Acoustic emission method in the delayed fracture mechanics of structural materials

In the long-term load (static, cyclic, shunting, etc.) much smaller than the destroyed one, as well as during the influence of physical and chemical factors in the material of the structural element, there is the emergence of new and diffusion of existing physical microdefects (vacancies, dislocations), their unification and formation mechanical defects (pores, microcracks). Further development of the last ones leads to the emergence of macrocracks and their subcritical growth. The result of such process is the loss of structural element integrity. All these processes together form the basis of the delayed fracture mechanics of structural materials. An energy approach was used for mathematical models construction of the structural materials delayed fracture under the mechanical loading and physicochemical factors influence. On this basis, the residual life of the structural elements could be determined. The application of this approach is demonstrated on the example of the high-temperature creep crack propagation. At the same time it should be noted that elementary acts in the delayed fracture mechanics are accompanied by the elastic waves emission (acoustic emission) and the new small size surfaces formation that could not be registered by the existing non-destructive testing. Therefore, in our opinion, the acoustic emission (AE) method is the only one that makes it possible to effectively investigate the structural materials delayed fracture kinetics. Nowadays devices are well fitted to capture the AE events parameters (amplitude and total impulses count), but their connection with the delayed fracture acts characteristics (mechanism and size of the new formed surface) are not known. In the previous studies, the authors theoretically found that the newly formed surface size for one AE event is proportional to its amplitude. This made it possible to determine the crack nucleation period as the sum of the AE events amplitudes multiplied by the corresponding constants. On the basis of this and the aforementioned energy approach, mathematical models were created here through the averaged values of the acoustic emission events parameters for the determination of the precritical periods of the delayed cracks growth during the long-term loads and high temperature influence. (C) 2019 The Author(s). Published by Elsevier B.V.