Non-intrusive Stress Measurement vs. Strain Gauges
In the turbine industry, blade vibration information is critical for validating engine designs and assessing engine health. Two methods for measuring blade vibration in turbo-machinery are strain gauge instrumentation and a non-intrusive stress measurement system (NSMS).
A strain gauge is an electrical conductor that experiences changes in resistance as it is strained. While strain gauges have traditionally been the standard method of obtaining blade vibration data, there are several limitations to this method. The attachment of strain gauges and routing wires directly to the blade can alter vibratory properties. The number of applied strain gauges must be restricted to limit the amount of influence on the vibration. As strain gauges can only deliver data for the blades to which they are mounted, this is a considerable drawback.
The Non-intrusive Stress Measurement System (NSMS), also often referred to as Blade Tip Timing (BTT), uses case-mounted probes to measure blade arrival times and calculate blade deflections. Given that NSMS probes are externally mounted on the engine case, there is minimal, if any, effect on blade vibratory characteristics.
In contrast to strain gauges, a non-intrusive stress measurement system measures every blade on every rotation for every probe. This provides valuable information on system behavior of a vibratory response that is not possible with a strain gauge, including full-spectrum nodal diameter content, mistuning, and insight into complex high-cycle fatigue (HCF) phenomena.
Instrumentation failure and the mortality rate of strain gauges are relatively high, particularly as technology advancements increase the speed and temperature of turbine engines. The robust quality of NSMS probes results in extended test periods and enables the use of this reliable technology for long-term engine health monitoring.
It is important to note that the fixed synchronous sample rate inherent to NSMS requires unique data analysis techniques. Our industry-leading Agilis experts have developed a suite of software for detailed analysis of NSMS blade vibration data. Over the last decade, Agilis c360® Vibration Intelligence Software has proven to be a reliable resource for the world’s premier engine manufacturers and operators.
While strain gauge applications are the conventional approach to obtaining blade vibration data, non-intrusive systems provide complete characterization of blade behavior when viewed with the sophisticated c360 analysis software. For more information about Agilis Measurement Systems or blade vibration analysis with our c360 Software, contact us at infomeasurement@agilis.com.
c360® is a registered trademark of Agilis Measurement Systems, Inc.