PSI - Issue 57

SEVEDE Théo et al. / Procedia Structural Integrity 57 (2024) 335–342 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 2 − ′ ( , ) ( ) 1 ( ) = ( , ) Therefore, the only parameters to know are the conductivity, the specific heat and the density of the material which can be easily obtained. The steady state regime consists of waiting the stabilisation of the temperature field which usually occurs when the thermal equilibrium is reached. The heat equation can then be written: ′ ( ) − ′ 2 ( ) 2 − ′ ( ) ( ) 1 ( ) = ( ) Compared to the transient regime, the heat exchange coefficients need to be known. The inconvenient of this method is that the stabilisation of the specimen thermal state can lead to such high temperatures that the original hypothesis of a small temperature variation could be challenged. In the framework of this study, the transient regime approach is considered. The 1D analysis strategy is applied to assess the thermal source profile using ML340 steel specimens. This can enable to evaluate different experimental parameters. The first parameter is the resolution of the IR images. Indeed, a high image resolution is needed to be able to identify the parameters of the chosen function and identify a reliable temperature field. The second parameter is the measurement noise level. Indeed, for the low stress amplitudes, the measurement noise level is not negligible comparing to the temperature elevation measurement and thus it remains difficult to correctly exploit the results. Therefore, the effect of noise needs to be reduced as much as possible, either by using a very precise measurement tool,or by using a thermalsignal large enough than the measurement noise, to have less influence on the average thermal behavior of the specimen. Once these precautions have been taken, it is possible to obtain a profile of the thermal source as a function of the axial position through the specimen length. 341 7 ̇ ( , ) − ′ 2 ( , ) 3.4. Application of the method to determine the thermal source from the temperature measurements results

Figure 4: thermal source distribution on a ML340 steel for different maximum stress amplitudes at the central point of the sample

Figure 4 represents the identified thermal source along an ultrasonic specimen made of ML340 steel. One can note that the thermalsource profile is heterogeneous in accordance with the applied stress amplitude wave profile through