Fatigue damage of welded high-strength steel details improved by post-weld treatment subjected to critical cyclic loading conditions

Abstract

High-frequency mechanical impact (HFMI) treatment has proven its potential in many aspects, however the improvement under the conditions of high stress ratios, stress ranges and variable amplitude loading (VAL) has not been sufficiently addressed. In that respect, the aim of this work is to verify the indications and trends in the recently published literature and to understand the local weld toe mechanisms related to several cyclic loading conditions. The investigations were performed for three types of welds subjected to high stress ratios, ranges as well as VAL in order to represent the real-life scenarios, where the aim was to include the state of compressive residual stresses, cyclic behaviour of material and eventually estimate the fatigue life.The induced beneficial stresses were relaxed after applying compressive overloads (OL). However, a tensile OL indicated a potential benefit, by reducing the extent of residual stress relaxation. With the increasing stress ratio (R) and maximum stress (σmax), the improve- ment due to the HFMI decreased 59% as compared with the cases for OL = 0.8fy. The estimated damage decreased rapidly from R = −1 to R = 0.2, however it decreased slowly from R = 0.2 to R = 0.5, indicating that the effect of HFMI was reduced for high stress ratios due to the stress relaxation. The remaining improvement was then governed by the modification of weld geometry and the strain hardening effect. The fatigue damage was decreased from as-welded (AW) to HFMI states. For R = −1, longitudinal attachments were the most improved joint with 86%, while transverse attachments and butt joints were improved by 73% and 61%, respectively. The fatigue damage was increased when relatively larger block loads applied. Hence, the HFMI improvement was reduced 20% for transverse and longitudinal attachments and 14% for butt joints. The damage for σmax = 0.6fy in HFMI was similar to the damage for σmax = 0.3fy in AW, despite a doubling of the maximum CAL, which indicated promising potential of the HFMI improvement.