Efficient immersion imaging of components with nonplanar surfaces

Ultrasonic array inspection of a component with a nonplanar surface can be achieved in immersion using a liquid layer to couple ultrasonic waves from an array probe into a solid structure. This paper presents an efficient way to compute the appropriate element time delays in immersion without compromising the measurement accuracy. In the proposed imaging process, the surface geometry is first measured ultrasonically by forming an image of the component surface in the couplant. This leads to a set of discrete points that define the surface profile of the component. The propagation time from an array element to a point in the component is then determined by a grid search of candidate ray-paths through each surface point to identify the one that yields the shortest traveling time. Propagation times in the component are first generated on a coarse mesh of points and then these values are linearly interpolated to find the propagation time to each image pixel. The computed propagation times are finally used to reconstruct an image of the component interior. An analytical model is developed to determine a relationship between estimated propagation time errors and their effect on the array inspection in terms of signal amplitude from a reflector. For nominally normal incidence inspection of a metallic component with a minimum surface radius of 30 wavelengths immersed in water, it is found that the surface of the component can be adequately described by points spaced by one wavelength and that delays can be computed on a coarse grid of points spaced at 3 wavelengths. With these parameters, the reduction in amplitude of a point target in the component is shown to be less than 1 dB.