Mechanical Tests Used for Yielding and Fracture

Most of the mechanical tests mentioned here are well described by Brown (1999). 

Very often uniaxial tensile tests are used for polymer characterization. In the case of thermosets, at temperatures below Tg, the stress-strain curve is a straight line (elastic region) and fracture occurs (due to flaws contained in the specimen) well before yielding (Fig. 12.1a). So other tests have to be 

After the op region, a strain-hardening phenomenon occurs. Usually the curves use nominal stress, = (load/initial cross section), and nominal strain, = dL/Lf,. 

But true stress (load/actual section) and true strain, calculated as the natural logarithm of the relative height, n(L/Lo), must be used to obtain better information on the material. The accuracy of strain measurements may be improved by the use of extensometers or strain gauges and actual cross section determination needs the use of double extensometers or stiffness correction of the machine and rigs (Cook et al., 1998). 

Nevertheless, a simple calculation of true stress is possible, assuming constant volume (which is not exactly true because the Poisson s ratio is 

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