A 3D Crack Evolution In Weld Metal, Base Metal And The Transitional Fusion Line Under A Mixed Fatigue Loading

Mahyar Asadi[1], Majid Tanbakuei Kashani[1], Mathew Smith[1], Chris Timbrell[2], Ramesh Chandwani[2], Arasch Rodbari[2]
[1] SKC Engineering (A division of Applus)
[2] Zentech International Limited

ESIA 14 – ISSI 2017 (Engineering Structural Integrity Assessment 14 in conjunction with the International Symposium on Structural Integrity 2017), Manchester, UK, 16-17 May 2017.

A limited number of automated algorithms and software are available that predict the 3D evolution of crack fronts in a mixed loading condition in welds in particular in the interface of weld and base metal i.e. fusion line. In this paper, the authors present a study of common low carbon steel pipe weld joints containing a crack detected in radiography films and embedded into a 3D FE pipe model that is constructed with different weld and base metal properties including different crack growth laws and fracture properties. Evolution of the detected crack front is predicted in 3D under mixed fatigue loading. This paper shows that a 3D model of crack growth captures the transient change of stress intensity factor along the crack front and therefore the immediate change in the direction of crack growth and the dynamic shape of crack can be predicted. A solution is also presented for handling the stress intensity factor on the boundary of weld metal and base metal when the crack front reaches the weld fusion line. From the structural integrity management viewpoint, the number of fatigue cycles, time for the crack to start growing, time to break to surface and leak-before-break, and the total time to final fracture are calculated. This paper shows that a fracture critical region such as welds with a high likelihood of service cracking or welding flaw can precisely be analyzed and life can be estimated to avoid early life failure in welded structures.