Shear-lag influence on maximum specific bending stiffness and strength of composite I-beam wing spar

Abstract

Beams with low height and high strength, used in modern unmanned aerial vehicles (UAVs) can be especially problematic considering the efficiency of material usage. This FEM study analyses the influence of shear-lag effect on maximum specific bending stiffness and strength of a composite I-beam wing spar with unidirectional carbon fibre-reinforced plastic flanges. Elliptical load distribution, which is characteristic of airplane wings, was used. For 12 different flanges, width-to-thickness aspect ratios and three different web skin thicknesses, stresses, and end deflections were observed. The flanges cross-section area and beam height were kept the same and constant in all cases. After a certain value of the flanges aspect ratio, the bending stiffness starts to decrease, because the shear-lag effect starts dominating over the still increasing moment of inertia of the cross-section. Maximum specific bending stiffness was reached at much lower aspect ratios of flanges than maximum bending stiffness. Maximum axial stresses start to increase in an almost linear fashion beyond a certain aspect ratio. The highest specific strength was found at relatively low aspect ratios. The influence of web shear stiffness on shear-lag magnitude was also analysed. Changing web stiffness in a selected range showed a negligible influence on shear-lag magnitude for all aspect ratios.