The Cumulative Deformation, Work-Hardening and Fracture of Magnesium Oxide at Room Temperature, Under Repeated Point Loading Conditions

Abstract

In earlier work¹, it was established that repeated traversais increase the density of defects in the dislocated volume in magnesium oxide beneath a softer lubricated metal slider, thus increasing the number of operating slip systems and giving rise to significant work-hardening. Further stress cycles ultimately lead to the formation of cracks and cause fragmentation. The number of stress cycles (N) required to cause this kind of fatigue fracture was found to be inversely related to the hardness of the slider. Furthermore, a plot of slider hardness:number of traversais resembled a conventional fatigue plot and indicated that the surface would not fail when deformed by softer materials below a certain limiting value of hardness. Further work² showed that the contact pressure transmitted to the crystal was directly related to the flow stress of the impressor and therefore dependent on the material used. When the contact pressure exceeds the critical resolved shear stress within the crystal, dislocations move and multiply in the harder solid. Thus, the flow stress of the softest material to achieve this effect may be used to estimate the critical resolved shear stress of the hard crystal.