Compliance, creep test

Studies have been conducted on creep compliance tests in which paint films were subjected to tensile loads of 4-7 psi (27.2-47.6 x 10 N/m ) and to 6% ozone for 505 h. A typical result for a high-quality emulsion-base paint is shown in Figure 13-1. Creep compliance is reduced by exposure to 6% ozone. If the effect is linearly related to ozone concentration, we might expect the same reduction in creep compliance at 0.1-ppm ozone in 3 x 10 h, or some 30,(XX) yr. Thus, reduction in creep compliance is not viewed as having a serious ozone contribution. 

The value of t/o can be obtained from a creep compliance test by waiting until the creeping flow reaches steady state. It can also be obtained from the dynamic mechanical data from the following relation... 

Creep compliance tests were conducted at a range of stresses at 170°C to measure the yield shess... 

In creep tests, the parameter of interest is the creep compliance, J, defined as the ratio of the creep strain to the applied stress, i.e. 

You have developed a new semicrystalline polymer, which has a typical activation energy for relaxation of Erei = 120 kJ/mol. You wish to know the creep compliance for 10 years at 27°C. You know that, in principle, you can obtain the same information in a much shorter period of time by conducting your compliance tests at a temperature above 27°C. 

Figure 3 shows the plot of the reduced tensile creep compliance, Dp(t), against t in logarithmic coordinates for the creep tests on Sheet I. A similar plot was made for the data obtained from Sheet II, and, in addition, for the relaxation data shown in Figure 1 after conversion to creep data using the relation (7) ... 

The master curves obtained from specimens cast from tetrahydro-furan solution at 2 and 4% strain, respectively, are slightly different. These differences, however, are probably within the experimental error. An idea of the reproducibility can be obtained from Figure 4, which shows the master curves of the creep compliances obtained on specimens cut from two sheets of Kraton 102 cast from benzene solution. Although the method of preparation appeared to be identical, there are noticeable differences between the two curves. Even larger differences exist between these curves and the master curve obtained from the relaxation data after conversion to creep. Again, there were no apparent differences in the method of preparation of the sheets from which the specimens for the relaxation and creep tests were cut. 

Creep Tests. All creep tests were run according to the procedure described in the Federal Highway Administration VESYS IIM Users Manual (15). The tests were performed on cylindrical samples 4 in. (10 cm) in diameter and 8 in. (20 cm) high. Creep tests of varying duration were performed at time spans between 0.1 and 1000 sec. There are a variety of ways to represent the creep compliance as a function of time, but perhaps one of the most useful is the power law form ... 

The creep-compliance technique has been used extensively by Sherman and co-workers for the study of ice cream, model emulsions, margarine, and butter (Sherman, 1966 Shama and Sherman, 1969 Vernon Carter and Sherman, 1980 Sherman and Benton, 1980). In these studies, the methodology employed was similar to that for ice cream, that is, the creep-compliance data on a sample were described in terms of mechanical models, usually containing four or six elements. Attempts were made to relate the parameters of the models to the structure of the samples studied. However, with increased emphasis on dynamic rheological tests and interpretation of results in terms of composition and structure, the use of mechanical models to interpret results of rheological tests has declined steadily. 

Strictly speaking, there are no static viscoelastic properties as viscoelastic properties are always time-dependent. However, creep and stress relaxation experiments can be considered quasi-static experiments from which the creep compliance and the modulus can be obtained (4). Such tests are commonly applied in uniaxial conditions for simphcity. The usual time range of quasi-static transient measurements is limited to times not less than 10 s. The reasons for this is that in actual experiments it takes a short period of time to apply the force or the deformation to the sample, and a transitory dynamic response overlaps the idealized creep or relaxation experiment. There is no limitation on the maximum time, but usually it is restricted to a maximum of 10" s. In fact, this range of times is complementary, in the corresponding frequency scale, to that of dynamic experiments. Accordingly, to compare these two complementary techniques, procedures of interconversion of data (time frequency or its inverse) are needed. Some of these procedures are discussed in Chapters 6 and 9. 

In practice, viscoelastic properties can be determined by static and dynamic tests. The typical static test procedure is the creep test. Here, a constant shear stress is applied to the sample over a defined length of time and then removed. The shear strain is monitored as a function of time. The level of stress employed should be high enough to cause sample deformation, but should not result in the destruction of any internal structure present. A typical creep curve is illustrated in Fig. 13A together with the four-element mechanical model that can be used to explain the observations. The creep compliance represents the ratio between shear strain rate and constant stress at any time t. 

In the Bueehe-Halpin theory the necessity of a strong filler-rubber bond follows naturally from the requirement of a low creep compliance. On the other hand the hysteresis criterion of failure, Eq. (32), does not make the need for filler-rubber adhesion immediately obvious. It is clear, however, that Hb cannot exceed Ub. In absence of a strong filler-rubber bond, the stress will never attain a high value the only way for Ub to become large would be for eb to increase considerably. There is no reason, however, why under these conditions eb should be much greater than in the unfilled rubber at the same test conditions and, in any case, it will be limited by the so-called ultimate elongation . This is the maximum value of eh on the failure envelope and is a fundamental property of polymeric networks. The ultimate extension ratio is given by theory (2/7) as the square root of the number of statistical links per network chain, n,... 

Devices are secured to the skin by use of a skin-compatible pressure-sensitive adhesive, usually based on silicones, acrylates or polyisobutylenes. These adhesives are evaluated by shear-testing and assessment of rheological parameters (Musolf 1987). Standard rheological tests include creep compliance (measurement of the ability of the adhesive to flow into... 

The four commonly used techniques to extract information on the viscoelastic behavior of suspensions are creep-compliance measurements, stress-relaxation measurement, shear-wave velocity measurements, and sinusoidal oscillatory testing (25-27). In general, transient measurements are aimed at two types of measurements, namely, stress relaxation, which is to measure the time dependence of the shear stress for a constant small strain, and creep measurement, which is to measure the time dependence of the strain for a constant stress. 

Abstract Based on the theory of irreversible process thermodynamics, non-linear stress-strain-temperature equations are derived, together with an expression for time-temperature equivalence. In addition, an equation of shift factor for time-temperature equivalence is also obtained. The parameters in the equations are experimentally determined and the main curves for creep compliance and cohesion of TOP granite are obtained by a series of creep tests. As a result, it is proved that both deformation and strength of the TOP granite follow the time-temperature equivalent principle. 

Uniaxial compressive creep tests were carried out for granite sampled from Three Gorges Project (TGP) site in China at seven temperatures from 20°C to 300°C in order to determine the parameters C , Ci, Cr and Cj in eq. (25). The curves of creep compliance vs. time in logarithmic coordinates... 

The creep compliance of rocks reflects their deformability whereas the cohesion reflects their strength. In Section 1, the expressions are derived for the time-temperature equivalence for rocks and the testing results obtained in Sections 2 and 3 have shown clearly that not only the deformability of the TGP granite rock but also its strength follow the time-temperature equivalent principle. The establishment of shift factor eq. (25) and the determination of its parameters make it possible to predict correctly the long-term mechanical response of rock at lower temperatures according to its short-term mechanical behaviour at higher temperatures. 

In the above discussion, six functions Go(w), d(w), G (w), G"(w), /(w), and J"(oj) have been defined in terms of an idealized dynamic testing, while earlier we defined shear stress relaxation modulus G t) (see Equation 3.19) and shear creep compliance J(t) (see Equation 3.21) in terms of an idealized stress relaxation experiment and an idealized creep test, respectively. Mathematical relationships relating any one of these eight functions to any other can be derived. Such relationships for interconversion of viscoelastic function are described by Ferry [5], and interested readers are referred to this treatise for the same. 

The viscoelastic behavior is evaluated by means of two types of methods static tests and dynamic tests. In the first calegtuy a step change of stress or strain is applied and the stress or strain response is recorded as a function of time. Stress relaxation, creep compliance, and creep recovery are static methods. The dynamic tests involve the imposition of an oscillatory strain or stress. Every technique is described in the following sections. 

Creep / relaxation test time dependent properties compliance... 

To test the stress—time superposition, the curve of creep strains shown in Figure 12.5 are presented as the dependence of creep compliance on log time in Figure 12.7. In the stress range from 10 to 30 MPa the difference between averaged compliance curves at different stress levels was less than the mean square error of the experimental data. Therefore, the dashed line in Figure 12.7 corresponds to the average compliance for stresses from 10 to 30 MPa. Thus, if the stress... 

Averaged experimental creep compliance data are represented by dots in Figure 12.12 as a function of log time. The tests show that even after 14 months the creep continued and deformations kept increasing. After tests of 10000 h the compliance of the materials examined exceeds the instantaneous elastic compliance by a factor varying from 3.8 (for pure PP) to 3.2 (for the 80% PP + 20% PLC blend). This shows that the inelastic strain of these materials manifests itself substantially methods of accelerated testing and prediction have to be subjected to very detailed scrutiny. 

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