Transient response of heated sandwich composite structures with different core materials

Sandwich composite structures are widely preferred in modern aircraft exterior and interior structures because of their lightweight and high strength capability. Aircraft structures might be exposed to harsh thermal environments during their operational life arising from aerodynamic heating, solar radiation or engine. The exposure to thermal environment affects the mechanical properties and modal parameters of aircraft structures. In this study, the effects of core structure on dynamic characteristic of sandwich composite structure is examined under transient thermal condition by using the thermal experimental modal analysis method. As test specimens, seven sandwich composite test plates are manufactured from aluminum, nomex and glassfiber core materials, which have different core structure parameters. The transient thermal environment is applied to one face of the test plate while the structure is excited with white noise vibration excitation. The responses of the test plates are recorded during heating process, and then analyzed by MATLAB to define modal parameters of the test plates at different temperature values. The test results showed that, under transient thermal environment, as the core cell size increased, the natural frequency decrease rate of the test plate increases and the damping of structure became more sensitive to temperature. The first natural frequency of the hexagonal and the OX-Core affected in same rate, but in the end of the test period, the second mode of the OX-Core was equal to initial value while the third mode decreased more than the third mode of hexagonal core. The damping ratio of the hexagonal core was more sensitive to thermal environment than the Ox-Core damping ratio. The study on the effect of core thickness showed that, the dynamic characteristics of sandwich structure became more sensitive to thermal environment as the core thickness increased. The change rate of natural frequencies and damping ratios were the highest for the thickest core. The results also showed that, each core material type affected from thermal environment differently and the dynamic characteristics of the test plate with Nomex core was the most sensitive while the glassfiber core affected less than the other core materials when heated with 3 °C/s heating rate. As the heating rate decreases, the change rates in natural frequency decrease for Nomex while increase for glassfiber and aluminum core. The change in glassfiber core is lower than other cores at each heating rate. The FRF responses of each test plates decreased under the effect of transient thermal environment.