Optimal FBG Sensor Placement in Composite Components: Algorithm and Sensor Type Trade Studies

Damage to composite rotor and airframe components is often not obvious by visual inspection, and periodic inspection using non-destructive evaluation methods is time consuming and increases the cost of ownership. An integrated sensor system that continuously monitors structural integrity can have a high payoff by reducing risks, costs, inspections, and unscheduled maintenance, while increasing safety. The goal of optimal sensor placement strategies is to find a set of sensor positions that maximize the probability of detection (POD) of damage. In our studies, Fiber Bragg Grating (FBG) sensors were used for the optimal detection of delaminations or cracks at ply drop locations in a layered composite structure (i.e., a helicopter flexbeam). In this paper, the effect of design parameter variation on the optimal placement of Fiber Bragg Grating (FBG) sensors in composite flexbeam structures was studied. In particular, a greedy sensor optimization approach was used to gauge the effect of variations of sensor properties, ply utilization, crack characteristics, and inter-sensor distance constraints. The methodology was used to compute (i) the minimal number of sensors, of various types, needed to obtain a given probability of damage detection (POD), (ii) the maximum POD that can be achieved with a given set of sensors, and (iii) tradeoffs between the use of high and low accuracy sensors. The advantages of using optimal sensor positioning versus uniform or random sensor positioning were demonstrated. Optimal sensor placement solutions are especially effective, and much better than uniform placement solutions, for a small number (i.e., 1 - 10) of sensors because the best sensor locations are selected by the optimization algorithm.