The authors focus on the construction of a structural health monitoring (SHM) system with an advanced grid structure (AGS) made of carbon-fiber reinforced plastic (CFRP). AGS is often applied to aerospace structures because the ribs carry only axial forces in the carbon fiber direction, making AGS structurally effective and lightweight, and because the repetition of many ribs in the AGS composition results in damage tolerance. The failure of a single rib hardly affects the fracture of the whole structure, making AGS a fail-safe structure. In this research, the authors have embedded multiplexed-fiber Bragg grating (FBG) sensors into an AGS rib in the longitudinal direction to measure mechanical strains of all ribs in order to detect the existence and regions of AGS rib fractures. Monitoring the change in riblongitudinal strains is the most effective SHM system for AGS. To confirm the proposal, the authors explore the following. First, various damage characteristics under low-velocity impact loading are investigated and it is verified that partial rib cracks are the most typical damage in AGS. An AGS is then fabricated with embedded FBG sensors and verified that the SHM system is able to measure all rib strains. Subsequently, it is analytically determined that the change in longitudinal-rib strains is the most appropriate mechanical feature for damage detection. Moreover, a statistical outlier analysis is introduced into the SHM system for automatic damage detection. Finally, AGS is established with the SHM system and verified experimentally. Results confirm that the existence of damage and its regions in AGS can be detected with the proposed SHM system.Keywords advanced grid structure (AGS) Á damage detection Á fiber Bragg grating (FBG) sensor Á impact rib fracture Á finite element analysis (FEA) Á statistical outlier analysis

A Ti-Ni shape memory alloy (SMA) foil and a small-diameter fiber Bragg grating (FBG) sensor were embedded simultaneously into a CFRP cross-ply laminate. When the specimen was heated, the recovery compressive force was generated from the embedded SMA foil, which homogenized the non-uniform strain distribution caused by cracks in the 90• ply. Then, the tensile stress in the 90• ply was relaxed and the occurrence of new transverse cracks was suppressed. This effect was evaluated with the embedded FBG sensor. When the specimen was heated, the deformed reflection spectrum of the FBG returned to its original shape, which suggested that Ti-Ni SMA foil was effective to suppress the damage. However, relaxation of thermal residual tensile stress in the 90• ply was also effective. The result of the 3D FEA suggested that the suppression of damage occurrence and growth was mainly caused by the relaxation of thermal residual tensile stress.

In this research, shape memory alloy (SMA) foils were embedded into carbon fiber reinforced plastic (CFRP) cross-ply laminates. When the tensile pre-strained SMA foil is heated, the SMA produces appropriate compressive stress (recovery stress) to suppress the transverse cracks in the laminates. In order to investigate the damage suppression effect, tensile loading-unloading tests were conducted for the CFRP cross-ply laminates. The test results implied that the recovery stress of embedded SMA foil suppressed the occurrence and the progress of the transverse crack. Then, to confirm that the damage suppression effect was caused by the embedded SMA foil, the onedimensional (1D) shear-lag model considering the behavior of SMA was newly derived. Using this model, the progress of transverse crack density was predicted probabilistically. The simulation results showed the same tendency as the experimental results. As a result, it was proved that the embedment of SMA foils into CFRP laminates is effective to suppress the occurrence and growth of the transverse cracks.