Multiple load path damage detection with optical fiber Bragg grating sensors

In this article, a new damage indicator is presented that can detect a (partial) load path failure for a multiple load path structure, based on variable amplitude strain response measurements by fiber optic Bragg grating sensors. Many (aircraft) structures have multiple load paths where after a (partial) failure of a load path the remaining structure can carry the limit load without catastrophic failure or too severe impact on the operational characteristics of the whole structure, until the structure is repaired, replaced, or modified. The damage indicator is defined as a ratio of the strain response summation of the current strain time variation measured at fiber Bragg grating (FBG) sensors and a reference strain time variation measured at the same FBGs. For application in real structures with varying loads and environmental conditions, it should be insensitive for the load time variation as well as temperature. The FBG strain response due to a load variation can be easily computed by means of a finite element analyses. In this way, an optimal number, location and orientation of strain sensors can be derived for complex structures. A test was performed on a box-shaped structure representative of a typical aircraft structure to validate the damage indicator, showing that the damage indicator is able to detect a damaged load path at an early stage of failure with few sensors per load path. From finite element analyses, the damage size can even be determined from the damage indicator value for a known damage scenario, making it a level 3 structure health monitoring system.