Computer assisted ultrasonic tomography

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 ultrasonic computerised-tomography. 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.