표면균열 무해화 기술에 의한 STS316L의 신뢰성 향상

Abstract

First study : The Effect of ultrasonic peening (UP) on tension fatigue limit of Ni 12% STS316L with a surface crack was investigated. UP was conducted on the specimens with a surface crack of aspect ratio, a/c=1.0 and 0.4, where a denotes the crack depth (a=0.1, 0.2, 0.3, 0.4 and 0.5 mm), and c denotes the half-length of surface crack. Tension fatigue tests were conducted under stress ratio, R=0.1. The result showed that the fatigue limit of the UP specimens having the crack below 0.3 mm in depth was almost same as that of the UP smooth specimens. Thus, the maximum depth of crack with various aspect ratios that can be rendered harmless was less than threshold stress intensity factor of the material. The estimated values were in good agreement with experimental values. According to the estimation method, a harmless crack assessment diagram was proposed. Second study: Cracks degrade the performance and reliability of steel structures; hence, researchers are extensively investigating solutions to the crack problem to ensure safety and easy maintenance of structures. A new technique has been developed in this work to render surface cracks harmless, which also reduces the fatigue limit by the influences of compressive residual stress caused by peening; this technique is referred to as surface crack non-damaging technology. This study aims to contribute toward reliability improvement of the of Ni 12% STS316 steel via the surface crack non-damaging technology. The crack depth that could be rendered harmless by ultrasonic peening(UP) was largely dependent on the compressive residual stress. Further, increased as the crack aspect ratio (As) and threshold stress intensity factor () increase. At large residual stress, the crack did not propagate and were rendered harmless. The relationship between , (the crack depth that can detected by nondestructive inspection), and (the maximum crack depth in the assumed safety factor) were compared.