Barely visible impact damage detection for composite sandwich structures by optical-fiber-based distributed strain measurement

The authors developed an impact damage-detection system for large-scale composite sandwich structures using an optical fiber network running throughout the structure. A Brillouin-based sensing system with high spatial resolution (pre-pump pulse Brillouin optical time-domain analysis (PPP-BOTDA)) was utilized for distributed strain measurement. The PPP-BOTDA sensing system can measure axial strain along the optical fiber by employing stimulated Brillouin scattering. The system realizes a spatial resolution of 10 cm, a sampling interval of 5 cm, and a sensing range of more than 1 km. Our previous study revealed that a non-uniform axial strain within centimeter spatial resolution broadens the width of the Brillouin gain spectrum, which is the output of the PPP-BOTDA. The specific response of the PPP-BOTDA was employed to detect non-uniform strain distribution along a residual facesheet dent in a damaged area. First, the response of the optical fiber sensor network, formed in the adhesive layer, was simulated to clarify the effectiveness and limitations of the proposed damage-detection technique. The system was then validated by an experiment. As the damage became larger, the width of the Brillouin gain spectra became broader. Consequently, the location and size of barely visible damage could be estimated. The system developed is quite useful for a first inspection of large-scale sandwich structures in aerospace and marine applications