Enhancing Image Repeatability with Ultrasonic Array

Abstract

Ultrasonic phased arrays are increasingly applied in the non-destructive evaluation
(NDE) of industrial structures. These transducers allow a range of different inspections
from a single location and can produce images for rapid visualization of the internal
structure. The use of phased array transducers enables the full-matrix capture (FMC)
acquisition technique which captures all available information regarding an inspection
at a given location. This technique enables the application of various imaging algorithms
in post-processing. One algorithm that is commonly referred to as the gold standard due
to its enhanced resolution and interpretability is called the total focusing method (TFM).
It produces images by focusing on every pixel of target regions and provides significant
performance advantages for NDE over traditional imaging techniques.
Having highly repeatable measurements is of interest in many engineering applications. This allows the potential of detecting small changes between inspections through
baseline subtraction, either to increase the detectability of small defects or to more
accurately characterise the growth of known defects. Investigation of the variation in
inspection conditions affecting image repeatability includes the study of positioning
inaccuracy, array element variability, and couplant wave velocity variability. A number of approaches are proposed in this thesis for determining the variations between
measurements, including a cross-correlation method and a time-of-flight method. The
principle of determining the variations is to use an ultrasonic array to capture FMC
from the testing component between inspections. A compensation algorithm is developed
and tested to improve the repeatability between measurements. This thesis investigates
the influence of varying conditions between inspections on TFM image repeatability and
aims to compensate for repeatability hence improving the sensitivity to small defects.
Experiments are conducted using different types of arrays including one-dimensional
(1D) arrays for two-dimensional (2D) images and 2D arrays for three-dimensional (3D)
space. The experimental results show a good agreement with a commonly used ray-based
model. The study shows positioning inaccuracy and couplant wave velocity variability are
the dominant factors affecting repeatability and that the effect of element variability is
negligible as long as TFM is used. The repeatability is profoundly sensitive to positioning
inaccuracy. Modest differences in wave velocity in the couplant can be tolerated, provided
that the relative difference in true couplant wave velocity and that assumed in imaging
calculations is preserved. Early-stage defects normally have a similar response amplitude
as microstructural noise, which is challenging to detect and characterise by conventional
techniques, such as the 6 dB drop method. By knowing the variations between inspection
i
conditions, the proposed compensation improves image repeatability, and hence the SNR
is improved by around 10 - 25 dB.

Date of Award	6 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul D Wilcox (Supervisor) & Anthony J Croxford (Supervisor)