Crazes shape

Processes that occur at a size scale larger than the individual chain have been studied using microscopy, mainly transmission electron microscopy (TEM), but optical microscopy has been useful to examine craze shapes. The knowledge of the crazing process obtained by TEM has been ably summarised by Kramer and will not be repeated here [2,3]. At an interface between two polymers a craze often forms within one of the materials, typically the one with lower crazing stress. 

TEM has been used to demonstrate that the craze normally fails at the material interface [4-6], In addition the fracture energy calculated from the craze shape tends to agree well with the macroscopic measure of toughness. 

The measure of the craze shape ahead of a propagating crack by Brown and Ward [38] appears consistent with the geometry of the plastic zone according to a Dugdale [33] model of a craze. For a precracked specimen under the remote load (Fig. 3), the craze is represented by a plastic zone similar to a strip at the tip of the crack. The profile of the plastic zone varies from zero at the location (a + Ac) to the value Acr at the crack tip. 

Thus, the Dugdale model has been fitted to the measured craze shapes and time-dependent mechanical parameters have been derived. Figure 3.25 shows and E as a function of crack speed indicating that in the crack speed range... 

Craze shape measurm. Very good At the lateral face No No Good except at craze tip... 

As discussed extensively in the previous sretions, some craze material properties may be inferred from the craze shape. The plots showing these properties are much more meaningful than the direct experimental values plotted previously. Nevertheless, they are calculated by means of models whose validity might be open to discussion. The simplest assumption that can be made is that the craze stress is constant along the craze boundary. This has been shown to be true for PMMA air crazes. Section 4.3 wilt be devoted to stress profile along these crazes in solvent vapors. 

Attempts have been made previously to use the craze shapes recorded by means of optical interferometry to calculate the craze stress distribution . There are at least two problems in the use of craze profiles obtained by interferometry for craze surface stress calculations ... 

The Fourier transform method has been used to calculate the craze surface stress distribution from craze shapes obtained by means of optical interferometry — the craze shapes are the same in air and in toluene gas, only their sizes vary — the craze surface stress is almost constant along the craze boundary — the craze fibril volume fraction remains constant in air and in toluene gas over the whole velocity range, despite the fact that at low velocity in toluene gas the craze length reaches 4 times the length in air — the optical interference setup may give valuable information on the variations of craze fibril volume fraction, but not on its absolute numerical value. 

The craze surface stress may be derived from interferometric craze shapes only in very particular cases, whde some doubts remain as to their validity. 

Fra Fraser, R. A., Ward, I. M. Temperature dependence of craze shape and fracture in polycarbonate. Polymer 19 (1978) 220-224. 

From the above analysis, we conclude that, if the craze shape remained constant with an unchanging aspect ratio p, the craze traction would always be less than the applied stress o-oo and the craze-tip driving force ... 

A quantitative analysis of craze shape and mass thickness contrast within the craze allowed Lauterwasser and Kramer [382] to derive the stress profile existing along a polystyrene craze. Kramer and his coworkers have extended this study to many other polymers, relating the mean density of craze material to entanglement density in the polymer glass and to toughness [395] without a basic change of preparation technique. 

G. L. Pitman and I. M. Ward, Eifect of molecular weight on craze shape and fracture toughness in polycarbonate. Polymer 20, 895-902 (1979). 

Figure 13.7 Schematic diagram of a craze. (Reproduced from Brown, H.R. and Ward, i.M. (1973) Craze shape and fracture in poiy(methyi methacrylate). Polymer, 14, 469. Copyright (1973) iPC Business Press.)...

Figure 13.9 The shear lips in polycarbonate. (Reproduced with pemission from Fraser R.A. W and Wardl.M. (1978) Temperature-dependence of Craze Shape and Fracture in Polycarbonate. Polymer, 19, 220. Copyright (1978) Elsevier Ltd.)...

The phenomenon of the loading and breakdown of the molecular strands has been studied by various methods. Optical and electron micrographs of crazes are shown in most of the cited references. Examples have been reproduced in Figures 9.8—9.10. Investigations of the craze shape by interference microscopy [15,155, 177] have also been discussed. At this point some results will be reported which have been obtained by thermal measurements [31, 50, 184—186], from analysis of the influence of molecular weight on crazing [11,15,65, 79, 146,178], through acoustic emission [174, 188], and by the ESR technique [189—190] respectively. 

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