ACOUSTIC-EMISSION DURING FATIGUE EXPERIMENTS ON FIRST YEAR SEA-ICE

Acoustic emission events were measured during the cyclic loading of ca ntilever beams of sea ice. The events were detected with resonant tran sducers, with bandwidth 20 kHz to 150 kHz, which are predominantly sen sitive to the vertically polarized component of the Rayleigh surface w ave. This confinement to the surface, along with the linear geometry o f the beam, make it possible to estimate a one dimensional source loca tion with only two transducers. We define an event magnitude and find that this magnitude increases as the load increases, with the largest energy release occuring during the fracture of the beam. The measureme nt of magnitude requires an estimate of the high frequency attenuation of the Rayleigh waves in sea ice. In the absence of existing data in the literature, we make our own estimate and compare this with suitabl y adjusted data for compressional waves. The sources of the events are precursors to the eventual fracture of the sea ice, having highest de nsity at the location of the final failure of the beam. We speculate t hat the emissions originate either from dislocation breakaway or the m icrocracking associated with this dislocation motion. By measuring the slope of the cumulative magnitude-frequency plot we may surmise that our data are consistent with the view that the system is organising it self into a stationary critical state.