Measuring moisture absorption in structural adhesive joints with fibre Bragg grating sensors

The increased joint efficiency, distribution of loads and decrease in stress concentrations have led to the increased use of adhesives for structural bonding. However, there are a limited number of techniques for verifying and monitoring the integrity and durability of adhesive bonds. This article studies the potential of estimating the curing and ageing of adhesive bulk samples with embedded fibre Bragg grating sensors through measuring the strain associated with hygroscopic expansion. This is achieved by relating the output of a fibre Bragg grating sensor to the deformation of the structure in which it is embedded. This work considers the possibility of mapping the changing structural resistance to mechanical loading (stiffness) of adhesive bonds as a function of time, under the influence of temperature and moisture as environmental factors. The goal is to map the influence of these environmental factors separately on the one hand, and their combined effect on adhesive bonds, on the other hand. This study subjects several bulk specimens to various environmental ageing loads. The swelling, associated to moisture absorption and that results in mechanical strain, is measured with fibre Bragg grating sensors. The moisture absorption behaviour at different temperatures and environmental relative humidity conditions determined in this way is verified using classical test methods (e.g. differential scanning calorimetry, gravimetric) on multiple fibreless specimens.