Ti 용접재의 피로크랙 진전거동에 관한 연구

Abstract

Fatigue fracture often occurs due to the fatigue crack in the machine and weling structure; and this is the result of severe operational environment, operational condition, and design condition caused by the large scaled and light weighted machine and structure in recent days. Therefore, safety and reliability should be secured to prevent the fatigue fracture of machine and structure, and the use of titanium has been increased in the metal material for the safety and reliability. The purpose of this thesis was to investigate the welding characteristics and the fatigue crack propagation behavior of titanium, commonly using power station, aircraft and ship. The experimented material was TIG welded in order to look over the characteristics according to the notch position and compare with other materials. We compared and reviewed th fatigue crack propagation behavior of nonwelded base metal and welded specimens having different notch position to evaluate the fatigue crack propagation behavior by welding condition. From these test, we reached conclusion as followings ; 1. 4 pass specimen has the highest hardness value and 5 pass and 7 pass specimens were the next in order. Also, the hardness distribution was very high in HAZ part rather than the welding zone, and it refers that this causes the decrease of fatigue life in welding part.. 2. At the same welding condition, 4 pass specimen which had the smallest number of welding pass had the highest tensile strength with 51kgf/㎟ and the tensile strength of 4 pass specimen was 10% higher than the one of 7 pass specimen, and its elongation reduced about 30%. 3. The initial fatigue crack propagation rate is slower in this order, boundary zone R＜ welding zone ＜ HAZ ＜ base metal. Especially boundary zone's growth speed is slower about 30 times than base metal's. 4. The cross section of fatigue did show brittle striation in overall; and, it is considered that the brittleness progresses along with the plane of cleavage, and the plastic deformation occurs as the stress gets increased. Therefore, it is thought that the crack bluntly occurred and its process stopped.