Modelling the stress response of cast iron pipes during pressure transients

Fatigue analyses used by water distribution network operators to monitor asset health and predict leaks rely on steady-state models that assume a linear pressure-strain relationship. This work experimentally investigated whether such a relationship remains valid for fatigue studies of pipes subjected to pressure transients. Two bespoke experimental procedures were developed to generate transients, shown to be extreme in the context of water distribution networks, through cast iron pipes. The synchronised recording of the hoop strain and internal pressure of twelve cast iron pipes was undertaken for eight repeatable transient events. The pipes contained idealised cases of external corrosion in the form of machined elliptical pits whose dimensions were informed by an extensive numerical study. Residuals from the regression analysis of recorded pressure and hoop strain data were nonlinear for all pipes tested, except the unpitted 50 mm diameter pipe. Deviation from a linear pressure-strain response was linearly related to the rate of pressure change, providing the mechanistic insight that the loading rate of transients on the pipe wall was the cause of the observed non-linearity. Such non-linear behaviour was observed irrespective of the pipe’s restraint. Pipes with a larger strain response per increase in pressure exhibited greater non-linearity. The severity of corrosion, therefore, contributed to the observed non-linearity. A multi-variable model incorporating the rate of pressure change showed statistical improvement over a model using solely pressure. However, the choice of model had no impact on the predicted stress cycles or, in turn, on the predicted fatigue life. Therefore, the applicability of a linear pressure-strain relationship for fatigue analysis has been shown for pipes before the initiation of fatigue cracks. Future fatigue studies on cast iron pipes prior to crack initiation can now focus on capturing pressure cycles, rather than the rate of pressure change.