What is stress relaxation?
The stresses created by constricted conditions (e.g. impeded thermal expansion between two fixed points) are absorbed by the visco-elastic behaviour of the plastic pipe until a tension/expansion equilibrium is created. This process is also termed relaxation. Critical operating conditions arise when varying loads are involved, and the periods of high temperature grow longer than the periods of low temperature. In particular, tensile stresses with long-term effects can, after frequent load changes, result in fatigue fractures. Stress peaks occur when an initial temperature variation affects an installation condition. If the temperature level remains nearly constant over a longer period, the heat stresses are reduced by relaxation to level = 0.50% of the maximum value. Heat stress generated by a varying thermal load and tensile stress having a long-term relaxing effect can be calculated.
