PSI - Issue 33

V. Di Cocco et al. / Procedia Structural Integrity 33 (2021) 1035–1041 Author name / Structural Integrity Procedia 00 (2019) 000–000

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From spectrathe content of austenite and martensite are quantified using as parameter the intensisty of austenitic and martensitic peaks, referred to noise their noise signal, and results are interpolated proposing a simple diffusional equation. Then a mechanical model wich takes into account the actual microstructure will be proposed. 3. Results and discussion In Fig. 1 the spectra at beginning conditions of investigated cycles and at maximum investigated strains, show the mean peak of austenite and of martensite. The position of the peaks change at different cycles, but the variation of the angles is negligible at cycles 50 and 100. As observed in a work of the authors [1], the main phenomena of microstructural evolution occur more evidently at the first fatigue cycles. For this reason the maximum values and the amplitude of the spectra observed at first cycle are much more different to the values observed at cycle 10.

a)

b)

Fig. 1. Austenite spectra observed at unload conditions: a) cycles 1 and 10, b: cycles 50 and 100

Considering the proposed model by Di Cocco and Natali the energy associated to the microstructure variation in differential form can be written as in (1) ݀ ܩ ൌ ݀ ߝ ൅ ௗ ஺ ஺ െ ௗ ெ ெ (1) where G is the energy associated with the microstructure variation, A is the volume fraction of austenite and M the volume fraction of martensite. This relation is valid in two hypothesis: The austenite - martensite transformation is perfectly reversible The transformations of austenite in martensite and of martensite in austenite occur without any intermediate phase formation. By minimization of the relation (1) the equilibrium of austenite and martensite quantity can be calculated by the relations (2) and (3) ܣ ൌ ஽௘ ష಴ഄ ଵା஽௘ ష಴ഄ (2) ܯ ൌ ଵା஽ ଵ ௘ ష಴ഄ (3) where the parameter C governs the microstructure cycle fatigue and D depends if the transformation is from austenite to martensite, or of martensite to austenite. For the considered alloy the values of C is 0.01 for the first cycle and the value of D is 700 in loading condition (transformation of austenite in martensite) or 70 in unloading conditions (transformation of martensite in austenite).