PSI - Issue 28

Costanzo Bellini et al. / Procedia Structural Integrity 28 (2020) 2283–2290 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Considering the structures near the threshold of stage II, the parameters of the austenitic cell and the martensitic cell were calculated. In addition, a fictitious material was considered in which the austenite-martensite phase transformation does not occur and the corresponding new cell parameter under deformed conditions was calculated (Fig. 7). The result showed that the transformed martensitic cell (monocline) is characterized by a size larger than its austenitic counterpart (cubic). This probably leads to a "relaxation" of the material during the transformation that is evident at the macro scale with the presence of a stage characterized by a low slope, sometimes close to zero (plateau).

Fig. 7. Excess of martensite dimension.

Using the thermodynamic approach proposed in the work of Di Cocco and Natali (2018), two simple formulas can be used to calculate the amount of austenite (A) and martensite (M) as a function of the deformation ε: ܣ ൌ ଵା஽ ଵ ௘ ష಴ഄ (1) ܯ ൌ ஽௘ ష಴ഄ ଵା஽௘ ష಴ഄ (2) The parameters C and D represent respectively the effect of the fatigue cycles and the type of test (loading or unloading). The parameter C considers the effect of the fatigue due to the number of cycles (its value range from 1 to 0.8 for 1 and 100 cycles, respectively). An interpolation of C values can be done by using a Wohler-like formulation, demonstrating a quasi-linear behavior in a log-log diagram, as shown in Fig. 8.