Bond strength and performance of optical fibre Bragg gratings sensors embedded in composite patch repairs for military aircraft

Abstract

Structural health monitoring (SHM) systems are recognized as a key element in a modern and efficient approach to structural lifecycle management. With novel applications of advanced materials, such as extensive use of fibre reinforced composites on aircraft, further advances in monitoring and inspection techniques are required. This is especially relevant for repairs on primary (i.e. load bearing) military aircraft composite structures, where traditional non-destructive inspection methods alone may be insufficient for certification of the repair. Optical fibre sensor systems with continuous in-flight monitoring are considered a promising approach to providing the necessary information on the status of the repaired area. Fibre optic sensor systems commonly involve the use fibre Bragg grating (FBG) sensors bonded to or embedded in the structure. As an element in a certified repair strategy, the limitations of the load transfer from the structure to the sensor must be known. Hence, extensive analysis and testing must be done before introducing any adhesive system/optical fibre combination as part of the SHM system. This paper presents experimental test results obtained for the interface properties of optical fibres with polyimide or Ormocer® coating embedded in an adhesive. Three different adhesives, which are all relevant for military aircraft primary composite structural repairs, are included in the test program. A modified fibre pull-out test set-up is applied for estimating the interfacial shear strength (IFSS) for each of the six combinations of adhesive and optical fibre, at two different nominal embedded lengths of 0.5 mm and 1.0 mm. For the polyimide coated fibre, the IFSS values obtained for the three adhesives from fibre pull-out do not differ significantly. The highest IFSS values are obtained at a nominal embedded length of 0.5 mm. The optical fibre with Ormocer® coating seems to have the strongest bond to the adhesives, but results in core pull-out instead of fibre pull-out. The obtained IFSS values for the core/coating interface of the Ormocer® coated fibre are not directly comparable with the IFSS values obtained for the polyimide coated fibre, but they still demonstrate the load transfer properties and the potential of the optical fibre for obtaining accurate strain measurements. As an overall conclusion, the polyimide coated fibre is considered the most appropriate for use in a fibre optic sensor system for SHM.