On the Fatigue Strength Improvement Factor for High Frequency Mechanical Impact Treatment Method

Nowadays, the most commonly applied post weld treatment method improving fatigue strength of welded structures is the High Frequency Mechanical Impact (HFMI) treatment method. However, the treatment process is already well-known and widely used, there are several unanswered questions about its impact on the mechanical properties and fatigue behavior of treated welded structures. For understanding the mechanical background of the fatigue properties of HFMI-treated, welded, normal and high strength steel structures, it is necessary to analyze fatigue test results from many different aspects. According to previous studies it can be observed that fatigue strength of HFMI-treated steel specimens increases with the yield strength of base material. However, the fatigue strength of as-welded details is independent of the steel grade; thus if yield strength increases, fatigue strength improvement factors (ratio between the fatigue strengths of as-welded and HFMI-treated specimens) of HFMI-treated steel specimens should increase as well. In this paper, the relationship between steel grade and fatigue strength improvement factors of HFMI-treated details is investigated by using previous experimental results. A large number of previous experimental results are revised by the authors; published test results were collected and re-evaluated. Using the analyzed measures, the effect of HFMI treatments was analyzed. Fatigue strength improvement factor related to HFMI is calculated for two different types of structural details (cruciform joints and longitudinal attachments). For both cruciform and longitudinal joints, it is observed that the improvement factor decreases with increasing yield strength.