The structure and formation of crazes

We have seen how the craze at the crack tip in a glassy polymer plays a vital role in determining its fracture toughness. Crazing in pol3oners also manifests itself in another way. When certain pol3nners, notably PMMA and polystyrene, are subjected to a tensile test in the glassy state, above a certain tensile stress opaque 

The interference bands on the fracture surfaces, which relate to the craze at the crack tip, were first observed by Berry [31] and by Higuchi [32]. Kambour confirmed that the PMMA fracture-surface layers were qualitatively similar to the internal crazes of this polymer, by showing that the refractive indices were the same [15]. Both surface layer and bulk crazes appear to be oriented polymer structures of low density, which are produced by orienting the polymer under conditions of abnormal constraint it is not allowed to contract in the lateral direction, while being extended locally to strains of the order of unity, and so has undergone inhomogeneous cold-drawing. 

Detailed studies have been made of the structure of crazes, the stress or strain criteria for their formation and environmental effects. These subjects now will be discussed in turn. 

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Electron Microscopic Results. The fundamental deformation step is the formation of crazes at the rubber particles (Figure 4). The crazes start directly at the interface between rubber particles and matrix in the equatorial zones around the particles, that is, in the zones of highest stress concentration. The structure of the amorphous material is transformed by local plastic defor-... 

Figure 6.13 Illustration of the structure of a nanocomposite (top) and formation of a nanoparticle-initiated craze under load (bottom) with the steps of debonding, nanovoid formation, and stretching of the voids up to fibrillation [16, 17]...

The mechanism how a rubber distributed in a network influences the rupture mechanism is not quite well understood yet. It is known that poly(vinyl chloride) forms shear bands when stress is applied and that parts of the rubber which are located in these shear bands may form crazes.13 It might well be that a network structure is efficient for the delocalization of stress energy only in combination with the formation of shear bands. Experimental work is needed to elucidate this further. 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

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