Multiple crazing

Multiple crazing in the matrix when the particle size of the dispersed particles is larger. 

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. 

Above 70-80 °C, the deformation remains mixed, but crazing is dominant and corresponds to CDCs, as revealed by the behaviour of the strain to craze and the strain rate dependence this temperature of occurrence of CDCs is denoted T223. Furthermore, the SDZs become less widespread and tend to be accompanied at the tips of the main crazes by regions of multiple crazing (Fig. 56)... 

Another point concerns the ji transition and the associated motions. Indeed, in PMMA the maximum of the /3 transition peak (Fig. 26) occurs around 10 °C, in such a way that the low temperature range corresponds to the low temperature part of the /3 transition, where the ester group motions are isolated, without any cooperativity with the main chain, and leading to multiple crazing, as described in Sect. 3.1.3.2. In contrast, in the case of MGIM76, the maximum of the f) peak is found around - 20 °C (Fig. 53), in such a way that the considered temperature range a is located in the high temperature part of the p transition, in which MGI-MGI cooperativity exists, as shown in [1] (Sect. 8.3). 

Shearing Shearing + crazing A Multiple crazing X Single craze... 

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. 

Fig. 8. Fracture toughness K, (at about lOjim/s crack velocity) for PMMA over a wide temperature range. Note the sharp increase of K, below the critical temperature = —20 C, below which multiple crazing occurs at the crack tip. Above the critical temperature, Kj remains almost constant versus temperature. From Ref courtesy of Chapman and HaU, Ltd.

The transition from a smooth fracture surface (a single craze has been broken in its midrib) to a rough surface (multiple crazing has occured at the crack tip) has been encountered in many polymers, only the temperature at which the transition occurs... 

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.

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%). 

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