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 () of the maximum depth of harmless crack ( ) was evaluated under three different conditions considering the threshold stress intensity factor ( ) 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.  by peening was analyzed using the relationship between  obtained by Ando's equation and  obtained by the sum of applied stress and residual stress. The plate specimen had a width 2 = 12 mm and thickness  = 20 mm, and four value of As were considered: 1.0, 0.6, 0.3, and 0.1. The  was larger as the compressive residual stress distribution increased. Additionally, an increase in the values of As and   led to a larger  . With a safety factor () 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  ,  (minimum crack depth detectable in non-destructive inspection),  (crack depth that reduces the fatigue limit to 1/), and As. Received 11 May 2021, revised 21 June 2021, accepted 9 July 2021 Corresponding author Ki-Woo Nam: +82-51-629-6358, namkw@pknu.ac.kr c 2021, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.