Exploitable results
Rubber-to-metal bonded components sometimes become unserviceable due to failure at or near the bond. There is little fundamental understanding of the mechanisms or even of the mechanics of such failure. Existing test methods show large variability and often produce failure away from the bond; the results cannot be related reliably to each other, to bond strength or to service performance. Consequently, the durability of bonds in service is uncertain. Thus, the objectives of this project were improved understanding of bond failure and the development of a more meaningful test method for near-bond failure, giving results which could be related to service performance.
For cohesive failure, where fracture occurs in the bulk of an elastomer, a fracture mechanics approach, relating the release of strain energy to the growth of a crack, has enabled laboratory test results to be related to performance of components of differing geometries. In order to assess whether this approach could be applied to failure at or near a bonded interface, constant force (and also constant rate) tests were systematically studied, with a range of geometries and types of stressing (peel tests at a range of angles, simple and pure shear tests, etc), in various environments (in air or ozone-enriched air, under water or solvent, at elevated temperatures), using different rubber formulations, bonding agents and various methods of preparation. For each formulation, the results from the different geometries could be interrelated approximately using the fracture mechanics approach.
The studies indicated that a constant force peel test would provide a suitable basis for a new test method. A suitable apparatus has been developed for carrying out such tests safely, on a routine basis, with automatic monitoring of the peel front appearance and location. Tests can be carried out under various loads and peel angles in gaseous or liquid environments.
Chemical and finite element studies indicate that near-bond failure occurs due to the mechanical constraint imposed by the rigid substrate, resulting in cavitation-like processes. Other areas of work have included the fatigue testing of components and the recovery and examination of articles after periods in storage or service, to establish what bond strengths are achieved in production and what changes occur subsequently.
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