Interaction between shear bands

Fig. 30a--c. Interaction between shear bands of type A with bands of type B a, and with spherulite boundaries perpendicular b and parallel c to shear band package A in PP 1320... 

Fig. 36a and b. Crack initiation as a result of interactions between shear bands (A, B) and coarse spherulite boundaries (SB) a before crack initiation (x = shear stress in the bands) b after crack initiation (C) ... 

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... 

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. 

In this context, some works investigated a series of blends obtained from hydroxy-propyl cellulose in lyotropic phase, and a new partial aHphatic polyimide (PI) or its poly(amic acid) precursor (PAA) [104]. The flow behavior of their mixed solutions in N,N-dimethylacetamide shows the effect of composition, temperature and shear rate on the orientation or mobiHty of the chain segments in the shear field. Specific interactions, such as the hydrogen bonds between PAA and the liquid crystalHne component, stabilized the resulted morphology. The band texture, typical for lyotropic HPC solutions, evidencing different intensities and dimensions is observed from atomic force microscopy images in PAA/HPC and PI/HPC blends (Figure 14.7) [104]. 

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