低 사이클 反覆引張을 받는 構造用鋼의 疲勞强度와 疲勞크랙 傳播에 관한 硏究 = Fatigue Strength and Crack Propagation of Structural Steel on Low Cycles Repeat Tension

다국어 초록 (Multilingual Abstract)

There has been many unexpected fractures of any steels used widely in many kinds of industries, which cause to a loss of lives and property. This is because the cause of fatigue demage which was not precisely considered In the conventional design.
Recently it has been considered that most of fractures occurred in the design of various mechanical structures such as pressure vessels, piping, etc. are the cracks caused by low cycle fatigue. Especially it is very important in presuming the life of notched structural steel to clarify the characteristics of the low cycle fatigue crack propagation of notched structural steel.
In this study, some experiments were carried out the domestic structural steel plates with "O"hole, "U" and "V" type notches under low cycle pulsating tension fatigue testing. And then the mechanism and behavior of low cycle fatigue were clarified through the consideration on the notch effect vs. fatigue strength of various notched specimens, the relations between crack propagation rate and stress intensity factor, and on the initiation of fatigue crack and its propagation, etc.
The results obtained in this study are summarized as follows;
(1) At the root radius R=0.25㎜ of "O" hole notch, it has higher fatigue strength and at R≥0.5㎜, almost the same in the S-N curve. But at the smaller "U" and "V" type notch root radius, it has the lower fatigue strength in the S-N curve.
(2) The initiation of fatigue crack was occurred at 45~55%, 20~50% and 10~30% through each total fatigue life of the "O" hole, "U" and "V" type notch. The "O" hole notch specimen has almost the same initiation of fatigue crack without regard of notch size, but the "U" and "V" type notch specimens have the lower initiation at the larger notch root radius.
(3) The "O" hole notch specimen has the lowest crack propagation rate and the incubation phenomenon of crack propagation rate occurred about 0.3㎜ of crack length.
(4) Between the crack propagation rate and stress intensity factor, the value of exponent "m" was 1.11∼2.34. At the smaller root radius of "O" hole notch, the value of exponent "m" becomes the larger, but at the larger root radius of "U" and "V" type notch the value becames the smaller.
(5) Propagating crack passes around or branches out on account of inclusions, with out passing through the inclusions.
(6) At the smaller root radius of "U" and "V" type notch, the shape of crack propagation became more intricate, but in the case of "O" hole notch, the opposite phenomenon occurred. And at the higher stress level of each notch, the crack propagation tend to be more intricate.