Hot Section Dynamic Strain Measurements
Dynamic strain measurements on gas turbine engine turbine components, especially airfoils are needed to substantiate the design life and to monitor or investigate issues such as HCF and other problems identified in the field. Thus resistive strain gages are installed on various engine structures in particular on gas turbine engine airfoils. These strain gages are then used in test to evaluate vibratory modes of interest; looking at both amplitude and frequency. The strain gages are often positioned to detect more than one vibratory mode (engine orders) using stress ratio information. Strain gages have seen wide application on fan, compressor, and turbine airfoils to make sure design intent is achieved, excessive strain levels are avoided, and margin of operational safety is assured.
Frequencies and amplitudes are both measurements of interest from the strain gages. Dependent on mode shapes and blade design the strain gages can be applied to both the pressure and suction side of the rotating and stationary airfoils. Dynamic strain measurements are then used in Goodman diagram analyses of total strain on a particular component to estimate fatigue life.
Up to about 700F (370C), foil backed commercially available strain gages are applied with epoxies that provide a minimal adhesive line and conform to the airfoil surface contour. The strain gages alloy selection is made depending on strain field, temperature, expected strain level, fatigue life, and wiring restrictions. The application epoxies are selected based primarily on temperature capability. For strain measurements greater than 700F (370C) found in gas turbine engine hot section application both flame sprayed aluminium oxide and high purity ceramic cements are used. The most widely used technique is wire wound strain gages, which are installed with flame sprayed high purity aluminium oxide.
Alternative technologies for dynamic strain measurements in gas turbine engines has included thin film deposited strain gages and more recently fiber Bragg gratings as well as Fabry Perot interferometers. Thin film dynamic strain gages are fabricated directly on the hardware using sputtered vacuum chambers and have dielectric as well as metallization material limitations. Thin film resistive strain gages have found reasonable acceptance within the OEM community for compressor airfoil test and evaluation.
Current Sensor Needs
Most importantly, the strain gage installations and associated leadwires must be able to survive the high temperatures found in gas turbine engines, handle the CTE mismatch, adhere under high centrifugal loading, and survive severe erosive and oxidizing gas path conditions. The overall thickness of the strain gage installation must be minimized. For epoxy installations .010” to .012” is expected. In contrast the flamespray installation is generally slightly thicker to .013”-.015”. Ultimately the thickness must be minimized to reduce cooling air disruption on film cooled airfoils and to reduce the installation effects. The strain gage installation increases the structures mass and stiffness modifying the structural frequency and damping behaviour of the component being tested. With the improvement in engine blade materials and coatings, the engine cycle temperatures and the airfoil surface temperature have increased to 2000F or more, which has a detrimental effect on strain gage life. For example in some applications, the flame sprayed turbine airfoil strain gage lives have become marginally acceptable, especially in the high pressure turbine on the pressure side of the airfoil where strain gages are located radially outward from the blade platform.
Nickel based alloy resistive strain gages with modified constituents suffer severe oxidation if not properly overcoated from the hot corrosive flowpath. In contrast platinum-tungsten gages have very high gage factors and its resistance changes as temperature increases, but also exhibits better oxidation resistance. Of primary concern for all development test in the flowpath is the possible delamination of the strain gages from the airfoils during engine operation. For example flame spray free filament strain gage installations, can delaminate when the surface temperatures exceed 1800F - 2000F.
Improvements in hot section strain gage technology will come from advances in both material and application technologies. Advancements in ceramic materials have demonstrated semiconducting materials that have potential but require development to yield a reliable instrument grade sensors having reliability and calibrations repeatability. Goals for engine airfoils application, especially rotating blades, would be dynamic strain gages survive for 20 hours at surface temperatures up to 2100F - 2200F. The base installation materials need to provide insulation to ground for the strain gage of >1 Megohm @ 2000F with an overall thickness of less than 0.015 inch.
Strategic Advisory Board (SAB) Members Addressing This Need
Aerodyn Engineering, Inc.
Cleveland Electric Laboratories Company, Inc.