Interaction between crazes and

Figure 19.9 TEM images and the corresponding sketch of the interaction between crazes and crystalline lamellae...

For ionomer samples with high ion content (more than 6 mol %), it is noted that fewer crazes are formed, and the direction of some of these is not perpendicular to the stress axis (Fig. 26a). Also, bifurcation of crazes is observed (Fig. 26b). In addition to these anomalies in craze structure, some shear character, such as localized shear deformation, also appears (Fig. 27a). For comparison, a TEM micrograph taken for a polycarbonate (PC) sample under the same experimental conditions is shown in Fig. 27b. PC is known to deform by shear deformation at room temperature In Fig. 27b, it is seen that shear bands are formed at approx. 45° to the stress direction. Similar structural features, although in smaller degree, are seen for ionomer samples with high ion content (Fig. 27a). Also, in these samples interactions between crazes and shear bands are noted (for example. Fig. 26a). Interaction effects have also been observed in other glassy polymers... 

The foregoing considerations about the mechanism of solvent cracking and solvent crazing suggest that the solubility parameter difference, as a quantitative measure of the interaction between polymer and solvent, will play an important part in these phenomena. 

We present recent results on the analysis of the interaction between plasticity and crazing at the tip of a preexisting crack under mode I loading conditions. Illustrations of the competition between these mechanisms are obtained from a finite element model in which a cohesive surface is laid out in front of the crack. 

Evidence for differences in activation volume and enthalpy between shear yielding and crazing has already been presented. In discussing kinetics, it is convenient to treat the two mechanisms as independent, and to calculate activation parameters for each process accordingly. It must be noted, however, that interactions do occur between crazes and shear bands under certain conditions, so that the kinetics cannot be regarded as completely independent. 

Localized shear yielding and crazing are competing mechanisms of brittle fracture under tensile loading. Two principal interactions can occur between craze and shear band in an early mechanism proposed by Bucknall et al. (171), Newman and Wolock (169), and Jacoby and Cramer (170). Later, however, Donald et al. (172,173) showed that interaction A cannot effectively lead to a stabilization of craze and only interaction B will stabilize craze growth and contribute to toughening of the polymeric material. 

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Looking at the structure of these crack tip plastic zones in more detail, it is found that the individual crazes are less straight compared to the low temperature crazes (Fig. 21). This indicates a more pronounced influence of the crystalline microstructure on craze formation. Figure 21a and b demonstrate for fine spherulitic, highly isotactic PP the interaction between the crazes and the microstructural features. Most of the... 

Analogous results have been found for other polymers. Crazing at crack tips exhibits exactly the same behavior. In order to evaluate the acceleration of crazing by action of a solvent, the interaction between the polymer and the solvent can be quantified by means of the solubility parameter, 5, defined as the cohesive energy density where AHy is the vaporization... 

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