PSI - Issue 42

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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approaches were used to analyze pre-strained specimen behavior. Nevertheless, both criteria provided more scattered values since they do not consider the back stress influence. During the investigation and research, the back stresses appeared as one factor that could be used to expose the difference between the last pre-strain cycles done by tensile and compression load. As shown in Figure 4 (a), the facture back stresses are indicated with crosses (pre-compression) and dots (pre-tensile). It was observed that pre-compression specimens tended to fail with smaller back stress, and it can be explained by dislocations pile up and annihilation and the Bauschinger effect. The specimens, in which the last strain was promoted by pre-compression (P3), start with a steep increase in back stress compared to the pre-tensile specimens (P4). Taking into account the behavior of the back stress change, equation (1) was elaborated, using the back stress rate dα , and adopting the tensile direction as positive. The back stress was expressed by the largest of the principal components and denoted by α 1 . In addition, a constant, μ , was added, and it was calibrated to minimize the ratio of the dispersion of the mean value for Weibull stress for each cyclic pattern of all nine specimens. The results of critical Weibull stress following the new equation (1) are exposed in Figure 4 (b), relating them with the damage degree.

Figure 4 – a) Back Stress vs. CTOD; b) Critical Weibull stress vs. Damage degree (back stress model); c) Fracture mechanism for specimens with last cycles that were executed by compression load

(1)

Two specimens that were pre-strained by the tensile load in the last cycle and another that was pre-strained by compression load in the last cycle have different performances in the CTOD fracture toughness test. Figure 4 (c) depicts the theory of failure mechanism from the end of the pre-strain cycle until the end of a CTOD test for a material that had the last cycle as a pre-compressive strain. Figure 4 (c) – (a) show that compressive stress is applied to the grains during the pre-straining process. When the CTOD test starts and this part of the microstructure are under tensile stress, the dislocations that were initially piled up by compression start to move in the opposite direction. Figure 4 (c) – (b) is regarded as the reversal movement of the dislocations. As the tensile load increases, the dislocations that moved in the opposite direction start to pile up again, as shown in Figure 4 (c) – (c), and this is thought to lead to microcrack formation. After the pile-up of dislocations is saturated, as shown in Figure 4 (c) – (d), the microcrack can progress to some extent in the pre-strained stage for pre-strained materials, until brittle failure. From this mechanism, it can be inferred that immediately after the load reversal occurs, as shown in Figure 4 (c) – (b), the dislocation pile up on the other side as shown in Figure 4 (c) – (c), which is the starting point for microcrack formation. The probability of microcrack initiation may be different before and after the saturation of dislocation pile-up, even if the incremental plastic strain is the same. In other words, the section where microcracking also occurs due to dislocation pile-up may have a higher probability of microcracking for the same plastic strain increment than the section where only dislocations accumulate after the dislocation pile-up is saturated, such as in Statistically Stored Dislocation (SSD), Ashby (1970).