Assessment of Different Fracture Regimes in Andesite Rock via Acoustic Emission, Wavelet Transform and Energy B-Value

Abstract

The damage process occurring in Andesite rock specimens subjected to uniaxial compression tests up to failure is investigated. During the test, the propagating elastic waves (Acoustic Emission, AE) due to micro and macro-crack growth are detected. Damage assessment is performed by determining and processing individual AE hits with a large number of overlapping transients with variable intensities. It is important to detect the sudden transition from diffused micro-cracking to localized macro-cracks which characterizes the catastrophic failure in brittle materials; these sudden transitions can be associated with critical values of AE parameters. It is known that different fracture regimes in brittle materials are connected with different characteristic frequencies of AE signals. The AE hits were analysed with the Continuous Wavelet Transform, and Wavelet Entropy was calculated in order to detect relevant scale (frequency) bands. A modification of the original b-value (Gutenberg-Richter parameter), was afterwards used to assess damage in different frequency bands. This modification, called energy b-value, is obtained (for each relevant frequency band) as the local slope of the log-log cumulative distribution of hits as a function of their AE energy. Local b-value minima indicate critical AE energy values. Thus, AE hits related to critical damage can be located along the test. Results were corroborated by comparison with the Cumulative AE (CAE) energy in each band. The hits with near-critical energy values preceded CAE jumps, which indicate coalescence of micro-cracks into macro-cracks.damage can be located along the test. Results were corroborated by comparison with the Cumulative AE (CAE) energy in each band. The hits with near-critical energy values preceded CAE jumps, which indicate coalescence of micro cracks into macro cracks.