PSI - Issue 30

L.A. Prokopyev et al. / Procedia Structural Integrity 30 (2020) 120–127 Prokopyev L.A. et al. / Structural Integrity Procedia 00 (2020) 000–000

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It is known from Buylo (2017) that significant part of acoustic impulses is emitted by a plastic zone that appears around of the crack tip under loading. Accordingly, there is a relationship between the characteristics of acoustic signals and the size and shape of the plastic zone. Note that the problem of determining the size and shape of the plastic zone by experimental methods is a difficult task; however, there are many studies on the theoretical aspects of the behavior of the stress-strain state at the crack tip with the plastic zone. Thus, to establish a theoretical relationship between the characteristics of acoustic signals and the parameters of fracture mechanics, considerable attention should be paid to the size and shape of the plastic zone. Finding the correlation of the plastic zone size around the crack tip on the stress intensity factor is an important task. As is known, exact determination of the size of the plastic zone that appears at the crack tip during its propagation is a difficult task. There are several approaches to estimate the size of the plastic zone under loading of cracked metal samples. One approach has been investigated by Bol'shakov (2016) that based to substitute the material yield stress in the equations of linear elastic fracture mechanics. In this case, the specific volume of the plastic zone can be represented as a function that depends on several parameters: yield stress, Poisson's ratio, stress intensity factor, T-stresses. In this dependence, the stress intensity factor characterizes the magnitude of the loading force, and the T-stresses, according to Meliani (2011) reflect the stress state biaxiality ratio. 2. Description of the calculation scheme for determining dependence of the plastic zone size around the crack tip on the stress intensity factor It can be assumed that T-stresses are the reason for the differences in the coefficients of the power dependence of the parameters of acoustic signals for the same values of the stress intensity factor. In this case, it is necessary to calculate the sizes of plastic zones for two different loading schemes with the same stress intensity factor values, and different T-stress values. Two thermal loading schemes of a steel sheet with a centrally located crack with the same stress intensity factor values and different T-stress values were selected. Thermal loading by cooling the local zone near the crack was selected for further use of the results of the work for non-destructive testing by the acoustic emission method. For five values of the cooling temperature t (-20  C, -30  C, -40  C, -50  C, -60  C), the stress intensity factor and T-stresses were determined by the finite element method. In order to determine the dependence of the crack tip plastic zone size on the stress intensity factor, a numerical simulation of the loading of a steel sheet with a centrally located crack was carried out in the ANSYS APDL parametric design language. Two loading schemes are shown on Fig.1 and Fig.2.

Fig. 1. Scheme No.1 for thermal loading of steel sheet with central crack.