Ultrasonic Nde Techniques for Integrally Fabricated Rotors

Solid-state bonding methods, e.g., diffusion bonding and pressure welding, are becoming common manufacture and repair techniques for gas turbine engine components. Effective NDE inspection techniques are crucial to the utilization of this approach due to the high stresses on the bond plane associated with jet engine operation. Recently we have examined ultrasonic techniques for assessing bond quality including leaky Rayleigh waves and critical angle longitudinal waves!1], for which the illuminating waves are nearly normal to the bond plane, and longitudinal waves at near grazing incidence to the bond!1]. Based upon preliminary theoretical analyses!1,2] of ultrasonic reflectivity from imperfect interfaces, it was found that the reflection coefficient for both longitudinal and shear waves increases to unity as the incident angle approaches grazing!1]. In contrast, the bond reflection coefficients for near normal incidence can be quite small, depending upon the degree of imperfection of the bond. A second drawback to the first two approaches mentioned is that surface roughness of the blade can cause scattering noise which dominates the signals reflected from the bond. The grazing incidence technique suffers less from these problems since the probe is oriented nearly normal to the surface of the blade and the interaction of the beam with the surface is minimized. In this paper we will first present the results of grazing incidence experiments which show the ability of the approach to distinguish bonds of different quality. These bonds were produced by systematically degrading standard fabrication procedures of major engine manufacturers. This is followed by a quantitative experimental study of the quasi-static spring model for the interaction of an ultrasonic wave with an imperfect interface. In this study, model samples with controlled interfacial disbonding were employed to demonstrate the high sensitivity of the grazing incidence configuration and the quantitative relationship of the measurements to the degree of disbonding.