AE fatigue experiments on tanks test samples with artificial pre-cracking

The acoustic emission (AE) method is well studied for its capability to detect and locate discontinuity that evolve in time, when a pressure tank is charged with a greater load than the usual one. When the discontinuity is a crack the AE is able to follow the nucleation and the propagation of the defect in the material. For this paper, experiment on pre-cracked samples with different notch shape are presented. The samples have been loaded with fatigue cycles and the acoustic activity during the tests has been recorded. The stress was always positive to keep the notched side in traction during the test. During the test AE signals become relevant when the load leads plastic strain near the notch. We noticed an increase of activity when the plasticity advances with a stepped shape. This stepped shape of the number of the events versus time agrees very well with the crack nucleation and its evolution to a fully developed crack. We applied the fractal analysis method to these experiments, to verify the correlation between the fractal dimension (Dt), the applied stress and the order/disorder of the material of the sample. The fractal analysis method is capable to follow the damage status of the sample, while fatigue test.