Craze collapse

Fig. 20 a Side-view of the crack-tip damage zone in a CT specimen of iPP with Mw of 455 kg mol-1 deformed at about 3 m s 1. b Oblique view of the damage zone showing the curved deformation front, c TEM micrograph of the collapsed fibrillar structure of the crack-tip craze, d Detail of structure at the craze-bulk interface [19]... 

The profound effect of water on tree growth that is so widely reported may be expected with the electrokinetic model on the basis of three principal effects. Water filling the crazes as they develop will help prevent their collapse. Water, as a good solvent for ionic species, will be an excellent medium to facilitate entry of surface-active agents, which, by a process similar to that of environmental stress cracking, will advance the void and craze formation caused by the electric field. Water, with its high relative permittivity, will distort and locally enhance electric fields in the neighbourhood of the voids and crazes where it accumulates. Whether one or other of these effects dominate in a particular situation depends on the exact nature of the specimen and its environment. 

All these are fast-drying (20 seconds or less) solvents, and in solid plastics would present a crazing problem. However, in foamed plastics this is not a problem. Aromatic hydrocarbon solvents, such as toluene and xylene, should not be used since they would cause a collapse of the cellular material. 

Fig. 8,34, Fracture surface of PMMA as observed in a scanning electron microscope. The structure originates from collapsed crazed matter. Micrograph obtained by Doll and Kdnczol [100]...

Additional proof for the crack growth subsequent to a craze follows from the surface structure of the broken sample. A thin film remains on the surface formed by the collapsed crazed matter, as can be observed by scanning electron microscopy. Figure 8.34 presents as an example a picture of the fracture surface of poly (methylmethacrylate), obtained for a resolution in the /im-range. The appearance of fracture surfaces may show considerable variations between different polymers, what is never observed, however, is a microscopically smooth surface, as it would result for a cleavage in a truly brittle fracture. 

Additional evidence for a crack growth behind a growing craze provides the surface structure of fractured samples. Images obtained by scanning electron microscopy show a thin him formed of collapsed hbrils on the fracture surface. Figure 10.46 presents as an example such an image, here of a fracture surface of poly(methylmethacrylate). 

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