PSI - Issue 42

3

Rafael Magalhães de Melo Freire et al. / Procedia Structural Integrity 42 (2022) 672–679 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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To analyze the cyclic plastic deformation effects on the fracture toughness behavior the following procedures were adopted: • Four-point bending tests were executed with two types of specimens as shown in Figure 1. Simulations were also carried out to obtain the necessary displacements in the machine to provide the required plastic deformation in the region of the elliptical arc notches; • CTOD specimens were extracted from the pre-strained material obtained in the four-point bending test; • The CTOD tests results were analyzed, and the Weibull Stresses were calculated by different methodologies, relating them to the material damage degree; • The MOTE of critical CTOD was calculated and a new approach and equation were elaborated to depict material behavior.

Figure 1 – a) Thin specimen dimensions; b) Dimensions of the thick specimen with side-groove

Figure 2 (b) shows that side-grooves in the middle of the specimen were designed in order to produce another stress-strain history highlighting the stress multiaxiality due to a local thickness reduction. The thick specimens represent thicker plates that were used to compare with thin specimens subjected to the same pre-strain cycles. Applying momentum during four-point bending tests, the top side of the specimen is strained by tensile load, while the deformation on the bottom of the specimen is provoked by compression, as shown in Figure 2 (a). Thus, the pre strain cycles were provided by changing the specimen position according to the local strain on the top and bottom parts, and CTOD specimens were extracted from the four-point bending specimens. In other words, a pair of CTOD specimens with opposite strain histories was manufactured from each four-point bending specimen, as Figure 2 (b).

Figure 2 – a) Four-point bending test; b) CTOD specimen manufacturing details

At the beginning of the four-point bending tests, the loading is applied on the specimen until the displacement measured by the clip-gage attains the specific value. After it, the machine is stopped and the specimen is turned upside down, and the load is applied again. Table 2 shows the pre-strain patterns considering a sequence of applied deformations. For each of these patterns, five specimens were used, and “as rolled” specimens represent the steel without any deformation in supplied standard condition. A 3-D FEM model was built on Abaqus (2018). Then the four-point bending test was simulated to assist the experimental tests, and, in the end, additional pre-strained CTOD simulations, P11 to P18 in Table 2, were conducted to propose a new equation for critical CTOD, which can represent the fracture toughness degradation due to plastic deformation.