A Note on Realistic Creep and Relaxation Testing

The testing of polymers requires unique understanding of the viscoelastic nature of polymers. For example in a creep test it is required to suddenly apply a constant tensile, compression, or torsion stress to a bar of material. The most common description of a uniaxial tensile creep test is shown in Fig. 5.5(a). Several questions may arise one of which is How is the load to be applied suddenly without causing dynamic effects. One answer is for the load to be applied as ramp input as shown in Fig. 5.5(b). Obviously, the latter case is not a correct creep test. How big an error is involved A solution of the differential equation representative of the material for the ramp input of Fig 5.5(b) can be obtained and it can be shown that the error in the strain output is negligible if the loading time, to, is small compared to the retardation time of the material, x. 

Similarly, the same difficulty occurs in a relaxation test. That is an ideal relaxation test is one where a sudden input of strain is required as shown in 

In relaxation testing, the stiffness of the specimen must be small compared to the stiffness of the load cell and testing machine. Of necessity the specimen is in series with both the load cell and testing machine and, therefore, the deformation in the specimen, the load cell and the testing machine are additive. As the load in the specimen decreases or relaxes, even in a fixed grip circumstance, the load also decreases in the load cell and/or the test machine. The deformation will then actually increase in the specimen to allow a decrease in the deformation (and load) in the load cell. For a very stiff specimen (such as a fiber reinforced composite), the change in load (or stress) recorded may reflect a redistribution of deforma- 

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