Testing force versus deformation measurements

The classic way that we 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 two 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,... 

When discussing the theory of rubber elasticity in Chapter 10, we were concerned with fairly large extensions or strains. These arose because polymer molecules could uncoil at temperatures above T. For materials used as structural elements (such as glassy polymers), we usually cannot tolerate strains of more than a fraction of 1%. Therefore, it is customary to employ measures of infinitesimal strain. In a tensile test, we usually take a specimen with tabs at the ends and stretch it, as shown in Figure 12.1. One end of the sample is typically fixed, whereas the other is moved outward at a constant velocity. The force F necessary to carry out the stretching deformation is monitored as a fimction of time along with the instantaneous sample length, L. From the measured load versus extension behavior, we can calculate the stress and strain as follows ... 

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