Embedded fiber Bragg sensors for damage identification in sandwich composites after impact

Abstract

This study investigates the use of fiber Bragg gratings (FBGs) for damage identification in sandwich composite structures subjected to multiple low velocity impact events. Initially, a set of twelve graphite-fiber-epoxy facesheet / Rohacell foam core sandwich composite specimens were tested to characterize their impact response. Each test consisted of repeated low-velocity impacts at a constant impact energy to measure the contact force and dissipated energy per strike and observe damage progression throughout the composite lifetime. The maximum contact force decreased slightly while the dissipated energy increased over the lifetime of the composite. Infrared thermography images were also taken of a specimen to provide comparison with a conventional non-destructive evaluation method for the detection of damage in sandwich composites. The infrared thermography detected damage to the impacted facesheet which was visible on the surface and highlighted shear cracking in the foam core (when imaged from the specimen side). The infrared thermography was not able to detect the extensive core crushing or facesheet-core interface damage present at the end of the laminate lifetime. Similar low-velocity impact testing was performed on sandwich composites with a single optical fiber embedded in the adhesive bond between the core and the faceplate. The presence of the optical fiber did reduce the average specimen lifetime but not the specimen stiffness. The final set of specimens included a FBG sensor in the embedded optical fiber. The peak wavelength of the FBG sensor was interrogated at 1 kHz throughout each impact event, while the full-spectral response of the FBG sensor was measured in between impact events. From the peak wavelength shift measurements, the interfacial residual strain was calculated. For all specimens this residual strain initially increased with strike number and then decreased. In some specimens, a final region of increasing residual strain was observed. These observations were combined with an analysis of the shape of the reflected spectrum and photographs of the sandwich composite surface to further understand the state of damage. The FBG response spectra were distorted due to transverse loading on the optical fiber present after fracture of the impacted facesheet, typically after crushing of the core in the vicinity of the impact. The form of the reflected spectra can thus be used as an in-situ indicator of the presence of core-crushing and potential core-facesheet debonding.