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Robust Methods for the Measurement of Bulk Residual Stress

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OBJECTIVE: Develop robust methods to measure bulk residual stresses in complex aerospace structural components. DESCRIPTION: Novel methods are sought for reliable bulk residual stress measurement in metallic aerospace components. These methods are desired to support the use of residual stresses for engineering benefit to include enhanced fatigue performance, stress corrosion cracking resistance, and/or enhanced damage tolerance. Aggressive performance and weight objectives are driving aircraft and engine manufacturers toward the use of advanced materials and structural concepts that may have inherent, process-induced residual stresses in localized, but critical areas. Certification of these components and structures will require that the influence of these residual stresses be properly accounted for during design. For example, the unitization of lugs and fittings with primary spars and bulkheads in airframe structure is being done in order to reduce part count, which, in turn, reduces the necessity for large numbers of fasteners and the associated hole preparation/mating requirements. Such unitization can be achieved through the use of large forgings which often have significant residual stresses in localized areas even after final machining. Since the primary structural elements for man-rated flight vehicles are typically designed based on damage tolerance concepts, this requires that fatigue crack growth analyses address the influence of residual stress on crack growth. Thus, robust methods to measure reliably the bulk residual stress will greatly facilitate the design and manufacture of unitized airframe structures. Similarly, engine designers are now considering the use of dual microstructure turbine disks to obtain optimum local properties while minimizing component weight. Hence, robust measurement techniques will benefit both aircraft engine and airframe components. Several methods exist for determination of surface and near surface residual stresses, including, but not limited to, x-ray diffraction, hole drilling, slitting, and speckle pattern interferometry. These methods are sufficient for near surface applications, but are insufficient for the determination of stress states throughout large components or bulk materials. Methods with enhanced capability for the determination of residual stresses at significant depths are available (e.g., neutron and high-energy x-ray diffraction techniques), but these typically require access to specialized physics facilities which greatly restricts their viability in a production environment. Desirable attributes of proposed techniques include: 1. Being capable of measuring bulk residual stresses in a wide range of aerospace structural alloys, especially Al and Ti airframe alloys (e.g., AA7085, AA 7050, AA 2024, Ti-6Al-4V). 2. Being capable of providing accurate measurements in challenging metallic microstructural environments to include large grains and high levels of crystallographic texture. 3. Having the ability to measure high residual stresses (i.e., stress values>50% of yield strength). 4. Having the ability to measure residual stresses in a high residual stress gradient; a rationale for estimating measurement error should be proposed. 6. Being capable of operating reliably in a production environment. 7. Nondestructive methods are desirable but not mandatory. In addition, the physics/mechanics underpinning the proposed method should be well understood and methods with some previous experimental proof of concept are favored. PHASE I: Demonstrate a prototype bulk residual stress measurement capability in a laboratory environment using a blind study to validate the method. Government furnished specimens will not be provided. With assistance from the TPOC verify relevance and viability of the approach with perspective users. Particular attention should be given in the proposal to the validation protocol of the technique. PHASE II: Develop and construct a fully functional demonstration system capable of performing surface residual stress analysis for a representative aerospace component such as an aluminum airframe component. With assistance from the TPOC demonstrate the capability for at least one relevant application with at least one prospective end-user. PHASE III: Military Application: The technology developed is applicable to the design of lighter weight, unitized airframe structure. Commercial Application: Commercial airliners and military transports have similar airframes; thus, the technology is applicable to the design of lighter airframe structure. REFERENCES: 1. J. Lu (Ed.), Handbook of Measurement of Residual Stresses, Prentice Hall PTR, Englewood Cliffs NJ, 1996. 2. M.B. Prime,"Cross-Sectional Mapping of Residual Stresses by Measuring the Surface Contour After a Cut,"Journal of Engineering Materials and Technology, 123, 162-168, 2001. 3. M.B. Prime, R.J. Sebring, J.M. Edwards, D.J. Hughes, and P.J. Webster,"Laser Surface Contouring and Spline Data-Smoothing for Residual-Stress Measurement,"Experimental Mechanics, 44(2), 176-184, 2004. 4. M.B. Prime,"Residual Stress Measurement by Successive Extension of a Slot: The Crack Compliance Method,"Applied Mechanics Reviews, 52, 75-96, 1999.
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