Submarine Certification Using Fiber Bragg Grating Sensors

In May of 2003, AITHER engineers planned, installed, and tested a fiber Bragg grating (FBG) strain sensor system to monitor a new submarine design undergoing American Bureau of Shipping (ABS) certification testing and to validate finite element analysis (FEA) predictions of the design. FBG sensors were chosen because of the known difficulties with using a large quantity of resistance strain gages (RSGs). After the submarine was designed using traditional formula-based techniques, detailed finite element analyses were performed to calculate both the global buckling behavior and local stress concentrations. Fiber optic triaxial, biaxial, and uniaxial gauge locations were selected based on classic submarine testing and analysis procedures. FBGs were placed on six optical fibers with two fibers (33 sensors) mounted internally to the hull and four fibers (64 sensors) mounted externally. Testing was performed by lowering the submarine to the design depth and recording strain measurements. The optical sensor signals were transmitted directly to the water’s surface and monitored topside by the Micron Optics Inc. si425 swept laser interrogator through more than 2000 feet of optical cable. Measured temperature-compensated strain values were compared to the FEM predicted strain values with excellent results. All 97 FBG sensors survived the installation, shipment, and testing of the submarine hull. To AITHER’s knowledge, this successful test represents the first time that FBG sensors have been used for certification testing of a submarine design and to validate FEA on a large-scale structure. For more information, refer to the Naval Engineers Journal paper entitled “Submarine Design and Certification Using Finite Element Analysis and Fiber Bragg Grating Sensors” in publications.