Crazing mechanisms

As above mentioned, SDZs appear in PMMA above 50 °C. Furthermore, it is also in this temperature range that the crazing mechanism shifts from CSC to CDC. 

Besides, the lower sensitivity of K c to the crack speed, observed in Fig. 30 at temperatures below - 20 °C, has to be related to the occurrence of the multiple craze mechanism (it does not require any slippage time, but only a time for fibrillation) and, so, to the isolated ft transition motions, as described above. 

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

Valuable information on the intrinsic craze phenomenon has been obtained from studies on pre-oriented material The effect of pre-orientation on craze initiation is central to the discussion of the molecular craze mechanism in Section 4.2. To facilitate the presentation of the experimental results, the following nomenclature is introduced. Quantities which refer to the pre-orientation and to the crazing experiment are labelled with indices one and two, respectively. If no index is used, the quantity refers to the result of both experiments. 

The toughness of PMMA is observed to increase slightly with increasing loading rate. For this material, only crazing takes place so that the variation of the toughness with the loading rate reflects the influence of the time dependent craze mechanism in the failure process. 

Crazing mechanisms and criteria have long been studied (Kausch, 1978 Michler, 1992), but further work is still needed because of limited number of experimental results under multiaxial stress and considerable differences in crazing behaviour under different conditions (Kawagoe, 1996). 

Characteristics of the Craze Mechanism. HIPS, as well as the ABS grades studied here, deform mainly by the formation of crazes. The reason is the strong tendency of matrix material to form crazes under load (3,12, 13). Details of the toughening mechanism have been reported recently (1-4). Therefore, only a brief review of the main points is given here, to clarify the difference between this mechanism and the shear mechanism. The processes... 

The section Toughening by the Multiple-Crazing Mechanism mentioned the effect of superposition of the local fields of stress concentration, which must also be considered. As shown in Figure 8, there is a remarkable increase of the local stress between particles by superposition if the interparticle distance is smaller than the particle diameter (A/D < 1, which corresponds to particle volume contents above 5%). 

Ternary blends, impact failed at 23 and —40°C, showed that whiting—typical of materials that fail by multiple crazing mechanism (1,47,110,111)—spreads over the whole failed surface. The SEM technique detected a developed system of pores whose size is between some fractions of a micrometer and several micrometers (Fig. 18.9). Evidently, pore generation is a consequence of severe crazing (1). As during modification of the PA6/g-PE binary blends by ethylene-propylene copolymer... 

For neat PP, the crazing mechanisms have been the object of many previous papers (41,49,50). It was shown that crazes resulting from early fragmentation of the crystalline stacks propagate in the amorphous phase between the lamellae along the radial directions as shown in Fig. 19.31. At higher strain, the cracks are rotated and become nearly parallel to the tensile direction. It is remarkable that although such... 

Phase distribution, dispersion, dissipated structure formation, flocculation and crazing mechanism, according to the new concept... 

In the separation of an adhering system at or near an interface, in terms of a craze mechanism, there will be four differences from separation within a bulk polymer. One is, that interfacial voids (or proto-voids) may exist. These can act as cavitation nuclei and interfacial craze formation, starting from such nuclei, would be orders of magnitude more rapid than crazing by homogeneous nuclea-tion. Such cavitation could also be more rapid than the processes that occur in the Taylor instability mechanism, particularly if it should happen that the voids at the interface formed a two-dimensional continuum. Patches of low-energy matter in the solid surface can also be loci of void initiation, even if no voids are present before loading. 

Next, we may make an estimate with respect to brittle separation by a craze mechanism. We can assume a fibril radius of the order of... 

An HIPS tensUe specimen extends to a strain of 0.56 by the multiple crazing mechanism. If the average extension ratio A = 4.2 in the fibrils, and the craze thickness is 1.2 fim in the full extended state, calculate the number of crazes per mm length of the stressed bar at fracture. What fraction of the HIPS is converted into crazes ... 

Polymeric systems can be classified as either brittle or pseudo-ductile. The first type tends to fail by the crazing mechanism and has low crack... 

Brittle polymers, for which < 0.15 mmol/cm and Coo > 7.5. They fracture by a dominant crazing mechanism and additionally exhibit a low crack initiation energy and a low crack propagation energy (resulting in low, both uimotched and notched, toughness). Examples are PS or PMMA. 

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