The Effect of Damages on Modal Frequencies of Flat Glass Fiber Reinforced Polyurethane Foam Sandwich Structures

In this research work, experimental and numerical analyses were conducted to ascertain the impact of damages on modal frequencies of sandwich structure. The structure consists of laminated E/glass epoxy face sheets with [0°/90°] s stacking sequences and polyurethane foam core. The fabrication processes involved are hand layup technique, mold filling process and the use of adhesives to assemble the structure. Damages introduced during fabrication, encompassing core damage, delamination between face sheets and core and impact damage. Experimental modal testing was conducted on four distinct specimens using a Fast Fourier Transform (FFT) analyzer under cantilever boundary conditions. The results reveal significant variations in modal frequencies and mode shapes in the structures. A consistent six-mode frequency bandwidth is observed within the 2000 Hz range across all specimens. The numerical analysis was performed by Finite Element Method (FEM). Experimental and numerical results were in a good agreement. It was observed that modal frequencies decrease due to the presence of damage in the fabricated structures.