Effect of increased yield strength, R-ratio, and plate thickness on the fatigue resistance of high-frequency mechanical impact (HFMI)–treated steel joints

Abstract

In 2016, the International Institute of Welding (IIW) published a recommendation for high-frequency mechanical impact (HFMI) treatment for improving the fatigue strength of welded joints. Since the publication of the HFMI recommendations, numerous of studies have been published with a considerable amount of new fatigue test data focusing on various aspects of the improvement; influence of base material yield strength, and loading and thickness effects. Since the data was scarce covering some of the aspects when the recommendation was published, re-evaluating the recommendation with these new test data, presented within this work, will further validate and extend the recommendations. The analysis reveals that the recommended improvement of fatigue classes based on the base material yield strength is well applicable. In addition, the reduction of fatigue classes for higher R-ratios up to the defined value of R = 0.52 is well considered. Finally, the practicability of the thickness correction factor is also confirmed by leading to a conservative fatigue assessment. The ratio of the statistically evaluated FAT class to the recommended value is shown to be conservative with a value of above one for almost every data set; however, even the ratio is below one in some minor cases, every single test data point in this study is assessed conservatively validating the applicability of the recommendation.

Effect of static load during HFMI treatment on fatigue strength and residual stress field of longitudinal atachment welded joints

“…Fatigue strength improvement by HFMI treatment is known to depend on the stress ratio of fatigue loading, meaning that the fatigue strength benefits from HFMI treatment decrease as the stress ratio increases [3][4][5][6][7][8][9][10][11]. The IIW recommendations for HFMI treatment take this effect into account as an additional reduction in the number of FAT classes [2].…”

Damage-Based Assessment of the Fatigue Crack Initiation Site in High-Strength Steel Welded Joints Treated by HFMI

“…For welded joints, fatigue strength improvement can be achieved by using postweld treatment in which the aim is to modify the weld toe regions to avoid fatigue crack development. Among others, high-frequency mechanical impact (HFMI) treatment has received much attention in the last two decades [1][2][3][4][5][6][7][8][9][10][11]. The application of the HFMI treatment introduces compressive residual stress in the weld toe and material hardening in the surface layer, simultaneously improves the local weld geometry, and removes typical weld imperfections.…”

“…The application of the HFMI treatment introduces compressive residual stress in the weld toe and material hardening in the surface layer, simultaneously improves the local weld geometry, and removes typical weld imperfections. The degree of the fatigue strength improvement of HFMI has been related with the material yield strength (f y ), namely, it increases together with the steel grade [9][10][11]. Therefore, the use of HFMI treatment for high-strength steel welded joints may lead to a superior fatigue performance [9][10][11].…”

“…The degree of the fatigue strength improvement of HFMI has been related with the material yield strength (f y ), namely, it increases together with the steel grade [9][10][11]. Therefore, the use of HFMI treatment for high-strength steel welded joints may lead to a superior fatigue performance [9][10][11]. The primary reason for this is the extended fatigue crack initiation and propagation periods within short crack lengths [12].…”

Fatigue strength assessment of TIG-dressed high-strength steel cruciform joints by nominal and local approaches

“…Concerning the latter one, methods mostly influencing the residual stress condition [6] are considered, like shot peening [7], hammer/needle peening [8], HFMI treatment [9] as well as further non-mechanical techniques, such as post-weld heat treatment [10]. Especially in case of the methods based on an improvement of the local residual stress condition, special attention must be laid on increased stress ratios [11][12][13] or variable amplitude loads [14][15][16] mostly due to a possible relaxation of the compressive residual stress condition [17,18].…”