Detection of BVID with Optical Fiber Sensors and Relationship to Composite Material Strength

– This paper investigates the effects of barely visible impact damage (BVID) on thermoplastic composites (TPCs) using coupon size specimens. Optical fiber strain sensors were used to investigate the best method for detecting the BVID event. Some of the specimens were tested after impact (CAI) to investigate the effect of the impact on static strength. The remaining impacted specimens were tested in fatigue to investigate the strength using post cyclic loading. The findings of this study can be applied to aerospace structures that use TPCs and regularly experience BVID. The composite specimens used for this study were made of Toray TC1225 LM PAEK and cut into 4 in. x 6 in. panels. The stacking sequence used for these panels was [45\ 0\ −45\ 90]4s. The impacts were performed following ASTM standard D7136 at energy levels ranging from 5J to 60J. Luna High-Definition Strain sensors were attached to 7 panels before impact and the Luna optical distributed sensor interrogator (ODiSI) measurement system was used to collect data. The optical fiber data showed that optical fibers are capable of detecting BVID on TPC materials and there was a consistent pattern of strain distribution after impact. Compression after impact testing data showed that the overall strength of the panels decreased by about 17 MPa for every 10 joules of impact energy. A significant reduction in fatigue life was observed for impact energies exceeding 50J.