A Basic Study on Growth Characteristics of the Small Surface Crack in 21/4 Cr-1 Mo Steel

J. Ocean Eng. Technol. 1987;1(1):104-110.

2 1/4 Cr-1Mo강의 작은 표면균열의 성장에 관한 기초적 연구

서창민,강용구

경북대학교 기계공학과,금오공과대학교 기계설계공학과

Keywords: fatigue crack, constant crack length curve, artificial fit, temper color method, crack face aspect, aspect ratio, semielliptical crack

핵심용어: 균열길이곡선, 템퍼칼라법, 균열면 형상, 형상비, 반타원형 균열

Abstract

Fatigue tests by axial loading (R = 0.05) were carried out to investigate fatigue crack growth characteristics of small surface cracks in 2 1/4 Cr-1 Mo steel at room temperature by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present test are determined as a function of the stress intensity factor range about a semi-elliptical crack, so that the application of linear fracture mechanics to the surface fatigue crack growth and to the fatigue crack growth into depth, and all the data obtained from tests were discussed in comparison with the data of Type 304 stainless steel and two type of mild steel under the same test conditions. The obtained results are as follows: 1)When the cycle ratios are same, surface fatigue crack length and its depth are almost same and fall within a narrow scatter band in spite of different stress levels. 2)Relations of the surface fatigue crack growth rate (da/dN) and fatigue crack growth rate into depth (db/dN) to its stress intensity factor range ($Delta K_{Ia}, Delta K_{Ib}$) can be plotted as a straight line at log-log diagram without dependence of stress level and coincide with the data of part-through crack in various steels.