In order to clarify the discrepancy of the fatigue life of welded joint specimens and that of welded structures, it is indispensable to detect the fatigue crack initiation life. For this purpose, the fatigue testing machine was stopped automatically by means of an electrical signal which was generated when the grille of copper foil on welded joints broke. Fracture of the grille corresponded to the crack initiation on the toe of weld of welded joints, because the grille adhered strongly on the weld zone through insulation coated on the weld zone. The grille was put in place by the vacuum deposition. The depth of the crack at the time of fatigue crack initiation was 0.2∼2mm on the fracture surface. This was well distinguishable by a colored part which appeared by heating the specimen after the fatigue testing machine was stopped. The above described treatment of specimens was found not to influence the fatigue life. So, this method is regarded as an effective way for detecting the fatigue crack initiation life. It was also revealed that the aspect ratio decreased with the increment of stress range. The smallest value of the aspect ratio was 0.05.
