SBIR-STTR Award

There is a need for a non-destructive inspection technique and methodology for tracking low cycle fatigue (LCF) life in aircraft engine disks to safely extend useful like and reduce maintenance cost. Recognizing that cracks occur only after the surface residual stress is in tension, one can correlate the observed residual stress with accumulated cycles and crack appearance. By measuring the residual stress, parts may be safely considered for service beyond the current "Safe Life" arbitrary criteria, and disk sets of similar residual stress can be matched for fewer scheduled maintenance inspections. FMA's NDT methodology wold quantify the expected life prior to any observed cracking, and thus yield a more economical regimen; more conservative that "Retirement for Cause", but with similar large benefits. The proposed Phase I effort will establish an initial statistical disk population data base and analysis in order to demonstrate the feasibility of the methodology to determine the probability and time to crack initiation. This will be accomplished by measuring surface residual stress via X-ray diffraction.

Keywords:
Low Cycle Fatigue Aircrasft Engines Turbine Disks

The primary objective of this Phase II project is the development of a production nondestructive test (NDT) for use as a quality control tool for newly manufactured shot peened disks, and in-service disks. The method involves residual stress measurements using X-ray diffraction. In a cooperative program with prospective users (United Airlines) and an engine manufacturer (Allison Engine Company), an initial statistical database of measurements will be generated for both new and used disks. A laboratory fatigue testing program to explicitly determine the relation between residual stress and LCF on sample specimens will be conducted by Allison. The disk and specimen data will be reviewed and statistically analyzed to provide a foundation and a process for determining the average life curve of engine disks. Identifying in-service disks that have residual stresses, or insufficient compressive stresses to endure another operating interval, would be an immediate benefit of this technology and could be started during the program. Additionally, its use as a quality control tool for new disks is a short-term reality. Eventually the process could be extended to tracking the aging process of disks and, in conjunction with conventional NDT, safely extend life and reduce life cycle costs.

Keywords:
RESIDUAL STRESS LOW CYCLE FATIGUE AIRCRAFT ENGINE DISKS