Matrix cavitation stress-whitened zones

Load Displacement and Fracture Behavior Over a Range of Test Speeds. As discussed at the beginning of this chapter, many authors have reported an apparent correlation between the toughening mechanisms and the development of a stress-whitened zone that diffracts light in the material. The occurrence of stress-whitening at 3-4% strain coincides with the appearance of both cavitation (6) and shear strain in the matrix (8). 

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. 

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. 

---