Dynamic test

FIGURE 2.10 Two common dynamic tests, (a) Torsional pendulum test and (b) vibrating reed test. (From Nielsen, L. E., Mechanical Properties of Polymers and Composites, Vol. 1, pp. 11-17, Marcel-Dekker, Inc., New York, NY, 1974.) 

FIGURE 2.11 Electromagnetic dynamic mechanical analyzer. (From Sepe, M. P., Advanced Materials and Processes Magazine, 4, 32-41, 1992.) 

Frequency and amplitude or force can be controlled. The sample can also be positioned in a temperature-controlled chamber to study the effects of temperature on mechanical properties. (A study of the parameters governing the DMA test is left to a homework problem.) 

When experiments are necessary to determine mechanical properties, the tests should be designed and conducted according to ASTM standards. ASTM is an abbreviation for the American Society for the Testing of Materials. For example, the DMA test above is compared with the ASTM D.790 flexural modulus test in Sepe. ° 

For a frequency of oscillation of a)/2 rHz, the stress cat any given time, h is given by  

The stress can be resolved into two parts, one in phase with the strain and the other 90° out of phase with the strain. 

Assuming these conditions apply to a torsion experiment, it also becomes possible to define two shear moduli, and G2. The first of these represents that part of the stress that is in phase with the strain, divided by the strain. 

The value of this latter parameter is proportional to the energy dissipated as heat per cycle, and is known as the loss modulus. The former quantity, Gj, is proportional to the recoverable energy, and is called the storage modulus. The two are combined to form the complex modulus, G related by the equation 

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After having proved the principles a dynamic test facility has been constructed. In this facility it is possible to inject 3 tracers in a flownng liquid consisting of air, oil and water. By changing the relative amounts of the different components it is possible to explore the phase diagram and asses the limits for the measurement principle. Experiments have confirmed the accuracy in parameter estimation to be below 10%, which is considered quite satisfactorily for practical applications. The method will be tested on site at an offshore installation this summer. 

A further excellent demonstration of the need to interpret experimental results in terms of a specific flux model is provided by the dynamic test... 

Thus dynamic tests will be influenced by the existence of the dead-end pores and, in principle, they offer the possibility of obtaining quantitative inforiuation on diffusion in these pores. 

Two different types of dynamic test have been devised to exploit this possibility. The first and more easily interpretable, used by Gibilaro et al [62] and by Dogu and Smith [63], employs a cell geometrically similar to the Wicke-Kallenbach apparatus, with a flow of carrier gas past each face of the porous septum. A sharp pulse of tracer is injected into the carrier stream on one side, and the response of the gas stream composition on the other side is then monitored as a function of time. Interpretation is based on the first two moments of the measured response curve, and Gibilaro et al refer explicitly to a model of the medium with a blmodal pore... 

Finally, it should be pointed out that all the results on dynamical testing presented here are correct only if the tracer gas is not significantly adsorbed at the solid surface. If it is adsorbed weakly, so that a linear isotherm is appropriate, the equations should be modified by the following replacements ... 

Toward New Explosive Molecules in Dynamic Testing Los Alamos National Laboratory, N. Mex., Mar. 1984. 

R. L. WooUey and G. J. Germaine, "Dynamic Tests of Hydrogen-Powered IC Engines," World Hydrogen Unergy Conference, Miami Beach, Fla., 1976. 

The shear modulus of a material can be determined by a static torsion test or by a dynamic test employing a torsional pendulum or an oscillatory rheometer. The maximum short-term shear stress (strength) of a material can also be determined from a punch shear test. 

Testing yields basic information about any materials (plastics, steels, etc.), its properties relative to another material, its quality with reference to standards or material inspections, and can be applied to designing with plastics. Examples of static and dynamic tests are reviewed in Chapter 2. 

Dynamic shear moduli are conveniently determined with automated equipment, for instance, with the torsion pendulum. However, moduli derived from dynamic tests are often higher than the results from static tests for lack of relaxation. Examples are shown in Table 3.3. Young s moduli of the polymers A, B, C, D, derived from tensile tests (frequency 0.01 Hz) are compared with shear moduli S determined with the torsion pendulum (frequency > 1 Hz). For rubberlike materials is 3S/E = 1, according to Eq. 

The mass of the network strands is, therefore, more appropriately deduced from static tests than from dynamic tests. 

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. 

Figure 7.9 Lag of strain behind stress in dynamic testing...

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Both static and dynamic tests are employed to evaluate the adhesion strength of cord-mbber composites. The major static tests used in tire industry are H-adhesion, 90/180° peel test, tire cord adhesion test (TCAT) and co-axial shear pull-out test (CSPT). Although these methods are... 

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