STRESS PROPERTIES OF HIGH DENSITY POLYETHYLENE PIPES UNDER CONSTANT PRESSURE IN THE LONG TERM

Abstract

Due to its durability, plastics pipes are normally used for water transport. It replaced the metal pipes .the long term hydrostatic pressure test according to ASTMD2837-02 and EN921 -1994 requires very long periods for testing from 1000~ 10000 hours ,which makes the test is expensive and consuming, the objective of this research to find a method to use a short –term test result predict the behavior of the plastic pipe for the long –term .for this purpose the short –term tests are carried at elevated temperatures 50oc ,60oc and 80oc and the curves of stress versus time were plotted . A time –temperature superposition approach is used to predict the creep curves at lower temperatures such as 20oc, 25oc and 30o c and 40o c. the stress curves are essentially parallel. This led to the thought that a shift factor aT , could be applied to move from one curve to another .it may be seen from chart. 1 that the three curves from 50oc ,60oc and 80oc are separated by a uniform distance log aT Williams ,landel and Ferry developed an empirical relationship for this temperature shift factor explain below: Log at = Where c1 ,c2 are constant Tref is a reference temperature.