Non-Destructive Testing of Industrial Materials Using Inverse Techniques

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Industrial quality control testing works straightforwardly when the quantity of interest can be measured independently. However, practical measurements often combine several quantities that then need to be separated. This can be done by using an "inverse" calculation. The "forward" calculation would be to sum all the target quantities, if they were known, to determine the combined measurement that would be made. The inverse calculation separates the various quantities and is much more difficult and mathematically sensitive. The two applications of inverse calculations explored here differ physically but they share substantial mathematical commonality and complementary features.***Log Sorting in Sawmills using X-Ray Computed Tomography:**To compete effectively in the international market, Canadian sawmills must produce the highest value products from the available raw material. This can be done by examining each log entering a sawmill, and strategically processing it for the highest value product. This sorting must be done well, else substantial wastage will occur either as high-quality material getting used for low-value products, or low-quality material failing to make high-value products.**X-ray Computed Tomography (CT) provides a promising way to identify the quality-controlling interior features of logs. CT measurements are well established in the medical field, but are not well suited for industrial use in sawmills because of their complexity, cost and mechanical sensitivity. Work has been ongoing here to develop a simplified, economical and rugged CT scanner design. The key to the approach taken is the observation that the measured logs and their internal features have very specific geometries, for example, logs are cylindrical, heartwood and sapwood are arranged axi-symmetrically, and knots are arranged radially. Thus, a specialized CT inversion scheme can be developed to take advantage of this advance information and thereby reduce the demand on both required measurement accuracy and computational size. Very promising results have been obtained in laboratory tests, and the present proposal is directed towards further functional developments such as spiral scanning and to practical demonstrations.**Practical Residual Stress Measurement in Industrial Components:**Residual stresses are locked-in stresses that exist in materials without the presence of any external loads. These stresses are important because they significantly affect the dimensional stability and material strength of industrial components. If not detected and controlled in manufacturing processes, substantial material wastage premature product failures can occur. The "hidden" character of residual stresses makes them difficult to measure reliably and requires the use of "inverse" evaluations.**Residual stress measurements are typically done by measuring the deformations that occur when some stressed material is removed, for example, by drilling a small hole. Optical techniques such as Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometry (ESPI) are attractive measurement techniques because they are non-contact, rapid and economical. They provide full-field optical data that allow sophisticated interpretation significantly beyond the more typical minimal data measurement approaches. Traditionally, DIC and ESPI have used monochromatic light. The proposed research is aimed at exploiting the larger data content available in multi-colour optical measurements. This allows 3-D deformation identifications from nominally 2-D data and enables more sophisticated evaluations of residual stresses, including some out-of-plane stress components that are not available with present measurement techniques.