荷重非伝達型十字溶接継手の疲労強度改善に及ぼす超音波衝撃処理(UIT)の影響に関する研究

Ultrasonic impact treatment (UIT) is a remarkable post-weld technique, which applies mechanical impacts in combination with ultrasound into welded joints and plastically deforms the weld toe, consequently introducing beneficial compressive residual stress near the treated area. Recent experimental studies demonstrate that acoustic softening of metals during ultrasonic impact (UI) may lead to increased plastic deformation and play an important role in the mechanism of UIT. In this study, a 3D simulation approach including thermo-mechanical welding simulation and dynamic elastic-plastic finite element (FE) analysis of UIT process of welded joints has been proposed. In the FE model, the actual process parameters and ultrasonic-induced material softening, which is appropriately adjusted to fit experimental results, are considered. The predicted residual stress distributions of non-load-carrying cruciform joints before and after UIT are compared with experimental results, showing a fairly good agreement with each other. In addition, an evaluation of fatigue strength of welded joints based on fracture mechanics has been carried out. The results not only clearly distinguish the difference of fatigue strengths of as -weld and UIT welds, but also show the influence of preload and stress ratio on the improvement, so that the proposed model may provide an effective tool to simulate UIT-process in engineering structures.