PSI - Issue 35

5

Martin Ferreira Fernandes et al. / Procedia Structural Integrity 35 (2022) 141–149 Martin Ferreira Fernandes et al. / Structural Integrity Procedia 00 (2021) 000 – 000

145

The average fatigue lives when the load holds of 10 s were applied at stress levels of 1000, 975, and 950 MPa reduced from 12299 to 1203, 23415 to 2334, and 83281 to 18475, respectively. The dwell-life debit was more pronounced for higher stress levels (i.e., 1000 MPa and 975 MPa) than the lower stress level (i.e., 950 MPa). The average dwell-fatigue life debits at 1000 MPa and 975 MPa were 10.2 and 10.0, respectively. However, the average reduction in fatigue life at 950 MPa was 4.5. Figs. 3b and 3c show the deformation curve measured during the first 400 dwell-fatigue cycles at a maximum stress level of 975 MPa and a hold period of 10 seconds. All the strain measurements were performed at the maximum stress level constant stages. The deformation results revealed that there was plastic strain accumulation during the dwell fatigue cycles. Equation 6 was obtained through the linear regression of the curve shown in Fig. 3c in the first 50 cycles, where is the strain and t is the time given in seconds. The strain rate, ⁄ or ̇ , can be obtained as the slope of the strain curve. The strain rate, calculated as the derivative of the strain function shown in Equation 6, during the first dwell-fatigue cycles at 975 MPa was 1.59 × 10 −2 −1 . As the strain rate increases, more slip systems are activated in the same period, and, consequently, the dwell-fatigue life decreases. Therefore, the dwell-fatigue damage was more pronounced during the first 50 cycles. ε=8.35×10 -3 +1.59×10 -5 t (6)

(a)

(b)

(c)

Fig. 3. (a) S-N curve of the dwell-fatigue data for load hold of 10 s. (b) Microdeformation x number of cycles and (c) Microdeformation x time for dwell-fatigue test at maximum stress of 975 MPa and a hold period of 10 s.