Use of In Situ SEM Multiple Cracking Test to Correlate Crack Propagation Mode with Acoustic Emission Signals in Thermal Oxide Scales: Application to Ni/NiO System

Multicracking tests are carried out in an SEM on nickel specimens preoxidized at high temperature. These tests are monitored by acoustic emission. By combining the analysis of the acoustic emission signals with SEM observations of the specimens, it is possible to find the signatures of the two active crack propagation modes. In mode I (propagation perpendicular to the metal–oxide interface), the acoustic emission signals have high amplitudes and short durations, whereas for propagation in mode II (along the metal–oxide interface), the AE signals have low amplitudes and long durations.