Force versus Deformation Properties

The classic way that ve perform force versus deformation measurements is to deform a sample at a constant rate, while we record the force induced within it. We normally carry out such tests in one of three configurations tensile, compressive, or flexural, which are illustrated in Fig. 8.1. We can also test samples in torsion or in a combination of frvo or more loading configurations. For the sake of simplicity, most tests are uni-axial in nature, but we can employ bi-axial or multi-axial modes when needed. 

We perform most tests uni-directionally, that is, we increase the deformation in only one direction, as shown if Fig. 8,2 a). Alternatively, we can perform a dynamic test in which the direction of the deforming force is reversed one or more times. In dynamic tests, the waveform of the applied deformation is often sinusoidal, as shown in Fig. 8.2 b), but many other modes are possible, including a sawtooth pattern, or a square wave, as sho vn in Fig. 8,2 c) and d), respectively. 

When the specimen is in the form of a strip, the gauge length is the distance between the upper and lotver jaws. When dogbones are tested, the gauge length is the length of the parallel portion of the specimen. 

In Fig. 8.4 a), the glassy amorphous polymer emends only a few percent before it breaks abruptly. The extension in the sample up to the point of failure is largely reversible, that is, the material behaves elastically. Polystyrene and polycarbonate, which are used to make CD jewel cases, exhibit this type of behavior. 

The examples that -we have shown here represent only a small fraction of all the variations possible. There is no such thing as a typical force versus elongation curve for polymers. Samples can break at extensions of only a fraction of a percent up to several thousand percent, with engineering stresses at break ranging from only slightly above zero up to more than 10 GPa, 

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Force versus displacement data are directly useful in comparing some simple mechanical properties of particles from different samples, for example, the force required to break the particle and the deformation at breakage. However, these properties are not intrinsic, that is, they might depend on the particular method of measurement. Determination of intrinsic mechanical properties of the particles requires mathematical models to derive the stress-strain relationships of the material. 

In amorphous state, solid polymers retain the disorder characteristic for liquids, except that the molecular movement in amorphous solid state is restrained. The movement of one molecule versus the other is absent, and some typical liquid properties such as flow are absent. At low stress, polymers display elastic properties, reverting to a certain extent to the initial shape in a relaxation process. However, they can be irreversibly deformed upon application of appropriate force. The deformation and flow of polymers is very important for practical purposes and is studied by a branch of science known as rheology (see e.g. [1]). The combination of mechanical force and increased temperature are commonly applied for polymer molding for their practical applications. The polymers that can be made to soften and take a desired shape by the application of heat and pressure are known as thermoplasts, and most linear polymers have thermoplastic properties. 

The modes of operation of a DMA are varied. Using a multifrequency mode, the viscoelastic properties of the sample are studied as a function of frequency, with the oscillation amplitude held constant. These tests can be run at single or multiple frequencies, in time sweep, temperature ramp, or temperature step/hold experiments. In multistress/strain mode, frequency and temperature are held constant and the viscoelastic properties are studied as the stress or strain is varied. This mode is used to identify the LVR of the material. With creep relaxation, the stress is held constant and deformation is monitored as a function of time. In stress relaxation, the strain is held constant and the stress is monitored versus time. In the controlled force/strain rate mode, the tanperature is held constant, while stress or strain is ramped at a constant rate. This mode is used to generate stress/ strain plots to obtain Young s modulus. In isostrain mode, strain is held constant during a tanpera-ture ramp to assess shrinkage force in films and fibers. 

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