Development of a stress limit surface for turbine blades under multimode forced vibration for use in engine testing

Conventional methods to estimate stresses due to the forced-response in Integrally Bladed Rotors rely on vibration measurement techniques that have various limitations and are not suitable for use when several natural modes are excited simultaneously. The accurate prediction of the blade peak stress due to multi-mode excitation cannot be established using these techniques alone and requires prior knowledge of stress distribution associated with each vibratory mode. These stress distributions can be obtained using commercial finite element software. With the availability of modal stresses, real time updates of the maximum stress can be obtained using a superposition approach which requires a spatial and temporal search. The paper presents a method to obtain a critical stress limit surface that can estimate the peak vibration stresses under multimode excitation conditions. This surface can be useful to program into test devices in order to ensure that engines are tested in safe conditions and also to provide real time estimate of stresses in turbine blades.