Impact-induced damage of laminated graphite/epoxy composites monitored using embedded in-line fiber etalon optic sensors

This paper describes the use of fiber optic sensors embedded into laminated graphite/epoxy composites for low velocity impact-induced damage assessment. The sensor system uses in-line fiber etalon (ILFE) sensors interrogated with path-matched differential interferometry (PMDI) and demodulated with a "single channel phase tracker." Uncoated and coated copper ILFE sensors were embedded at the mid-plane of [90(2)/0(4)/OF(0)/0(4)/90(2)] composite plates subjected to transverse impact (The OF(0) denotes an optical fiber orientation of zero degree). The ILFE sensor response was compared to a resistance strain gage bonded to the back side of the specimen and to the load cell mounted on the impactor. The strain measured with the metal coated ILFE sensor and the load cell response were used to plot a hysteresis strain-load curve to assess the impact-induced damage. The result shows that the similar to 1 J impact energy starts to initiate the composite damage, and that the area contained within the hysteresis loop area is approximately proportional to the delamination area evaluated by X-ray radiographs. X-ray NDE indicates that the embedded ILFE sensor does not alter the structural integrity based on the nacre-scale observations. In addition, the impact test showed that the sensor embedded in the middle layer had higher survivability than the resistance strain gage bonded to the rear surface of the plate.