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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a test frequency f= 1 Hz, to limit the temperature increment below 5 °C. An increasing value of the maximum applied nominal stress was adopted, ranging from 10 MPa up to 16 MPa, with a stress step Δσ= 1 MPa and a number of cycles per block equal to 3000. Constant amplitude (CA) fatigue tests, with the same stress ratio and frequency of the stepwise tests, were performed to derive the S-N curve of the welded material. Infrared camera FLIR A40 was used to monitor the specimen’s surface temperature. For the static tensile tests, a sample rate of 1 image per second was adopted, with a temperature measurement range between -40°C and +120°C; while, for the fatigue tests, 1 image every 30s was acquired. During all the tests the maximum temperature value of a rectangular measurement area, placed on the entire length of the specimen’s reduced section, was recorded.

(b)

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Fig. 3. a) welding process, according to UNI 10520 and 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 of the temperature signal for each stress 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

To obtain a clear temperature signal during static tensile test, adiabatic conditions must be assured. The applied stress is reported versus the specimen’s surface temperature variation, estimated as the variation respect time zero (ΔT = T i – T 0 ). In Fig. 4, in the initial part of the ΔT-t curves it is possible to distinguish an almost linear temperature trend; then, it deviates from the linearity up to a minimum value. It is possible to draw two linear regression line, for the first linear decreasing trend (phase I, ΔT 1 fit point series) and the latter for the plateau region (phase II, ΔT 2 fit point series), not taking into account the temperature values near the slope change (Experimental Temperature series). According to the intersection point, the limit stress could be linked to the macroscopic stress that leads to the first irreversible plasticization phenomena within the material. The limit stress has been evaluated on three tests, obtaining a value equal to 13.9±0.1 MPa.