Using distributed fibreoptic sensing to monitor repaired structures reinforced with steel‐patches

Adhesively bonded steel patches present a novel approach to strengthen fatigue cracked steel bridges or similar structures. After the repair, the new weld is covered with an adhesively bonded steel patch, redistributing parts of the load and reducing the overall stress in the repaired structure. Unfortunately, this covers the weld permanently and therefore any form of direct, visual inspection is no longer possible. Hence arises the demand for a technical solution to reliably determine the condition of the repaired weld and its reinforcement. Fibre optic sensors using Rayleigh backscatter offer a spatial distributed strain measurement from which a deformation profile can be generated. This deformation profile allows to determine the load distribution between patch and structure and can be used to identify possible changes in the load distribution which can represent a change in the mechanical properties and therefore a possible defect. Having not only the information about the absolute value of the strain but also its distribution, the expected damage can be located and measured. The deformation profile also allows to separate different forms of failure for example a broken weld, adhesive failure within the patched area or sub surface migrating corrosion.