Fatigue and leakage behaviour of corroded cast iron pipes

This research explored the hypothesis that different corrosion pit and multiaxial stress alignments cause a significant change in the fatigue strength and leakage behaviour of grey cast iron pipes. Destructive laboratory experiments were used to characterise the fatigue strength and failure mechanisms, including leak-to-burst behaviour. The experiments subjected the pipes to cyclic internal pressure and/or bending loads to cause stresses acting in different directions for a range of artificial pit shapes, including long, narrow pits for the first time. These unique experiments revealed that small cracks in cast iron pipes can close and stop leaking even under high positive water pressures, which has significant implications for leak modelling and detection. Small corrosion pits with stress alignments that caused a high stress concentration were shown to extend the leak-to-burst interval by about two orders of magnitude due to stress concentration effects of the pit. Such pits will progress to catastrophic burst behaviour more slowly than from small initial cracks under uniform wall loss or lower stress concentration pits. From a leakage onset perspective, the fatigue strength reduction caused by long, narrow corrosion pits was found to change by about 60% depending on the applied stress direction, showing the importance of considering pit-stress alignment in damage assessment and modelling. This research has found that corrosion pit shape, and the relative alignment of the pipe wall stresses, can substantially affect the ability of cast iron pipes to withstand cracking, as well as affecting the stability of leaking cracks once formed. These findings can be utilised to improve cast iron pipe condition assessment and leak detection techniques.