PSI - Issue 33

4

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

751

Fig. 2. Temperature trend during a static tensile test

The use of high precision IR sensors allows to define experimental temperature vs. time diagram during static tensile test in order to define the stress at which the linearity is lost. In 2010, Clienti et al. (Clienti et al., 2010) for the first time correlated the damage stress σ lim related to the first deviation from linearity of ∆T temperature increment during static test (end of phase I) to the fatigue limit of plastic materials. If it is possible during a static test to estimate the stress at which the temperature trend deviates from linearity, that stress could be related to a critical macro stress σ lim which is able to produce in the material irreversible micro-plasticity. This critical stress is the same stress that, if cyclically applied to the material, will increase the microplastic area up to produce microcracks, hence fatigue failure. 3. Materials and Methods Static tensile tests and stepwise fatigue tests were carried out on dogbone specimens made of C45 steel with a cross section of 12mm x 6mm (Fig. 3a). All the tests were performed with a servo-hydraulic load machine MTS 810 (Fig. 3b) with a maximum load of 250 kN. The static tests were performed under stress control adopting three different stress rate: 200 MPa/min, 400 MPa/min and 800 MPa/min. For each stress rate, three specimens were tested. The stepwise fatigue tests were conducted on two specimens with a stress ratio R= -1 and a test frequency f= 20 Hz. An increasing value of the maximum applied stress was adopted, ranging from 180 MPa up to 320 MPa, with a stress step Δσ= 20 MPa and a number of cycles per block equal to 20000. The infrared camera FLIR A40 was used to monitor the specimen’s surface temperature. For the static tensile tests, a sample rate of 2 image per second was adopted with a temperature measurement range between -40°C and +120°C, while for the stepwise fatigue tests a sample rate of 2 image per minute was adopted with a temperature measurement range between 0°C and +500°C. The specimen surface was deeply cleaned from rolling oil by means of absorbent paper; then it was coated with high-emissivity black paint (emissivity up to 0.98). During all the tests the maximum temperature value of a rectangular measurement area, placed on the entire length of the specimen’s reduced section, has been recorded.