Stretch forming of beams of non-uniform section

Abstract

Stretch-forming operations are used widely for the fabrication of airframe and automotive structural components, in a variety of materials. The parts may have single- or double-curvatures, such as in the case of the wing-skin panels and window frames of aircraft. In many applications, the workpiece - usually of uniform cross-section - is subjected to a suitable tension and then wrapped around a die of the desired shape. Deformation occurs mainly by bending at the fulcrum point of the die surface. Compressive buckling and springback are avoided by applying sufficient tensile load to stretch the material and move the neutral surface towards the centre of the forming die. In this work, the emphasis has been on the consideration of the forming of T-section beams in such materials as 7075 Aluminum: this type of beam is usually used in the building of an aircraft fuselage. A work-hardening material has been considered, where the stress-strain relationship is given by σ=Kϵ n , this relationship having been found to give a close fit to experimental stress-strain data for 7075 Aluminium. The variation of the applied bending moment with the axial load for the fully-elastic, elastic-plastic and fully-plastic conditions is given, both in analytical and graphical forms.