General

Prior to the general use of fusion welding for fabrication of structures and pressure equipment, the primary joining method was riveting. Parts made by riveting are discontinuous, such that a fracture which starts in one plate does not necessarily spread into its neighbours. Fusion welding, on the other hand, creates monolithic structures and a running crack can result in catastrophic failure. This possibility became a dramatic reality during and immediately after World War II with the break up and loss of numbers of Liberty ships and T2 oil tankers. Subsequently there has been a less frequent but nevertheless persistent incidence of catastrophic brittle failure, and one of the major objectives of welding research and technology is to contain and, if possible, eliminate this problem. There has been some success in this endeavour, and catastrophic brittle fracture is now a comparatively rare mode of failure, as will be seen in Chapter 6.

Welds also give rise to stress concentrations that may predispose the joint to fatigue failures. The welding residual stress may act as a driving force for various modes of stress corrosion cracking, and differences in chemistry or metallurgical structure may result in selective corrosion of the weld or HAZ. The elevated temperature properties and in particular creep strength and ductility may be lower than that of the parent metal, such that joints may suffer premature failure in high temperature service.

In reviewing these potential problems it is convenient to start by discussing fast crack propagation in plates since...