Multiplicity of crazing

As a consequence, the overall penetrant uptake cannot be used to get direct informations on the degree of plasticization, due to the multiplicity of the polymer-diluent interactions. The same amount of sorbed water may differently depress the glass transition temperature of systems having different thermal expansion coefficients, hydrogen bond capacity or characterized by a nodular structure that can be easily crazed in presence of sorbed water. The sorption modes, the models used to describe them and the mechanisms of plasticization are presented in the following discussion. 

Craze Nucleation Theory. In various ways it has been suggested that the role of the rubber particles is that of stress concentrators. Thus, Schmitt and Keskkula (33) believe that the multiplicity of stress concentrators (i.e., a multitude of weak points) produce a large number of small cracks rather than a few large ones more energy is needed to propagate a large number of small cracks, and stress fields of the various... 

How does crazing in polymers differ from crack initiation in ceramics or silicate glass, in respect of (i) The orientation of the craze or crack plane with respect to the principal stresses (ii) the energetics of craze or crack growth (iii) the likelihood of multiple craze initiation ... 

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 ... 

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... 

Ductile failure is mostly associated with extensive shear yielding. However, as will be shown later, multiple stable crazing as observed in rubber-modified polymers can lead to the same ductile failure mode. There is very little understanding of which molecular parameters favor ductile fracture. Quenching of the melt seems to favor ductile fracture. Typical test variables leading to a ductile fracture are plane stress, elevated temperature, low strain rates, absence of aggres-... 

Fig. 4). Semibrittle materials show a small volume of plastically stretched polymeric material ahead of a crack tip on a micrometer scale, eg, in the form of crazes or shear bands. Macroscopic fracture is quasibrittle or semiductile, occasionally with necking. Larger plastic deformation involves large volumes on a mesoscopic scale (10-/um scale), including formation of larger or multiple crazes or shear bands. In this sequence (from top to bottom in Fig. 4), the plastic deformation involves larger volumes, yielding an increase in the amount of energy absorption (an increase of toughness).

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

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