small artificial defect(미소인공결함) = A Study on the Behavior of Fatigue Crack Growth for Structure Carbon Steel(S43C, S50C)

국문 초록 (Abstract)

본 실험에서는 자동차의 전륜 구동축재(S43C, S50C)에 대해서 미소 원공결함을 가공한 비열처리재와 고주파열처리재의 피로시험편에 회전굽힘응력을 가하여 피로강도와 피로크랙의 발생 및 전파거동에 관하여 실험한 결과 다음과 같은 결론을 내렸다.
비열처리 S43C재 및 S50C재의 피로한도는 각각 98.1MPa, 107.9MPa이었고, 열처리재의 경우 각각 147MPa, 156MPa로서 열처리재는 비열처리재보다 약 50Mpa 증가하였다. 응력이 피로한도의 110%에서 120%로 증가하는 경우 비열처리인 S43C재 및 S50C재에서 피로수명의 감소율은 각각 약 49%, 약 45%였으며 또한 열처리재의 경우는 각각 약29%, 약 22%로서 비열처리재보다 열처리재가 피로수명 감소율이 낮게 나타났다. 평활재에서 표면 미시크랙의 발생은 비열처리재인 S43C 및 S50C에서는 수명비(N/Nf) 0.2정도에서 처음 관찰되었으며, 동일한 수명비에서 관찰된 미시크랙의 수는 S43C가 많았고, 평균 크랙길이는 S50C가 길었다. 피로크랙길이와 파단수명비의 관계 log(2a)=A+B(N/Nf)식에 본 실험재료를 적용한 결과 응력수준에 관계없이 잘 일치하였다

다국어 초록 (Multilingual Abstract)

In this paper, an experiment to investigate the behavior of fatigue crack propagation in induction hardened structural carbon steel(S43C, S50C) with artificial defect are conducted by the rotary bending fatigue test in the room temperature. The summarized results are as follows:
The fatigue limits obtained from experiments of as-received S43C and S50C were 98.1MPa and 107.9MPa respectively. The fatigue limit of heat-treated materials were 147MPa and 156MPa respectively. It means that the heat-treated specimens showed about 50MPa higher fatigue limits than the as-received material. When the applied stress was increased from 110% to 120% of fatigue limit, the decreasing rate of fatigue life was around 49% and 45% respectably in the as-received S43C and S50C, in case of the heat-treated materials, the decreasing rate of fatigue life was around 29% and 22% respectively. It means that the heat-treated materials had lower decreasing rate of fatigue life than the as-received materials. The initiation of surface micro crack was observed at 0.2 of fatigue life ratio in as-received S43C and S50C. The number of micro cracks in S43C were more than that of S50C, but the average crack length in S50C was longer than that of S43C at the same fatigue life ratio. Experimental results obtained from fatigue test were evaluated with the equation log(2a)=A+B(N/Nr). The relationship between crack length and cycle ratio showed a good agreement to the experimental results.