Craze mechanism, characteristics

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... 

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

It is interesting to remark that like most mechanical parameters the crazing stress exhibits viscoelastic characteristics, decreasing with increasing temperature and with decreasing strain rate (which is an indication that it is better to speak of a crazing strain). We come back to the discussion of crazes in Sect. 13.5.5. 

When a crack propagates in polystyrene at low crack velocities, the craze ruptures close to its median plane by a mechanism having the approximate characteristics of viscous flow. Each fracture surface is then covered by a thin layer of craze. At higher crack velocities, however, failure occurs along the boundaries between the craze and the adjacent bulk polymer by practically brittle fracture (I). The change in fracture... 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

Similarly, for values of Z/Z < 1 and N/Ne > 1, the only possible failure mechanism is simple chain scission without onset of crazing. These two failure mechanisms give low values of Cjc of the order of a few J/m2 and thus are characteristically observed for weak interfaces. 

Optical KLenents. Problems which are common to many solar-related optical elements Include dirt retention, cleaning, surface abrasion, and photodegradation. A common feature of some of these problems Is that the deleterious effects occur at an Interface. Ultraviolet radiation, atmospheric components, mechanical stress, etc., can have a profound effect on performance by changing surface characteristics. The lifetimes of UV stabilizers can be limited by exudation permeability can cause harmful reactions at Interfaces and mechanical properties can be Influenced by surface crazing. In other applications mechanical behavior of the bulk polymer Is critical and virtually all applications require that the polymer system withstand multiple environmental stresses simultaneously. 

Based on the evidence so far, it seems probable that impact strength in rubber-modified plastics depends on the relative importance of crazing and shear yielding induced by the rubber particles,f as weU as on other parameters such as the rubber characteristics (below). As mentioned above, however, extrapolation from low-strain-rate to high-strain-rate tests cannot be made quantitatively. Research into the deformation mechanisms at high strain rates should be fruitful, though difficult to accomplish. 

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