Amorphous critical strain rate

The selection of the dominant deformation mechanism in the matrix depends not only on the properties of this matrix material but also on the test temperature, strain rate, as well as the size, shape, and internal morphology of the rubber particles (BucknaU 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). The properties of the matrix material, defined by its chemical structure and composition, determine not rally the type of the local yield zones and plastic deformation mechanisms active but also the critical parameters for toughening. In amorphous polymers which tend to form fibrillated crazes upon deformation, the particle diameter, D, is of primary importance. Several authors postulated that in some other amorphous and semiciystalline polymers with the dominant formation of dUatational shear bands or extensive shear yielding, the other critical parameter can be the interparticle distance (ID) (the thickness of the matrix ligaments between particles) rather than the particle diameter. 

Although amorphous elastomers are found to tear steadily, at rates controlled by the available energy for fracture G (as shown in Figs. 13 and 14), strain-crystallizing elastomers do not tear continuously under small values of G, of less than about lO J/m for natural rubber for example (see Fig. 15). Nevertheless, when small stresses are applied repeatedly, a crack will grow in a stepwise manner by an amount A/ per stress application, even though the corresponding value of G is much below the critical level [5]. 

Fig. 5.21. Schematic illustration of craze formation in the creep test on transparent amorphous thermoplastics. Visible crazes occur at a certain time and strain dming the creep test. These times are indicated in the creep curves measured at different stresses. The connecting line of these points provides a curve which describes the strain limit at which craze formation occnrs as a function of time or deformation rate respectively. An extrapolation of the cnrve towards great times yields the critical limiting strain for craze formation...

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