PSI - Issue 57

Lewis Milne et al. / Procedia Structural Integrity 57 (2024) 365–374 Lewis Milne et al. / Structural Integrity Procedia 00 (2019) 000 – 000

371

7

500

Q355B 20Hz Q355B 20kHz Run Outs

450

400

Lower bound – 100MPa increase at 20kHz

350

Upper bound – 136MPa increase at 20kHz

300

250

Stress Amplitude (MPa)

200

1,00E+04

1,00E+05

1,00E+06

1,00E+07

1,00E+08

1,00E+09

Number of cycles to failure

Figure 6 - Evaluation of discrepancy between the 20Hz and 20kHz SN curves at the upper and lower bounds for the comparison for Q355B ste el

Using this method, the average discrepancy in the SN curves between ultrasonic and conventional frequency was evaluated as 145MPa for the S355JR and 118MPa for the Q355B. To take into account the difference in strength of the materials, the discrepancy magnitudes were normalized relative to each material’s corresponding yield strength. This provided an average discrepancy in the SN curves of 37% and 28% of the yield strength for S355JR and Q355B respectively. The observed discrepancy in the results between the two test frequencies was therefore 1.32x larger for the S355JR than the Q355B based on these normalized values. As the S355JR was tested with a larger risk volume at conventional frequencies, as opposed to Q355B which was tested with the same geometry at both frequencies, it is proposed that this 1.32x increase in discrepancy for the S355JR is caused by the additional influence of size effects between the two tests. This would therefore suggest that the frequency sensitivity is being overestimated for S355JR due to the size effects. The second comparison method was through the use of the frequency sensitivity parameter , as defined by Bach et al. (2018). To evaluate based on test data fora given material, equation 1 is evaluated between pairs of datapoints at an equivalent numbers of cycles to failure for both test frequencies. In this investigation, was evaluated for five different specimen pairs for both Q355B and S355JR, and the average frequency sensitivity parameter value, ̅ , was determined for each. The average frequency sensitivity was therefore evaluated as ̅ = 0.0664 for S355JR and ̅ = 0.0356 for Q355B. As expected, the frequency sensitivity parameter is higher for S355JR than for Q355B. For comparison, ̅ was also evaluated for a number of ferritic-pearlitic steels in literature. The comparison of these results plotted against the ferrite volume content of the steels is given in Figure 7.

0,1

c15e (Bach et al., 2018)

S275JR (Gorash et al., 2023)

c45e (Bach et al., 2018)

0,08

̅

S15C (Guennec et al., 2015)

Q345 (Liu et al., 2016)

0,06

S355JR (Current Investigation)

c60e (Bach et al., 2018)

0,04

Q355B (Current Investigation)

0,02

Wheel Steel A (Li et al., 2019)

0

Average Frequency Sensitivity,

0

10

20

30

40

50

60

70

80

90

100

-0,02

Figure 7 – Average frequency sensitivity, ̅, vs ferrite volume content for a range of ferritic-pearlitic steels Ferrite Vol %