HFMI effectiveness at high stress ratios (R = 0.55–0.70) in four-point bending of high-strength steel butt welds

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 addresses that gap by quantifying HFMI effectiveness at R = 0.55 and R = 0.70 under constant-amplitude four-point bending. We tested 93 transverse V- and X-butt welds in S355J2 + N, S460NL, S690QL, and S960QL at 5 Hz to failure or 1×10⁷ cycles (51 specimens at R = 0.55; 42 at R = 0.70) in both as-welded and HFMI-treated conditions. HFMI shifts the S–N curves upward by up to six FAT classes and increases fatigue life typically by ≈ 3–10× (ranging from ≈ 1× to > 30× at R = 0.70 and up to > 200× at R = 0.55), with the largest gains observed for high-strength grades. The benefit persists to R = 0.70 provided the nominal maximum stress remains at or below yield (elastic regime); cases with local plastification show reduced gains. Elastic HFMI data scatter by only ≈ ± 0.19 decades around draft IIW design lines, supporting a cautious extension of HFMI design rules toward R ≤ 0.70 and supplying bending-mode evidence for components operating at elevated mean stress.