한국해양공학회지 (Oct 2021)
Reliability Improvement of Offshore Structural Steel F690 Using Surface Crack Nondamaging Technology
Abstract
Microcracks can rapidly grow and develop in high-strength steels used in offshore structures. It is important to render these microcracks harmless to ensure the safety and reliability of offshore structures. Here, the dependence of the aspect ratio (As) of the maximum depth of harmless crack (ahlm) was evaluated under three different conditions considering the threshold stress intensity factor (ΔKth) and residual stress of offshore structural steel F690. The threshold stress intensity factor and fatigue limit of fatigue crack propagation, dependent on crack dimensions, were evaluated using Ando's equation, which considers the plastic behavior of fatigue and the stress ratio. ahlm by peening was analyzed using the relationship between ΔKth obtained by Ando's equation and ΔKth obtained by the sum of applied stress and residual stress. The plate specimen had a width 2 W = 12 mm and thickness t = 20 mm, and four value of As were considered: 1.0, 0.6, 0.3, and 0.1. The ahlm was larger as the compressive residual stress distribution increased. Additionally, an increase in the values of As and ΔKth(l) led to a larger ahlm. With a safety factor (N) of 2.0, the long-term safety and reliability of structures constructed using F690 can be secured with needle peening. It is necessary to apply a more sensitive non-destructive inspection technique as a non-destructive inspection method for crack detection could not be used to observe fatigue cracks that reduced the fatigue limit of smooth specimens by 50% in the three types of residual stresses considered. The usefulness of non-destructive inspection and non-damaging techniques was reviewed based on the relationship between ahlm, aNDI (minimum crack depth detectable in non-destructive inspection), acr N (crack depth that reduces the fatigue limit to 1/N), and As.
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