Finite Analysis of the Cold Expansion of Aircraft Fastner Holes

Enhancing the fatigue performance of aging aircraft structures is of significant concern for military and civil operators worldwide. Inducing permanent compressive stresses in the region surrounding fastener holes, using hole cold expansion, is one such method. The beneficial effect derived from this process is entirely dependent on the magnitude and distribution of the residual stress surrounding the hole, therefore identification of accurate residual stress profiles is critical. This research focused on the development of finite element simulations of the hole cold expansion process. It evaluated the residual stress fields developed from FEA models of different levels of complexity. This included two and three dimensional uniform expansion simulations and three dimensional simulations with the expansion developed by contact with a rigid mandrel, with and without the inclusion of a lubricated sleeve. The residual compressive stress profiles were shown to vary significantly through the thickness of the workpiece and were also strongly influenced by the direction of mandrel motion. Therefore the 2D and 3D uniform expansion models were unable to accurately capture the residual stress. The inclusion of the sleeve was important when friction was introduced into the simulation as it prevented direct axial deformation from the mandrel. The use of a kinematic hardening law was required to accurately capture reverse yielding effects near the hole surface. Defence Technology Agency Auckland, New Zealand EXECUTIVE SUMMARY Background Enhancing the fatigue performance of aging aircraft structures is of significant concern for military and civil operators worldwide. Cost effective measures are required that do not lead to a significant increase in weight, the wholesale premature replacement of structural components or overly arduous inspection and maintenance intervals. Inducing permanent compressive stresses in the region surrounding fastener holes, using hole cold expansion, is one method of enhancing fatigue performance. The split-sleeve hole cold expansion process, developed by Boeing in the late 1960s, has been successfully used on aircraft structure for over thirty years. This fatigue enhancement method was included in the centre wing upgrade as part of the RNZAF C-130 Life Extension Programme. This process involves pulling an over-sized tapered mandrel, pre-fitted with a lubricated split sleeve, through a fastener hole. The combined diameter of the mandrel and sleeve is sufficiently greater than the hole diameter so as to develop a prescribed amount of plastic deformation around the hole. Upon mandrel removal the reaction of the elastically deformed material on an annulus of plastically deformed material immediately surrounding the hole creates a compressive residual stress field. In comparison to other techniques, this method develops a large, controllable residual compressive zone with high compressive stresses. Accurate assessment of the residual stress profile surrounding a cold expanded hole is critical because the magnitude and distribution of the residual stress is directly related to the fatigue performance of the hole. Residual compressive stresses have the effect of reducing the stress concentration after the application of tensile and bearing loads to the fastener hole and therefore reduce the effective stress intensity factors for cracks emanating from the hole. A number of analytical models, experimental techniques and numerical simulations have been developed to identify the residual stress field induced by the cold expansion process. Analytical models and experimental measurements of the residual stress field are limited by the fact that they are unable to predict the significant through thickness variation of residual stress. Therefore, research has focused on developing numerical simulations using finite element analysis (FEA) tools. Advancements in FEA and computing technology have meant that increasingly complex simulations can be performed effectively and efficiently. Sponsor RNZAF Director of Aeronautical Engineering (DAE) Aim The current research focuses on the development of finite element simulations of the cold expansion process. It looks to compare and quantify the residual stress fields developed as the level of complexity of the finite element model increases. The following models are included in the study: • 2D plane stress/strain simulation with uniform expansion applied to hole • 3D simulation with uniform expansion applied to hole • 3D simulation with expansion developed by contact with axially drawn rigid mandrel • 3D simulation with expansion developed by contact with axially drawn rigid mandrel and including a deformable sleeve.