Dynamic mechanical testing torsion pendulum

Previous studies of ionomers have shown that dynamic mechanical tests are most sensitive to the presence of large clusters. For this reason, dynamic mechanical test was performed using a computerized torsion pendulum (28). The frequencies varied from ca. 3 Hz for the glassy region to ca. 0.1 Hz for the low modulus region. The heating rate was usually 0.6°C/min with a temperature control of 1°C. 

In dynamic mechanical tests, the response of a material to periodie stress is measured. There are many types of dynamic mechanical test instruments. Each has a limited Irequency range, but it is generally possible to cover frequencies from 1(E to 10 cycles per second. A popular instrument for dynamic mechanical measurements is the torsion pendulum (Figure 13.6A). A polymer sample is elamped at one end, and the other end is attaehed to a disk that is free to oscillate. As a result of the damping characteristics of the test sample, the amplitude of oscillation decays with time (Figure 13.6B). 

Part of the work performed on a sample will be converted irreversibly into random thermal motion by movement of the molecules or molecule segments. This loss passes through a maximum at the appropriate transition temperature or relaxation frequency in the associated alternating mechanical field (torsion pendulum test). A similar effect is obtained by the delayed response of the dipoles with dielectric measurements. Therefore, dielectric measurements can be made only on polar polymers. According to the frequency used, the glass-transition temperatures measured with dynamic methods lie higher than those obtained by quasistatic methods (see Section 10.5.2). 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

Torsion property As noted, the shear modulus is usually obtained by using pendulum and oscillatory rheometer techniques. The torsional pendulum (ASTM D 2236 Dynamic Mechanical Properties of Plastics by Means of a Torsional Pendulum Test Procedure) is a popular test, since it is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial processes or easily cut from fabricated products. 

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. 

Dynamic-Mechanical Measurement. This is a very sensitive tool and has been used intensively by Nielsen (17) and by Takayanagi (18). When the damping curves from a torsion pendulum test are obtained for the parent components and for the polyblend and die results are compared, a compatible polyblend will show a damping maximum between those of the parent polymers whereas the incompatible polyblend gives two damping maxima at temperatures corresponding to those of the parent components. Dynamic mechanical measurement can also give information on the moduli of the parent polymer and the polyblend. It can be shear modulus or tensile modulus. If the modulus-temperature curve of a polyblend locates between those of the two parent polymers, the polyblend is compatible. If the modulus-temperature curve shows multiple transitions, the polyblend is incompatible. 

The loss factor, tan 8, can be measured with the aid of dynamic-mechanical experiments (such as the torsion pendulum). The deformation in such a test varies as indicated in Figure 7.13 the damping follows from the logarithmic decrement , A, it can be easily shown that... 

A convenient method for determining transition times and transition temperatures of polymeric materials is dynamic mechanical analysis. One type of instrument which is particularly suitable for polymeric solids is the freely oscillating torsion pendulum (TP). Advantages of the TP include its simplicity, sensitivity, relatively low frequency ( 1 Hz) which permits direct correlation of transition temperatures with static nonmechanical methods (e.g., dilatometry and calorimetry), and its high resolution of transitions A major disadvantage of the conventional TP is that test temperatures are limited by the inability of materials to support their own weight near load-limiting transition temperatures. 

Torsion pendulum n. A device for performing dynamic mechanical analysis in which a sample is deformed torsionally and allowed to oscillate in free vibration. Shah V (1998) Handbook of plastics testing technology. John Wiley and Sons, New York. 

Standard Test Method for Dynamic Mechanical Properties of Plastics by Metals of a Torsional Pendulum... 

Even though torsion pendulum tests run at elevated temperatures provide very useful information regarding the dynamic mechanical properties of plastics, one must not forget that it is, nevertheless, a short-term test. The data obtained from creep modulus and creep rupture tests conducted at elevated temperatures must be relied on for long-term effects. 

Test method for dynamic mechanical properties of plastics by means of a torsional pendulum Practice for determining resistance of synthetic polymer material to fungi. Practice for determining resistance of plastics to bacteria... 

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.)... 

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