Investigation on fracture behaviour of turbine blades under self-exciting modes

This paper investigates the failure mechanisms of turbine blades occurring during similar conditions of turbine operation. Optical techniques are utilised to identify critical frequencies that may be the cause of blade fractures. For that purpose, time average, stroboscopic holographic interferometry and holographic moire are utilised to record vibration patterns. Strain maps are computed from stroboscopic and holographic moire patterns. Stress distributions are computed under the assumption of linear elasticity. It is found that stress trajectories deduced from holographic measurements match well with the initial trajectories of crack observed for the in-service broken specimens. This fact allows the excitation modes causing blade fractures to be identified. Finite element analyses carried out on models corrected in view of the experimentally observed vibration modes provide the values of stress causing fracture initiation. The paper discusses the failure mechanisms of the blades on the basis the experimental evidence and finite elements results. The observed failure modes seem to be a form of the principal normal stress fracture criterion. An argument based on damage accumulation explains the observed behaviour. Further evidence on failure mechanisms is gathered from scanning electron microscope images of the fractured specimens.