Transition from Ductility to Brittleness in the Micromechanics of Impact Damage of ZnS and ZnSe Ceramics

Time series of acoustic emission (AE) pulses are excited by dropping a load onto samples of ductile-brittle ZnS and ZnSe ceramics. AE activity is recorded in two frequency windows of 100–200 and 400–800 kHz. In both ceramics, low-frequency emission occurred from the moment the load is applied; the high-frequency signal appears with a time delay of 100–150 μs. Statistical analysis of the pulse series reveals a qualitative difference in emissions in the specified frequency zones. The energy distribution of pulses emitted in the 100–200 kHz range follows a random poissonian-type dependence, whereas the AE series in the 400–800 kHz range shows a correlated accumulation of microdamage. Activity in the low-frequency region appears at the initial stage of failure (plastic flow) and is attributed to dislocation glide. When the ultimate deformation is reached, a brittle accumulation of interacting microcracks occurs. The described procedure for analyzing the process of damage development under impact loading allows one to determine the transition point from the disordered degradation of the structure of ductile-brittle materials to cooperative, brittle destruction.