The authors tried to construct a structural health monitoring (SHM) system to identify damage in composite grid structure
called Advanced Grid Structure (AGS) by using two types of guided waves, compressional and flexural waves, propagating
along ribs of AGS. Fiber Bragg grating (FBG) sensors network embedded in AGS is utilized as their receivers. AGS is
defined as trussed structures whose ribs are made of Carbon fiber reinforced plastic (CFRP). AGS has often been applied
to aerospace structures because it is structurally effective and redundant. The authors had reported one possible method
of SHM of AGS by monitoring of static strain distribution with embedded FBG sensors network in AGS. In this paper,
we propose another possible method of SHM of AGS. We utilized two types of guided waves, compressional and flexural
waves, for damage diagnosis. First, we verified our proposed system experimentally. The results confirmed that FBG
sensors could measure both elastic waves and two types of guided waves were generated with the proposed system. Then,
some basic characteristics of compressional and flexural wave propagations were clarified experimentally to find that
compressional wave has directionality and flexural wave has isotropy. Based on the characteristics, the authors proposed
two types of damage diagnosis methods with compressional and flexural waves, respectively. Moreover, those proposed
methods were applied to two specific damage types, rib crack and debonding between ribs and skin. The specific damage
diagnosis methods were verified experimentally to find that damages could be detected by those methods.
In this research, the authors target on the construction of structural health monitoring system of Advanced Grid Structure (AGS) made of Carbon fiber reinforced plastic (CFRP). AGS has often been applied to aerospace structures because of the following advantages: (1) Since ribs carry only axial forces, the weakness in the transverse direction of the CFRP unidirectional laminates is negligible. (2) AGS has damage tolerance because the fracture of a rib hardly affects other ribs, namely AGS is a fail-safe structure. In this research, in order to detect existence and regions of rib fractures in AGS, we embedded multiplexed fiber Bragg grating (FBG) sensors into AGS in rib longitudinal directions for measurement of strains. Monitoring of the change in rib longitudinal strains is the most effective SHM system for AGS. In order to confirm our proposal, we carried out following discussions. First, we analytically revealed that the change in rib longitudinal strains was the most sensitive signal for damage detection because of AGS's structural redundancy. Then, we introduced a statistical outlier analysis technique into the SHM system for damage recognition. Finally, we established AGS with the SHM system and verified experimentally. The result of the test showed that damage existence and regions in AGS could be detected with the proposed SHM system.
Grid structures are the structures made of the trusses consisting of simple ribs. Especially, the structure which uses carbon fiber reinforced plastic (CFRP) unidirectional composites as ribs is called advanced grid structures (AGS). Highly Reliable Advanced Grid Structure (HRAGS) is one of the AGS in which fiber Bragg grating (FBG) sensors are embedded in the longitudinal direction of the ribs in order to detect various damages that appear in the composite grid structures. In this research, the authors tried to identify the damage location in AGS from the structural strain distribution measured by FBG sensors embedded in all ribs. When some damages appear in the AGS, the structural strain distribution in the AGS changes accordingly. Considering the tendency of change, the damage location was identified. At first, FBG sensors were embedded into AGS and three point bending test was examined. The result showed that these embedded sensors could detect the strains applied to the corresponding ribs. Then, low velocity impact test was carried out, which revealed that only fiber breakage was appeared in the AGS. Moreover, three types of models for finite element analysis (FEA) were proposed and compared with the experimental result. According to the comparison, the authors selected beam element model (BEM) for damage-location identification in this research. Furthermore, strain distributions in the structure including damages were calculated with this model. The result proved that the identification of damage location could be realized.