Reliability Approach to Fracture Assessment for Defects in Welded Joints

Abstract

In order to make fracture assessment for defects in weldments with respect to brittle fracture, a few methods have been standardized. Those present assessments are all conducted in a deterministic manner. Therefore it is not clear which level of reliability defects evaluated acceptable during a given in-service period correspond to from the viewpoint of brittle fracture.
This paper presents a probabilistic method to evaluate the reliability for defects to survive during a given in-service period, considering defects in welded joints of marine structures subjected to wave loads as reference. Though in the case of probabilistic evaluation it is necessary to take both the probability characteristics of fracture driving parameter caused by the randomness of wave loads and the probability characteristics of fracture toughness caused by the scatterness of material properties into consideration, in this paper only the former was taken as random variable assuming that the longterm distribution of wave loads is the exponential distribution and wave loads occur independently of appearence sequence. The variation of the fracture stress limit with the growth of crack size and the exceeding probability of the required fracture stress were taken into consideration to propose the formulation of the reliability analysis.
Discussions were carried out based on numerical results. Defects evaluated acceptable by present assessment methods have the different reliability depending on the given conditions despite the same in-service period. Even if the given conditions are same, fracture probability of accepted defects increases in substance with the in-service period. Therefore it is required to consider the concept of reliability in fracture assessment for defects.