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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Fig. 3. a) dob-bone specimen geometry; b) experimental setup

The specimen rupture instant has been taken as the reference time for synchronizing the data. To clean the temperature signal from noise during static tests, a locally weighted scatter plot smooth filter (rlowess) with a data span of 10%, was adopted. To find the limit stress, two linear regressions were performed on the temperature data set of the Phase 1 (thermoelastic trend) and Phase 2 (plateau region). The limit stress could be found as the relative stress level of the intersection point between these two straight line. For the stepwise fatigue tests, the stabilization temperature was estimated as the average value with one standard deviation of the temperature signal for each block, excluding the point in the transition from one stress level to the other. Generally, these temperature points are within a span of 30% in the initial part and of 10% in the final part of the stress level block. The specimen’s surface temperature variation ΔT was estimated as the difference between the instantaneous temperature and the initial value of temperature recorded at time zero.

4. Results and discussion 4.1. Static tensile test

In static tensile test, the stress rate has to be choose properly in order to assure adiabatic conditions. In the following figures, the applied stress is reported versus the specimen’s surface temperature variation, estimated as the difference between the instantaneous temperature and the initial temperature of the surface recorded at time zero (ΔT = T i – T 0 ). For all of the adopted stress rate is reported only one graph as an example, considering that the other tests exhibit the same thermal behavior. For a stress rate equal to 200 MPa/min, the temperature trend has not been reported since it showed a slow test velocity which allows the specimen to exchange heat with the surrounding environment, i.e. the energetic release is not adiabatic. For a stress rate of 400 MPa/min (Fig. 4), in the initial part of the ΔT-t curve it is possible to distinguish an almost linear temperature trend. At a certain point, it deviates from the linearity reaching a minimum value. 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). The stress value, related to the intersection point of the temperature regression lines, could be linked to the macroscopic stress that leads to the irreversible plasticization phenomena within the material. The limit stress has been evaluated on three tests, obtaining a value equals to 222.2±4.0 MPa.