A fracture mechanics based approach to ultrasonic testing of fretting fatigue cracks

This thesis reports a study with the aim of compiling a complete nondestructive evaluation scheme for characterising the early stages of fretting fatigue cracks on the surface of a thick aluminium plate. The size and orientation of the cracks is sought for use in fatigue life analyses. A stress and fracture mechanics analysis is first conducted in order to provide information about the parameters that affect the cracks shape. The mathematical model of the contact and crack problems was formulated and discussed. A technique making use of the distributed edge dislocations was developed and implemented for the easy determination of mode-I and mode-II stress intensity factors and the new results were evaluated against known data from the literature. Very good precision was achieved with less computing time than other numerical methods. The present model can also take into regard material and traction distribution effects, the implications of which are included in the investigation. The shape of the fretting fatigue cracks was determined to be a kinked one, starting at an angle to the surface and developing perpendicularly to it. The conclusion is that fretting fatigue can adequately be described by a stress and fracture mechanics analysis which provides the distribution of the tractions and the fracture parameters. The non-destructive testing work concentrates on the use of ultrasonics techniques for the crack shape and length determination. After a general background study, an ultrasonics spectroscopy method was developed and the Rayleigh wave scattering by different kinked surface slots was experimentally investigated. Different ranges of ratios d/lambda, which express the relation between the defect size and the wavelength, were considered and the reflected signals were analysed in both the time and the frequency domain. A spectral ray study provided data for the anticipated magnitudes of the signals from every feature of the crack. The implications for the inverse nondestructive characterisation of fretting fatigue cracks are discussed and it is found that the selection of the transducer wavelength in relation to the first angled part of the crack is of paramount importance for the crack characterisation.