Residual Stress Measurements Program to Support Condition Based Maintenance (CBM) of Critical Rotating Components of Propulsion Systems

The fatigue properties of metals used in modern turbine engines are improved by cold working processes, such as the various forms of peening. Peening imparts compressive residual stresses (RS), which inhibit, or prevent, the initiation and propagation of cracks in critical locations of turbine engine components. The problem is that the RS state of critical rotating components, whether peened or not, is seldom known. This is complicated by the fact that RS relaxes with operational usage; thus, later in the component’s life cycle it becomes possible for the RS to become tensile—a potentially dangerous condition for critical components. RS is a key factor that must be fully understood to ensure the requisite fatigue life. Currently, NAVAIR has no means of reliably and nondestructively measuring RS in the tight locations of critical rotating components. Proto has considerable experience in measuring RS in numerous components of several USAF engines, including the F-22 engine, and is recognized as the world’s leader in x-ray diffraction RS technology. Proto proposes to prove the feasibility and demonstrate its robust capability to perform RS measurements in the lab as well as the depot environment of FRC-SE. This proof-of-concept will be performed on a current fighter engine. Proto plans to develop and demonstrate a database management system, tailored for RS data, to archive, analyze and develop RS trends for input to fatigue life prediction. Proto further proposes to conduct laboratory testing and analyses to develop the RS/fatigue relationship for use in prognostics or life prediction in Phase II. The RS measuring capability proposed by Proto will contribute directly to Condition Based Maintenance by enabling direct measurements and supporting quantitative decision making in materiel disposition matters, such as to accept, rework or reject, for these high value engine components.