부산대학교 도서관

Abstract : First study : Failure analysis of SCM435 high strength bolt have been studied including macroscopic and microscopic fracture observation, Energy Dispersive X-ray Spectroscopy, Vickers hardness test and applied stress evaluation. cracks (ratchet marks) were generated by the repetitive loads acting on the bolts, initial stress of bolt and the stress concentration. The applied stress was found to be slightly higher than the fatigue limit of the material. The mold maximum temperature is about 130 ℃, and it was fracture by the cyclic stress because the strength is lower than room temperature. The initial stress of bolt must be removed, and the mold temperature during the process must be maintained by room temperature. Bolts are recommended to be peened to improve fatigue limit.Second study: Since the injection mold is repeatedly used for mass production, fatigue phenomenon due to cyclic stress may occur. The structure is damaged the surface or interior due to cyclic stress or strain. This study analyzed the failure of SCM435 high-tension bolts connecting the upper and lower parts of a three-stage injection molding machine. The bolts undergo an accurate heat treatment to prevent the formation of chromium carbide, and must prevent the action of dynamic stresses. Bolts need accurate tightening and accuracy of heat treatment, and screws are required compression residual stress due to peening. Third study analyzed the causes of failure of SCM435 bolts that fix the springs of automobile air brakes that have been fractured during use. The cause of failure was analyzed using SEM, EDS, metallogical microscope and Vickers hardness tester. In the fracture, the ratchet mark began at the outer boundary of crack origin, and the grains at crack origin were found to have clear intergranular corrosion. One SCM435 bolt was subjected to a stress of 398 MPa, it's a stress of about 80% of the fatigue limit. As a result of such a large applied stress, cracks occurred at the corrosion origin and were fractured. In order to prevent the SCM435 bolt from fracture, it is necessary to use the correct composition, the accuracy of heat treatment, preventing damage by external impact, preventing corrosion of the damage part by moisture, and introduction a compressive residual stress by peening.