Creep behaviour uniaxial tension

The most common technique employed to date has been that of creep in uniaxial tension. It was shown above that with the inclusion of lateral strain measurements this is a powerful technique giving access to up to 6 independent creep compliance functions. This is more than for any other known method. It further has the overwhelming advantage over many methods, such as say torsional or flexural creep, that the stress is sensibly uniform over the working volume of the specimen. This advantage is paramount in studies of materials displaying non-linear behaviour in creep since analysis of the non-uniform stress situation in non-linear systems is not well developed. Attempts to overcome the non-uniform stress situation in torsion, by recourse to, say, torsion of thin walled tubes, lead to severe difSculties in specimen preparation in oriented materials, when anisotropy of behaviour is to be studied. 

In the original paper [20], the authors reported work on the uniaxial tension of plasticized poly(vinyl chloride), sulphur vulcanizates of butyl rubber and polyisobutylene. Very successful predictions were made at extension ratios of up to 5. Zapas and Craft [21] applied their formulation to multistep stress relaxation and creep and recovery of both plasticized poly(vinyl chloride) and pol3dsobuty-lene. McKenna and Zapas applied a modified form of the model to the torsional deformation of poly(methyl methacrylate) [22]. McKenna and Zapas [23] have used the model in analysis of the tensile behaviour of carbon-black-fllled butyl rubbers. 

There appears to be no information on the uniaxial creep of polymers used as structural adhesives such as is available referring to the creep of adhesive joints in lap-shear or torsion. The latter is reserved for Chapter 7 where the few data that are available are given. An apparent exception is the careful study of a nylon-epoxy adhesive (FM 1000) by Shen and Rutherford (1972). These authors achieved something approaching uniaxial stress using a cylindrical butt joint in direct tension. However, what they called creep was simply the delayed elastic response. They likened the adhesive to a metal in its behaviour and used the classical but inappropriate concept of separating the creep behaviour into three stages as was done many years ago for metals. 

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