Verified and Validated Software for Three-dimensional (3-D) Fatigue Crack Growth and Stable Crack Extension

Development of a software tool capable of predicting crack growth in structures with complex geometries for accurate structural life predictions and residual strength assessment is proposed. The custom crack growth simulation tool, CRACK3D, will be used as the basic code for further development and implementation. First, further enhancement of the CRACK3D module for computational geometry and topology will be performed so that CRACK3D can handle most of geometric features present in modern aircraft structures. Second, CRACK3D will be modified to contain built-in advanced computing techniques, including high performance computing FORTRAN/C++ coding, parallel computation option for running on distributed memory system and shared memory systems. Third, the three-dimensional virtual crack closure technique, 3D VCCT, will be used to calculate stress intensity factors for complex crack geometries under arbitrary mixed mode loading conditions. Fourth, the alternate superposition-based method will be used to take into account the effect of residual stress on fatigue crack growth rates. This method is simple and relatively accurate. A crack closure based approach can also be implemented for improving the accuracy of the alternate superposition-based method if accuracy becomes a concern. Finally, experimental validation of the capabilities will be performed using either existing literature or simple coupon experiments. BENEFIT: Successful completion of the proposed effort provides the foundation for conversion of CRACK3D into a form that would provide AF personnel with an efficient and effective simulation tool for assessing selected flaw criticality. Specifically, the following results are anticipated. (1)A finite element formulation of VCCT with 10-noded tetrahedral elements, implemented in CRACK3D, for simulating three-dimensional crack growth using unstructured meshes. (2)A program module for re-meshing local regions around critical crack fronts and containing internal material boundaries, for modeling crack growth in multi-material structures. (3)A program module of computational geometry and topology for updating structural geometry and topology due to crack growth in structures with arbitrary, complex geometries.(4) A theoretical model and simulation methodology of fatigue crack growth in structures under mixed mode loading conditions, implemented in CRACK3D, for prediction of fatigue crack path under general loading conditions, and (5) An enhanced software package, CRACK3D, for calculation of stress intensity factors under mixed mode loading conditions and fatigue crack growth simulation, in addition to simulation of stable tearing crack growth. Taken together, completion of these developments provides the foundation for future Phase II efforts to make the 3D fracture analysis methodology both effective and relatively easy to use. The modified version of the software package CRACK3D is intended for commercialization at the completion of the Phase II effort. To achieve this goal, various strategies will be used to publicize CRACK3D and to demonstrate its capabilities and applications. Included in this effort will be development of a relatively easy-to-use interface to allow users to utilize the power of the methodology with minimal FE meshing effort.