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The main aim of the present work is to study the effect of axial preload on the durability of aerospace fastened joints. For that purpose, experimental and numerical approaches are adopted to demonstrate the beneficial effect of axial preload on the fatigue life of representative aerospace fastened joints. In the experimental approach, an innovative tightening technique, based on iterative tightening, is used to reduce the scatter habitually induced on preload when the conventional torque tightening technique is employed. The main benefit, besides the use of this alternative tightening technique, is to allow an accurate preload to be introduced without geometry or stiffness modification, thus enabling the preload effect to be determined precisely. The results show the effect of preload on the static resistance, fatigue life and failure mode of fastened joints. In the numerical approach, a 3D finite element model is developed and the analysis of its behavior confirms and explains experimental observations. Analysis of longitudinal stresses through critical paths shows that the effect of high preload is beneficial because it reduces hydrostatic pressure and shear stress amplitude, thus leading to longer fatigue lives. The correlation between experimental observations and the numerical results is studied using SWT and Crossland multiaxial fatigue criteria.

Bolted joints are one of the most common elements in aerostructures. Currently, the preload applied to join the parts together is achieved by applying torque to the bolt head or to the nut. In the case of shear joints, an emerging opportunity to optimise structural joints is to apply the preload more accurately. Up to now, the preload effect is most often neglected due to the large scatter on its value due to torque tightening. The aim of this article is to describe the effect of preload on shear joints, for this reason, a numerical and experimental approach has been adopted to demonstrate its influence. In the experimental approach, iterative tightening was used to reduce the scatter on preload. The benefits of preload on high load transfer double lap shear joints will be discussed. In the numerical section, an approach of predicting fatigue life of shear joints is presented. The effect of preload/friction of coefficient is also studied. Finally, practical aspects like alternative tightening techniques and a focus on an industrial application case are also presented.

International audience; A number of experimental studies have demonstrated that pre-tensioning fasteners significantly increases the fatigue life of shear joints. Fatigue life prediction for clamped joints is still a complex and unresolved issue because of the mixed nature of load transfer, which partly occurs by frictional forces. This article presents a calculation method based on a multiaxial-type critical-plane criterion, which determines the fatigue life and fatigue crack initiation site. For this purpose, the values of hydrostatic pressure and amplitude of maximum alternating shear stress on a critical octahedral plane are determined at each point of the structure and for each loading increment, and are used to predict the fatigue life of the joint. A dedicated experimental study was conducted within materials & processes airbus laboratory to validate the numerical results. In addition, a dimensionless parameter Rs is introduced to quantify the proportion of load transferred by friction and by bearing of the bolt, and allows the optimal preload to be approximated, above which value more clamping should not give further improvement. This value characterizes the shift of the crack location from the net section to the outside of the compressed area.