Volume-initiated crazes

This study (34) implies that a right dispersion of rubber particles may permit optimum stress field overlap that affords lower craze-initiation stresses and therefore can rapidly dissipate the strain energy in the HIPS. A more homogeneous spatial distribution of rubber particles allow for a uniform development of crazes. Prevention of the strain localization phenomenon to avoid the detrimental situation, where crazes prefer to develop in certain areas and quickly lead to a catastrophic crack, could result in a larger total volume of crazed material. Further, Donald and Kramer (22) discovered no crazes nucleating from an isolated rubber particle with diameter smaller than 1 urn because of an insufficient size of stress-enhanced zone. Since Sample-A has a small average particle size it should contain a large number of small rubber particles. Two small rubber... 

With the above considerations on initiation and inactivation of craze fronts we now develop relations for q, the steady state active craze front length per unit volume. Here it is necessary to distinguish two separate cases sinface initiation limited crazing and volume initiation limited crazing. The first category applies to the cases of most homopolymers and to many block copolymers with phase dimensions too small to result in stress concentrations in the requisite volume element that was mentioned above. In these instances crazes are observed to initiate from sources at... 

Volume initiation limited crazing. Consider a large volume V of a polymer with internal heterogeneities of approximately spherical shape with a radius R and an initial density of Hq/V (m ). The rate of increase of craze front length under a distant stress of in this case is... 

In the first attemped quantitative model proposed by Lazzeri and Bucknall [52], it was predicted that only particles >0.25 p diameter were capable of cavitating imder a given set of conditions, lending some support to the concept that there is an optimum rubber particle size for the toughening. They also showed that cavitated rabber particles can initiate crazes and dilatational bands in the matrix. They propose that the sudden conversion of rubber particles into the mechanical equivalent of voids (by cavitation) has the indirect effect of accelerating the volume expansion in the matrix through the formation of dilatational shear bands. 

A good dispersion of rubber particles appears to favor the nucleation and growth of a large number of thick crazes uniformly distributed in the polystyrene matrix. This is believed to be an efficient source of energy absorption for the material under mechanical loading. The concepts of stress field overlap and critical volume of stress concentration zone for craze initiation were introduced to explain the observed mechanical behavior of HIPS. 

In discussing shear deformation, it is convenient to distinguish between the initial elastic and viscoelastic response of the polymer to the applied load and the subsequent time-dependent response. However, the distinction is somewhat arbitrary and is not as fundamental as that between elastic volume response and crazing. Viscoelastic shear deformation continues throughout the period under load. The observed time-dependence of lateral strain reflects both generalized viscoelastic relaxation and shear band formation. Since crazing consists simply of displacement in the tensile stress direction, it makes no contribution to lateral strain therefore —e specifically measures deformation by shear processes. 

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... 

As demonstrated in Figure 2, the specific volume of PC increases notably when crazes II are initiated, resulting in a loss in density of approximately 8% at the rupture of the specimen. This is comparable to that found for high impact polymers where the number of crazes is increased artificially by the incorporation of rubber particles. 

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