The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part I - experimental analysis and creep characterization

The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. In viscoelastic pipes, typically made of polyethylene (PE), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. This is a result of the retarded deformation of the pipe-wall. This effect has been observed in transient data collected in a high-density PE pipe-rig, at Imperial College (London, UK). Several transient tests were carried out to collect pressure and circumferential strain data. The pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. The creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. A good agreement between the creep functions was observed. Creep tests are important for the characterization of the viscoelastic behaviour of PE as a material; however, when PE is integrated in a pipe system, mechanical tests only provide an estimate of the actual mechanical behaviour of the pipe system. This is because creep depends on not only the molecular structure of the material and temperature but also on pipe axial and circumferential constraints and the stress-time history of the pipe system.