Comparative Ib-value and F-function analysis of Acoustic Emissions from elementary and structural tests with marble specimens

Acoustic Emissions data recorded while marble specimens were subjected to various types of mechanical loading are analysed in the direction of detecting possible common characteristics that could be considered as fracture precursors. The tests carried out were either elementary (three-point bending and uniaxial tension) or structural ones (pure shear of marble blocks interconnected by means of metallic connectors). The data were analysed in terms of the Ib-value (improved b-value) and the F-function, paying special attention to the time interval just before fracture. The temporal variation of both the Ib-value and the F-function were plotted using logarithmic time scale, in terms of the time-to-failure parameter (t_f − t) (where t_f is the time instant of failure), since the specific way of presenting the data offers a deeper insight to what happens during the very last time interval before macroscopic fracture. The analysis indicated that both the Ib-value and the F-function provide clear indices designating entrance into the critical state, i.e., the state of impending fracture. In addition, it was concluded that a power law governs the behaviour of the Ib-value in the time interval before fracture and the exponent of this law seems to be somehow related to the loading scheme.