PSI - Issue 57

4

SEVEDE Théo et al. / Procedia Structural Integrity 57 (2024) 335–342 SEVEDE Théo/ Structural Integrity Procedia 00 (2019) 000 – 000

338

The Self-Heating method is based on the measurement of the temperature elevation at the surface of the specimen as a result of the intrinsic dissipation due to the micro-plasticity phenomenon. In this method, successive cyclic loading blocs of 10 4 cycles are performed with increasing the stress amplitudes, and the temperature evolution relative to each stress amplitude is registered using IR camera with a recording frequency of 100 frames per second. Finally, the 1D temperature elevation profile is obtained assuming a mean radial temperature projection in the cross section of the specimen. Figure 2 shows a diagram of a stress amplitude loadingbloc applied at a materialpoint of the specimen and the corresponding temperature elevation profile. One can note two steps for each loading block: firstly, the stress amplitude is applied for the specified numberof cycles and then the cyclic loading is interrupted in order to cool the specimen down to the initial equilibrium temperature. This protocol is repeated for each loading block, and the temperature evolution field relevant to each stress amplitude is recorded. The temperature field is measured on the longitudinal area of the specimen using an infrared camera. The recording parameters were selected in such a way to enable a high special resolution and maximize the number of pixels in the zone of interest of the sample. Fig.3 represents the temperature rise field obtained on a ML340 steel sample under a maximum stress amplitude (in the center of the sample) of 982 MPa, after 0.1s of cyclic loading. One can note that the temperature elevation field presents a heterogeneous profile through the sample length which requires to be considered for a relevant identification of the thermal source. The temperature rise profile is related to the stress amplitude profile along the sample length with a maximum in the center and equals zero at the extremities. In order to apply the self-heating method, the 1D approach is necessary to implement an appropriate methodology to determine the thermal source from the temperature elevation field. 2.2. Temperature Field measurement

Figure 3: temperature elevation profile through a ML340 specimen under a maximum stress amplitude at the centre point of 982 MPa