Creep measurements tensile

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. 

It should be noted that in the case of fibre symmetry (in which there are only 5 independent creep functions) tensile creep measurements on specimens cut at only three angles to the fibre axis (say 0°, 45° and 90°) will give three independent combinations of the five, say S22it [2S23(t)+S44(t)] and SssU). Whilst if lateral contraction measurements are also carried out during creep then all 5 functions, including the... 

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. 

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 ... 

Values of modulus determined in tension or flexure at one or more temperatures are provided in tables of data supplied by manufacturers. Whilst these single-point data are useful for materials selection, they are obviously inadequate for detailed design of load-bearing components. Here the engineer must look for information about time-dependence, which is usually obtained firom tensile creep measurements. Most manufacturers handbooks contain sets of creep curves obtained over a range of applied stresses, for strains up to about 0.03, as shown in Figure 8.13 some handbooks also include creep curves for one or more elevated temperatures. 

Creep measurements involve measuring a constant tensile or flexural load to a respective specimen (as discussed previously) and measuring the strain as a function of time. In a typical creep plot, percentage creep strain is plotted against time. The apparent creep modulus at a particular time can be calculated by dividing the stress by strain at that particular time. Creep compliance is determined by dividing the strain by stress. For a tensile test, the simplest way to measure extension is to make two gauge marks on the tensile specimen and note the distance between the marks at different intervals. However, accurate measurement of extension requires an optical or laser extensometer. In a flexural measurement, the strain is usually calculated with the help of a linear variable differential transformer system. 

To prevent unrealistically short creep rupture times, one needs to maintain stress loads below about 60-70% of the measured tensile strength at the same test chamber temperature. This is an important criterion for selecting appropriate weights for accelerated tensile strength determination. 

Tension probes measure tensile properties such as stress and strain of thin films and fibers in addition, creep and stress relaxation measurements can be carried out in the tensile mode CLTE and hygroscopic expansion can also be measured in this mode (Prime et al. 1974). 

Fig. 2.10. The small-strain tensile-creep compliance versus creep time of poly(vinyl chloride) quenched from 90 °C to 20 °C and aged at 20 dz 0.1 °C for a period of time in days (indicated above the curves), after which each individual creep measurement was performed. The reduced curve on the extreme right was obtained by shifting the individual creep data to the longest-aging-time (1000 days) response as indicated by the arrow. From Struik by permission [44].

Figure 8.6 Photograph of the extensometry system of Clayton, Darlington and Hall (1) upper arm of tensile extensometer (2) specimen (3) brass contact pieces on lateral extensometer arms (4) lower arm of tensile extensometer and (5) glass plates with shoulders resting on ends of lateral extensometer arms. (Redrawn from Clayton, D., Darlington, M.W. and Hall, M.M. (1973) Tensile creep modulus, creep lateral contraction ratio, and torsional creep measurements on small nonrigid specimens. J. Phys. E., 6, 218. Copyright (1973).)...

Clayton, D., Darlington, M.W. and Hall, M.M. (1973) Tensile creep modulus, creep lateral contraction ratio, and torsional creep measurements on small nonrigid specimens. J. Phys. E., 6,218. 

FIG. 26 Comparison of steady-state or plateau elongational viscosities of polyethylene melts from homogeneous isothermal elongation tests (unfilled s mibols) and transient tensile viscosities evaluated from converging flow (filled symbols). Unfilled symbols with tick denote tensile creep measurements. T = 150°C. 

Figure 3.10 Plots of logjjg versus loge for a low-density polyethylene at 150 °C (O) the averaged value of several measurements carried out at constant elongation rate, ( ) constant elongation rate experimental data with corrections for the inflnence of the interfacial tension, and (A) with tensile creep measurements. Refer to Chapter 5 for details of the experimental methods to obtain steady-state elongational flow data. (Reprinted from Laun and Munstedt, Rheologica Acta 17 415. Copyright 1978, with permission from Springer.)...

The Mtinstedt tensile rheometer (MTR) is an end-separation device in which the sample is stretched vertically in a cylindrical oil bath [ 199]. This is an improved version of the universal extensional rheometer described by Mtinstedt etal. [177]. The basic idea is illustrated in Fig. 10.23. The specimen is fastened by an adhesive to small metal plates, one of which is attached to a force transducer at the bottom of the bath, and the other is coupled to a pull rod that is vertically displaced by a toothed belt driven by a motor. This instrument has been used for a number of important studies [ 158,191 ]. The MTR can reach strain rates of 5 s and can be used for creep measurements. The temperature is limited to about 220 °C because of the physical properties of the silicone oil used to fill the bath. 

This envelope may be used to describe relaxation, creep or constant strain rate measurements. A change in strain rate or temperature only shifts a point along the failure envelope, which is thus dependent only on the structural characteristics of the elastomer. The ultimate properties of rubbers are mainly governed by their viscoelastic properties, and reduced master curves can be obtained for tensile strength and strain as a function of time to break. The failure process is a non-equilibrium one, developing with time and involving the consecutive rupture of the molecular chains. The ultimate properties can then be predicted from creep measurements. ... 

In creep measurement a constant tensile stress (o) is applied to the sample and the time-related strain 8 (t) is measured. Stress and strain are related by ... 

Abstract Polymeric solids have tensile creep compliance, compression creep compliance, flexural creep compliance, and tangential or incremental tensile and compressive compliance. While these compliance values would all be numerically the same in a given metal, th will all be numerically distinct in polymeric solids. This paper investigates vriiy these compliance values vary in polymeric solids and presents experimental data to indicate the magnitude of this variation between several of these compliance values in iso-polyesters and vinylesters. The tensile creep compliance is found to be 15% greater than the flexural creep compliance in vinylesters while the incremental compliance at 3700 hours in a creep test is found to be approximately 25% less than the initial compliance on loading for iso-polyesters. The measured tensile and flexural creep compliances may be used to calculate the compressive creep compliance. 

Col2] Vickers hardness measurements, tensile tests, creep-rupture tests Hardness, stress, tensile strength... 

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