Yuki Ono

This paper investigates the primary factors influencing the fatigue behavior of butt-welded joints connecting normal- and high-strength steels of varying thicknesses. The research focused on dissimilar joints composed of 20 mm S690QL and 15 mm AH36, with similar joints (20 mm S690QL and 15 mm S690QL) included for comparison. Comprehensive experimental measurements of fatigue strength, misalignment, local weld geometry, and local material hardness were carried out for both joint types featuring thickness steps. Fatigue tests and fracture surface analysis were conducted to identify failure locations for crack initiation. Fatigue strength assessments using the structural stress and effective notch stress approaches were carried out to evaluate the influence of geometry-dependent stress concentration on fatigue failure behavior. The experimental results indicate that the failure locations varied depending on the specimen type. For the dissimilar joints, the locations were identified at the weld toe, specifically at the boundary between the heat-affected zone and weld metal in S690 side, which exhibited lower microhardness values. A comparison of local stress ranges, accounting for the effects of misalignments and local weld geometries, showed higher stress ranges at the failure locations. The scatter of fatigue test results was reduced significantly when the local stress ranges were considered. Consequently, the study confirms that the interaction of local material properties and stress concentrations from global and local weld geometries influences the fatigue failure behavior of high-strength dissimilar welded joints with thickness steps.

This study experimentally performed direct measurements of shot velocity using an ultra-high-speed camera. Two nozzle types and distance between nozzle and specimen were prepared, and shot velocity was measured from shot movement captured by image processing. Measured shot velocity agrees well with a value generally reported in previous studies, despite the shot velocity increases with increasing the distance between the nozzle and the specimen in case of a straight nozzle. Then, a numerical simulation on shot peening was conducted using RADIOSS, reflecting the shot velocity measurement results. The simulated residual stress was compared with measurement results obtained on base materials near weld toes. As a result, the shot peening analysis successfully captured the depths associated with the minimum compressive residual stress and the plastic region. Based on this analysis model, this study investigated the influence of shot angle, friction, and material hardening rules on the residual stress distribution. The investigation results showed that the shot angle and friction might be key influencing factors for better estimating the amount of compressive residual stress. Thus, the shot peening angle to weld toes should be certainly controlled.

This study investigates the influence of surface integrity and the localized fatigue phenomena on the initiation and propagation of short fatigue cracks in high-frequency mechanical impact (HFMI)-treated welded joints. The treated surface region, characterized by a compressive residual stress field, smooth notch geometry, and work hardening layer, improves welded joints’ fatigue strength. However, how these surface conditions influence the fatigue damage process zone during short crack initiation and growth is not yet well known. Therefore, this study systematically investigates the influence of different surface characteristics on fatigue life modeling of HFMI-treated welded joints made of high-strength steel. This is achieved using a non-local continuum damage mechanics-based approach of crack growth and elastic–plastic finite element simulation, explicitly modeling treated surface conditions. The simulated fatigue life is first verified with experiments and then applied to various surface conditions. The simulation results show that most of the fatigue life is spent until a crack size of 0.2 mm. The compressive residual stress field greatly extends both short crack initiation and propagation life, with its degree of contribution highly dependent on loading history and residual stress change. The role of the work hardening layer is mainly concentrated on improving fatigue life during short crack initiation and the very beginning of short crack growth.

This research studies the influence of high-peak loads on local relaxation of residual stress and fatigue damage in high-strength steel welded joints treated by high-frequency mechanical impact (HFMI) treatment. The joint behavior is simulated with elastic–plastic finite element analyses that account for the combined effect of geometry, residual stress, and material properties. This simulation uses two treated geometry models: with or without surface roughness on HFMI groove, and two material properties: S690QL and AH36 structural steels. The results show that surface roughness and load history, including high-peak loads, significantly influence fatigue response. It is revealed that the model neglecting the surface roughness cannot represent the amount of residual stress change and fatigue damage at less than 100 µm depth from the surface. In addition, the local yield strength in the HFMI-treated zone affects the plasticity behavior near the surface imperfection under the high-peak loads, which provides comparatively different fatigue damage between S690QL and AH36 in some cases. As a result, this study provides the further understanding needed to develop a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

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.