Abstract High-frequency mechanical impact (HFMI) is among the most effective post-weld treatments for improving fatigue performance. However, the current IIW Recommendation provides design guidance only up to a stress ratio of R = 0. 52, leaving high-R applications without codified rules. This paper quantifies HFMI effectiveness at R = 0. 55 and R = 0. 70 under constant-amplitude four-point bending. A total of 92 transverse V- and X-butt welds in S355J2+N, S460NL, S690QL, and S960QL were tested at 5 Hz to failure or runout. The fatigue life improvement factor λ reached mean values of ₌₄₀₍ λ mean ≈ 5. 6 for S960 and ₌₄₀₍ λ mean ≈ 2. 2 for S690 at R=0. 55 R = 0. 55. These values represent conservative lower-bound estimates, as runouts were treated as failures at the test termination limit. The benefit persists to R=0. 70 R = 0. 70 provided that the nominal maximum stress remains below the yield strength; cases exceeding the yield strength show reduced gains. Crucially, all HFMI data points—including those loaded beyond the yield strength—lie above the draft IIW design lines.
Kepka et al. (2026) studied this question.