Abstract
Imaging defects in austenitic welds presents a significant challenge for the ultrasonic non-destructive testing community. Due to the heating process during their manufacture, a dendritic structure develops, exhibiting large grains with locally anisotropic properties which cause the ultrasonic waves to scatter and refract. When basic imaging algorithms, which typically make constant wave speed assumptions, are applied to datasets arising from the inspection of these welds, the resulting defect reconstructions are often distorted and difficult to interpret correctly. However, knowledge of the underlying spatially varying material properties allows correction of the expected wave travel times and thus results in more reliable defect reconstructions. In this paper, an approximation to the underlying, locally anisotropic structure of the weld is constructed from ultrasonic time of flight data. A new forward model of wave front propagation in locally anisotropic media is presented and used within the reversible-jump Markov chain Monte Carlo method to invert for the map of effective grain orientations across different regions of the weld. This forward model and estimated map are then used as the basis for an advanced imaging algorithm and the resulting defect reconstructions exhibit a significant improvement across multiple flaw characterization metrics.
Highlights
In many countries, infrastructure is ageing and cannot be replaced due to global financial pressures
When many grains and orientations are present in a complex sample it can sometimes be difficult to visually assess the likeness of the reconstructed map with the known material geometry, when we are primarily interested in the effective medium which will provide sufficient correction for defect imaging but may not accurately represent the structure well
It was demonstrated that use of this model within the reversible-jump Markov chain Monte Carlo (rj-MCMC) inversion framework could be successfully used to generate an approximation of the material map which in turn could be exploited by a delay and sum style imaging algorithm
Summary
Infrastructure is ageing and cannot be replaced due to global financial pressures. Industry is presented with the challenging problem of safely maintaining their infrastructure and extending its life span. Ultrasonic non-destructive testing (NDT) involves the transmission of mechanical waves through industrial components to facilitate the detection of interior damage and offers an economically and environmentally desirable solution to this challenge. Typically arranged in linear arrays, are deployed to carry out these inspections, resulting in large volumes.
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