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Analytical Tool for Design and Repair of Engine Hardware for Robust High Cycle Fatigue Performance

Description:

 
 

TECHNOLOGY AREA(S): Air Platform, Materials/Processes

ACQUISITION PROGRAM: JSF, Joint Strike Fighter

OBJECTIVE: Develop a robust analytical tool for the design and repair of high cycle fatigue (HCF)-resistant integrally bladed rotor (IBR)/blisk airfoils.

DESCRIPTION: Premature failure due to high cycle fatigue or in-service damage has plagued Naval Aviation throughout its history. This issue, for some aircraft propulsion systems, has resulted in a limited mission capability. Good design, inspection, and maintenance practices have mitigated most safety-related risk from HCF, but maintenance costs and asset readiness are still a significant issue for aircraft engine operators. An analytical tool that can accurately portray the risk of current hardware and estimate the benefit of available surface treatments would allow the recent trend of reduced sustainment costs to continue.

Over the past decade, sustainment cost reductions have resulted from the application of compressive residual stress through surface treatments (e.g. shot peening, laser shock peening, low plasticity burnishing etc.) to compressor airfoils to improve HCF performance and tolerance to in-service damage. Generally, these “upgraded” airfoils were developed using simplified, deterministic analytical tools (or models), and components were qualified through extensive fatigue testing. Having a sophisticated analytical tool that is able to incorporate the residual stress fields of available surface treatments while incorporating probabilistic design for a more robust HCF design or for repair optimization is necessary to lower the sustainment costs and maximize the service life of IBRs/blisks.

Because probabilistic design systems are relatively new, their application to airfoils for increased HCF resistance is not accounted for. For example, much of the engineering and qualification of blade improvements, which relies on compressive residual stress imparted by surface treatments, has been deterministic (i.e. empirical, time and cost intensive). A more representative and efficient process for design of HCF resistant airfoils and airfoil repairs is needed, particularly given the cost savings that would result from extending the service life of IBRs/blisks. The design process for HCF-resistant airfoils and robust airfoil repairs should optimize deterministic surface treatment modeling while incorporating probabilistic design systems.

HCF resistance is more critical at this time because the HCF issue is more complex in the integrally bladed rotors (IBR)/blisk configuration found in propulsion systems as opposed to its bladed disk predecessor. The bladed disk configuration benefits from increased airfoil damping from the dovetail joint. However, the welded joint of an IBR/blisk configuration allows aeromechanical “cross-talk” between airfoils which may allow an airfoil that is not excited by its environment to become excited due to response of an adjacent airfoil. The welded joint of an IBR/blisk also nullifies the ability to remove blades following in-service damage which necessitates aggressive airfoil repairs to minimize scrapping of these costly, integral components.

HCF limits are reflected in engine design analysis using engineering models that are calibrated to coupon test data, and HCF performance is validated and certified through component and full-scale tests. Original engine manufacturer (OEM) design practices reflect variability in factors that drive HCF failures (like operational loads, usage cycles, and in-service damage) and in factors that provide HCF resistance (like strength, microstructure, and surface quality). The aforementioned sources of variability are robustly included in probabilistic design systems. As for airfoil repairs, or blends, they leave a given IBR/blisk in a unique condition relative to the design basis that does not benefit from the design practice of a pristine airfoil.

The envisioned analytical tool would be able to deterministically and accurately incorporate the residual stress profile (including the equilibrating tensile residual stress) as a function of depth in the airfoil post surface treatment while incorporating the probabilistic variability innate to airfoils (e.g. manufacturing, material, in-service damage, blend accuracy etc.) to produce a bounded distribution of benefits in the form of a failure rate. The failure rate enhancement due to a surface treatment should be apparent. The application of the analytical tool would be during the design process or as-needed in response to an in-service repair. At this time it is not necessary to model the surface treatment process but rather the residual stress output of any of these processes. Potentially there is opportunity to model the process for an all-inclusive analytical tool which would require an industrial partnership. The analytical tool should produce accuracies within 10% of the actual stress and maintain, or better, the mesh size of the parent finite element method (FEM) model. The analytical tool should leverage, where applicable, modern computer aided design and analysis software available in the market. Residual stress relaxation through loading or thermal effects shall also be considered.

A few applications that an analytical tool will facilitate:

 

- Component life assessment

 

- HCF design or surface treatment definition following a service-revealed HCF deficiency

 

- Airfoil serviceable limit expansion

 

- Airfoil repairable limit expansion

Collaboration with a major engine OEM is highly recommended, but not required.

PHASE I: Determine project feasibility and develop an analytical tool for the design of HCF resistant IBR/blisk airfoils. The analytical tool should be able deterministically model a depth-wise residual stress profile resulting from available surface treatments within the component's current FEM model. Demonstrate that the analytical tool can accurately portray the subsurface residual stress profile of a component with a surface treatment. The residual stress of that component should be confirmed with a conventional measurement technique (such as x-ray diffraction). Develop the probabilistic elements such as an understanding of geometric, material, processing, and airfoil damage variation, and a means to capture variation in an integrated design process. Demonstration that the probabilistic elements are understood should be provided but need not be integrated with the deterministic, analytical tool at this time.

PHASE II: Develop and validate an integrated deterministic/probabilistic analytical tool that can be adopted during the design process for HCF-resistant IBR/blisk airfoils and airfoil repairs. Demonstrate the architecture of the analytical tool and how it will be integrated into the design process of new or legacy IBR/blisk airfoils. Develop data and experience that show the degree to which the new design process would save sustainment costs. Demonstrate full deterministic/probabilistic analytical tool capabilities.

PHASE III DUAL USE APPLICATIONS: Finalize technology and assist the Navy in integrating the probabilistic design capability for HCF resistant IBR/blisk airfoils and airfoil repairs. Develop airfoil repair services, potentially in coordination with the OEM, that rely on the probabilistic design capability. Private Sector Commercial Potential: Emerging commercial aviation fleets have also committed to the use of integrally bladed disks in their compression systems. The analytical tool developed under this effort is directly applicable as an element of the design process of commercial turbine engines.

Residual stress is an industry-wide phenomenon that has attention in applications such as -- but certainly not limited to -- railways, bridges and gun barrels as well as processes such as welding, heat treatment and cold work.

REFERENCES:

  • Prevey, P., et al. (2002). Improved Damage Tolerance in Titanium Alloy Fan Blades with Low Plasticity Burnishing. International Surface Engineering Conference, Oct 7-10, Columbus, OH
  • DeWald, A. T., & Hill, M. R., (2009). Eigenstrain-Based Model for Prediction of Laser Peening Residual Stresses in Arbitrary Three-Dimensional Bodies Part 1: Model Description. The Journal of Strain Analysis for Engineering Design 44.1 1-11. doi: 10.1243/03093247JSA420
  • Coratella, S., et al. (2015). Application Of The Eigenstrain Approach To Predict The Residual Stress Distribution In Laser Shock Peened AA7050-T7451 Samples. Surface and Coatings Technology 273 39-49. http://dx.doi.org/10.1016/j.surfcoat.2015.03.026

KEYWORDS: Model; Residual Stress; high cycle fatigue; probabilistic design; in-service damage; integrally bladed rotor

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