Micromechanics of impact damage of ductile (ZnS) and hard (MgAl2O4) ceramics

The time-resolved generation of the electromagnetic emission (EME) and acoustic emission (AE) excited by the point impact loading of ductile ZnS and hard MgAl2O4 ceramics was detected. It has been found that the EME generation precedes the AE activity in loaded ZnS ceramics, while the EME signal lags the AE generation from MgAl2O4. This dissimilarity of temporal patterns was explained by the difference between the mechanisms of the excitation of electrical charges in loaded ductile and hard ceramics. The EME from loaded ZnS is caused by the motion of charged dislocations, while , the EME effect in MgAl2O4 is due to the annihilation of opposite electric charges emerged on microcrack edges. The AE energy yield from growing microcracks in both ceramics as well as the EME activity in MgAl2O4 were found to be random (Poissonian-like), which is typical of the localized fracture of solids occurring without long-range interactions between perturbed structural localities. The energy distribution in EME time series generated in ductile ZnS prior to cracking followed a power law, which is characteristic for the correlated motion of charged dislocations at the stage of plastic flow. The temporal patterns of damage accumulation at different structural levels were specified through physical and mechanical properties of tested materials.