Structural integrity of quasi-isotropic composite laminates with embedded optical fibers

Embedded optical fibers are considered for use in structural monitoring of aircraft fiber-reinforced composite structures, such as wings and stabilizers. However, it is crucial that these optical fibers do not themselves significantly degrade the load-bearing ability of the composite. An embedded optical fiber, having a diameter an order of magnitude larger than the reinforcing fibers of a composite, alters the microstructure of the composite and gives rise to perturbations in the stress fields. The present study is an experimental investigation into how the introduction of embedded optical fibers affects the failure modes of a composite laminate. Quasi-isotropic composites with two realistic embedded optical fiber configurations were investigated, i.e. closely spaced optical fibers and optical fiber loops. Closely spaced optical fibers may be necessary near connectors and in some sensing schemes making use of multiple sensors. Similarly, changing the direction of an optical fiber is necessary in some sensing schemes and in cases where a single optical fiber is to be used to monitor a larger area. Static tensile and compressive experiments, as well as fatigue experiments, were conducted to study effects of the embedded optical fibers. Extensive post-failure fractographic examinations were carried out to identify if any failure modes were related to the presence of the optical fibers. Both room temperature dry, and high temperature (90 degrees C) wet conditions were considered. It was found that the embedded optical fibers did not initiate fiber-dominated failure in the composite, and therefore did not cause any significant deterioration of the composite strength, neither in tension nor in compression. However, the damage evolution process was affected by the optical fibers as they were preferred sites for matrix cracking, especially in the hot-and-wet condition. In fatigue, damage grew from the edges of the specimens, not from the optical fibers, and the fatigue life was not affected by the optical fibers.