In this research, the authors developed a smart sandwich structure with damage detection and suppression functions using a small-diameter fiber Bragg grating (FBG) sensor and shape memory alloy (SMA) honeycomb core. Impact damage was detected using the change in the reflection spectrum of the FBG sensor due to strain change along a bend of a facesheet caused by core buckling. After the damage detection, in order to heat the SMA honeycomb, a voltage was applied to a nichrome wire embedded in the facesheet. The core buckling disappeared and the bend of the facesheet was relieved. These abilities could also be confirmed by theoretical and numerical simulations.
Impacts create internal damage in foam core sandwich structures and this internal damage can cause severe reduction in strength. Therefore, the aim of this study is the impact identification of foam core sandwich panels by using embedded fiber Bragg grating (FBG) sensors. FBG sensors were embedded between the skin-core interface and strain response during impact was used to identify the impact location. Strain measurement system consists of a light source, FBG sensors, arrayed waveguide grating (AWG) and photo detectors. The accuracy of the system was demonstrated by indentation test using form core sandwich panels with FBG sensors. From the result, this system was able to measure strain accurately. And then we investigated the relation between strain and distance from sensors to the indentation point.
The authors proposed fiber-optic-based damage monitoring of carbon fiber reinforced plastic (CFRP) bolted joints. Optical fibers were embedded along bolt holes and strain change along the optical fiber induced by internal damage was measured by a Brillouin Optical Correlation Domain Analysis (BOCDA), which is a high spatial resolution distributed strain sensing system. This study began by investigating damage modes of CFRP bolted joints after bearing failure. Effective embedding positions of optical fibers were then proposed and their feasibility was evaluated by finite element analysis simulating the damage propagation in the bolted joint and consequent strain change. Finally, verification tests were conducted using specimens with embedded optical fibers at various positions. It was clearly shown that damage could be detected using residual strain due to fiber-microbuckling (kinking) damage or permanent deformation of neighboring plies. Furthermore, damage size and direction could be estimated from the change in the strain distribution. The system developed is quite useful for a first inspection of large-scale composite structures in aerospace applications.
Composite structures are optimized in shape by terminating specific plies gradually. However, stress concentration at the ply drop-off can cause interlaminar delamination from the ply edge. In this study, Ply Curving Termination (PCT), which is introduced by locally curving fibers at 0° ply edges, is used to enhance tensile strength of the ply drop-off. To clarify the dependence of the PCT strength on the curved fiber angle (PCT angle), static tensile tests using ply drop-off specimens are conducted, followed by detailed damage observations using X-ray CT. The results indicates that the strength significantly increases with PCT but the fracture behavior differs depending on the PCT angle. Interestingly, specimens with large PCT angles fail from the position where the fibers curve, away from the ply edge. This PCT-specific failure mechanism is then elucidated using finite element analysis and the virtual crack closure technique, indicating that shear lag caused by the stiffness change due to fiber curving is important. Finally, using the revealed PCT failure mechanism, PCT angles that are the most effective in enhancing tensile strength is discussed.
Indentation loading or low-velocity impacts on foam core sandwich structures can leave only barely visible damage on the face sheets while causing notable damage in the core due to the thin face sheets and relatively weak core. In practical applications, the measurement and damage detection can also be notably affected by environmental conditions and relaxation of the foam core. To detect the damages, a Rayleigh-scattering-based fiber optic strain monitoring system with high spatial resolution was applied to foam core sandwich structures. Damage detection ability of the system was therefore tested by indentation tests on sandwich beam and panel specimens while also considering the aforementioned effects. Finite element analysis verified by the measurements was used to provide a method to estimate the damaged core area from the measured strain data. Finally, the monitoring ability of the system was demonstrated by low-velocity impact tests on a large scale sandwich panel. The used system provided detailed strain data during and after indentation and impact tests. Due to the high resolution of the system, the damage location and size could be estimated from the obtained strain data even when barely visible dent remained on the face sheet, thus demonstrating the damage detection ability even at various environmental conditions and after relaxation of the foam core.
Dimpling in the composite face sheets of honeycomb sandwich structures due to mismatch in the thermal expansion coefficients of the constituent materials was studied with emphasis on its monitoring and prediction. Strain distributions along optical fibers embedded in the face sheet were monitored during manufacturing. Dimple formation and in-plane strain distributions in the face sheets were studied using finite element analysis, and an analytical model based on the beam theory was constructed to predict the dimple depths from the strain data. A system using twin optical fiber sensors was proposed to accurately measure the dimpling-induced strains. The usability and performance of the system was evaluated using small scale specimens and finally on a more realistic large-scale specimen. The system could measure the strain changes due to dimpling of the face sheets and provided decent prediction of the dimple depth distribution along the sandwich panels.
