The strength of highly elastic materials

TLDR

Repeated stressing of vulcanized rubber can cause crack propagation and failure, but a fatigue limit exists that is not correlated with viscoelastic behavior and varies only slightly with chemical structure. The study discusses why the fatigue limit T0 and gross strength properties are influenced by different structural aspects of the material. T0 is approximated by calculating the energy needed to rupture polymer chains across the crack path, derived from chemical bond strengths while ignoring secondary forces. Below a critical strain severity, crack propagation ceases, defining a fatigue limit T0 of about 5 × 10⁴ erg/cm², and theory and experiment agree within a factor of two.

Abstract

Under repeated stressing, cracks in a specimen of vulcanized rubber may propagate and lead to failure. It has been found, however, that below a critical severity of strain no propagation occurs in the absence of chemical corrosion. This severity defines a fatigue limit for repeated stressing below which the life can be virtually indefinite. It can be expressed as the energy per unit area required to produce new surface ( T 0 ), and is about 5 x 10 4 erg/cm 2 . In contrast with gross strength properties such as tear and tensile strength, T 0 does not correlate with the viscoelastic behaviour of the material and varies only relatively slightly with chemical structure. It is shown that T 0 can be calculated approximately by considering the energy required to rupture the polymer chains lying across the path of the crack. This energy is calculated from the strengths of the chemical bonds, secondary forces being ignored. Theory and experiment agree within a factor of 2. Reasons why T 0 and the gross strength properties are influenced by different aspects of the structure of the material are discussed.