Dynamic—mechanical testing

Cre and stress-relaxation measurements conespond 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 

F nre 18.14 Stress in a linear spring and a linear dashpot in response to a sinusoidal applied strain. 

If a sinusoidal strain, y = y sin a t (where co is the angular frequency, radian/s) is applied to a linear spring, since t = Gy, the resulting stress t = Gy sin cot is in phase with the strain. For a linear dashpot, however, because the stress is proportional to the rate of strain rather than the strain, x — gy = jjcoy cos cot, the stress is 90° out of phase with the strain. These relations are sketched m Fig. 18.14. 

G s — storage modulus (in-phase component) and an out-of-phase or loss modulus is defined by 

The complex modulus G is the vector sum of the in-phase and out-of-phase moduli  

Tests such as the tensile strength and tear strength tests evaluate polyurethanes to destruction. When polyurethanes are used in a practical situation, the aim is for them to have as long a life as needed in the application. Stress, strain, and shear are applied to the polyurethane at various frequencies and at different temperatures. There may also be dynamic variations on top of a static load, for example, vibrations on a loaded isolation pad. 

ISO 2856 and ASTM D2231 outline general requirements for dynamic testing but with no descriptions of actual method or equipment. The tests are divided into three main groups  

These tests form the classical methods of testing, but the use of dynamic mechanical analyzers are becoming more popular, especially those that can operate from subambient temperatures to 150°C and above. 

Resilience is one of the most common rebound tests. It is fundamentally a deformation of the material for half a cycle. The rebound resilience basically is the ratio of the indentor after to before impact expressed as a percentage. There are a large number of methods for this test, including the following. 

ASTM D1054 Standard test method for rubber property-resilience using a Goodyear-Healey rebound pendulum 

FIGURE 15.15 Quantities in dynamic testing (a) rotating vector diagram (b) stress and strain. 

Comparison of the real (in-phase) and imaginary (out-of-phase) parts of Equation 18.40 gives 

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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. 

The simplest dynamic system to analyse is one in which the stress and strain are changing in a sinusoidal fashion. Fortunately this is probably the most common type of loading which occurs in practice and it is also the basic deformation mode used in dynamic mechanical testing of plastics. 

Dynamic mechanical tests measure the response or deformation of a material to periodic or varying forces. Generally an applied force and its resulting deformation both vary sinusoidally with time. From such tests it is possible to obtain simultaneously an elastic modulus and mechanical damping, the latter of which gives the amount of energy dissipated as heat during the deformation of the material. 

Die Tg can be determined readily only by observing the temperature at which a significant change takes place in a specific electric, mechanical, or physical property. Moreover, the observed temperature can vary significantly, depending on the specific property chosen for observation and on details of the experimental technique (for example, the rate of heating, or frequency). Therefore, the observed Tg should be considered to be only an estimate. The most reliable estimates are normally obtained from the loss peak observed in dynamic mechanical tests or from dilatometric data (ASTM D-20). 

Dynamic mechanical tests, which are the other major group of testing techniques, tend not to be subject to such wide variation in results as nondynamic ones, and hence statistics tend not to be used to the same extent. 

Dynamic mechanical testing (4.5) was employed as a probe to investigate the influence of ionizing radiation on the mechanical properties of an epoxy since it can provide several types of information (e.g. E, E, E" and tanj). 

Cured Olixomer Properties. The T, of the cured specimen ms not obvious by DSC, however the T was observed at 383°C (Q maac) by dynamic mechanical testing of a torsion bar. The T obtained by this method has been driven up in temperature by the slow heating rate of 2°C/minute used in the test, the actual T is somewhat lower... 

Dynamic Mechanical Tests. Plasticizer efficiency, can be measured, not only be the lowering of T , but also by temperature dependence of typical dynamic mechanical properties, such as modulus and damping. 

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. 

Dynamic mechanical testing of the epoxy matrix/epoxy sizing blends was done on a DuPont model 983 DMA interfaced to a 9900 model controller. Stoichiometric mixtures of DER 383 and DACH were first prepared, and then sufficient size was added to produce the desired concentration on a weight percent basis. The mixture was degassed and poured into silicone RTV-664 [10] molds that contained four cavities, 3.2 x 12.5 x 60 mm. The specimens were cured at room temperature for 16 h in a desiccator, placed in a forced convection oven, and ramped to 80°C at 5°C/min and held at that temperature for 2 h. The samples were allowed to cool to room temperature and then removed from the mold. The specimens were replaced in the oven on a metal sheet and postcured at 175°C for 2 h. The free surface of the specimens was ground on a Struers Abramin polisher using 320 grit SiC paper and water to produce parallel faces on the specimens. 

Since dynamic mechanical tests measure the response of a material to an applied stress at different temperature and frequency, they measure the transition of the material from glassy to leathery to rubbery state. If the frequency is kept constant and low (about one cycle/sec), the results are related to measurements of transition by other techniques. Thus, some cross-checking is possible. 

The results of the dynamic-mechanical tests carried out on the blends and on the corresponding pure CPVCs are given in Figures 5, 6, 7, and 8. 

E = storage modulus determined in a dynamic mechanical test, GPa... 

Dynamic Mechanical Testing - Film properties such as impact resistance and the cure response of thermosetting resins are conveniently investigated by dynamic measurements in which an oscillatory or torsional strain is applied to the sample with the stress and phase difference between the applied strain and measured stress being determined. In the present study, a Rheovibron Viscoelastometer was used which employed a sinusoidal strain at a... 

The values of the various softening temperatures can now be related to the shape and the position of the E(T) curves, discussed in Chapters 3 and 4. A satisfactory comparison is hardly possible, because the time scales of the tests differ too much E(T) curves have, in most cases, been measured by dynamical-mechanical tests at a time scale round one second, whereas the determination of softening temperatures extends over several minutes. In principle, however, the picture presented in Figure 8.1 is valid (see also Qu. 8.3). 

Bucknall, in addition to micrographic studies, confirmed polymer miscibility via dynamic mechanical testing that the Tg peaks in these systems became intermediate to the pure components indicating mutual compatibility [107-109]. This result will be discussed in the following section. 

Dynamic-mechanical testing of cross-linked samples are often carried out with high precision on specimen strips in torsion mode, e.g., with a Rheo-metrics Dynamic Analyzer II (RDA) with a sample size of 28x10x2 mm. Here, temperature-and strain sweeps are performed in a displacement range from 0.01% to about 5% strain and a frequency range between 0.1 and 100 Hz. Dynamic mechanical testing of uncross-linked samples can be made, e.g., with a Rubber Process Analyzer RPA 2000 (Alpha Technologies) from 0.28% to 350% strain at various frequencies and elevated temperatures. 

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