Damage Tolerance Analysis Of A CrackedAttachment Lug Using BEM

Abstract

A detailed 3D analysis using BEM (Bounday Element Method) was performed to determine the stress distribution and SIFs of a corner crack of the fuselage to wing attachment lug of a medium range military transport aircraft.The contact pin-lug has been studied with non-linear contact analysis.Based on this study a crack growth prediction has been performed with a proper analytical model. For the above prediction, both constant and variable amplitude loads were considered. An experimental correlation has been made for both crack growth and final fracture of the component. As a result of the present work it can be concluded the industrial feasibility of fracture mechanics analyses using BEM. In fact a good agreement has been reached with the available experimental results. Also it can be concluded that the interference-fit pin installation causes a significative decrease in the stress intensity factor range and hence improvements in the fatigue crack growth life. The elasto-plastic CORPUS model will be used to predict crack growth evolution. Nomenclature : a crack depth length \i shear modulus C surface crack length V Poisson's ratio DLL design limit load Q2 Bcasy linear order element DK S.I.F. range Q38 Bcasy reduced quadratic order element E Young's modulus SIF stress intensity factor KI mode stress intensity factor DISCUSSION Structural fatigue often initiates in areas of high stress concentration caused by the bearing load of pins. The above causes a small crack initiation time and crack growth life. The main purpose of this report is to validate the Fracture Mechanics tool proposed by ALENIA. The method uses BEM for SIFs evaluation and an elasto-plastic crack propagation Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 562 Localized Damage model for the life prediction under spectra and costant amplitude loads. A threedimensional BEM analysis using BEASY code, an engineering analysis system based on the Boundary Element Method has been performed in order to determine the stress distribution in the selected component in presence of a crack and under contact conditions. Even if a number of studies and analytical results are available in literature, the capability of a full BEM analysis gives more understanding results. Hereafter the contents, method and the results of the analysis are described. The component selected for the analysis is the fuselage to wing attachment of a medium range military transport aircraft for which results are known from an experimental test program. LUG COMPONENT DESCRIPTION Fig.l shows the detailed geometry of the component. The lug is made of 7075-T73511 forged aluminum alloy for which Young modulus is 71 GN/m^ and Poisson ratio is 0.3, with a hole diameter of 25.5 mm. The pin and the bushing are both of 155-PH steel with Young modulus equal to 210 GN/m^ and Poisson ratio to .33. The lug component has been tested with an initial corner flaw of 1.27x1.27mm under a variable amplitude spectrum coming from in service monitoring. The same test article was also tested with constant amplitude load cycles. The test apparatus used consists, mainly, of a rigid beam fixed on the ground on which both the test article and the hydraulic jack have been installed. The upper end of the test article has been constrained through fasteners to a strong back while the lug lower end has been joined to the stroke of the hydraulic jack. BOUNDARY ELEMENT MODELS BEM is an integral differential equation method based on the formulation reported below : N U'ltlS. in which I are segments in which boundary S is subdivided and C% is a costant linked to the source point, while u and t are displacements and tractions vectors. BEASY is an engineering analysis system based on this method. The surface description of the model was realized directly in BEASY-IMS, the interactive modeling environment of BEASY software, and post-processing ensures an easy way to visualize results, in particular stress intensity factors. The model was divided into three distinct zones or substructures:the main body of the lug, the lug upper side and the pin. This feature makes easier crack tip modeling as well as full model handling (see fig. 2). Different models have been realized with various crack sizes.In Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533