Measurement of the Fracture Toughness of the Metal-Composite Interface in Composite Repair Systems for Steel Pipework

This paper describes the fracture mechanics approach behind the water pressure ‘blow-off test for characterising the failure behaviour of composite repairs on steel tubes. The technique enables the fracture toughness of the metal/composite interface to be determined at the point where failure initiates. Surprisingly the values found for the interfacial fracture toughness are typically an order of magnitude lower than those found in other fracture mechanics tests. A further procedure was therefore developed, in which the water pressure was replaced by a mechanical load from a steel probe. This latter test was carried out with and without water present.