PSI - Issue 57

Grégoire Brot et al. / Procedia Structural Integrity 57 (2024) 53–60 G. Brot et al. / Structural Integrity Procedia 00 (2023) 000–000

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tested. This material has a bimodal microstructure, i.e. it contains both primary α grains and α + β lamellar grains. Its mean grain size is 10 µ m.

Fig. 1. Geometries of the test specimens: a) tested at f load = 15 . 67 Hz, b) tested at f load = 800 Hz (in mm).

All SH experimentation was done at room temperature with a load ratio of R σ = − 1. Testing with a loading frequency of f load = 15 . 67 Hz was performed on one specimens of the five LPBF material grades and on one wrought specimen. SH testing was also done at f load = 15 . 67 Hz on specimens that were previously deformed in tension until ε pl ≈ 4 %. Testing with f load = 800 Hz was conducted on one sample of P 1 650 ◦ C, P 1 920 ◦ C and P 1 1020 ◦ C, i.e. grades with di ff erent microstructures but the same porosity level. Some experimental parameters such as the number of cycle per loading step or the IR camera acquisition frequency were adapted depending on the loading frequency f load (Table 1). All SH tests were conducted until specimen failure. During all tests, temperature of black-painted specimen was measured with an infrared (IR) camera SC7000 from FLIR. Cooling pauses were done in between loading steps so that specimen’s temperature reaches a value close to the initial temperature at the previous step. When analysis thermographic data, a 0D approach was used. Data processing was done on the the mean surface temperature on the the specimen gauge noted T ( t ). Evolution of T ( t ) is presented for some specimens on Figure 2. f load (Hz) σ 0 ∆ σ (MPa) Number of cycles per loading step f IR (Hz) Lock-in thermography 15.67 50 or 100 25 6000 99 Yes 800 150 25 100 000 10 No Table 1. Self-heating test parameters: f load is the stress loading frequency, σ 0 is the stress amplitude during the first loading step, ∆ σ is the stress amplitude increment between two loading steps and f IR is the acquisition frequency of IR camera. For tests at f load = 15 . 67 Hz, the used testing machine is a servohydraulic one from Schenck with a maximal load capacity of 100 kN. To avoid recording too large IR data, two IR films of 60 s are automatically captured at the beginning and the end of each loading steps. Films at the beginning record the last 5 s of the cooling pause to assess the initial temperature and the first 860 load cycles (Figure 2.a). Films at the end of steps recorded the last 860 cycles and the first 5 s of cooling pause to estimate the thermal time constant. In parallel to the temperature field, the lock-in force signal was recorded by the IR camera. Krapez et al. (2000) give more details on lock-in thermography use in the context of fatigue limit assessment of metallic materials. In order not to influence demodulation of temperature signal, the value of frequency was chosen not to have common divisor with the electric supply frequency ( f AC = 50Hz) nor the with the IR camera acquisition frequency (Table 1). Thermal steady-state is reached at the end of all excepted last loading steps. As f IR >> f load , T ( t ) is well sampled and can be demodulated (Figure 2.b). Signal processing of T ( t ) extracts the steady state temperature increment ∆ T stab , the initial heating speed per cycle S heat , i , the amplitude of the first harmonic of temperature signal A f and the one of the second harmonic A 2 f . S heat , i is assessed using a linear interpolation of T ( t ) on the first 860 cycles ( ≡ 55 s). The used demodulation method is the one presented in Krapez et al. (2000). For tests at f load = 800 Hz, the used testing machined is a 1 kHz high-cycle fatigue machine from MTS with a 25 kN maximal load. The load applied on specimen is determined by subtracting dynamic e ff ects determined with an accelerometer placed near the load cell. With both fatigue machines, the imposed stress amplitude is reached in about 1 s at the beginning of loading steps. Number of cycles per step was set to 100 000 in order to study stress