Non-Collinear Inspection for the Detection of Damage Precursors

Abstract

Fatigue and creep damage are two of the most common modes of failure in engineering structures, and both are extremely difficult to detect in their early stages. Conventional ultrasonic approaches have to date been unable to detect the early stages of these damage types, due to the lack of any apparent changes to the gross structure on the scale of the wavelength. An alternative to conventional NDT techniques is the use of nonlinear techniques, a series of approaches that rely on the nonlinear nature of material responses. This paper examines how a non-collinear mixing method may be employed to measure the inherent nonlinearity in a material and hence detect precursors. The fundamental theory is described and it is shown how this can be used to predict the nature and amplitude of the generated response. Experimental results are given that indicate the sensitivity of the approach to damage precursors. This is then used to show how a pure amplitude measurement is perhaps not the best approach to measure nonlinearity and presents a more robust measurement technique that is more practically applicable. Finally the suitability of the approach and its limitations for monitoring are commented upon.