Monitoring of aeronautical composites with embedded FOBG sensor: Part II—Strain response under fatigue and alternate bending loading spectrum

The novel manufacturing concept for embedding fiber optic Bragg grating (FOBG) sensors allowed for the damage-free embedding of the FOBG fiber to the laminate structure. In this article, coupons with embedded FOBG fiber were tested under alternating bending spectrum and for different peak loads ranging from 30% and up to 60% of ultimate tensile strength at their bottom surface. The differences between the strain measurements of the surface-attached strain gauge and the embedded sensor were almost 2.4% and approximately the same for all the applied peak loads. Application of 65,000 fatigue cycles on several coupons was assessed to simulate the fatigue-loading of the aircraft structures over their life span. The fatigued-coupons were tested afterwards at the same alternate bending-loading spectrum, and their strain measurements were compared against the respective loading spectrum without prior fatigue. The strain measurement differences at peak loads was of the order of 3.0% for the small-applied loads and exceeded 7.0% for the high-applied loads. Fatigue damage after 65,000 cycles was not essentially accumulated in the sensing area of the FOBG sensor, and therefore, its current exploitation in real aircraft structures, for example, a 2-m-long Ω-stringer from carbon preimpregnated fibers from an Aerospace Industry with monitoring capabilities, is proved.