TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: PMA-261, H-53 Heavy Lift Helicopters; Joint Strike Fighter

OBJECTIVE: Develop and demonstrate an innovative tool to prognosticate crack growth that integrates a next generation crack growth model with a robust finite element code.

DESCRIPTION: Current crack growth formulations are often applicable over a short growth regime and require extensive calibration with test data. The calibration parameters are dependent on the stress or R-ratio and hence have to be generated for multiple R-ratios. Newer methodologies are now being proposed that improve on these short comings. One such method is the "unified crack growth model" proposed by Vasudevan et al. They have proposed a novel two-parameter approach that can characterize crack growth from initiation to final failure. Validation of the theory with test data is the subject of ongoing research. However, the work already completed has demonstrated that for shorter intervals, which still are longer than can be characterized by conventional one-parameter models, the two-parameter model can reliably account for R-ratio and span multiple growth regimes. Since the material can be calibrated at a single R-ratio, the test burden is much smaller. The innovation that is sought in this work is to select a novel method such as the one described and integrate it with a robust finite element program. The advantage of coupling a finite element analysis with a crack growth program is that the stress intensity factors at the growing crack front can be accurately determined throughout the analysis for complicated crack shapes and under complex loadings. The crack growth parameters are functions of the stress-intensity factor at the crack tip. Thus this integration is necessary to get the model "out of the lab" by facilitating the validation of the crack growth model in real world situations. Implementing crack growth capability in any finite element has its unique numerical challenges. Commercial software vendors have started implementing progressive damage growth capabilities in their products. These efforts can be leveraged in this work, however, innovation is needed to make the programs more robust and ensure convergence with minimal user effort.

PHASE I: Demonstrate the feasibility of integrating crack growth models with finite element software and develop a plan for implementation. Leverage existing software where feasible. Provide a concept demonstration.

PHASE II: Develop the necessary algorithms and produce prototype software based on the recommended approach. Demonstrate use of the prototype tools through creation of an analytical model of a selected structural component that has undergone prior testing. Verify that the prototype software provides appropriate results by correlating available test data on the selected component. Compare the analytical results with results from other modeling approaches.

PHASE III: Implement the validated algorithms and method in a released version of the software. Apply this analysis tool to structural analysis applications on aircraft program structural improvement and development efforts.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: This crack growth algorithm and method of solution, as implemented in the analysis software, will be directly applicable to all commercial aerospace developers.

REFERENCES:
1. Vasudevan AK, Sadananda K, Crack-tip driving forces and crack growth representation under fatigue, Int. J Fatigue, 2004, (26), pp 39-47.

2. Nooroozi, AH, Glinka, G, and Lambert S, A study of the stress ratio effects on fatigue crack growth using the unified two-parameter fatigue crack growth driving force, Int j Fatigue, 2007, (29), pp 1616-1633.

3. Wu, Z., Glinka, G., and Jakubczak, Calculation of Stress Intensity Factors for Cracks in Structural and Mechanical Components subjected to complex stress fields, J. ASTM International, 2004, (1), no. 9, pp 23-32.

KEYWORDS: Fracture; Crack Growth; Fatigue; Finite Element Analysis; FEM; Life Prediction

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