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

Abstract

The authors developed an impact damage-detection system for large-scale compositesandwich structures using an optical fiber network running throughout the structure. ABrillouin-based sensing system with high spatial resolution (pre-pump pulse Brillouinoptical time-domain analysis (PPP-BOTDA)) was utilized for distributed strainmeasurement. The PPP-BOTDA sensing system can measure axial strain along theoptical fiber by employing stimulated Brillouin scattering. The system realizes aspatial resolution of 10 cm, a sampling interval of 5 cm, and a sensing range ofmore than 1 km. Our previous study revealed that a non-uniform axial strainwithin centimeter spatial resolution broadens the width of the Brillouin gainspectrum, which is the output of the PPP-BOTDA. The specific response of thePPP-BOTDA was employed to detect non-uniform strain distribution along a residualfacesheet dent in a damaged area. First, the response of the optical fiber sensornetwork, formed in the adhesive layer, was simulated to clarify the effectivenessand limitations of the proposed damage-detection technique. The system wasthen validated by an experiment. As the damage became larger, the width of theBrillouin gain spectra became broader. Consequently, the location and size ofbarely visible damage could be estimated. The system developed is quite usefulfor a first inspection of large-scale sandwich structures in aerospace and marineapplications.