Technical report | Travelling Wave Excitation System Verification through Mistuned Blisk Analysis

Abstract

An acoustic travelling wave system (TWS) was verified through direct comparison of numerical and experimental results of a tuned and mistuned research blisk (integrally bladed rotor). The system, developed by the Defence Science and Technology (DST) Group, is used for characterising mistuning of blisks, which can contribute to premature High Cycle Fatigue (HCF) failures in gas turbine engines fitted in modern military aircraft. The finite element method (FEM) was applied and a modal analysis was conducted to determine numerical values of which to compare to experimental data obtained from TWS tests. A 12-bladed, flat, stainless steel research blisk was analysed using the TWS, with results showing promise with less than two percent error through comparison of modes (natural frequencies) of the two blisk cases examined. System improvements, limitations and possible sources of error are also discussed within this report.

Executive Summary 

Due to the wide use of integrally bladed disks (blisks) in modern gas turbine engines for lighter weight and higher performance, High Cycle Fatigue (HCF) increasingly becomes a cost and safety driver. It is important to be able to understand the behaviour of blisks, particularly mistuning caused by variations of material or geometric properties between blades – which contributes to premature HCF failures.

A travelling wave system (TWS) is one of methods for testing the vibration modes (resonant frequencies) of blisks in order to characterise mistuning. TWS testing is generally simple and low in cost, in comparison to traditional spin testing, that provides a non-contact solution replacing strain gauges which may otherwise change the dynamic response of a blisk. Therefore, the Defence Science and Technology (DST) Group  have developed a TWS to enhance its capability in this field.

A finite element analysis (FEA) was applied using ANSYS 17.1 to analyse the differences between tuned and foreign object damaged (FOD) research blisks. FOD was examined due to its occurrence in aircraft engines and has direct relevance to mistuning. Three 3D CAD models were produced with different locations of damage in each modelled blisk in order to examine the worst-case-scenario of FOD.

Experimental work was undertaken using the TWS with multiple repeated tests for each engine order (EO). Modes extracted from the FEA were compared with those from the experimental analyses for the tuned and mistuned blisk case in order to assess the system. Mode matching for both sets of results ensured that the mode frequencies were properly correlated.  It was determined that there was less than two percent error between numerical predictions and experimentation; proving the system is usable for real in-service blisks.

Outputs determined allow DST to enhance capability to provide independent airworthiness advice to the Australia Defence Force (ADF) concerning HCF and HCF-related failures. The verified TWS allows for testing of real, in-service blisks to contribute to the reduction of HCF-related events in aircraft engines.