Interpretation of fracture mechanisms in ductile and brittle materials by the Acoustic Emission Technique

Nowadays, the measure of the damage phenomena inside a structure is a complex problem that requires the use of innovative Structural Health Monitoring (SHM) and non-destructive investigation methodologies. The non-destructive method based on the Acoustic Emission (AE) technique has proved highly effective, especially to predict fracture behavior that take place inside a material subjected to mechanical loading. Objective of the research is to use the Acoustic Emission monitoring to evaluate the fracture propagation process during tensile tests, three-point bending (TPB) tests and compression tests. The most representative AE parameters have been measured by sensors in order to obtain detailed information on the wave propagation velocity, signals localization as well as on the dominant fracture mode. As a matter of fact, the waves frequency and the Rise Angle are used to discriminate the prevailing cracking mode from pure opening or sliding. Moreover, the cumulated number of AE events and their amplitude are used to compute the signal energy. For the three-point bending tests on concrete beams, the energy dissipated to create the fracture surfaces and the energy emitted and detected by the AE sensors have been compared on the basis of their cumulative value at the end of the test and their rate during the process loading, in order to investigate on their correlation. A numerical simulation of the mechanical response of the TPB tests has been also performed on the basis of the cohesive crack model. This approach has permitted to obtain a step-by-step evaluation of the crack propagation and a more detailed analysis of the mechanical energy dissipation rate during the loading test. In addition, a dedicated in-situ monitoring at the San Pietro - PratoNuovo gypsum quarry located in Murisengo (AL) - Italy, is started and it is still in progress, developing the application aspects of the AE technique, which has been widely studied from a theoretical and experimental point of view by some Authors in the safeguard of civil and historical buildings. Preliminary laboratory compression tests on gypsum specimens with different slenderness (λ=0.5, λ=1, λ=2) were conducted to assess the validity and efficiency of the system in view to a permanent installation for in-situ monitoring. Currently the quarry is subjected to a multiparameter monitoring, by the AE technique and the detection of the environmental neutron field fluctuations, in order to assess the structural stability and, at the same time, to evaluate the seismic risk of the surrounding area.