A testing method was to he developed based on previous
experience on computer tomography. It has been known for some
time that an image of a cross-section of an object could be
reconstructed using projections or scans at different angles
describing 180 degrees
around the cross-section. Sound velocity,
attenuation and sound reflection are the parameters used in
The object under investigation in this work is a cylinder
of acoustic properties like plastic enclosed in a steel casing.
The task was to find defects in the cylinder and these defects
were of a nature such, that echoes received from them are low
and therefore the pulse echo technique cannot be used in their
detection. A reconstruction method, based on the "Algebraic
Reconstruction Techniques" (ART) is used here to detect these
defects. The main achievements are that these recionstructions
are obtained from constrained-scans, due to total reflection of
ultrasound near the edge of the field, and a technique of obtaining
an image with no background speckles.
square An extensive computer simulation was done using a mathematical
model of the object, which generated scan data similar to
that obtained by actual experiment. Different processing techniques
were tried to improve the image quality and testing
reliability especially to remove the above mentioned problems.
A successful method was chosen such that it will be easy to
implement in a real testing situation. Successive marking of
defect-free areas of the different scans resulted in a well defined
defect structure. No-scan areas were identified by experiment
and both the no-scan areas due to total reflection and those
due to different modes of propagation received the same treatment.
Using a marking technique together with the known coordinates
of these areas an integrated image was obtained.
The implementation of the system was done using a microcomputer
together with a commercial flaw detector. The flaw
detector was coupled to a digital counter to measure the transit
time. The datawere acquired by interfacing the counter to the
microcomputer. Data acquisition, control and processing together
with the reconstruction programme constituted the software which
was developed for this system.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.