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.