This study examined the fatigue strength of cruciform joints and longitudinal joints fabricated with laser-arc hybrid welding, in which tack welds were installed at the connection. The cruciform joints were tested under tensile cyclic loads and out-of-plane bending cyclic loads. In the longitudinal joints, four-point bending fatigue tests were performed on plate girders fabricated with the hybrid welding. In addition, finite element analysis was performed on the cruciform joint to investigate the effect of tack welds on stress concentration at weld toes. The test results revealed that the fatigue strengths of the hybrid welded joints are equivalent to those of arc welded joints with or without tack welds and satisfy the fatigue design curves specified in JSSC recommendation. Moreover, the analysis results indicated that tack welds have little influence on stress concentration at weld toes in cruciform joints fabricated with laser-arc hybrid welding.
To achieve both welding productivity and quality, in-process welding quality control technology is required. The authors are developing method to detect welding defects directly during welding by using image sensing technology for production lines. To detect the welding defects directly, observing the welding phenomena just near arc where the base metal is melted and defects should occur is important. Conventionally, high speed camera with laser external light source were used for observing the welding phenomena just near arc, however the cost is high and the equipment is complex for production lines. In this paper, the observation results and the required time resolution of welding phenomena just near the arc were verified by comparison with two observation method, using only an industrial CMOS camera without using an external light source and using high speed camera with laser light source. By comparison with observation result, the welding phenomena just near the arc could be observed by using only CMOS camera from backside when the molten pool flowed under the arc. And the required time resolution for observation the phenomena just near the arc was as follows, fluctuation of the arc: over 100Hz, fluctuation of the molten pool: over 60Hz, droplet transfer: over 600Hz.
The aim of this study is to confirm that ductile crack growth resistance of cracked components can be predicted by simulation model with the proposed damage model, which is derived by unit cell analyses with an initial void and reflects void growth behavior until ductile crack initiation. Critical strain under constant stress triaxiality history, which is used in the damage model as ductility of the material, could be determined properly using tensile test results by inverse analyses. Tensile tests using notched round bar specimens, which simply represent stress triaxiality history similar to cracked component, were conducted for two different steels and the proposed damage model was found to be validated for evaluation of ductile crack initiation limit. Then, ductile crack growth tests of two types of bend specimens with a machined notch and a fatigue pre-crack were conducted for a mild steel. It was demonstrated that ductile crack growth resistance could be reproduced using simulation model with the proposed damage model with high accuracy and the influence of plastic constraint to ductile crack growth behavior could be simulated.
High strength steel sheets have been developed for both reducing automobile weight and keeping passengers’ safety. Delayed fracture due to diffusible hydrogen is known as one of problems in arc welds of high strength steel plates, which is often prevented by applying pre/post heating processes. However, there is little reported regarding hydrogen diffusion behavior during rapid temperature change in resistance spot welds of steel sheets. Hydrogen diffusion behavior is affected by thermal and stress fields. Therefore the effect of rapid change in these fields needs to be properly considered for elucidation of hydrogen diffusion behavior in resistance spot welds. This study aims to clarify the delayed fracture mechanism of resistance spot welds. The hydrogen diffusion simulation model considering the effects of thermal and stress changes during welding is constructed. It is assumed in the model that hydrogen solubility and diffusivity in the nugget are larger than those of base metal. A change in fracture pattern dependent on hydrogen environment can be well explained using this model. The numerical simulation predicts that high residual stress leads to hydrogen concentration at the edge of the nugget after welding. This concentration would be the main factor of the delayed fracture in the nugget when the nugget diameter is small. In this situation, hydrogen is introduced from antirust oil, water and contamination on sheet surfaces to the nugget during welding. Numerical simulation assumed in actual use is also performed, in which the hydrogen is introduced from the sheet surfaces. Hydrogen concentrates at the edge of the nugget in relatively mild hydrogen environment, whereas in severe environment it highly concentrates in the HAZ around the nugget before concentrating in the nugget. These calculated results can well explain the transition of fracture pattern from the nugget to the HAZ in the experiment.