack growth in magnesium single crystal
The analysis of crack growth in magnesium single crystal was performed using molecular dynamics simulation with Embedded Atom Method (EAM) potentials. The twinning process at the crack tip was analyzed. Four specimens with increasing sizes were used to investigate the influences of material length scale on crack growth of magnesium single crystals. Furthermore, the effects of temperature, and the loading strain rate were also verified. The specimens were subjected to uniaxial tension strain up to the total strain level of 0.2 with a constant strain rate. In the simulation of each specimen, the averaged stress strain curve was monitored. The simulation results show that the specimen size, loading strain rate, and temperature strongly influence the peak stress point at which the twin nucleated and subsequently the crack grew. The initial slope of the averaged stress strain curve is independent of the loading strain rate and temperature. Moreover, high temperatures induce increased atomic mobility, and thereby atom reorganization, which, in turn, releases the stress at the crack tip.
Author(s): Tian Tang, Sungho Kim, Mark F. Horstemeyer, Paul Wang
Corresponding Author: T.Tang
Background information of the EAM potential can be found here:
The size effects on the crack growth were studied using four specimens (from 1 through 4) of increasing size (ranging from from 4884 to 229,900 atoms) - generated by increasing the width and the height of the specimen (maintaining the thickness, t, constant). As shown on Figure 3, a center crack was introduced in each specimen by removing the atoms from the perfect crystals and maintaining the ratio of the initial crack length to the width of the specimen to a/w = 0.1. The top and bottom boundaries are free surfaces. There is about 1 nm deep atomic surface layers at the top and bottom boundaries that were fixed for applying Mode I cyclic loading. In all simulations, the specimens were equilibrated at 100 K by running 2000 timesteps before applying uniaxial tension loading. The uniaxial tension strain loading was applied along the y axis up to the total strain of ey = 20%. For the sake of eliminating the stress oscillation resulting from the sudden loading employed on the top and bottom boundary, the loading was applied such that the velocity was linearly distributed along the y-direction from the bottom to the top. Except in the investigations of strain rate effects, the uniaxial tension loading was applied at a constant strain rate of 109 Hz. All simulations were performed at a constant temperature of 100 K except in the studies of the effects of temperature.
According to the simulation results, the following conclusions were obtained:
The authors acknowledge the Center for Advanced Vehicular Systems at Mississippi State University and the Department of Energy for supporting this research.
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