Patent ID: 7673388

Claim:
A method of forming a fracture start portion of a ductile metal part for forming the fracture start portion of the ductile metal part on an inner circumferential face of a through hole by irradiating laser to an opposing position of the inner circumferential face of the through hole of the ductile metal part having a predetermined through hole and by forming a plurality of recess parts at a predetermined interval from one opening to the other opening of the through hole, comprising: forming a recess part by irradiating the laser onto the inner circumferential face of the through hole while defocusing from the inner circumferential face of the through hole of the ductile metal part by a predetermined amount while supplying an assist gas to a position of the laser irradiation, wherein the defocusing of the laser is conducted such that the shape of the recess part becomes widest at the opening of the recess; and forming a series of the recess parts linearly separated from each other by a predetermined distance on the inner circumferential face of the through hole of the ductile metal part by moving the laser linearly at a predetermined speed from the opening on one side to the opening on the other side of the inner circumferential face of the ductile metal part while irradiating the laser onto the inner circumferential face of the ductile metal part at a predetermined pulse, wherein the predetermined distance between recess parts is such that an outer circumference of an opening of a recess part does not contact an outer circumference of an opening of an adjacent recess part.