構造用鋼의 疲勞크랙 發生 및 傳播에 關한 硏究 = Fatigue Crack Initiation and Propagation of Structural Steel

다국어 초록 (Multilingual Abstract)

There have been a number of investigations on the basic study for the fatigue problems, but it has not yet been clarified enough for the mechanism of fatigue failure. In spite of numerous investigations on the fatigue behavior of mild steel, it is seldom to obtain any information on the reports about the behavior of fatigue crack propagation and its mechanism of spring steel, which is widely applicable data for the structural design.
This study aims to clarify the behavior and basic mechanism of fatigue crack propagation through observation and study such as the fatigue crack propagation, its propagation rate, stress intensity factor and fractograph on spring steel by using a rotary bending fatigue testing machine. And also mild steel tested its behaviors in comparison with spring steel.
The results obtained in this study for spring steel are summarized as follows;
1) The initiation of fatigue crack for spring steel occurrs after 10∼20 percent of fatigue life in case of stress level such as fatigue limit.
2) The following relation is obtained from the present experiment on the fatigue crack length, normal stress and repeated cycles:
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3) The summarized relationship between crack propagation rate and stress intensity factor of the spring steel is as follows:
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4) In the crack initiation in the vicinity of the transverse hole, the slip band is generated and accumulated toward the maximum shearing stress direction and finally grown to the micro-crack.
5) The spring steel has very few micro-cracks and a great influence of nonmetallic inclusions and alloying elements on crack propagation. On the other hand, it has a tendency of more rapid propagation along or through the boundary of the matrix, pearlite grains and inclusions, as compared with the low carbon steel. These phenomena obviously come from the raised stress level above the endurance stress.
6) It is founded that several cracks is appeared in great value of the ratio between acting stress and fatigue limit, and a few crack is appeared in small value of those. This tendency assured at fracture surface both spring steel and mild steel in case of unnotched specimen.
7) Unpropagated fracture area is positioned to the center of bar section and its area becomes larger as the stress level is increased. However, at low stress level, it is located to the side of bar section with a shrunk area of fracture. The slewing appears on the fracture surface, which is induced to the anti-direction of the revolution.

목차 (Table of Contents)

• Abstract
• 1. 緖 論
• 2. 實驗方法
• 2.1 試 驗 片
• 2.2 實驗裝置
• 2.3 實驗方法
• 3. 實驗結果 및 考察
• 3.1 應力集中係數
• 3.1.1 圓孔노치 周邊의 應力과 應力集中係數
• (1) 模孔貫通試驗片의 斷面係數
• (2) 模孔非貫通試驗片의 斷面係數
• (3) 修正應力集中係數
• 3.1.2 修正應力集中係數와 노치 係數
• 3.2 波勞限度曲線
• 3.3 波勞크랙 傳播 및 傳播速度曲線
• 3.4 波勞크랙傳播速度와 應力
• 3.5 波勞크랙傳播와 應力擴大係數
• (1) 波勞크랙傳播速度와 應力擴大係數사이의 關係
• (2)σn-m 및 σn-C의 關係
• 3.6 波勞크랙의 發生과 傳播의 微視的觀察
• 3.6.1 波勞크랙의 發生過程
• (1) 스프링 鋼
• (2) 軟 鋼
• 3.6.2 波勞크랙의 傳播
• (1) 스프링 鋼
• 1-1) 介在物이 있을 경우의 크랙傳播
• 1-2) 腐蝕된 組織에 있어서의 크랙傳播
• 1-3) 크랙傳播의 連帶現象
• 1-4) Slip band의 Pile up에 의한 塑性域
• (2) 軟 鋼
• 3.7 波勞破斷面
• (1) 平滑試驗片의 波勞破斷面
• (2) 模孔試驗片의 波勞破斷面
• 4. 結 論
• 5. 參考文獻