VNR Method - Maximizing the Signal-to-Noise Ratio at Cryogenic Temperatures of Structure-Critical Fasteners

Aircraft structures are commonly assembled using a large number of fasteners, bolts, and rivets. In defect detection, it is essential to choose inspection parameters that maximize the signal-to-noise (S/N) ratio, i.e. the response from the defect divided by the average response from the competing grain noise.  On the other hand, for material characterization it is often useful to choose inspection parameters that maximize the backscattered grain noise.  The signals generated during an Eddy-Current Testing (ECT) inspection allow for the localization and dimensioning of defects in the material being examined. A noise free ECT signal from a defect generally has an “eight” shaped Lissajous figure in the impedance plane having the key characteristics of the phase angle and amplitude.  However, in practice, ECT signals are corrupted by several forms of noise, which can interfere with the analysis of the acquired signal, reducing the correct/misreading ratio and increasing the duration of the inspection. Generally, the material noise can produce significant distortions in the Lissajous figure and may cause misreading of the phase angle and amplitude.  In some cases, the distortions are so extensive that making an evaluation is not possible.  A novel method was conceived, named Victor Noise Reduction (VNR) enabling an inspector to detect small flaws not previously inspectable by conventional NDT methods. In this patent registered technique  the material is cycled through cryogenic temperatures.