Propagation Characteristics of Fatigue Microcracks on Smooth Specimen of <TEX>$2_{1/4}$</TEX> Cr-1 Mo Steel

J. Ocean Eng. Technol. 1990;4(2):100-111.

Propagation Characteristics of Fatigue Microcracks on Smooth Specimen of $2_{1/4}$ Cr-1 Mo Steel

Suh Chang-Min,Woo Byung-Chul,Jang Hui-Rak

$2_{1/4}$ Cr-1 Mo강의 평활재상의 미소한 표면피로균열의 성장특성

서창민,우병철,장희락

경북대학교 기계공학과,경북대학교 대학원,경북대학교 대학원

Keywords: Suface Fatigue Crack, Major Crack, Crack Growth Rate, Stress Intensity Factor, Crack Depth, Cycle Ratio

핵심용어: 표면피로균열, 주균열, 균열성장속도, 응력확대계수, 균열깊이, 반복수비

Abstract

In this paper, fatigue tests were carried out at stress test levels of 461 MPa, 441 MPa, and 431 MPa by using smooth specimen of$2_{1/4}$ Cr-1 Mo steel with the stress ratio(R) of 0.05. The initiation, growth and coalescense process of the major cracks and sub-cracks among the fatigue cracks on the smooth specimen are investigated and measured under each stress level at a constant cycle ratio by the replica technique with optical microscope. Some of the important results are as follows: In spite of the difference of stress levels, the major crack data gather into a small band in the curve of surface crack length and crack depth against cycle ratio N/Nf. The sub-crack data, however, deviate from the band of the major crack. The growth rates, da/dN, of major and sub-crack plotted against the stress intensity factor range, ${Delta}K$, have the tendency to be compressed on a relatively small band. But it is more effective to predict fatigue life through major cracks. The propagation behavior of surface microcracks on the smooth specimens coincides with that of the specimen having an artificial small surface defect or through crack.