A Study of Resonance Coincidence in Bladed Discs

Abstract

The failure in service of just one or two out of a batch of nominally identical units poses two urgent problems. First, it is necessary to establish the immediate cause of the failures but then, and equally important, it is essential to determine whether these isolated cases represent a few ‘rogue’ units or whether they herald the subsequent failure of many others. This latter problem is examined in relation to the vibration loads borne by turbomachinery blading. The possibility that minor dimensional variations between one unit and another can result in them exhibiting significantly different vibration characteristics is studied in two detailed cases. One example shows that the shortening of all the blades on one disc by 3 per cent can worsen their resonant response levels under operating conditions by 50 per cent, and thereby drastically shorten their fatigue life. The presence of small variations between the blades on a disc—blade detuning—can also increase response levels by as much as 90 per cent. It is concluded that apparently similar bladed disc units could well exhibit quite dissimilar vibration characteristics in service, and consequently have widely differing fatigue lives.