InstallationDamage: Effect on Lifetimes of uPVC Pipes subjected to Cyclic Pressure

ReportNo WSAA 68

October 1993

SYNOPSIS

Asurvey of field installation damage in Australia has shown that when pipes areinstalled in water reticulation systems they can have significant levels ofdamage or notches. These notches can be in the order of 50% of the wallthickness 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.

Experimentalinvestigations of the effect of both simulated sharp and blunt notches ofvarying depth, on uPVC pipes subjected to a representative cyclic pressureregime, shows that there is a relationship between the depth and radius of thenotch and the cycles to failure.

For a100 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 arepresentative pressure surge, a fatigue crack threshold exists, below whichcracks do not grow and consequently pipe lifetimes in excess of 100 years canbe expected for pipes installed under such cyclic pressure conditions. Forlower pipe classes, the fatigue crack threshold still applies but for a similarpressure range lower levels of installation damage are allowed.

Forpipes with blunt notches, a crack initiation period exists compared to pipeswith sharp notches. However the majority of installation damage consists ofessentially sharp notches and consequently crack initiation times would be veryshort for the majority of pipes with installation damage.

Fordifferent pipe sizes and pressure regimes to the ones investigated, thelifetimes will depend upon several factors including the depth of the notch,the notch tip radii, the characteristics of the applied stress and the facturetoughness of the pipe polymer. These lifetimes can be predicted by theoreticalmodelling of the experimental notch depth/cycles to failure relationship, via afracture mechanics analysis. Refinement of this model to take into accountchanges in parameters such as crack growth rates, notch tip radii, stressintensity factor, test frequency, fracture toughness, residual stress, meanstress and crack tip ductility as the crack grows through the pipe wall, allowsthe lifetimes of pipes to be predicted.

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