Structurally Integrated Fiber Optic Damage Assessment System For Composite Materials

Abstract

Progress toward the development of a fiber optic damage assessment system for composite materials is reported.This system, which is based on the fracture of embedded optical fibers, has been characterized with respect to the orientation and location of the optical fibers in the composite.Together with a special treatment, these parameters have been tailored to yield a system capable of detecting the threshold of damage for various impacted Kevlar /epoxy panels.The technique has been extended to measure the growth of a damage region which could arise either from impact, manufacturing flaws, or static overloading. The mechanism of optical fiber fracture has also been investigated.In addition, the influence of imbedded optical fibers on the tensile and compressive strength of the composite material has been studied.Image enhanced backlighting has been shown to be a powerful and convenient method of assessing internal damage to translucent composite materials.

45585 Damage assessment for composite smart structures

“…This in tum depends upon the overall layup and the direction of the applied load. This observation is consistent with our understanding that through-the-thickness matrix cracks constitute the principal fracture mechanism for embedded optical fibers [8]. In general the optical fibers should be perpendicular to the reinforcing fibers in the ply that separates the optical fibers from the region of damage to be sensed.…”

“…The simplest form of such a system would involve the interrogation of the optical fibers by a source of light and disruption in their transmission due to their fracture used to determine the location and extent of damage. We have undertaken extensive measurements to demonstrate the viability of employing imbedded optical fibers for assessing impact induced damage in Kevlar/epoxy samples [8,9]. We have also shown that load induced growth of a damage zone resulting from fatigue, mishandling, excessive load or flaws created during manufacture could also be detected by this system [ 10].…”

“…In the first place the greatest sensitivity to damage arises when the optical fibers are embedded as close to the surface of maximum tensile strain as possible and sandwiched orthogonally between a pair of collinear plies [8]. When this is not possible the orientation leading to the best damage response for optical fibers embedded between a given pair of plies is determined by the most likely direction for transverse matrix crack propagation.…”

“…Another factor contributing to the relative insensitivity of the optical fibers to damage was their alignment of the optical fibers parallel to the reinforcing fibers. We have found this orientation to shield the optical fibers from the matrix cracks associated with a region of damage [8]. The use of a two dimensional photodiode array, a video camera and a computer was suggested [12] to permit the output of all of the optical fibers to be viewed simultaneously.…”

“…The translucent nature of Kevlar/epoxy (made mostly from Fiberite HY -E 17714AA/4560 unidirectional prepreg tape) allowed us to map the location of each optical fiber imbedded in panels of this material by means of the diffuse laser radiation emitted by the optical fibers (especially in the vicinity of bends) [8]. This was made easier because we used multimode optical fibers (Coming-1517 multimode with a cladding diameter of 125 jlm and a 50 jlm core) for this damage sensing work.…”