Nucleation and growth of small fatigue cracks in filled natural rubber under multiaxial loading

This paper presents and discusses observations of crack nucleation and small crack growth in a filled natural rubber compound subjected to multiaxial loading. A hollow cylindrical specimen was used in which simultaneous axial and shear strains are produced. The loading path types investigated include axial, torsion, proportional axial-torsion, and nonproportional axial-torsion loadings. It is shown that cracks appear and grow in a particular orientation during a typical fatigue test. The nature and evolution of these cracks with applied cycles were studied by direct observation. The observed failure plane behavior is compared to predictions based on the calculated cracking energy density. Effects of crack closure, load phase angle, crack density, and crack face shearing (mode II), as well as aspects that distinguish between ‘‘nucleation’’ and ‘‘growth’’ processes in rubber are discussed.