PSI - Issue 33

Dario Santonocito et al. / Procedia Structural Integrity 33 (2021) 724–733 Santonocito et al./ Structural Integrity Procedia 00 (2019) 000–000

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4.2. Stepwise fatigue test

Stepwise fatigue test under loading control have been performed on the same specimens adopted for the static tensile test and the superficial temperature has been monitored with an infrared camera. The adopted testing frequency allowed to maintain the temperature increment below 5 K, avoiding thermal failure of the specimens. In Fig. 6 are reported the temperature signal versus the real applied stress level on the specimen and the recorded stabilization temperature. As it is possible to observe, for the stress level below 13.35 MPa, there is no temperature increment, or they are minor than 0.16 K. As the stress level reached 14.25 MPa, higher temperature increment, respect the initial temperature, are registered, greater than 0.56 K. for the last stress level, no stabilization temperature is reached; the temperature signal increases up to the specimen failure.

Fig. 6. Stepwise fatigue test

Reporting the temperature increment vs. the relative applied stress level (Fig. 7), it is possible to obtain a series of point with a bilinear trend. The fatigue limit can be estimate as the change in the slope of the ΔT-σ curve, where the energy release of the material is higher compared to the heat generated by internal material dumping. By performing the linear regression of the temperature points below 13.35 MPa and above 14.25 MPa, and making their intersection, a value of 14.0 MPa is estimated.

Fig. 7. Fatigue limit assessed by Thermographic Method.