APPLICATION OF CORRECTION COEFFICIENTS TO THE SIGNAL AMPLITUDES FROM REFLECTORS IN ULTRASONIC NON-DESTRUCTIVE TESTING REFERENCE BLOCKS

Abstract

Ultrasonic non-destructive testing (NDT) is one of the most widely used methods for the technical diagnostics of complex engineering structures. A critically important aspect of this method is not only the ability to detect the presence of a defect, but also to quantitatively determine its size, which directly influences the assessment of an object’s technical condition and the decisions regarding its further operation. In traditional ultrasonic NDT systems, the most common approach to estimating defect size is the amplitude method, which is based on comparing the amplitude of the signal reflected from a defect with the amplitude of a reference reflector in a test block. Despite its popularity and apparent simplicity, the amplitude method has significant metrological limitations, primarily associated with the poor reproducibility of amplitudes obtained from nominally identical defect simulators in calibration or reference blocks. Long-term experience in verification, calibration, and production of such blocks shows that signal amplitudes may differ by 4–10 dB even when geometric and technological requirements are met with high precision. Such a deviation is critically important, as it changes the estimated reflecting area by several times, leading to incorrect evaluations of defect size. The main causes of this phenomenon include acoustic inhomogeneity of materials, variations in the actual geometry and surface roughness of the defects, wave and diffraction effects, differences in the formation of the acoustic beam by the transducer, and the influence of ultrasonic attenuation in real materials. Collectively, these factors give rise to the problem of low reproducibility of amplitude parameters, which is one of the key obstacles to improving the accuracy and stability of ultrasonic NDT results. This article proposes a new approach to compensating for such discrepancies, based on comparing the amplitude of the signal reflected from a real flat-bottom hole with that of a signal reflected from a quasi-infinite surface, whose amplitude is stable and reproducible in similar materials. Based on an analysis of acoustic-path equations for the far field, a mathematical model of an amplitude correction factor is formulated. The developed model makes it possible to determine the difference between the theoretical and actual amplitude ratios and to compensate for uncontrolled factors that arise during real measurements. In addition, the study includes an analysis of the sources of uncertainty associated with the proposed correction.