Viscoelasticity dynamic mechanical testing

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Dynamic mechanical tests measure the response of a material to a periodic force or its deformation by such a force. One obtains simultaneously an elastic modulus (shear, Young s, or bulk) and a mechanical damping. Polymeric materials are viscoelastic-i.e., they have some of the characteristics of both perfectly elastic solids and viscous liquids. When a polymer is deformed, some of the energy is stored as potential energy, and some is dissipated as heat. It is the latter which corresponds to mechanical damping. 

The development of constrained-layer damping materials through the use of dynamic mechanical testing and mathematical modeling has been described. It has been shown how different types and loadings of fillers will affect the measured viscoelastic properties of chlorobutyl rubbers. It has then been shown how these changes will affect the damping performance of these materials in constrained layer structures. 

Dynamic mechanical tests have been widely applied in the viscoelastic analysis of polymers and other materials. The reason for this has been the technical simplicity of the method and the low tensions and deformations used. The response of materials to dynamic perturbation fields provides information concerning the moduli and the compliances for storage and loss. Dynamic properties are of considerable interest when they are analyzed as a function of both frequency and temperature. They permit the evaluation of the energy dissipated per cycle and also provide information concerning the structure of the material, phase transitions, chemical reactions, and other technical properties, such as fatigue or the resistance to impact. Of particular relevance are the applications in the field of the isolation of vibrations in mechanical engineering. The dynamic measurements are a... 

As mentioned earlier, the DMTA technique measures molecular motion in adhesives, and not heat changes as with DSC. Many adhesives exhibit time-dependent, reversible viscoelastic properties in deformation. Hence a viscoelactic material can be characterized by measuring its elastic modulus as a function of temperature. The modulus depends both on the method and the time of measurement. Dynamic mechanical tests are characterized by application of a small stress in a time-varying periodic or sinusoidal fashion. For viscoelastic materials when a sinusoidal deformation is applied, the stress is not in phase with displacement. A complex tensile modulus E ) or shear modulus (G ) can be obtained ... 

The Takayanagi model was developed to account for the viscoelastic relaxation behaviour of two phase polymers, as recorded by dynamic mechanical testing. " It was then extended to treat both isotropic and oriented semi-crystalline polymers. The model does not deal with the development of mechanical anisotropy on drawing, but attempts to account for the viscoelastic behaviour of either an isotropic or a highly oriented polymer in terms of the response of components representing the crystalline and amorphous phases. Hopefully, comparisons between the predictions of the model and experimental results may throw light on the molecular processes occurring. 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

Fig. 32. Simple relationship between stress (a) and strain (e) in dynamic mechanical tests illustrating the role of the phase angle, showing (a) Hookian elastic response, (b) viscoelastic response, and (c) vectorial representations of complex oscillatory stress/strain response (74).

In dynamic mechanical tests, the reduced chain mobility due to the crystalline domains is detected by the shift of the damping peak to higher temperatures, that is, the increase of Tg, as already stated for DSC analyses. In addition, as shown in Figure 9.9, the DMTA evidences an increase of the elastic component of the viscoelastic behavior of the polymer as a significant reduction of the damping peak (data from Ref. 32). 

In the dynamic mechanical tests, either a vibrational force or a deformation is applied to the specimen, and then the sinusoidal response of either the deformation or force is measured, respectively. The dynamic mechanical properties are measured as a function of frequency at a constant temperature or as a function of temperature. The temperature dependence of dynamic viscoelasticity is conveniently used by the plastic industry to characterize solid polymers. Recently, various kinds of equipment for measuring dynamic viscoelasticity are commercially available and widely used for scientific and practical purposes. 

Golden, H.J., Strganac, T.W., Schapery, R.A., An Approach to Characterize Nonlinear Viscoelastic Material Behavior using Dynamic Mechanical Tests and Analyses , Journal of Applied Mechanics, Vol. 66, pp. 872-878, 1999. 

The viscoelasticity of soft materials is probed via several types of experiment. In stress relaxation measurements, the strain is held constant and the decay of stress is monitored as a function of time. Usually the rate of decay decreases as time progresses. In creep experiments, the stress is held constant and the increase in strain is monitored. Generally, the strain increases rapidly at first and then the rate of increase becomes smaller. In addition to these measurements, many commercial rheometers are able to perform dynamic mechanical testing, where an oscillatory strain is applied to the specimen. If the frequency of deformation is (o and t denotes time, we can write the strain as... 

The incorporation of fillers to elastomeric compounds strongly modifies the viscoelastic behavior of the material at small strains and leads to the occurrence of a non-linear behavior known as Payne effect [49] characterized by a decrease in the storage modulus with an increase in the amplitude of small-strain oscillations in dynamic mechanical tests. This phenomenon has attracted considerable attention in the past decade on account of its importance for industrial applications [50-54]. The amplitude AG = G q—G ) of the Payne effect, where G q and G aie the maximum and minimum values of the storage modulus respectively, increases with the volume fo-action of filler as shown in silica-filled PDMS networks (Figure 4.7a). At a same silica loading, the PDMS network filled with untreated silica displays a much higher G value than the treated one and is much more resistant to the applied deformation (Figure 4.7b). 

Creep and stress-relaxation measmements correspond to the use of step-response techniques to analyze the dynamics of electrical and process systems. Those familiar with these areas know that frequency-response analysis is perhaps a more versatile tool for investigating system dynamics. An analogous procedure, dynamic mechanical testing, is applied to the mechanical behavior of viscoelastic materials. It is based on the fimdamentally different response of viscous and elastic elements to a sinusoidally varying stress or strain. 

The viscoelastic behavior of polymer fibers is complex. Experimentally, it is important to perform simple laboratory tests from which information relevant to actual in-use conditions can be obtained. The viscoelastic characterization of polymer fibers often consists of condncting mechanical tests that are similar to those discussed in Chapter 15, but are modified so as to enable the observation of the time dependency of fibers response. Three most important viscoelastic tests are creep, stress relaxation, and dynamic mechanical testing. 

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. 

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 measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

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