Whitened zone toughness

Figure 13. Schematic of the structure of the stress-whitened zone perpendicular to the fracture plane, a Tough fracture at high speed, b Tough fracture at low speed.

Han, Y.-C., Yang, Y.-M., Li, B.-Y., Feng, Z.-L. Comparison of the crack growth method and the crack stress whitening zone method for the fracture toughness determination of phenolphthalein poly (ether ketone). Angew. Makromol. Chem. 225 (1995) 131-138. 

According to more recent theories, the toughness of high impact polystyrene is caused by flow and energy dissipation processes in the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and in the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution in impact polystyrene. Stress maxima create zones of lower density, called crazes (3), in which the polystyrene molecules are extended parallel to the direction of stress. Macroscopi-cally craze formation appears as whitening the flow processes result in irreversible deformation (cold flow). 

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