Acoustic emission monitoring of mode I fracture toughness tests on sandstone rocks

A series of mode I fracture toughness tests employing the semi-circular bend (SCB) and the pseudo-compact tension (pCT) methodologies have been performed with two sandstone types and specimens of 50 and 100 mm in diameter. The tests were complemented with the concurrent monitoring of acoustic emission (AE) activity using a multi-sensor array. With the obtained data, we compare and analyse the two testing methods, the mechanical evolution along experiments and the significance of different acoustic emission parameters. The hypocenter location of the AE events allows a better understanding of the processes occurring in the tests and the analysis of properties like amplitude or peak frequencies are valuable complements. Crack modes are assessed using a parameter-based strategy (comparison of the average frequency, AF, and rise angle, RA), and the potential of the Ib-value to discriminate cracking processes in fracture toughness tests is explored. Results show that the pCT method provides more comprehensive and detailed results, both in terms of mechanical information and acoustic emission data. We discuss the applicability of the generally accepted models of small-strain brittle material failure typically applied to compressive tests (including or not including AE data) and further methodological questions of interest when considering AE monitoring in fracture toughness tests.