On the condition assessment of cast iron trunk main: The effect of microstructure and in-service graphitisation on mechanical properties in flexure

The mechanical properties of cast iron used for trunk mains in the water industry have been studied. Specimens have been sourced from nine different pipes, which had been in service for up to 150 years before failure. The bulk microstructures of each cast iron have been analysed with regard to the graphite flake morphology and size. The stress–strain behaviours in tension and compression have been derived from specimens loaded in flexure. Flexural strength data have been obtained for 30 specimens from each pipe (three batches of 10 from different locations along the length and around the circumference of the pipe) and these data have been analysed using Weibull methods. The depth of graphitisation visible on the fracture surface of each sample has been measured. It is shown that the strength of the cast iron samples decreases with increasing depth of graphitisation. When the layer of graphitised material is of reasonably uniform thickness, the strength reduction is modest, but where the section loss is more local, the strength reduction is more significant — for some samples there is a reduction in strength of more than 50% when the depth of graphitisation exceeds 4 mm. Simple strength-of-materials and fracture mechanics approaches are shown to provide reasonable bounds for the data.