It is known to perform non-destructive testing by computing images of the material within a structure from ultrasound reflections within the structure, for example to determine the location of defects like cracks, holes etc. This involves measuring reflected signals, obtained with respective combinations of transmitter and receiver positions. These measurements are processed to obtain image values for different positions in the structure. A known algorithm to compute the image value for an imaged position involves computing synthetic reflection measurements of a synthetic focused transmission directed at the imaged position, by summing measured reflected signals obtained using different transmitter positions multiplied by phase factors that compensate for the differences in phase delay from the transmitter positions to the image position and difference in phase delay from the image position to the receiver positions.
FIG. 1 shows the resulting synthetic signals as a function of receiver position (horizontal) and time between transmission and reception (vertical, time increasing downward) for a simulated structure. Amplitude reflections occur along curves. Each curve corresponds to a reflection point, and in the curve the points correspond to the different travel times to the different reception positions. In the figure the top corresponds to the surface of the structure under test closest to the transducers and halfway a straight line corresponding to the back surface can be distinguished. In between hyperbolic curves for different reflecting points can be a seen. The image intensity for the imaged position is a sum of synthetic signal values along the curve.
This imaging method requires a selection of the phase factors to be used for different imaged positions and the shape of the curve of each imaged position. Assuming constant ultrasound propagation speed throughout the structure, the phase factors and the curves can be computed using geometrical considerations.
However, this method does not give reliable results in structures that comprise anisotropic and/or inhomogeneous materials, such as coarse grained or fibrous materials. In welds, for example, part of the material may have crystallized and other parts may be amorphous, which results in unreliable imaging. Similarly, in material made of fibers in a matrix of resin, the results may be unreliable.
FIG. 2 shows an ultrasound image obtained for an exemplary simulated structure of such a material wherein three point reflectors are present (indicated by circles). Instead of the points, stars are visible. The inventors have found that the unreliability of results of the known method for coarse grained or fibrous materials is at least partly due to anisotropy in the ultrasound propagation speed that results from the presence of grains or fibers. The structure contains three isolated reflectors, but in the image the reflectors are smeared due to the effect of anisotropy.