PSI - Issue 47

R. Nobile et al. / Procedia Structural Integrity 47 (2023) 176–184

179

4

R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

(a) (b) Fig. 3. (a) Four-wire connection technique for measure electrical resistance; (b) Scheme of the electrical connections where R s and R c are the resistance of the specimen and the resistance due to contact, respectively. 2.3. Thermographic monitoring procedure The tested specimens were monitored at regular time intervals during the fatigue tests using a FLIR A655sc thermal imaging camera, equipped with an uncooled micro-bolometric sensor that provides 14-bit data up to 50 frames per second, as shown in the setup of Fig. 4a. The surface of the specimen to be monitored was suitably painted with a high temperature matt black paint, proper for thermal analysis. The thermal sequences were acquired at regular intervals of 5000 cycles with a resolution of 640 x 240 pixels, using a frame rate of 100 Hz and an acquisition time of 10s. The thermal data obtained were previously processed with the commercial software Research-IR which allows to convert the thermal sequences from ".seq" files into ".mat" or ".csv" files and subsequently processed in Matlab with specific analysis routines. A damage parameter has been proposed by the authors in this study (Nobile et al. (2022)) based on the thermal contrast of irreversible thermoelastic sources to study the evolution of fatigue damage in selected areas. Assuming initially undamaged conditions, the D diss parameter is related to the dissipative heat release which provides interesting information on the material degradation as the fatigue life percentage varies (N/N tot ) expressed by the following relationship: ( ) = [ − 0 0 | − − 0 0 | ] = [ 0 | − 0 | ]∗100 (1) where T 0 [K] is the absolute temperature at medium load detected in the initial conditions of the undamaged specimen. The two terms of Eq. (1) are related to the chosen regions of interest (ROI) (Fig. 4b), damaged and undamaged, at a given instant of the fatigue life. The D diss parameter represents the evolution of the damage associated with a normalized contrast evaluated at the average load level in the selected damaged and undamaged areas.

(a) (b) Fig. 4. (a) Thermographic setup and (b) Example of thermal map for A3 specimen with ROIs at 70 % of fatigue life.