PSI - Issue 57

Lewis Milne et al. / Procedia Structural Integrity 57 (2024) 365–374

370

6

Lewis Milne et al. / Structural Integrity Procedia 00 (2019) 000 – 000

(a)

(b)

Figure 5 – (a) Fracture surface and (b) Fracture origin for the S355JR 380MPa UFT sample

7. UFT Fracture Surface Analysis One of the characteristic features of VHCF failure is often considered to be the transition from surface fracture origination to fracture initiation from defects within the body of the material (Hong and Sun 2017). For all of the UFT tests in the current investigation, however, failure appeared to originate at or near the surface of the specimen, with no clear defects being observed at the origins. SEM images of the fracture surface of a representative UFT sample are provided in Figure 5. This is consistent with other high-ferrite steels tested in literature, where it was observed that the fracture did not transition to subsurface defects but instead changes from athermal slip band formation to intergranular crack formation along grain boundaries at the surface (Pu et al. 2019; Torabian et al. 2017). As such, it is proposed that this is the case for the steels in this investigation. 8. Evaluation of Frequency Sensitivity Due to the lack of definitive fatigue limit results for the Q355B material, analysis of the frequency sensitivity was made based on the finite life regime. Two different methods were used for this comparison, as adapted from the papers outlined in section 1. The first comparison method to be applied was an empirical comparison based on the Average Discrepancy between the SN curves at the two test frequencies, which was previously applied by the authors to S275JR steel (Gorash et al. 2023). In the previous investigation, the discrepancy in stress amplitude between the conventional and ultrasonic SN curves at the same number of cycles to failure was evaluated along the full length of the SN curves, from 10 4 to 10 10 cycles, and averaged out to obtain the average discrepancy value. This average discrepancy could then be subtracted from the UFT data to allow comparison between the two test frequencies. This method was applied to both the Q355B and S355JR results in this investigation, with one modification. To ensure that the comparison was not being skewed by extrapolated values, the discrepancy between the ultrasonic and conventional frequency SN curves was only evaluated where there was an overlap in test data. As such, the longest lasting specimen at conventional frequencies was taken as the upper bound, and the shortest lasting specimen in the UFT test as the lower bound for the comparison. The corresponding range used for comparison is shown for Q355B in Figure 6.