Approximate Modal Identification of Lightly Damped, Highly Symmetric Bladed Disks

Abstract

Turbine blade cracking or failure caused by high-cycle fatigue is a concern in both aerospace and aircraft engines. Because of manufacturing advances leading to integrally bladed disks, nonuniformities and damping associated with the blade-to-disk interface have been considerably reduced. In systems such as these, bladed-disk assemblies can behave as lightly damped, rotationally periodic structures in which many resonant conditions, involving response in all ofthe blades, exist. Because of the nonuniform, aerodynamic forces on the blades, excessive vibration response in the high-frequency regime can be a problem and, thus, identification of resonant conditions in this region of the spectrum is important in avoiding operation at potentially damaging speeds. We present a simple modal identification technique that can help locate potentially harmful resonant conditions and aid in the validation of computational models of highly tuned, bladed-disk assemblies across a wide frequency regime. The experimental approach used to acquire the necessary measurements in a timely manner is also described. An example is presented using data acquired from an impulse test of a turbine and includes results up to 20 kHz.