AbstractPartly unsaturated natural rubber hydrochloride films undergo stress cracking in ozonized oxygen. The origin of cracks and the correlation between stress distribution and rate of crack growth have now been studied. Cracks are initiated at foreign particles and/or surface fissures of microscopic dimensions. Fracture is promoted only if the shape of the incipient crack fulfils certain geometric conditions. These films become highly birefringent when stretched beyond the elastic limit; specific birefringence increases linearly with strain. It follows from stress optical studies that large plastic deformations take place ahead of the crack, whereas the film is deformed only elastically. Cut growth experiments confirm these conclusions. Fractography reveals that cracking consists mostly of a series of discontinuous events on a microscopic scale. The influence of crack length and polymer modulus on the critical stress, necessary for the onset of ozone cracking, is predictable from the Griffith criterion. The maximum stress at the tip was calculated from the observed dimensions of the crack and found to surpass the yield stress of the polymer. Mechanical brittle fracturing and ozone‐induced stress corrosion have been compared. The remaining empirical element, which determines the rate of cracking, is the rate of hardening on ozonization of the fresh surfac
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