AFRL Completes Dual-Use Initiative for Nonintrusive Stress Measurement
By Plans and Programs Directorate, AFRL/XP
/ Published December 12, 2006
WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL, in partnership with Williams International and Hood Technology Corporation, managed a Dual-Use Science and Technology initiative to advance Nonintrusive Stress Measurement System (NSMS) technology for small turbine engines. Exploiting initial NSMS research conducted by the Arnold Engineering Development Center, the team developed and demonstrated a suitable NSMS for use in current and future generations of small gas turbine engines. The team used the high-pressure compressor of the XTL-87 Joint Expendable Turbine Engine Concept technology demonstrator engine to baseline system design requirements for the effort.
The primary goal was to apply a version of Hood Technology's successful large-engine turbine blade measurement system to Williams International's smaller gas turbine engines. Hood expanded the system's sensor capability by decreasing the sensor's size, improving its resolution, and expanding its operational temperature range, enabling the measurement system to detect the position of extremely small airfoils at the high shaft speeds and discharge temperatures typical for small turbine engines. The resulting small-engine NSMS can detect damaging resonance conditions and provide accurate data related to blade stress.
The system uses case-mounted laser probes to measure a blade's time of arrival to the probe location. Engineers compare the recorded arrival time to a known disk rotational position to infer the blade deflection. They can then relate the deflection to a blade's vibration mode shape and its associated stress. AFRL's secondary program objective was to develop a blade stress measurement system with a service life longer than that of the surface-mounted strain gage and slip ring system currently used in turbine engine blade measurements. A key aspect of this NSMS capability is that it may be capable of providing data throughout the entire service life of a typical unmanned air vehicle application.
Engineers conducted a number of rig and engine tests to characterize the performance of the small-engine NSMS, including consecutive tests of the strain gages, slip rings, and optical probes. With this technology now sufficiently proven, Williams International includes it as a standard practice for the development and certification of both military and commercial engines, with or without parallel strain gage corroboration.