Axial Tensiometers

For such axial tension experiments, the results are normally plotted as stress-strain curves. If the rate of strain is fixed and constant, the x-axis (independent variable) can be 

FIGURE 16.2 An automated materials testing apparatus typically applies a known stress on a dogbone-shaped sample and measures the resulting stretch (strain). A fixed or linear strain can also be programmed with the required force (or stress) measured as the sample is stretched. 

FIGURE 16.3 Stress-strain curves for different samples of block copolymers of caprolactone with -butyl acrylate tested at (a) room temperature and (b) 70 °C. Polymer compositions were 0, 20, 39, 50 and 71 wt% n-butyl acrylate from top to bottom in (a). Reprinted with permission from Referenece 1. Copyright 2001 National Academy of Sciences, USA. 

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This chapter covers some of the methods and instruments used to determine the mechanical properties of polymers. Examples of instrument designs and typical data generated in these measurements will be introduced. In particular, automated axial tensiometers (to find elastic modulus, yield stress, and ultimate stress), dynamic mechanical analyzers (to determine storage and loss moduli), and rheometers (to measure flow viscosity) will be introduced. This chapter considers the principles behind the devices used to establish and measure the properties of viscometric flows. One of the common techniques used to determine viscous flow properties, PoisueiUe (laminar) flow in cylindrical tubes, is also important in technical applications, as polymer melts and solutions are often transported and processed in this manner. The time-temperature superposition principle is also covered as a way to predict polymer behavior over long timescales by testing materials across a range of temperatures. 

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