PSI - Issue 28

M.R. Tyutin et al. / Procedia Structural Integrity 28 (2020) 2148–2156 TyutinM.R./ Structural Integrity Procedia 00 (2020) 000–000

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of the stress and the total number of acoustic emission signals ∑ N AE on the relative strain, and a sharp decrease in the AE intensity Ṅ AE . The beginning of the pre-fracture stage (IV-th stage) upon reaching a load close to the maximum and corresponding to the kink point on the ∑ N AE -ε* dependence, after which ∑ N AE remains constant until fracture. The boundary between the II and III stages passes through the kink point of the deformation curve of material determined according to [30,31], in which the slope of the deformation curve changes. In the region of transition from stage II and within stage III, the angular coefficient of the ∑ N AE -ε* dependence also changes. 3.2. Evaluation of physical properties of specimens by magnetic nondestructive methods Figure 3 shows the stress-strain diagrams, the self-magnetic field intensity H r , the magnetic field intensity H EC assessed by eddy current method, and the coercive force H C on ε*. The deformation dependence of the percentage of the martensitic phase resulting from a deformation-induced martensitic transformation in unstable austenitic steel is also presented in Fig.3c. Upon fracture of low-carbon steel specimens, the following development of magnetic characteristics is observed: at the strain hardening stage the eddy current parameter H EC and the coercive force H C increases (Fig.3a). At ε* = 0.3, maximum is observed on the deformation dependence of H EC (Fig.3a). The changes in the H EC and H C values in this deformation range (ε* = 0.3–0.45) are slowed down (Fig.3a). With further deformation, a significant increase in damage is observed. The coercive force increases monotonically, and the eddy current parameter decreases. Before the final fracture of the specimen, there is a drop in the H r (Fig.3a). A sharp change in the intensity of the self-magnetic field H r during testing of medium-carbon steel specimens occurs at the stage of plastic flow and reaches a minimum at the yield point (Fig 3b). Further, the parameter H r changes slightly, reaching its maximum at the ultimate strength, and decreases before the fracture of the specimen (Fig.3b). From Fig.3b, it can be seen that the eddy current parameter H EC decreases to a minimum value at ε * = 0.6, after which this parameter grows to a maximum value corresponding to the ultimate strength of the material. The coercive force H EC increases monotonically in medium-carbon steel specimen (Fig.3b). In stainless steel specimen, self-magnetic field intensity H r changes dramatically at the plastic flow stage similar to changing this parameter during the deformation of low-carbon and medium-carbon steel specimens (Fig.3c). It should be noted that in the initial state, stainless steel specimens are non-magnetic, therefore the characteristics of H EC and H C practically do not change at this stage (Fig.3c). When loading specimen, deformation-induced martensitic transformation takes place, and the magnetic properties of this steel appear, as can be seen from the graph on Fig.3c. When deformation of ε* = 0.5 is reached, a sharp increase in the coercive force and martensite content is observed. It should be noted that the growth of the eddy current parameter begins earlier – at ε* = 0.3. As seen in Fig.3c, the eddy current parameter begins to grow earlier than the coercive force. The higher sensitivity of the eddy current method is since the surface layer of the specimen is controlled. While the coercive force is measured in the volume of the material, damage in which is lower than in the surface layer. When testing bainitic steel specimens, changes in most physical properties are observed at an earlier stage of tension at ε * ~ 0.2-0.4 (Fig.3d) [29]. At the relative deformation ε * = 0.4, the coercive force and the eddy current parameter decrease, and a kink point appears on the self-magnetic field intensity curve (Fig.3d). After reaching the ultimate strength, an increase in the coercive force and a drop in the eddy current parameter are observed. The fracture stages identified by changes in acoustic emission parameters (Fig.2) can also be connected with the changes in magnetic characteristics (Fig.3). At the boundaries of the fracture stages, either change in the slopes of the magnetic properties curves or inflection point on these curves are observed. Our study showed that the sensitivity of the applied methods significantly depends on both the stage of deformation and the structure of the material.