This paper describes the use of fiber optic sensors embedded into laminated [90{sub}2/0{sub}4/OF(0)/0{sub}4/90{sub}2] graphite/epoxy composites for low velocity impact detection and impact-induced damage assessment (The OF(0) denotes an optical fiber orientation of zero degree). 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". Initial impact tests comparing surface-mounted ILFE and resistance strain gages using an aluminum plate were favorable. The ILFE sensors were then embedded at the mid-plane of [90{sub}2/0{sub}4/OF(0)/0{sub}4/90{sub}2] composite plate subjected to transverse impact. The ILFE sensor response was compared to a resistance strain gage bonded to the back side of the specimen, comparison of the sensors showed that, as expected, higher transverse loads provided more dominant membrane stresses. An unexpected result of this series of tests was that the sensor embedded in the middle layer had higher survivability than the resistance strain gage bonded to the rear surface of the plate. In a final series of experiments, copper coated ILFE sensors embedded in composite plates were investigated for their potential as damage sensors. The strain of 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 the ～1 J impact energy starts to initiate the composite damage, and the hysteresis loop area is approximately proportional to the delamination area evaluated by X-ray radiographs. Finally, a control group of composite specimens without embedded optical fiber sensors was also tested under low velocity impact. X-ray NDE indicates that the embedded ILFE sensor does not alter the structural integrity based on the macro-scale observations.
展开▼
