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Advanced Non-Destructive System to Characterize Subsurface Residual Stresses in Turbo-machinery Components


Compressive surface treatments are frequently used in turbo-machine components to add a factor of safety to their component life. The residual stress (RS) profile that is imparted to metallic components can vary by application, service use, time, and environment. The US Navy is interested in non-destructively measuring the subsurface residual stress field in metallic engine components, specifically in titanium and Inconel® alloys. The current industry standard for measuring RS is x-ray diffraction (XRD) which is limited to measuring surface stresses. In order to measure subsurface stresses with XRD, the component must be destructively evaluated. Because of this, a subsurface stress for a production component is unable to be confirmed except for the few candidates used for quality assurance. Even when these candidates are analyzed, XRD-measured RS does not correlate well with the design model showing the difficulty in modeling subsurface stress. Extensive measurements of surface RS have been performed during recent years confirming that surface RS relaxes at critical locations of engine components with operational usage. These components are designed to initially have compressive RS at key locations; however, as the RS relaxes with usage, it may approach a tensile condition and the component no longer benefits from the intended factor of safety. An NDI system that provides quantitative, subsurface measurements of RS at critical locations of turbo-machinery components of the propulsion system is sought. Such technology will provide the ability to implement subsurface stresses to a design model, confirm that the design intent was met, as well as offer the ability to actively monitor the life remaining for a component due to operational stress relaxation. Critical components of concern include fans, disks, and blisks/integrally bladed rotors (IBR). Critical locations tend to be inside surfaces of bolt-holes, bores, slots and fillet radii. In order to be a valuable design tool, the new NDI device must be capable of the following: • Resolving stress within 10% of actual stress value (emphasis will be placed on method of validation); • Resolving stress location within 10% of modeled local mesh size; • Discerning stress values in each element of the model used to life the component; • Results for stress magnitude and location should be repeatable to less than 10%, using ASTM F1469 as a guideline; • Through component measurement for typical gas turbine engine components is desired. The ability to measure residual stress at depths customary to advanced surface treatments (~0.150 inches) is necessary. Components can be moved or manipulated to achieve this requirement; • Measuring Titanium- or Inconel®-based alloys common to cold section components with ability to expand to other materials; • Producing a favorable return on investment; • And be safe for the user. Attributes such as time per measurement, measurement environment and system size should be addressed. Time per measurement and system size should be minimized as much as possible while the measurement environment should be practical (e.g., humidity, component cleanliness, etc.) The ability to quantify the percentage of each grain orientation (e.g., 100, 111 planes) for cubic and hexagonal structures in a specific volume and discern the stress of a specific orientation is not required but would be encouraged. The desire for this solicitation is to develop a technology with the ability to measure subsurface stresses for use in component design, and the ability to be developed into a manufacturing quality control tool as well as a portable, field inspection device. Establishing a working relationship with relevant original engine manufacturer(s) (OEM), while not required, will greatly enhance probability of success. PHASE I: Demonstrate feasibility and proof-of-concept of proposed NDI system capable of quantitatively, nondestructively and reliably measuring and tracking surface and subsurface RS at critical locations of turbo-machinery components. Provide preliminary design for a system. PHASE II: Develop, produce, validate and implement a robust and rugged NDI RS measurement prototype system based on the results of Phase I. The prototype should be capable of obtaining the necessary subsurface RS data nondestructively and tracking it for comparative analyses during the life cycle of each individual component. Integrate the system to develop life management methodology and validate life management algorithms for application to be used on engine components. Provide detailed design for a system. Perform a demonstration of the developed NDI system. PHASE III: Mature the system for field use by making the system robust, rugged and ensuring ease of use for the operator in both Navy and commercial applications. Perform any final testing and commercialize and transition the technology for field and Original Equipment Manufacturer usage.
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