PSI - Issue 75

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

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To ensure reliable fatigue performance, stress limits must also be observed. For nominal stress-based design, the stress range must not exceed 1.5 times the yield strength, and the maximum tensile stress should remain below the yield strength. For compressive loads, the minimum stress must be greater than –0.8 times the yield strength. For structural hot-spot stress, the stress range should not exceed twice the yield strength. These limits ensure that the beneficial residual stresses induced by HFMI are maintained throughout the structure’s service life. Updated S–N curves reflecting these improvements are provided in the guideline. HFMI-treated welds follow a steeper slope in the S– N diagram, with a slope of m = 5 up to 10⁷ cycles and m′ = 9 beyond that point, compared to the as-welded condition where the slope transitions from m = 3 to m = 5. This allows for more accurate life predictions, especially under variable amplitude loading. Figure 4 exemplifies this for FAT 80 as welded and HFMI 4 steps up to FAT 125. The highest allowable fatigue strength class after HFMI treatment, based on the nominal stress method, is FAT 180 for R ≤ 0.15. For higher R -ratios, the allowable FAT class is reduced.

Figure 4. S-N curves for an example of FAT 80 as welded and HFMI treated 4 steps up to FAT 125. For N > 10 7 cycles, the dashed lines represent constant amplitude loading without a fatigue limit and the solid lines are for variable amplitude loading. 3. HFMI Treatment for Retrofitting Existing Structures HFMI treatment is a proven method for extending the fatigue life of welded steel structures already in service, provided defects are small and the process is properly controlled. Experimental studies show that HFMI remains effective even when applied to pre-fatigued joints with small surface cracks. Cracks up to 1.5 mm deep have been shown to stop propagating after proper HFMI treatment, resulting in significant life extension. This makes HFMI suitable as a crack-arresting technique, provided that proper inspection and application procedures are followed. The retrofitting process begins with non-destructive testing (NDT), typically using dye penetrant or magnetic particle methods, to detect surface-breaking cracks. If cracks are found, their depth and sharpness are assessed to determine suitability for HFMI. Cracks that are short and blunt ( ≤ 1.5 mm) may be treated directly, while larger or sharper flaws require grinding or welding repair before HFMI is applied. Once suitability is confirmed, the structure's remaining fatigue life is estimated using S-N curves adapted for HFMI-treated welds. Care must be taken to ensure the treatment does not shift failure to other locations, such as the weld root or untreated areas. HFMI should be applied directly to the weld toe using an approved procedure. The IIW general recommendations for retrofitting of welded structures [10] provides a maintenance and inspection procedure for life management of welded steel structures, shown in figure 5 (left). The new chapter included in the update version of the IIW HFMI recommendations serves as a complement to Miki [10], figure 5 (right).