Strain-based health assessment of bonded composite repairs

Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous through-life assessment of the repair’s efficacy. This paper describes an SHM technique for the detection of debonding in composite bonded patches based on relative strain measurements. Two types of repairs known as the external doubler repair and the scarf repair are examined in this paper. A finite element study showed that the strain in the debonded region changes significantly compared to the undamaged state. An experimental investigation was conducted, using carbon/epoxy repairs to carbon/epoxy substrates and fibre Bragg grating sensors for the measurement of strain, to validate the theoretically predicted results for the external doubler repair. Debonds were detected by means of a differential strain approach, where two sensors were strategically positioned so that their strain differential increased with the disbond length. A threshold value may be set to indicate imminent critical repair failure. The experimental measurements agreed well with the numerical findings, indicating that the proposed scheme has great potential as a simple monitoring technique for composite bonded repairs.