Bolted joints with radial pretension for thick-walled composites structures

Bolted connections are generally used to reversibly join two thick-walled components made of fibre reinforced plastics. Fields of application for many decades by now are multi-stage rockets for transportation of satellites, connection of aircraft wing and fuselage, or the root connection of rotor blades of wind turbines. Due to their anisotropic material properties, bolted connections of composite structures are significantly more sensitive to the failure mode bearing. Especially when high static and cyclic loads are to be transferred this sensibility leads to an increase in material mass and costs, because the wall-thickness at the connection has to be increased to reduce the stresses. It is state of the art to axially pretension a bolt by means of a bolt head and a nut to increase the load-transfer capacity of the joint. A novel connection concept is to apply a radial pretension between the hole and the bolt by means of clamping bushes. This concept principle is similar to shrink-fit joints of metallic components. The clamping bushes are used to expand the borehole to a defined diameter and thereby create a radial pretension. The diameter-related tolerance between bolt and borehole thereby is completely compensated, which lowers the requirement for drilling or milling the borehole to an exact diameter. The radial pretension can be combined with an axial pretension or an axial support of the composite structure’s borehole. Finite element calculations and first experiments show, that a radial pretension significantly increases the fatigue bearing strength of structures made of glass-fibre reinforced plastic. Axial pretension mainly increases the static bearing strength. A combination of radial and axial pretension has not been tested yet, but is supposed to both increase static and fatigue bearing strength. For further investigation of this concept the following parameters will be investigated in detail: level of pretension, choice of material (glass and carbon fibre, different layups) and design of small, simple and strong clamping bushes.