Strain Fields in Adhesive Joints of Composite Patch Repairs

Abstract

Composite patch laminates adhered to damaged metal marine and civil engineering structures are an attractive alternative to repair by welding. This is a preferred method when hot work, such as metal grinding and welding, needs to be avoided due to fire and explosion hazard in the oil and gas industry or traffic interruptions for maintenance and capacity upgrading of civil infrastructures as bridges. Composite patches can easily be made in a wide range of geometries and their light weight and little space requirements make them easy to apply. Previously research has concentrated on the strength of adhesively bonded joints and the damaged metal´s stress concentrations. In addition, many analytical solutions to calculate the interfacial stresses in the bond line between the metal substrate and the composite patch have been developed. However, when a patch repair in a real application shall be evaluated the long-term performance is critical to understand. Knowledge of the long-term behavior of the patch repair has been limited and could lead to rejection of this repair method for critical applications. This thesis describes the critical axial strain field in the adhesive and composite laminate for a composite patch repair notched metallic beam. The study has used a novel measurement method with Distributed Fiber-Optic Sensing of a normal optical telecom fiber combined with an Optical Backscatter Reflectometer interrogator to measure accurately down to a millimeter a continuously axial strain field inside the adhesive and composite laminate. This system has made it possible to measure and understand the axial strain field behavior during quasi-static loading and fatigue loading. Damage growth in the patch repair has been accurately measured inside the laminate and 3D plots of the axial strain field through the patch thickness. This method provides new possibilities to evaluate the strain field behavior of an adhesively bonded composite patch repair allowing a better understanding of the long-term properties.