Cavitated rubber particles, stress-whitened zones

This study of stress-whitening in rubber-modified epoxies showed that the size of the whitened zone at the root of a notch decreases with increasing rubber content. Stress-whitening has been shown to be caused by hydrostatic stress. Two different species of stress whitening were found. One is reversible by heating and is deduced to be due to matrix cavitation the other is irreversible by heating and is due to highly cavitated rubber particles and shear bands. 

Low-Speed Deformation. Dijkstra et al. (13, 19) and Janik et al. (18) showed that in samples fractured in a slow-speed notched tensile impact, the stress-whitened zone has two layers. Far from the fracture plane a cavitated structure is present, with cavities in the rubber particles (Figure 12, top). In particular, the bigger particles seem to be cavitated and the particles less than 100 xm are not. In the layer next to the fracture plane, the cavities are strong-... 

Crack Stability. At low test speeds, stable crack growth with an extended stress-whitened plastic zone and crack blunting occur by the same mechanisms as those involved in the kinetics of the plastic zone, namely, rubber cavitation followed by shear deformation of the matrix. The ability of the matrix to shear is controlled by its relaxation behavior, which therefore determines its plasticity and the deformation imposed on rubbery particles distant from the notch. 

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