Hot-Section Dynamic Rotating Strain

Dynamic stress measurements on turbine blades are needed to substantiate the blade design life and to monitor or investigate field applications. The strain gages are installed on various areas of the blade root and airfoil depending on the vibratory mode of interest. The gages are often positioned to detect more than one vibratory mode using stress ratio information. The strain gages are installed on turbine airfoils, both with and without thermal barrier coatings, and on the pressure and suction side of the airfoils. The most widely used technique is wire wound strain gages, which are installed used flamespray.

Strategic Advisory Board (SAB) Members Addressing This Need

Aerodyn Engineering, Inc.

Cleveland Electric Laboratories Company, Inc.

Current Sensor Needs

The strain gages and leadwires must be able to survive very high temperature, centrifugal loading, and erosive gas path impingement. The overall thickness of the flamespray must be minimized to reduce cooling air disruptions on film cooled airfoils and to minimize the affects of vibratory damping on very small airfoils. With the improvement in turbine blade materials and coatings, the airfoil surface temperature has increased, which has had a detrimental affect on the strain gage life. Turbine strain gage life has become marginally acceptable, especially on the pressure side of the airfoil with strain gages located radially outward from the blade platform. Nickel alloy strain gages suffer severe oxidation and Platinum Tungsten gages have very large gage factor and resistance changes as temperature increases. Flamespray delamination often occurs, as the surface temperatures exceed 1800F - 2000F.

Future applications need dynamic strain gages that can survive for 10-20 hours at surface temperatures up to 2100F - 2200F. The installation materials need to provide insulation to ground of 1 Megohm @ 2000F with an overall thickness of less than 0.017 inch.

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