Multiple crazing mechanism

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. 

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... 

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 ... 

Electron microscope studies have shown that the toughness of ABS polymers is caused largely by multiple craze formation (1,2). The rubber particles appear both to initiate and to control craze formation, so that impact energy is dissipated in the production of numerous small crazes (3). However, this theory does not exclude the possibility of contributions from other mechanisms. The observation that many ABS polymers tend to neck during a tensile test suggests that shear mechanisms are also significant. 

Figure 8. Three-stage mechanism of multiple crazing (a) stage 1 stress concentration and craze initiation at rubber particles (b) stage 2 superposition of stress fields (small interparticle distance, high rubber volume content) and formation of broad craze bands and (c) stage 3 limitation of crack length and crack stopping at rubber particles.

It is well known that the main mechanisms of inelastic deformation are shear yielding and multiple crazing in the rigid matrix phase, as well as cavitation in the soft dispersed phase in rubber-toughened plastics and multiphase polymers [42]. Eor many years, these mechanisms have been studied using microscopy techniques. 

Crazing is a mechanism of plastic deformation that is extremely localized. Even when the number of crazes in the sample is substantially increased, as in the case of multiple crazing in rubber-toughened blends, their early stages of development... 

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