UC San Diego

Structural health monitoring is the process of detecting damage to a structure, where damage can be characterized as changes to material/mechanical properties including but not limited to plastically deforming the material or the modification of connections. Fiber optic cables with fiber Bragg gratings have emerged as a reliable method of locally measuring strains within a structure. During the manufacturing of composite structures, the fiber optic cables can be embedded between lamina plies, allowing the ability to measure strain at discrete locations within the structure as opposed to electrical strain gauges, which must typically be applied to the surface only.

The fiber optic sensors may be used to see if the local strain at the sensor location is beyond desired limits, or the array response may be mined to determine additional information about the loading applied to the structure. The work presented in this thesis is to present novel and potential applications of FBG sensors being used to assess the health of the structure.

The first application is the dual application of the FBG sensor as a method to determine the strain around a bolt connection as well as the preload of the fastener using a single fiber optic sensor. The composite material around the bolted connections experience stress concentrations and are often the location of damage to the structure from operational cyclic loading over the lifetime of the structure. The degradation can occur more quickly if the fastener is insufficiently tight to transfer load properly.

The second application is the ability to locate the impact location of a projectile with damaging and non-damaging energy. By locating and quantifying the damage, the sensor array provides the basis for a structural health monitoring system that has the potential to determine if the damage is extensive enough to replace, or if the part can be salvaged and retrofitted.