Imaging Anisotropic Components with Ultrasonic Arrays

Abstract

An ultrasonic array NDE method was deemed most appropriate for the in situ inspection of the single-crystal turbine blade. Ultrasonic array probes consist of many individually wired ultrasonic elements within a single transducer package. The elements can be fired individually or in groups with different timings or phase shifts. This allows an array probe to steer and focus the ultrasonic beam in a way that would otherwise require a great number of different single element transducers. Furthermore, array probes enable ultrasonic imaging of a component from a single inspection location. Ultrasonic imaging involves converting raw ultrasonic data collected by the elements in an array into an image that attempts to accurately represent the internal structure of a component. This allows ultrasonic data to be analysed in a more intuitive manner and reduces interpretation errors. The overall aim of imaging is to improve the probability that a defect is detected by an inspector whilst minimising the chance that a design feature is not mistaken for a defect which could lead to the component being needlessly withdrawn from operation. In this section a novel ultrasonic array imaging algorithm known as the Total Focusing Method (TFM) is introduced. The TFM algorithm is shown to offer many benefits over conventional imaging methods. The algorithm works by post-processing the ultrasonic data, which has been collected using a procedure known as full-matrix capture (FMC), to synthetically focus ultrasonic energy at every point within the image. The FMC process involves transmitting an ultrasonic wave from one element whilst receiving on all other elements in the array. This process is then repeated until the data from every transmit-receive element combination has been collected. The anisotropic behaviour of single-crystal components is shown to significantly distort the images produced with the TFM algorithm. If uncorrected, this distortion could lead to missed defects, false-calls and ultimately an unreliable inspection. Therefore, the TFM algorithm is adapted for imaging anisotropic materials using the models developed in Chap. 2. Simulated and experimental imaging results are presented for both linear and 2D ultrasonic arrays for isotropic and anisotropic (single-crystal) components. The effect of beam profile variation due to the orientation of a single-crystal and the effect of the misalignment of the crystal on imaging performance are demonstrated.