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Total Fatigue Life Assessment of Complicated Structures


OBJECTIVE: Develop innovative concepts for coupling analytical and/or numerical techniques to evaluate total fatigue life of structures involving complicated two-dimensional (2D) and three dimensional (3D) cracks of planar and non-planar shapes. DESCRIPTION: Cracks in aircraft structural components are of complex shapes and subjected to complicated load histories. Too often, crack growth analysis is carried out by oversimplifying the problem through the use of simple crack shapes and one dominant loading. This results in less rigorous stress intensity factor (SIF) values that are needed in crack growth analysis and therefore unrealistic crack growth life values. Accurate SIF values are needed for predicting reliable total life assessment of complex structures under complicated loading histories. A number of numerical techniques such as Finite Element Method (FEM), Boundary Element Method (BEM) and a few others have been used in SIF evaluation for cracks of complex shapes and geometries. The primary disadvantages of these methods are that very fine meshes are required to accurately capture the strengths of singularities near the crack tip. Additionally, when crack growth is considered, in high-gradient residual as well as load-induced stress-fields, such fine meshes have to be continuously regenerated making these methods computationally very prohibitive. These solution techniques are not suitable for mixed mode crack growth problems. Fundamentally novel and innovative methods are sought for the analyses of non-collinear mixed mode fatigue crack-growth in two-dimensions, and non-planar mixed mode crack growth in three dimensions, of arbitrarily shaped embedded and/or surface flaws in complicated geometries of thin and thick-section aerospace structural components. Analytical and/or numerical methods should be capable of generating accurate SIF values under various loading conditions involving load redistribution, mulit-element cracking, and complex residual stress-fields in an efficient way with less computational complexities. Innovative coupled analytical and/or numerical techniques may increase the overall accuracy of the SIF solution to reliably predict total life of structural components efficiently. PHASE I: Develop an innovative approach for coupling analytical and/or numerical techniques and demonstrate the feasibility of the developed technique. PHASE II: Fully develop the concept into a prototype fatigue life and crack growth methodology that can be used in conjunction with commercial finite element analysis software packages. PHASE III: Transition the developed fatigue life assessment and crack growth analysis package working with government and industry partners. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Methods and techniques developed can be folded into commercial finite element analysis packages as a module for broad use in a wide variety of industrial applications in estimating the life of a variety of safety critical structures.
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