An analytical approach to objectively sizing cracks using ultrasonic phased array data

摘要

Ultrasonic phased array systems are becoming increasingly popular as tools for the nondestructive evaluation of safety-critical structures. The data captured by these arrays can be analysed to extract information on the existence, location and shape of defects. However, many of the existing imaging algorithms currently used for this purpose are heavily reliant on the choice of threshold at which the defect measurements are made and this aspect of subjectivity can lead to varying defect characterisations between different operators. To combat this, the work presented here uses the Born approximation to derive a mathematical expression for the crack size given the width of the pulse-echo response lobe of the frequency domain scattering matrix. These scattering matrices can be easily extracted from experimental data if the location of the flaw is known a priori and so the method has been developed exclusively for the objective characterisation of flaws. Due to the analytical nature of this work, conclusions can be drawn on the formula's sensitivity to various experimental parameters and these are corroborated using synthetic data. The sizing of a subwavelength crack is undertaken and it is shown that examination of the scattering matrix correctly captures the crack form of the defect and outperforms the standard TFM in this regard (the nature of the defect is obscured by side lobes in the TFM image). It is also suggested that the derived formulae could potentially be used to inform and optimise array design.