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Tear Strength of Stretched Rubber

1978; American Chemical Society; Volume: 51; Issue: 1 Linguagem: Inglês

10.5254/1.3535725

1943-4804

A. N. Gent, H. J. Kim,

Tribology and Wear Analysis

Abstract Highly stretched rubber sheets can be split apart quite easily, indicating that the originally isotropic material becomes “fibrous” in character—i.e., much weaker for a tear running in the direction of extension than for one running at right angles to it. This anisotropy of strength is shown most clearly by elastomers which crystallize on stretching, for example, natural rubber compounds; it is not so evident in rubber compounds which do not undergo strain-induced crystallization. Also, it is apparently more pronounced for compounds containing reinforcing fillers, notably carbon black, which stiffen and strengthen elastomers. Indeed, carbon-black-filled natural rubber compounds show anisotropy of strength even after the extension is removed. The test sheet then regains its original dimensions more or less completely, but it remains softer for subsequent deformations in the direction of pre-stretching, and weaker for a tear running in this direction. Although unfilled natural rubber compounds and filled and unfilled noncrystallizing elastomers are also softened to varying degrees by prestretching, they do not appear to show anisotropy of strength to any significant degree when the prior extension is removed. This phenomenon is apparently unique to filled, strain-crystallizing, elastomer compounds. Differences in strength in different directions, induced either by a previous deformation or by a present one, are of considerable practical interest. They are probably responsible for the phenomenon of “knotty” tearing, when a running tear spontaneously deviates from the expected path, at right angles to the principal tensile stress, and turns instead into the stress direction. As a result, stress concentrations at the tear tip are greatly reduced, and the tear stops altogether until the applied stress becomes high enough for a new tear to break ahead. This stick-slip pattern of tearing is characteristic of filler-reinforced elastomers; indeed, it appears to be the principal mechanism of reinforcement because their tear strengths are not much greater than those of unfilled elastomers when knotty tearing is prevented. Thus, reinforcement by fillers is closely associated with, and may depend upon, anisotropy of tear strength in the deformed state.