The tear behaviour during the bursting of natural rubber balloons under 100%, 300% and 400% biaxial strain has been investigated in detail by means of high speed cinematography. The tear velocity was observed to be approximately three times the finite elastic wave velocity during unloading (the retraction velocity), in each case. A theoretical analysis is provided for the case of uniform biaxial strain, relevant to the present work, leading to formulae for the retraction and elastic wave velocities. Observed retraction velocities were consistent with theoretical predictions based on the static stress-strain curve in biaxial tension. It is suggested that tear velocity is determined by the elastic properties of the rubber in a localised region of high strains just around the tip.

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The mechanics of puncture have been studied experimentally and theoretically, by means of fracture mechanics. When a sharp cylindrical indentor penetrates rubber, a starter crack initiates as a ring on the rubber surface before puncture occurs. By treating this as militating puncture, an equation has been derived for the energy of puncture. The elastic energy stored in the rubber is considered in terms of the energy beneath and surrounding the indentor. An equation for the energy beneath the indentor is determined with the aid of a model experiment based on the biaxial stretching of rubber by inflation. The energy stored in the rubber surrounding the indentor is calculated using elasticity theory. The magnitude of these contributions is assessed for different indentor sizes and different rubber vulcanizates, The theoretical approach is shown to be verified by a series of experiment for sharp indentors. The values of puncture energy so obtained were found to agree well with the catastrophic tearing energy obtained from trouser tear tests. For blunt indentors which do not cause ring crack formation, other considerations are needed. These are discussed together with experimental results for hemispherical indentors.