PSI - Issue 33

Giacomo Risitano et al. / Procedia Structural Integrity 33 (2021) 748–756 Risitano et al./ Structural Integrity Procedia 00 (2019) 000–000

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4.2. Stepwise fatigue test

A second series of test was performed on two specimens adopting a step increment of the maximum applied stress with R= -1 and test frequency f= 20 Hz. In Fig. 6 the superficial temperature evolution is reported versus the number of cycles and the applied stress. The temperature has been evaluated as the difference between the instantaneous temperature and the temperature at the beginning of the fatigue test. For stress levels equal to 180 and 200 MPa it is not possible to assess in a clear way a temperature increment. As the stress level increases from 200 MPa to 220 MPa, it is possible to observe a temperature rise with stabilization of the trend. With higher stress level (>240 MPa) the temperature experiences a very high further increment without plateau region till the specimen failure. Both of the tested specimen never reached stress level higher than 280 MPa. The Energy Parameter Φ has been estimated for the two tests as the numerical integral of the subtend area of the temperature curve vs the number of cycles, showing value with the same order of magnitude (~2x10 6 Cycles·K).

Fig. 6. Stepwise fatigue test on C45 applying the Thermographic Method.

Due to the sudden higher temperature increment it has not been possible to evaluate the S-N curve of the material with the procedure proposed by Fargione et al. (Fargione et al., 2002). On the other hand, it has been possible to evaluate how the temperature experiences an increment for stress level in the stress range between 220 and 240 MPa. This indicates that fatigue damage begins for those stress levels and it is in agreement with the value of the limit stress found by means of static traction test applying the STM.

4.3. Comparison with other tests

The limit stress obtained by the STM could be compared with the fatigue limit obtained with alternate traction compression tests in literature, conceivable as the more damaging dynamic load condition for the material. Szala and Ligaj (Szala and Ligaj, 2016), performed CA fatigue tests, with stress ratio R= -1, obtaining the S-N curve of a C45 steel. The fatigue limit evaluated with a 50% probability of survival at 2x10 6 cycles is equals to 210 MPa. Curà and Gallinatti performed a stair case procedure, with R= -1, for the same medium carbon steel (Curà and Gallinatti, 2011). They report a fatigue limit equals to 239±9 MPa. Within the Energy Method group of the Italian National Machine Design Society (AIAS), a round robin has been performed between several universities (Barbagallo et al., 2021; Colombo et al., 2020). The aim of this round robin was to compare the different energetic test procedures available for the rapid estimation of the fatigue limit of