Craze-growth experiments

All the craze-growth measurements in both types of PS and in PMMA at both 293 K and 253 K over a range of applied stress are summarized in Fig. 11.15, where the tensile stresses aao were normalized by an appropriate tensile reference plastic resistance uo for all types, which was derived from compression-yield 

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Having set out a detailed model of craze growth, we now compare its predictions with recent experiments in two principal areas 1) the effects of entanglement density and 2) the effects of temperature. 

This type of experiment was extensively used in very early experimental works on craze growth. Doll and coworkers used it with optical interferometry to evaluate... 

The log (V ) versus log (to) plot is a straight line, with a slope of — 1, if the activation enthalpy of the craze growth and breakage are the same in a constant stress experiment at variable temperatiues. 

Table 11.7. Craze inception strain (ecj) and craze inception stress (oci) for polymers [41]. The Gci values were calculated from ecj and Young s modulus (E) values by assuming linear elasticity up to the inception of crazing and thus using Equation 11.1, and rounded off to the nearest integer. E and yield stress (ay) values from Ref. [41] are also listed, and differ (in some cases by a significant amount) from the values of these properties listed in Tables 11.1 and 11.6 for many of the same polymers since they are from different experiments. E, Gci and Gy are in MPa. The stress for failure via crazing (ac) is usually significantly larger than GCj since it also includes the effects of craze growth and propagation, but it is not yet as well-quantified as Gci.

Argon and Salama 1976). As Fig. 11.10 shows, in the temperature range of 100 130 °C of experiments over which the value of the product Doy remains constant, ay varies considerably as in all glassy polymers, where the value of D shows a compensating change consistent with the craze-growth model discussed in Section 11.8. 

Comparison of the craze-growth model with experiments... 

Comparison of the predictions of the craze-growth model in PS/PB diblock copolymers with experiments... 

For comparison of the predictions of the craze-growth model of the PS/PB diblocks with experiments, the material parameters given in Table 11.4 were used for 293 K and 253 K. For the bulk modulus Kc and Poisson s ratio Vc the values were calculated from the elastic moduli of PS and PB using the theory of Chow... 

An example of a study conducted using a tensile stage in the SEM is the evaluation of the ductile failure of poly(vinyl chloride) [534] in which stamped dumbbell shaped pieces of polymer from 1 mm thick sheets were extended to a neck in an Instron tester and then strained in the SEM. Low accelerating voltage was used for imaging of the uncoated specimens. These experiments showed that, after neck formation, fracture occurs by crack propagation from a flaw or cavity within the surface craze. The in situ deformation of amorphous polymers by shear deformation and craze growth has been observed in optical microscope studies by Donald and Kramer [509]. Thin films of various... 

However, experimentally it has been observed that craze growth occurs faster than craze initiation and so it is difficult to explain growth with such a model as it is essentially the same mechanism as initiation. Also, some of the features of the craze predicted from such a mechanism, such as a closed cell structure formed from the formation of isolated voids, are inconsistent with experiments that clearly show that the craze has an open structure. 

Figure 16 shows a typical evolution of fringe pattern for the case of a crack that grows into an immobile craze. This kind of experiment allows a clean observation of crack growth. However, it has not been used at all, because a steady-state growing crack-craze system will be more efficient from a practical point of view, as shown in Sect. 3). Nevertheless, it will be encountered again in the case of an oscillating crack-craze system described below. 

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