The introduction of a beneficial compressive residual stress field around fastener holes to mitigate fatigue problems is commonly referred to as “cold working”. Among several technological processes developed over the last 40 years to achieve an effective compressive stress field, the “Split Sleeve Cold ExpansionTM “ process, invented at the Boeing Company and marketed by Fatigue Technology Inc. (FTI), is, at the moment, accepted as a standard practice for new aircraft production and aging aircraft refurbishment. Split Sleeve Cold Expansion is accomplished by pulling a tapered mandrel, pre-fitted with a lubricated split sleeve, through a hole. After a final reaming, the desired annular zone of residual compressive stresses is obtained.
Despite the widespread in service application of this technique, the positive effect of the process is not yet accounted for in structural design. Therefore, a large research program was developed in the framework of a cooperation between the Department of Aerospace Engineering at University of Pisa and EnginSoft S.p.A. Florence to obtain a quantitative assessment of the residual stress field that can be introduced in a correct fatigue life calculation for both crack initiation and crack propagation.
Actually, Finite Element simulations of the Split Sleeve Cold Expansion process using a general “kinematic hardening” material model give rise to a non conservative compressive residual stress field around the hole, inconsistent with experimental measurements.
Numerical difficulties are mainly connected with the highly positive and negative strains recorded in the material surrounding the hole during the cold working process. So a complete tensile-compressive stress-strain curve of the aeronautical aluminium alloy 7075-T73 was experimentally obtained: the material follows a generic monotonic s-e curve, but exhibits a remarkable “Bauschinger” effect during the unloading phase. The optimization code ModeFRONTIER was consequently applied in order to better describe the actual material behavior: a valuable optimized ANSYS Chaboche material model was achieved by superimposition of five kinematic hardening models.
The three dimensional nature of the technology was taken into account by considering only 3-D complete ANSYS FEM models. Despite more time consuming, the direct modeling of the mandrel, its drawing across the hole and the presence of the lubricated split sleeve highlight some peculiar effects of the process, experimentally observed. Moreover, the results of fatigue and propagation tests performed on “dog-bone” specimens, thickness 2.3 mm, with a central cold worked hole, diameter 4.83 mm, were consistent with FEM distributions of residual stresses, thus showing the effectiveness of FEM simulation in process design. |