ABSTRACT

Offshore structure, as well known, is subjected to a number of loadings, such as permanant, live and environmental loads, during its lifetime. The dominating loading is random in nature. Welded joints usually become the critical parts of the structure. This paper presents the comparative study on the fatigue crack growth behavior of welded joint of A537 steel under random fatigue loading and the influences of load ratio as well as frequency on the fatigue crack propagation. Based on the test results the following equation: da/dB = 1.71 × 10−10(∆Krms)4.54 is proposed for estimating the fatigue crack growth of welded joint.

INTRODUCTION

Off shore structure, generally, consists of large amount of welded components. Significant differences in composition, microstructure and mechanical properties exist between welded joint and base metal. In addition, complicated residual stresses also accompany with these welded components and welds often have stress concentrations caused by toes and roots, as well as possible defects that act as sites for crack formation. Therefore, damage or fracture occurs frequently at welds or in the vicinity of welded joint while fatigue load applying to the component. The use of fracture mechanics to predict the fatigue crack growth at welded joints has been gaining increased attention in the last two decades (Lohne, 1979; Agerskov et al,1992 and Dijkstra et al 1993). In addition, the influence of welding residual stresses on the fatigue crack propagation has gained more and more attention. But as the complication is inherent in the welded joints and furthermore material, welding condition, post: weld heat treatment and type of specimen etc. for fatigue testing are different with each other in those tests, no consistant result was obtained from those studies.

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