Numerical and Experimental Comparison of Impact Damage Detection by Using Distributed Fiber Optics on a Stiffened Composite Panel

Abstract

The scope of the activity is the development of an SHM system based on distributed fiber optic sensors able to detect, localise and measure, the debonded skin/stringer extensions and delamination areas on a composite stiffened panel generated by low energy impacts.

The panel is sensorized with 1.2m long fiber optic interrogated with “Rayleigh Backscattering” technology. The fibers are bonded along the edge caps of each stringer and also on the skin according to a specific grid layout. The strain data are then processed by a proprietary SHM algorithm, developed in the SMAF (SMart AirFrame) project of PRORA (Italian Aerospace Research Program). The algorithm named LHEO is based on the identification of high local gradients correlation in time and space domain corresponding to post-impact residual strain discontinuities. Through the use of this tool, the algorithm allows to detect the size and position of damage occurred in the structure without the need of healthy reference. Then, in order to provide an estimation of the surface extension of the impact effect on the skin, a dedicated subroutine, named EB “Expanding Bubbles” is developed. This method uses as input the parts of the sensor network characterised by positive outcomes given by the LHEO algorithm and generates progressive circular regions around each sensor, computing the ratio of border points over total points falling inside the circle. The results were expressed in terms of deviation with respect to the NDI reference coming from C-SCAN measurements. A good agreement was achieved for both the kind of damages.