Our expert team has significant experience determining the creep properties of polymer materials and products at Smithers' rubber and plastic testing labs.
Linear elastic materials and viscous materials have predictable linear behavior when load is applied for a period of time - the same is not true for polymers. When load is applied there is instantaneous elastic deformation followed by progressive permanent creep deformation. The degree of creep deformation depends on the time that the material is under load and the rate of deformation varies with temperature and chemical environment.
On releasing the load there is some elastic recovery, the rest of the relaxation is again dependent on time and the environment. Ultimately the material will not return to its original state and there is a degree of hysteresis. If load is re-applied the material will deform and creep again and the permanent deformation becomes cumulative.
To predict the behavior of plastics under long term load the creep characteristics of the material must be measured. Creep displacement (strain) is measured over extended periods of time - two weeks, 1000 hours or longer - for a given constant load (creep stress). These measurements can be repeated at a number of stress levels and temperature may be used to accelerate the rate.
It is important to understand the actual service conditions that are being represented by the material tests in terms of maximum operating temperature and the chemical environment as these can have dramatic effects on the creep behavior.