Abstract
M.Ing.
This dissertation deals with the structural integrity analysis of a low pressure (LP) turbine with transverse cracking. Cracks are initiated in the centre ring keyway of the shaft by a fretting mechanism and propagate during barring operation. The cracks arrest during normal operation when they are still relatively shallow. The aim of the calculations is to predict the transition where cracks start propagating by high cycle fatigue during normal operation of the shaft.
Most influencing factors, including continuum mechanics, mathematical modelling, material behaviour, service loads and industry experience, are studied in detail as a precursor to the case study.
The calculated results show that the case study is an example where the application of fracture mechanics on the crack-free stress field leads to erroneous results. There is a significant redistribution of stress in the presence of the crack so that the actual stress ratio, as calculated from three dimensional cracked models, varies significantly compared to the crack-free model.
Calculated results, together with carefully researched material properties, confirm the postulated crack growth during barring operation and predict limiting crack sizes where high cycle fatigue would ensue during normal operation. The case study concludes that the shaft has a significant remaining life and that it can be returned to service with periodic non-destructive examinations.