Creep apparatus tensile

The following constraints are therefore relevant to the design of tensile creep apparatus if it is to be versatile in application to a wide range of materials and orientations ... 

Buckley and McCrum have recently published work on the anisotropy of creep obtained using a tensile creep apparatus based on precision measurement of clamp displacement. Creep strains up to a maximum of 0 1% were used. The creep compliance was subject to error limits of 5% but the scatter of points on a given creep curve was always less than 0-5% provided creep was terminated after 60 s below room temperature or after 180 s above room temperature. Details of the apparatus have not yet been published. 

The time dependence of the shear compliance 544(f) was studied over a range of draw ratios in cold-drawn LDPE by Qayton using torsional creep apparatus. In general, good agreement was obtained between this directly measured shear compliance and that calculated by inserting the tensile creep data above into eqn. (8) (see Qayton et al ). It should be mentioned that the measurement of S44. for a material with fibre symmetry is one of the cases where the difficulties mentioned in Section 10.3.2 are least severe, since torsion occurs about an axis which is perpendicular to a plane of isotropy in the specimen. 

A detailed examination of the problems of direct measurement of 544(f) and 566(f) in highly cold-drawn LDPE with fibre symmetry was carried out by Ladizesky and Ward using torsional creep apparatus. They found only a small dependence of 544(t) upon time at high draw ratio, in agreement with the above mentioned studies on tensile and torsional creep. The direct determination of 566(fX by experiments in torsion was, however, shown to require a complicated double extrapolation procedure reasonable agreement then being obtained between the measured values and those derived from the measurements of 5t 1 and 5i2 during tensile creep. 

Creep. Creep measurements were carried out in a Frank apparatus preheated to the desired temperature by an air bath, using tensile microspecimens 0.1 cm thick, obtained by milling, as described by ASTM D-1708. 

Fluoriuated Ethylene Propylene Copolymer Thermoplastic copolymer of tetrafluoroethylene and hexafiuoro-propylene. Has deereased tensile strength and wear and creep resistanee, but good weatherability, dielectric properties, fire and ehemical resistance, and friction. Decomposes above 204°C (400°F), releasing toxic products. Processed by molding, extrusion, and powder coating. Used in chemieal apparatus liners, pipes, containers, bearings, films, eoatings, and cables. Also called FEP. 

Polychlorotrifluoroethylene Thermoplastic polymer of chlorotrifluoroethylene. Has good transparency, barrier properties, tensile strength, and creep resistance, modest dielectric properties and solvent resistance, and poor processibibty. Processed by extrusion, injection and compression molding, and coating. Used in chemical apparatus, low-temperature seals, films, and internal lubricants. Also called PCTFE. 

Sensitivity is such that movements of <0-25 //m can be readily detected and accurate tensile strain measurements made over the range OT to > 5% if required. The system has been proved over a number of years by inter-laboratory comparisons, especially with the highly sophisticated apparatus for creep in isotropic materials developed by Turner. Stability for long term creep measurements has been proved for periods up to 6 months. Full details of the apparatus proving trials are given by Qayton et al ... 

Middleman [M24], Goldstein [G8], Furuta et al. [Fll], Lobe and White [LI4], Toki and White [T7], Montes et al. [M37], Osanaiye et al. [OlO], and K. J. Kim and White [K8a]. The apparatus (with constant-temperature chamber) may be placed in a tensile tester and operated in a mode with a fixed velocity V giving a constant shear rate. It may, on the other hand, be used in a creep mode with hanging weights. This provides constant stress experiments. At low stress levels one needs to compensate for the weight of the central member which exerts a gravitational stress [OlO]. At very low stresses one may accurately determine the yield value of rubber-carbon black compounds. Osanaiye et al. [OlO] have made measurements at shear stresses below the yield value. 

Figure 6.2 High-temperature tensile creep testing apparatus. (Reproduced from Leaderman, H. (1962) Large longitudinal retarded elastic deformation of rubberlike network polymers. Trans. Soc. RheoL, 6, 361. Copyright (1962) Society of Rheology.)...

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