Abstract
Carbon Fiber Reinforced Plastic (CFRP) composites are expected to be used in liquid hydrogen (LH2) tanks for hydrogen-powered aircraft and reusable launch vehicles due to their superior specific strength and specific stiffness and their suitability for weight reduction. However, in cryogenic environments such as LH2 temperature, large thermal stresses occur due to the difference in coefficient of thermal expansion (CTE) between 0° and 90° layers as well as between carbon fiber and matrix resin. In addition, low temperature embrittlement occurs at the fiber/resin interface and in the resin itself. Therefore, matrix cracks propagate at much lower tensile strains than at room temperature condition. Since matrix cracks are pathways for gas leakage, it is important to monitor their initiation and evolution. Fiber optic sensors are lightweight, small in diameter, and flexible, making them easy to embed in CFRP. They have the advantage of having little effect on the mechanical properties when embedded in CFRP. Fiber Bragg Grating (FBG) sensors can measure strain with high accuracy, and their application to structural health monitoring is being investigated. In general, FBG sensors are used for single-point measurement or quasi-distribution measurement by multiplexing multiple FBGs on a single optical fiber. On the other hand, a distribution measurement method that applies optical frequency domain reflectometry (OFDR) to FBG sensors has been developed. However, previous studies have been limited to measuring the spatial distribution of strain, and there are no examples of detailed measurements of the location of matrix cracks in CFRP. This study investigates the applicability of the OFDR-FBG method for the detection of matrix cracks in CFRP laminates. An FBG sensor of 100 mm gauge length is embedded in a cross-ply CFRP specimen. The specimens are subjected to tensile loading. Transverse matrix cracks in the 90° ply are observed at the edge of thespecimen using an optical microscope and an X-ray inspection system. The results confirm that matrix crack propagation can be successfully detected as a strain change at low matrix crack densities. The matrix crack positions detected by the FBG sensor are directly compared with those obtained from the microscopic observation, and the difference is generally less than 2%. It is demonstrated that the matrix cracks can be detected with high accuracy using FBG sensors based on the OFDR method. The OFDR-FBG system can detect cracks with high accuracy in areas where the crack density is relatively low. On the other hand, when a large number of cracks are generated, it is difficult to distinguish individual cracks due to the flattening of the measured strain.
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