船體 構造用鋼 熔接材의 疲勞크랙 傳播特性 및 環境疲勞壽命 豫測

Abstract

Welding is the most general work of various structures and junction processes such as ships and bridges. Many studies on welding part have been carried out, but many parts to study are remained yet due to the complicated effect such as geometrical condition and load form. The strength evaluation of the most weakest junction part is required for the safety design of all structures. Most of all, in order to enhance the reliability and safety of the welding part, whose use is the highest, it is very important to establish the efficient structure manufacturing technology by studying and investigating the evaluation of fatigue strength in various environments. This study analyzed the relations of ΔK, da/dN, and ΔKth according to the welding methods of SMAW, FCAW, and SAW. In addition, the conclusions below were obtained by manufacturing the specimen of fatigue life, which is the similar condition as CT specimen, and illuminating the fatigue limit and fatigue life in two sorts of corrosion environment experimentally. The following are the results of this study. 1. In the stage II, the value of stress intensity factor range was the highest in SMAW welding method of stress ration R=0.1, and appeared under the sequence of FCAW and SAW, and as the completion section of stress intensity factor was low, threshold stress intensity factor was lowly formed in da/dN - ΔK. 2. The fatigue crack growth rate(da/dN) according to the welding methods was examined in the range of ΔK = 20 MPa √m and ΔK = 15 MPa√m. In result, it was found that fatigue crack growth rate was highest in SMAW (Base metal 3. In the early range of fatigue crack growth, the threshold stress intensity factor(ΔK_(th)) was the highest in SMAW and the subsequent high sequences are FCAW, SAW, and Base metal. The result of investigation is that as respective stress ratio increases, threshold stress intensity factor decreases, and it proves that the life of material should be extended by using welding method, suitable for the using condition of low stress ratio in consideration of the reduction of fatigue life in the facility of heavy load. 4. As a result of observing the fatigue life by dividing the increase of stress ratio into low stress ratio zone(R=0.1→R=0.3) and high stress ratio zone(R=0.3→R=0.6), the fatigue life of each welding method is sensitively worked in high stress ratio, judging from the fact that the width of life reduction increases in the high stress ratio zone compared to the width of life reduction in the low stress ratio zone. 5. In the fatigue limit of welding methods before corrosion, the welding of SMAW and FCAW shows the same fatigue limit compared to Base metal, and SAW holds the lowest fatigue limit value. 6. As a result of comparing the fatigue life of SMAW and FCAW with Base metal, in the relation of crack length(a) and cycle number (N), it appeared that fatigue life of SMAW welding method was highest. 7. After 90 days immersion in corrosion solution, 3.5% NaCl solution and 6%FeCl₃ solution, the fatigue limit was formed under the sequence of Base metal> SMAW> FCAW> SAW, and in comparison with the case prior to corrosion, it appeared that the reduction of width of fatigue limit increases in 6% FeCl₃, rather than 3.5% NaCI. It is judged that it is because of stress concentration, by the pitting, generated in the metal surface. 8. The fatigue limit had a same tendency as 90 days immersion after 180days immersion in the corrosion solution 3.5% NaCI and 6%FeCl₃, but the gap of fatigue limit according to the each welding method was extended, and it was found that fatigue life is sensitively changed in accordance with the change of fatigue limit, while SAW welding method has the value of lowest fatigue limit. ９. As a result of predicting fatigue life by using material constant, obtained from the experiment and applying to the expectation of Nisitani fatigue crack growth rate, the expectation was matched with the range of mean relative error, ±10 %. It is judged that it will be used as an important basis in the future strength evaluation. 10. The attachment of a fretting oxide to the all CT specimen was confirmed microscopically. As a result of observing the fracture surface with SEM, normal striation was formed in the crack growth, and it was progressed perpendicularly towards the operation direction of stress.