Stress relaxation and creep are the same phenomenon except that one looks at it from a different perspective. Polymers under constant stress creep because the material is constantly attempting to relax the stress. In the same vein, plastics stress-relax under constant strain because the stretched polymer chains are constantly attempting to creep or flow to relieve the stress caused by the stretching. The ideal way to generate stress relaxation data is to take a specimen, put it under a particular strain in a tensile machine and monitor the stress as it decays with time. The problem is that the machine is tied up for long periods of time --- but, it is by far the best way to develop accurate stress relaxation data.
An alternative approach is to use the creep modulus data to estimate the stress relaxation at some point in time. A word of caution, however: compare the apparent creep modulus to the original short term modulus of the material --- donÆt compare the time-dependent creep modulus to the modulus at 100 hours, the typical origin for a logarithmic curve (this is not the modulus at zero time, but the creep modulus at 1 hour --- by this time most polymers will have relaxed about 20% of the initial load). Keep in mind that flexural creep data may be somewhat conservative, and somewhat misleading at strains significantly beyond 5%.
In a paper entitled, ôPull-Out Forces on Joints in Polyethylene Pipe Systemsö, the authors, J.L. Husted and D.M. Thompson, listed the following relationship between apparent modulus and time: Modulus as % of Time Initial (Instantaneous) Modulus
6 min 0.1 hour 100% 1 hour 1.0 hour 80% 10 hours 10 hours 67% 4 days 100 hours 51% 6 weeks 1000 hours 39% 1.1 years 10,000 hours 30% 11 years 100,000 hours 24% 50 years 438,000 hours 21%
These data were developed for polyethylene, however, interestingly enough, these same estimated relaxations have worked well for predicting stress relaxation in other plastic materials including other polyolefins, acetals, and PVC. Even more useful is the fact that these same apparent relaxations work well as a guideline for estimating the relaxation of polymers at various temperatures, when given the short-term modulus of the material at that temperature.