The effect of friction on crack propagation in the dovetail fixings of compressor discs

Abstract

Stress frozen photoelasticity has been used to model dovetail compressor blade fixings. During loading a known coefficient of friction was applied and the effect of the variation of this parameter on crack initiation and propagation was investigated. Data were recorded from the specimen using an automated computer aided polariscope based on the method of phase stepping. Isochromatic and isoclinic data were collected and used to determine the stress distribution, the stress intensity factor and the crack propagation direction. The method to predict the direction of crack propagation has been improved so that photoelastic data can be used reliably for this purpose. Three values of the coefficient of friction were used for two different dovetail geometries. It was found that the initial values of the mode II stress intensity factors were higher for a lower friction coefficient. An increase in crack length produced a corresponding decrease in the mode I stress intensity factor and a decrease in the mode II value. It was concluded that the coefficient of friction influenced crack growth at all stages of crack growth because it affects the relative levels of the mode I and mode II stress intensity factors. This has an effect on the direction of the maximum principal stress direction and so on the direction of crack propagation.