PSI - Issue 75

Gary B. Marquis et al. / Procedia Structural Integrity 75 (2025) 530–537 Marquis, Barsoum & Leitner / Structural Integrity Procedia (2025)

533

4

4. Fatigue Strength Improvement and Assessment To evaluate the HFMI fatigue strength improvements, three main fatigue assessment methods are employed: the nominal stress method, the structural hot-spot stress method, and the effective notch stress method. The nominal stress method is most used and is applicable to design classes FAT 50 to FAT 90. The structural hot-spot stress method is better suited for complex geometries where stress concentrations are more pronounced, while the effective notch stress method provides the highest resolution for detailed local analysis, especially in advanced fatigue-critical applications. One significant update concerns yield strength adjustments. For steels with yield strength exceeding 355 MPa, fatigue strength can be increased by one FAT class for every 200 MPa increase in yield strength, up to a maximum of eight FAT classes for steels up to 1300 MPa. These relationships are graphically presented in Figure 3.

Figure 3. Recommended increase in number of FAT classes as a function of yield strength for R ≤ 0.15 when fatigue assessment is based on nominal stress or structural hot spot stress. Another key factor influencing fatigue performance is the load stress ratio, or R-ratio. As the R-ratio increases, the beneficial effect of HFMI is reduced due to the diminishing stabilizing effect of compressive residual stresses. The guideline introduces a reduction factor, f R , to account for this effect. For example, an R-ratio of 0.35 leads to a fatigue strength reduction of approximately 21%, corresponding to f R = 0.79. If the R-ratio exceeds 0.52, fatigue improvement must be validated by experimental testing. Table 1 shows the recommended maximum increase in number of fatigue classes and the maximum improvement factor, f st , at N = 2·106 for different ranges of yield strength at R ≤ 0.15 for nominal and structural hot-spot stress methods. Similar recommendations exists for the effective notch stress method. Table 1. Maximum improvement factor at N = 2 ⋅ 10 6 for different ranges of yield strength at R ≤ 0.15. f y [MPa]

Increase in # of FAT classes

Improvement factor, f st

235 ≤ f y < 355 355≤ f y < 550 550 ≤ f y < 750 750 ≤ f y < 950 950 ≤ f y ≤1300

4 5 6 7 8

1.60 1.80 2.00 2.25 2.55