Stress-concentration zone, craze initiation

Particles whose diameters are significantly greater than Dopt are less effective for craze initiation. The stress-concentration zone is much broader than the optimum thickness of crazes. Therefore, several thinner crazes having a total volume smaller than the volume of a craze with a thickness equal to Dz are initiated (3,19). 

Craze initiation Each rubber particle initiates a stress concentration field Gp. in its surrounding, with the highest values in the equatorial zones here, crazes start and propagate perpendicular to the tensile direction. 

Figure 13.9 Sequence of events in a croid formation, (a) Initial state at the crack tip. (b) Cavitation ofthe rubber particles dueto loading head of the crack tip. (c) Cavitation of rubber particles near the already cavitated particles due to stress-concentration effect. The croid is forming, (d) Croids are propagating ahead ofthe crack and inside the craze-like damaged zone many shear bands develop between cavitated rubber particles. (Sue, 1992 with kind permission from Kluwer Academic Publisher.)...

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. 

The experimental results clearly reveal that the size of the stress-concentration zone, that is, the diameter of the rubber particles, is the primary factor for the initiation of crazes. Therefore, the effectiveness of the different-sized rubber particles can be shown in an effectiveness curve, which gives the probability of the particles to initiate craze (19, 22). Starting from Dmin for craze initiation, the effectiveness of particles of different sizes increases up to a maximum and then drops at larger diameters. 

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