PSI - Issue 34

Dario Santonocito et al. / Procedia Structural Integrity 34 (2021) 211–220 D. Santonocito et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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In all the three tests, the temperature signal exhibits a first linear trend, due to the thermoelastic effect (ΔT 1 temperature series), then it deviates from the linearity reaching a plateau region (ΔT 2 temperature series). The transition between the two trends can be assessed by performing the linear regression of the two temperature series and making their intersect ion. The corresponding value of the stress, the limit stress σ lim , can be related to the first plastic damage introduced in the material. While test 1 (Fig. 5a) and test 2 (Fig. 5c) show the same temperature trend, with a gradual transition between Phase I and Phase II, the second one (Fig. 5b) has a more marked transition, with a value of limit stress considerably lower than the others. The average limit stress, for the three considered tensile tests, is equal to σ lim = 24.4±3.5 MPa. 4.3. Stepwise fatigue tests To verify the initiation of plastic damage within the material and to assess its fatigue limit, a rapid procedure has been carried out by increasing in a stepwise way the stress level during a fatigue test and monitoring the specimen temperature with an infrared camera. In Fig. 6 are reported the applied stress level vs the temperature trend for two of the tests performed. For each stress level, the stabilization temperature has been estimated. For the last stress level, when the specimen approaches to failure, no stabilization is reached by the temperature signal.

(a)

(b)

Fig. 6. Stepwise fatigue tests, with stress ratio R= 0.1 and test frequency f= 1 Hz.

It is possible to report the stabilization temperature vs. the applied stress level (Fig. 7), and observe a bilinear trend, with a knee region. By performing the linear regression of the points below and above the knee region, and making the intersection of the two straight line, it is possible to assess the fatigue limit in a rapid way by adopting the TM. While the first two stepwise fatigue tests show a clear bilinear trend (Fig. 7a,b), the third one has a very smooth transition near 27 MPa (Fig. 7c). Despite all, it is possible to perform the linear regression. For the three stepwise fatigue tests considered, an average value of the fatigue limit equal to σ 0,TM = 26.5±1.1 MPa has been estimated. The value of the limit stress, found with the STM, is in good agreement with the one of the fatigue limit found by TM. The limit stress is a more conservative fatigue parameter compared to the fatigue limit for a positive stress ratio of 0.1. Compressive loads are a more severe fatigue damaging conditions for the specimens. Both the testing procedure allow to reduce the overall testing time and amount of material to be tested.