PSI - Issue 28

Giacomo Risitano et al. / Procedia Structural Integrity 28 (2020) 1449–1457 G. Risitano et al./ Structural Integrity Procedia 00 (2019) 000–000

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In the initial part of the ΔT-t curve it is possible to distinguish the linear trend of the temperature, then it deviates from the linearity reaching a plateau region. It is possible to draw two linear regression line, the former for the first linear phase (early stage of the temperature signal, ΔT 1 fit point series) and the latter for the second phase (last stage before the sudden increase in the temperature signal, ΔT 2 fit point series), not taking into account the temperature values near the slope change (Experimental Temperature series). Solving the system of equations, it is possible to determine the intersection point of the two straight lines. The corresponding value of the nominal applied stress could be related to the macroscopic stress that leads to the irreversible plasticization phenomena in the material. The average limit stress evaluated on three tests, is equals to 222.2±4.0 MPa. It is in good agreement with other work on the same material, with a stress ratio R= -1, by several authors (Curà and Gallinatti, 2011; Szala and Ligaj, 2016).

Fig. 4. Temperature evolution vs. stress vs. time during a static tensile test on C45.

From the finite element simulations, the first stress invariant and the plastic work per unit of volume of the cross section area have been evaluated during the load increment. In Fig. 5 are reported the experimental superficial temperature of the specimen (not filtered) and the estimated FEM temperature vs. the applied nominal stress. The experimental temperature trend experiences a deviation from the linearity of the thermoelastic effect for a stress value below the yielding stress of the material (~435 MPa). After the yielding stress has been reached, the temperature experiences a high further temperature increment till the specimen breakage. The FE simulated temperature trend shows a perfect linear trend up to the yielding point, then the plastic work per unit of volume suddenly increases, leading to an abrupt positive change in temperature, reaching asymptotically the final ultimate stress.