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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Regarding the results from the FEM model of pre-strained and CTOD specimens simulations, thin and side groove specimens showed similar behavior to the previous one, and it was not possible to conclude that thicker plates would be more impacted under pre-strain cycles considering these additional patterns. The last cycle that was pre compression still shows a higher impact in comparison to the specimens that were pre-tensioned by the last pre-strain cycle. This confirms that pre-strain patterns that finish with pre-compression are more severe and need to be accounted for the fracture toughness loss. As the MOTE estimation method developed by Ozawa et al. (2022) requires the estimation of the Weibull distribution for each pre-strain pattern with respect to the critical CTOD, it was necessary to use the Weibull distribution related to Weibull Stress from the experiments to obtain the shape parameter of the critical CTOD Weibull distribution. For the shape parameter from the Weibull Stress Weibull distribution, m = 10.6, while the shape parameter of the critical CTODWeibull distribution was assumed to be m = 2, according to the well-established theory about its value for cleavage fracture mentioned in Freire et al. 2021 and Rugieri et al. (1992). The Weibull distribution for the experiments was obtained following the Weibull stress equation 1 which considers the back stress. The scale parameter, σ w,u , in the Weibull distribution related to Weibull Stress was determined as σ w,u = 1414 MPa, using the method of least-squares. Then, it was used to obtain the scale parameter of the critical CTOD Weibull distribution. In other words, the scale parameters from the cumulative probability of failures with the Weibull stress as a parameter and with the critical CTOD as a parameter were matched, using the method of least-squares.

(2)

For m = 2 in the cumulative probability of failures with the critical CTOD as a parameter, δ cr,MOTE equation is obtained as in equation 2 that was given in Ozawa et al. (2022), and it was used to calculate δ cr,MOTE for each additional pre strain pattern. The relationship between δ cr,MOTE and material damage degree D obtained for each additional pre-strain pattern is shown in Figure 5 (a). The previous experiment data was added in the graphic with the additional pre-strain patterns, then it is confirmed that fracture toughness reduces under pre-straining conditions and the pre-compression in the last cycle is more harmful since the pre-tensioned material showed lower reduction for the critical CTOD.

Figure 5 – a) Fracture toughness with additional pre-strain patterns vs Damage Degree; b) Cyclic pre-strain graphic with the division of elastic and plastic regions, considering the region of the inverse movement section of dislocation, which can be eliminated from the analysis Manson-Coffin equation and the Miner rule were used to relate the material damage with the plastic strain at the fatigue crack tip. As the material damage value is not directly obtained from the experiment, a simplified equation to evaluate the fracture toughness reduction can be formulated considering macro parameters. To be simple and accurate, a new equation for CTOD must include some findings from the test results as follows: • Fracture toughness decreases as plastic strain is applied, but eventually converges to a constant value.