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Optical fiber sensors have attracted considerable attention in health monitoring of aerospace composite structures. This paper briefly reviews our recent advancement mainly in Brillouin-based distributed sensing. Damage detection, life cycle monitoring and shape reconstruction systems applicable to large-scale composite structures are presented, and new technical concepts, "smart crack arrester" and "hierarchical sensing system", are described as well, highlighting the great potential of optical fiber sensors for the structural health monitoring (SHM) field.

Sandwich structures with advanced composite facesheets are attracting much attention as a solution to maximize the potential of composite materials. However, the composite sandwich structures are prone to damage, such as impact damage and debonding. Although these damages are difficult to detect using conventional nondestructive inspection method, they cause significant reduction in the mechanical properties. Hence, several researchers have attempted to detect and suppress the damages using smart sensors and actuators. In this paper recent developments on smart technologies to improve reliability of the composite sandwich structures are reviewed. First, the state-of-the-art sandwich technology in aerospace application is presented. Next, typical damages in composite sandwich structures are described, which is essential to effectively apply the smart technologies to sandwich structures. Then, smart technologies which have been applied to sandwich structures are briefly shown with focusing specific properties of sandwich structures. It includes damage detection using dynamic response, wave propagation and optical fiber sensors. Finally, a smart honeycomb sandwich concept is also presented. State-of-the-art sandwich technology in aerospace structuresSandwich concept has wide range of advantages and potentials over quasi metal-derived design. Since the facesheets are continuously sustained by the core, the global and local stiffening can be achieved. Moreover, the integral simple stiffening reduces the complexity in analysis, manufacturing and maintenance, resulting in cutting the total life cycle cost of the structure. Introduction of sandwich structures in fuselage shells reduces the noise level inside the cabin, because of high damping properties of the core materials, and thus increases passengers' comfort. In this application, an integration of thermal isolation is also possible.

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding.

This study develops a fiber-optic-based technique for in situ characterization of direction-dependent cure-induced shrinkage in thermoset fiber-reinforced composites. A procedure is established to embed fiber Bragg grating (FBG) sensors in composite out-of-plane directions and to measure key through-thickness chemical cure shrinkage directly under practical curing conditions. First, sensitivity of the proposed method is evaluated through comparison with a standard technique (i.e. thermo-mechanical analysis (TMA)), and the effect of sensor tail length on measurement sensitivity is discussed considering shear-lag effect. Next, combined with double-sided vacuum bagging and demolding during curing, FBG sensors embedded in through-thickness and in-plane directions clarify direction-dependent cure-induced shrinkage in autoclaved unidirectional carbon/epoxy. Finally, the feasibility of characterizing through-thickness shear strain, which is important in complex-shaped parts but cannot be measured using conventional techniques, is confirmed. The developed technique will be a powerful tool for evaluating cure shrinkage in complex-shaped parts and for validating process-simulation tools based on internal strain.

In thick thermoplastic composite laminates, nonuniform temperature distribution arises in the through-thickness direction during high-rate manufacturing processes. This causes the so-called thermal skin-core effect. The surface region solidified in advance constrains shrinking of the inside region, so nonuniform residual stress/strain distribution arises in the through-thickness direction. This study quantitatively clarified this mechanism and identified the amount of residual stress/strain by utilizing fiber optic–based internal strain measurement and process simulation. First, in-plane transverse strain of thin carbon fiber/polyphenylenesulfide laminates was measured using fiber Bragg grating sensors to determine two key parameters for stress/strain simulation; thermal/crystalline shrinkage strain and composite stiffness. Abaqus-based simulation using these properties was then performed to calculate stress/strain distribution in thick laminates. The simulated strain agreed well with the measured value and it was confirmed that the residual stress developed in a relatively low temperature range. In addition, transverse three-point bending tests were conducted to validate the amount of residual stress calculated by the simulation. The bending strength increased by the thermal skin-core effect and the amount of strength increase coincided with the simulation, confirming the validity of the simulation. Extension of the proposed approach to the evaluation of the morphological skin-core effect is also discussed.