Installation Damage: Effect on Lifetimes of uPVC Pipes Subjected to Cyclic Pressure
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report No UWRAA 68
A survey of field installation damage in Australia has shown that when pipes are installed in water reticulation systems they can have significant levels of damage or notches. These notches can be in the order of 50% of the wall thickness and although the radius of the notch tip varies considerably, in over50% of cases they can be considered as sharp, i.e. less than 25 mm notch tip radius.
Experimental investigations of the effect of both simulated sharp and blunt notches of varying depth, on uPVC pipes subjected to a representative cyclic pressure regime, shows that there is a relationship between the depth and radius of the notch and the cycles to failure.
For a 100 mm, Class 20 pipe containing sharp notch damage less than 1.1 mm deep (corresponding in the cases examined to a stress intensity factor of 0.21 MPaÖm), subjected to are presentative pressure surge, a fatigue crack threshold exists, below which cracks do not grow and consequently pipe lifetimes in excess of 100 years can be expected for pipes installed under such cyclic pressure conditions. For lower pipe classes, the fatigue crack threshold still applies but for a similar pressure range lower levels of installation damage are allowed.
For pipes with blunt notches, a crack initiation period exists compared to pipes with sharp notches. However the majority of installation damage consists of essentially sharp notches and consequently crack initiation times would be very short for the majority of pipes with installation damage.
For different pipe sizes and pressure regimes to the ones investigated, the lifetimes will depend upon several factors including the depth of the notch, the notch tip radii, the characteristics of the applied stress and the facture toughness of the pipe polymer. These lifetimes can be predicted by theoretical modelling of the experimental notch depth/cycles to failure relationship, via a fracture mechanics analysis. Refinement of this model to take into account changes in parameters such as crack growth rates, notch tip radii, stress intensity factor, test frequency, fracture toughness, residual stress, mean stress and crack tip ductility as the crack grows through the pipe wall, allows the lifetimes of pipes to be predicted.