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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Foam core sandwich structures are rather vulnerable against transverse loading which can leave only barely visible damage in the structures. Environmental conditions can further affect the loading response and thus the formation of damage. To study this, uniaxial compression-tension loading tests were conducted at 30, 60 and 80℃ temperatures using polymethacrylimide (PMI) foam specimens conditioned at three distinct humidity conditions (dry, ambient and wet) before testing. Effect of the conditions on the mechanical properties and loading response of the foam was studied. Strength and stiffness of the foam degraded with increasing temperature and humidity with the combined effect of these conditions being even more severe than either one alone. Indentation tests of polymethacrylimide foam core composite sandwich beams with composite face sheets were also conducted at the same temperature and humidity conditions. The indentation response was seen to correlate with the compressive response of the foam material. [ABSTRACT FROM AUTHOR]