Investigation of the Long-Term Creep Modulus for Buried Polyethylene Pipes Subjected to Constant Deflection

The tests reported in this paper are simulating the stress relaxation in constantly deflected pipe rings of various high density and medium density polyethylene grades. The pipe rings are deflected to various amount so that constant strain values from 0. 8 % to 3. 7 % are achieved. The force which is needed for keeping the rings deflected is continuously measured during a period of 10,000 hours. Exptrapolation gives the long-term creep modulus up to 50 years with the strain as parameter values. The relation between creep modulus, stress and time under constant strain is discussed and compared with the creep modulus under constant stress conditions. Finally the ring stiffness for polyethylene pipes subjected to constant deflection is given as a function of time.