Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

Abstract

The failure assessment diagram (FAD) method has been widely accepted to evaluate the extent to which cracks may affect structural safety. The usage of this FAD method has been validated and included in [1]-[3]. The structure under investigation, described in four fully welded T-joint (BCC5) specimens, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. The four specimens were modeled under different displacement loading using a finite element analysis program Ansys and SolidWorks software. In this work, the application of a FAD (Lr, Kr) using maximum stress, cumulative stress ranges, and the last half-cycle stress range was investigated. The results are showing that all the points were lying outside the FAD curve except for the BCC5D specimen point was inside FAD when using maximum stress.
 Conclusions made that the cumulative stress gives Lr and Kr are extremely large and hence predict failure too early. With the Crack Tip Opening Displacement (CTOD) of the test specimen assumed to be about 1mm rather than 0.1mm it was found that, if a FAD is to be used to indicate failure, then both Lr and Kr should be based on the maximum stress. It appears that the FAD methodology does help to predict the final failure (which is the usual application in such cases). This represents more effectively the structural behavior and would be more easily used by designers.