Distributed internal strain measurement during composite manufacturing using optical fibre sensors

After manufacturing a composite element, it is expected to have residual stresses which affect the quality and the mechanical properties of the composite. In this work, a distributed optical fibre sensor was embedded in an epoxy carbon fibre panel, measuring the development of residual stresses by monitoring in-situ and in real-time the full manufacturing process, from the resin infusion to the curing cycle, and the changes experienced within. An optical sensor interrogator detects and measures changes in strain and temperature along the optical fibre with high precision and accuracy, making it possible to obtain a full strain/temperature profile. Data acquired from the embedded sensor led to track and characterize the strain profile at every stage of the manufacture process. This work showed that the infusion stage is highly related to the residual strain after the curing process. Moreover, the residual strain is slight higher close the edge of the structure.

Simultaneous measurement of physical parameters using FBGs embedded in unidirectional and bidirectional composite materials

A new sensing head based on embedded fibre Bragg gratings on composite plates was designed. The sensing head was experimentally tested and compared against results simulated using finite element methods, with good agreement. The obtained results make it possible to obtain temperature and strain/curvature measurements simultaneously. By embedding the FBG sensors in the part being measured, the optical fibre is protected from damage and moisture and the material can be closely interrogated. Temperature and strain sensitivities both change when sensors are embedded at a bidirectional interface, while unidirectional embedded sensors keep the thermal sensitivity, resulting in a composite patch that can sense its own alterations in real time, by using a composite material having two distinct sections with different reinforcement orientations.