The role of visco-elasticity on the crack growth behaviour of rubber.
This thesis concerns crack growth phenomena in rubber. It is widely known that a relationship exists between the magnitude of the stored energy release rate available to drive a crack, called the tearing energy (T), and the resultant crack growth rate. For rubbers this basic relationship is said to be a characteristic of the material. The magnitude of T is related to both the visco-elastic losses and the crack tip diameter (d). However the actual size of d and its relationship with the visco- elastic losses is not clear. This thesis examines the crack growth behaviour in relation to d and the visco-elastic losses for a wide range of rubbers, whose visco-elastic properties are altered either by swelling in a liquid, altering the test temperature or the cross-link density and by the incorporation of fillers. Static, constant T, crack growth tests were carried out. These revealed that two different crack growth processes exist. For the fast crack growth process, T is determined by variations in the visco-elastic losses alone. For the slow crack growth process, T is determined by variations in both the visco-elastic losses and d. It is proposed here that the factors, which alter d, are associated with cavitation ahead of the crack tip for unfilled materials and with strength anisotropy for carbon black filled materials.
In cyclic crack growth tests, the crack growth per cycle, dc/dn, can be considered to result from the sum of time and cyclic dependent crack growth components. For the first time, the detailed magnitudes of the contribution of each of these components to dc/dn have been determined, for a wide range of materials and mechanisms responsible for this behaviour are postulated. Also crack growth tests, both static and cyclic, were extended to very large extensions. Lastly this investigation revealed that the tensile strength for both crystallising and non- crystallising rubber can be predicted using the tearing energy concept for a variety of loading regimes.