Abstract
Vibration and stability studies have been undertaken on glass fibre reinforced polymer composite unstiffened and stiffened plates to optimise their dynamic properties. Boundary conditions, stiffeners and orthotropy of the material add to the complexity of a mathematical solution and to overcome this problem experimental and analytical studies were undertaken. The former method was carried out by impact hammer and an FFT digital signal analyser and the latter method was undertaken using finite element computer software. The current research concentrated upon the procedures and possible techniques available to optimise the dynamic properties of the plate without introducing weight penalty with the object of achieving an efficient structural performance coupled with an economic design. It has been shown that most of the increase in frequency and critical buckling load was directly related to the increase in stiffness of the stiffener and its position on the plate structure. The mode shapes have provided information regarding the most advantageous position for the setting of the stiffeners; they must be positioned away from nodal lines. The effect of the stiffener was significant for the fully clamped and clamped/free plates where only bending modes of vibration are present. However, for the completely free plates, where both bending and torsional modes of vibration could occur, the effect that the stiffeners have on the torsional modes was minimal. To locate precisely the position of the stiffener may be difficult when the plates are subjected to in-plane compressive loads, because higher order mode shapes may interchange. The mass-saving advantage which has been obtained in this research has shown that the stiffened plates with top-hat stiffeners were seen to have higher natural frequencies, within a specific vibration mode, compared to stiffened plates with rectangular stiffeners (blade).