The study intended for clarify that dominant factors of fatigue strength of T joint which welded by laser arc hybrid welding. Fatigue test has been conducted using 5 types of HT780 T joints fabricated under different welding conditions respectively. Dominant factors for fatigue strength of T joint are Frank angle and curvature at toe of T joint fillet. The fatigue strength is increasing inverse proportional to Frank angle and proportional to curvature at toe. Heat-filler feeding ratio affects Frank angle and curvature at toe. The former would be decreased and later should be increased by heat-filler feeding ratio increasing. In the study, fatigue strength of the one type of joint shall be recognized as JSSC fatigue strength class D. All types of joint in the study can be deal as at least JSSC fatigue strength class E. Frank angle of these joints are smaller than 47 degree and curvature of toe are larger than 1.3mm. These quantitative results of the study are very useful for quality control of practical fabrication. That secures the fatigue strength of Non-load carry T joint welded by laser arc hybrid welding.
