표면균열 무해화 기술에 의한 탄소강의 유지 보수 합리화 및 신뢰성 향상에 관한 연구

Abstract

ABSTRACT : This study evaluated the properties depending on the carbon content using 0.44 wt% carbon steel, 0.61 wt% carbon steel and 0.57 wt% carbon steel (HV470, HV570). The fatigue limit was evaluated based on the size of microcracks. Using the threshold stress intensity factor and compressive residual stress distribution by shot peening(SP), the crack aspect ratio (As) dependence of the harmless maximum crack depth () and crack depth, which reduces the fatigue limit of non-SP smooth materials by 25% or 50%, was evaluated. The fatigue limit remained almost constant up to a certain limit regardless of the carbon content. However, the fatigue limit decreased rapidly as the crack size increased. As the crack aspect ratio decreased, the fatigue limit of the depth (point A) also decreased significantly. In addition, the fatigue limit ratio was lower for steels with high carbon content. The fatigue limit ratio of the depth decreased significantly because the crack propagation in the depth direction was fast as the crack aspect ratio decreased. On the other hand, the fatigue limit ratio of the surface cracks increased as the crack aspect ratio decreased. The harmless crack depth () of 0.44, 0.57 and 0.61 wt% carbon steels were all determined in depth. The compressive residual stress distribution significantly affects the harmless maximum crack depth (), and the larger the compressive residual stress, the longer the . The crack aspect ratio (As) had an effect on , and as it decreased, the value of tended to decrease. Because both 0.44 wt% (As < 0.6) and 0.61 wt% carbon steels indicated > , can be rendered harmless, but cannot rendered harmless because both steels indicated < . Meanwhile, for 0.57 wt% carbon steel (HV470, HV570) indicated > (, ), and can be rendered harmless. Hence, for the SP performed in 0.57 wt% carbon steel, a and can ensure the safety and reliability of steels. Because the 0.44 wt% carbon steel indicated (, ) < (, ), the crack depths of and can be detected using NDI. Meanwhile, because 0.61 wt% carbon steel indicated < , crack depth of can be detected. In contrast, because 0.61 wt% carbon steel indicated (, ) > and > , the crack depths of and cannot be detected using NDI. Because 0.57 wt% carbon steel (470HV, 570HV) indicated (, ) > (, ), the crack depths of and could not be detected using NDI. Hence, more sensitive NDI technology must be applied to cracks that cannot be detected.