On the Modal Analysis of a Cracking Compressor Blade

Abstract

On the Modal Analysis of a Cracking Compressor Blade The paper deals with issues related to reliable recognition and prediction of health of steel compressor blades. It outlines symptoms that indicate the blade cracking, and identification methods that can be used during operation, maintenance and repairs/overhauls of turbine jet engines of the SO-3 type. An optical method involving laser measurements of blade vibration while on the excitation source is presented in details as it is implemented in the WZL-3 (Military Aviation Works No. 3). The method has been verified in practice and some findings from the verification process are discussed. Next, new testing capabilities offered by the method are presented, including identification of early symptoms of material fatigue (the strengthening and the weakening of the material) and the crack initiation. The topics under discussion have been illustrated with some examples taken while measuring modal properties typical of both defect-free and faulty blades as well as from fatigue tests conducted according to the LCF and HCF profiles. It has been proved that the analysis of resonance responses for specific modes of operation allows of reliable differentiation between material's fatigue and cracking.

High Sensitive Methods for Health Monitoring of Compressor Blades and Fatigue Detection

“…The measurement system gives almost laboratory accuracy. That's why it let [23]: precise identification of blade modal properties in measured frequency range; metrological factors influence analysis on recorded resonance characteristics;modal parameters trends analysis be observed during fatigue tests. …”

“…The cracking propagation and blade break-off occur at limiting decrease in frequency [23]. The change of the blade frequency also results from erosion and the corrosion.…”

“…The second stage of blade damage can be observed, for example, in “a resonance curve,” (RC) measured by a laser point head during modal frequency testing [3, 23]. This approach is sufficient to solve overhaul problem (level II) with experimental modal analysis method.…”

“…An experimental modal analysis [17–21], which has been used in Poland as a sensitive NDE method during overhaul blade tests since 2008 and the method of the high-frequency identification of blade modal properties [3, 22, 23]. …”

Crack propagation methodology under complex loadings

“…Consequently, it can induce a reduction of the natural frequencies of the structure. This observation has instigated numerous works on the non-destructive identification of cracks using vibrational analysis [32][33][34]. Regarding crack propagation, a frequency shift may lead to unpredicted LCF/HCF interaction.…”