SBIR-STTR Award

The Air Force has been placing increased emphasis on probabilistic methods for predictions of design reliability of fracture critical engine components, including metallic turbine engine blades and disks. Current state-of-the-practice for these methods typically include a significant amount of conservatism in crack initiation and fatigue crack growth and inspection design criteria due to uncertainties in material properties, fatigue performance, crack growth analysis, stress analysis, residual stresses, damage mechanisms, and nondestructive inspection, among others. We propose to develop and demonstrate a new probabilistic life prediction methodology that will significantly reduce uncertainty and conservatism by employing an accurate mechanics based crack growth analysis. We will combine an existing high fidelity 3D crack growth simulator (FRANC3D) with an existing probabilistic life prediction code (DARWIN). Both codes are recognized as being the most mature and the most capable codes in their areas of specialization (high fidelity crack modeling and probabilistic life prediction , respectively). The new methodology is expected to reduce conservatism in probabilistic life predictions, thus extending component lives or inspection intervals. The proposed effort includes the involvement of a major engine OEM.

Benefit:
Current probabilistic methodologies for setting fatigue lives and inspection intervals for metallic engine components include a significant amount of conservatism due to uncertainties in the, among other things, crack growth analysis. The proposed effort will combine a high fidelity crack growth simulator (FRANC3D) with a probabilistic fatigue life calculator (DARWIN). The resulting tool and methodology is expected to reduce conservatism in probabilistic life predictions, thus increasing the predicted mean time to failure. For a constant relative probability of failure this will extend the allowable component life and inspection intervals. Extending component fatigue lives and inspection intervals will yield significant costs saving over the lifetime of the engine. The resulting methodology can be used in non-engine applications such as airframes, land and sea based turbines, and terrestrial vehicles.

Keywords:
Fatigue, Fracture Mechanics, Probability, Crack Growth, Life Prediction, Engine Components

Over the past few years, the Air Force has increased the emphasis on probabilistic methods for design reliability predictions of fracture critical engine components, including metallic turbine engine blades and disks. A significant shortcoming and potential source of conservatism of most current life prediction tools and methodologies is the reliance on stress intensity factor values obtained from highly idealized component and crack geometries. In Phase I, we demonstrated that this shortcoming could be overcome by making an existing high fidelity 3D crack growth simulator work together with an existing probabilistic life prediction code. The resulting prototype software tool was used to perform a probabilistic life prediction for a geometrically complex engine component. For Phase II, we propose to develop the prototype software into a tool suitable for routine production use. This will include creating a unified graphical user interface, adding additional features, performing sensitivity studies that will assess accuracy/efficiency tradeoffs and develop best practices, and performing a series of analyses that include advanced life prediction topics.

Benefit:
The successful completion of Phase II will result in a high fidelity probabilistic fatigue life prediction tool for metallic turbine engine components. Such a tool will allow engine manufacturers to reduce uncertainty and conservatism in fatigue life assessments. This means that for current engine designs, component lives or inspection intervals can be extended with no increased risk of failure. For new engine designs, the tool can be used to help find the optimal point among performance, efficiency, and life cycle cost. The tool can be used by the government to perform high accuracy fracture risk assessments independent of the manufacturers. The tool can also be applied to other applications with fracture critical components, such as airframes and power generation turbines, among others.

Keywords:
Fatigue, Fracture Mechanics, Probability, Crack Growth, Life Prediction, Engine Components