Abstract
This study aimed to identify the fatigue crack initiation site of high-frequency mechanical impact (HFMI)-treated high-strength steel welded joints subjected to high peak stresses; the impact of HFMI treatment residual stress relaxation being of particular interest. First, the compressive residual stresses induced by HFMI treatment and their changes due to applied high peak stresses were quantified using advanced measurement techniques. Then, several features of crack initiation sites according to levels of applied peak stresses were identified through fracture surface observation of failed specimens. The relaxation behavior was simulated with finite element (FE) analyses incorporating the experimentally characterized residual stress field, load cycles including high peak load, improved weld geometry and non-linear material behavior. With local strain and local mean stress after relaxation, fatigue damage assessments along the surface of the HFMI groove were performed using the Smith–Watson–Topper (SWT) parameter to identify the critical location and compared with actual crack initiation sites. The obtained results demonstrate the shift of the crack initiation most prone position along the surface of the HFMI groove, resulting from a combination of stress concentration and residual stress relaxation effect.
Highlights
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
The element size was set to about 0.1 mm around the high-frequency mechanical impact (HFMI) groove, and the size was gradually increased towards the other global parts
Six simulation results are presented as parameters: the residual stress either as HC or LC and the material properties as BM-1, BM-2 or BM-3
Summary
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. 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 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]
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