Walking-gate spatial signal averaging

The microprocessor-based Autoscan-II ultrasonic inspection system for the detection of fatigue cracks in military aircraft utilizes an advanced signal-processing technique for reliable detection of 0.030-in. fatigue cracks under installed fasteners. This signal-processing technique takes advantage of the fact that crack signals 'walk' in time as the transducer inspects around a fastener. Bolt size, scan depth, and angle of incidence of the ultrasonic shear wave are used to determine the 'walk rate' of a defect in the inspection zone. With this geometric information, the signal-processing technique (walking gate) will enhance any pulse-echo return 'walking' at the computed 'walk rate'. The output of the walking gate will have an improvement as large as 10:1 in spatial resolution, a signal-to-noise improvement of well over 10 for certain geometric conditions, and a minimum of 8:1 improvement for random noise. These improvements are in addition to any gain made by conventional signal averaging and are realizable in many other applications. Extensions of this signal processing into conventional shear-wave C-scans and other component geometries will be discussed as well as the hardware implementation of the system.