Time-dependent tests

Samples subjected to time dependent testing in creep or dwell fatigue experiments (exposure) were subjected to cyclic tensile loading at room temperature until failure (post exposure). Elastic and creep strains from exposure experiments as well as strain to failure during post-exposure... 

Chemical cross-linking reactions and sol/gel transitions can also be monitored using a time-dependent test. 

The idea of using mathematical modeling for describing materials behavior under loads is well known. Some physical phenomena, which can be observed in materials during testing, have time dependent quantitative characteristics. It gives a possibility to consider them as time series and use well developed models for their analysis [1, 2]. Usually applied... 

This is an example of a paired data set since the acquisition of samples over an extended period introduces a substantial time-dependent change in the concentration of monensin. The comparison of the two methods must be done with the paired f-test, using the following null and two-tailed alternative hypotheses... 

Fig. 7. Toxicity of chlorine to aquatic organisms, (a) Time-dependent mortaUty (50%) of four example species in various levels of total residual chlorine in the laboratory, where for A, A.losa aestivalis and B, Salmogairdnerii r (correlation coefficient of the curve) = —0.96 and for C, P/euroneetesplatessa and D, Salmo trutta r = —0.98. (b) A summary of chlorine toxicity to freshwater species, indicating overall no-effect thresholds for acute and chronic exposures. Numbers indicate where more than one test yielded the same result. A different summary figure appHes to marine organisms because of differences in the...

In summary, then, design with polymers requires special attention to time-dependent effects, large elastic deformation and the effects of temperature, even close to room temperature. Room temperature data for the generic polymers are presented in Table 21.5. As emphasised already, they are approximate, suitable only for the first step of the design project. For the next step you should consult books (see Further reading), and when the choice has narrowed to one or a few candidates, data for them should be sought from manufacturers data sheets, or from your own tests. Many polymers contain additives - plasticisers, fillers, colourants - which change the mechanical properties. Manufacturers will identify the polymers they sell, but will rarely disclose their... 

Reference electrodes at the test points may only be needed part of the time, depending on the mode of operation of the protective systems (e.g., for monitoring or for permanent control of potential-controlled protection current equipment). Potentiostatic control is always preferred to galvanostatic systems where operational parameters are changing. 

The reader is encouraged to use a two-phase, one spatial dimension, and time-dependent mathematical model to study this phenomenon. The UCKRON test problem can be used for general introduction before the particular model for the system of interest is investigated. The success of the simulation will depend strongly on the quality of physical parameters and estimated transfer coefficients for the system. 

When a voltage is applied to a dielectric (insulator), a current passes that decays with time owing to various polarization mechanisms [ 133]. Conductivity is always time-dependent. This general time dependency affects conductivity measurement for nonconductive liquids, where the peak initial current is used to calculate conductivity. Test methods are given in 3-5.5 and... 

Figure 2.5-2 depicts the force of mortality as a bathtub curve for the life-death history of a component without repair. The reasons for the near universal use of the constant X exponential distribution (which only applies to the mid-life region) are mathematical convenience, inherent truth (equation 2.5-19), the use of repair to keep components out of the wearout region, startup testing to eliminate infant mortality, and detailed data to support a time-dependent X. 

The probability of the redundant system failing is the product of the single system time-dependent unavailability which includes approximately linearity between test and repair. There are two ways of performing the surveillance tests on these two systems sequential and staggered testing. 

Endurance Burn Test A test in which a flame arrester is subjected to a stable flame on the surface of the arrester for a length of time (depends on whether UL 525 or the USCG protocol is used) until the highest obtainable temperature is reached on the ignited side or until the temperature on the protected side has a temperature rise of 100°C. 

The electrical resistance of a coating on a pipeline will depend upon the effectiveness of its initial application to the metal and on its condition at the time of testing absorption of moisture may decrease the resistance by as little as 10% during a 2y-year period. The leakage resistance of a given section of pipeline can be determined by the following procedure ... 

Dynamic loading in the present context is taken to include deformation rates above those achieved on the standard laboratorytesting machine (commonly designated as static or quasi-static). These slower tests may encounter minimal time-dependent effects, such as creep and stress-relaxation, and therefore are in a sense dynamic. Thus the terms static and dynamic can be overlapping. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Stress relaxation tests are alternative ways of measuring the same basic phenomenon in viscoelastic polymers as creep tests, Le. the time-dependent nature of their response to an applied stress. As such, they have also been of value in understanding the behaviour of these materials. The essence of stress relaxation tests is that strain increases with time for a given stress, so that if stress is decreased with time in a controlled manner ( relaxed ), a state... 

The general mode of operation in dynamic tests is to vary the stress sinusoidally with time. A viscoelastic solid in which the viscous element is that of a Newtonian liquid (as defined earlier) responds with a sinusoidal strain of identical oscillation frequency. However, because of the time-dependent relaxation processes taking place within the material, the strain lags behind the stress, as illustrated in Figure 7.9. 

Visco-elastic substances can be described with the spring/shock-absorber model of Kevin and Voigt, and have phase displacements of 0° to 90°. In analogy to other time dependent processes in physics, the oscillation tests are evaluated with complex arithmetics. Obtained are the complex quantities ... 

E. Vindimian, B. Vollat, and J. Garric. Effect of the dispersion of oil in freshwater based on time-dependent daphnia magna toxicity tests. Bull Environ Contamination Toxicol, 48(2) 209-215, February 1992. 

Computer program MOTIF Evaluation of time spectra for NFS of synchrotron radiation, allows fully automatic fits of experimental data. Has been tested on several Unix platforms by fitting NFS time spectra of Fe-57, Sn-119, Eu-151, Dy-161, Ta-181 in various compounds with different time-independent and time-dependent hyperfine interactions... 

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