Creep in 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. 

Tensile creep, with or without lateral strain measurements, has been augmented by torsional creep, but usually creep is then limited to very small strains hopefully to avoid the non-linear behaviour. The results are therefore limited. Torsional creep measurements do seem, however, to constitute the only known method at this time for evaluating Sssit) and Se6(t) for oriented sheet materials with orthorhombic symmetry. 

Tensile and torsional creep methods will be discussed below. For discussion of the very specialised techniques which have been used for studies of anisotropy of compliance in fibres and monofilaments, such as the Hertzian contact technique by Hadley et al and Pinnock et readers are referred to the original papers and to Ward. It should be noted that such methods are not well adapted to creep measurement and are mainly used for determination of isochronous parameters. They suffer from all the limitations, referred to above, associated with non-uniform stress situations. 

Experimental techniques for creep measurements on isotropic materials are highly developed and have been discussed in detail by Turner and coworkers. Discussion here will be limited to the special requirements associated with studies of anisotropy. 

Since it is necessary to cut samples in a variety of directions in the plane of the sheet the overall length of such samples can be severely limited by the dimensions of the drawn sheet. If drawing procedures are to be kept within the scope of normal laboratory facilities the thickness of the drawn sheet may be less than 1 mm and the width or length less than 5 cm. It follows that creep specimens cut from drawn sheet may be both short and lacking in rigidity when compared with conventional isotropic specimens. The lack of rigidity is aggravated for relatively soft materials such as low density polyethylene (LDPE). 

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