Integrated fracture mechanics approach to analyse fatigue behaviour of welded joints

Abstract

Current fracture mechanics methods for fatigue assessment, including those that consider thresholds for crack propagation, are based on long crack behaviour. The present work is concerned with an attempt to predict the fatigue strength of welded joints using a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, which is a function of crack length. The fatigue strength of butt welded specimens stressed transversely was analysed. Experimental results from the literature were used for comparison. Estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks, and the applied stress distribution along the crack path obtained from simple finite element models. The influence of plate thickness, initial crack length, and reinforcement angle on fatigue strength of butt welded joints was analysed. Results show good agreement with experimental trends.     

Inclusion engineering: a keytool in achieving steelmaking operations stability at OneSteel Whyalla

Abstract

A new billet casting machine and ladle metallurgy furnace were commissioned at OneSteel (formerly BHP) Whyalla steelworks in June and October of 1999. The production of low Mn/Si ratio grades using, at first, an oxygen reheating station, and then later a ladle metallurgy furnace, provided many challenges to the achievement of stable steelmaking and casting operations. The multiphase equilibrium model as developed by the Commonwealth Scientific Industrial Research Organisation (CSIRO) has been used to solve complex metallurgical problems of practical importance in the production of silicon killed steels. Subtle changes to ladle practice for dissolved oxygen content combined with calcium treatment dictates inclusion composition and liquidus. Inclusion composition control has been a key requirement for making open cast product with good surface quality, and also for the control of refractory stopper erosion for submerged cast grades.     

Short fatigue crack growth behaviour of a ferritic steel weld metal

Abstract

The present study deals with the short fatigue crack growth behaviour of a ferritic steel (nuclear grade SA 333 Grade 6 steel) weld metal at two load ratios R of 0.1 and –1. Single edge notched weld joint specimens were tested at different stress levels. The acceleration and deceleration in short crack growth with increasing stress intensity factor range was explained in terms of microstructure of weld metal. Acicular ferrite and grain boundary ferrite acted as barriers to crack growth. Non-propagating cracks were observed due to multiple cracks and blocking by ferrite grain boundaries. Transition crack length, the limiting crack length above which the crack exhibited a typical long crack behaviour, was determined to be 1 mm at R=0.1 and 1.5 mm at R=2 1.     

Investigation of coating failure on the surface of a water ballast tank of an oil tanker

The current crude oil tanker is constructed as a double-hull structure which consists of an oil tank and a water ballast tank whose surface is coated with epoxy paint to prevent corrosion. Since the cracks that developed in the epoxy coating have caused corrosion of the interface of the water ballast tank, the identification of the parameters for crack development is important. In addition, the moisture absorption by the epoxy coating can cause deterioration of bond strength, which results in delamination of the coating and accelerates the corrosion at the interface. In this study, after the mechanical and thermo-mechanical properties of epoxy paints were measured, the residual stresses induced by the temperature change and cure shrinkage were calculated by the finite element analysis, which were compared with the experimental results. Also, the pull-off tests were performed to investigate the deterioration of the bond strength of epoxy coatings due to moisture absorption. It was found that the thermo-mechanical properties such as the coefficient of thermal expansion and glass transition temperature of the coating materials had dominant effects on the crack resistance rather than the cure shrinkage; the moisture penetration to the bonding interface caused interfacial failure and a significant deterioration of bond strength.     

Effect of impurities on intergranular fracture of cadmium during thermocycling and creep

Abstract

The paper is devoted to an analysis of experimental data obtained as a result of investigation of an influence of the impurity nature on the failure of a model material of a high purity under condition of its thermocyclic treatment and high temperature creep. The fact that impurities promote the process of cavity formation was known long ago. However, since investigations were conducted with commercial materials, which had a variety of impurities, it was impossible to investigate the influence of each chemical element separately on the phenomenon in the presence of other additives. Experimental studies are described that have clarified what sort of impurities are responsible for the phenomenon of cavity formation, in what way they act in combination, over which temperature interval, whether neutralisation of them is possible and how this neutralisation can be achieved.