Three-dimensional flaw reconstruction using a real-time X-ray imaging system
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A system has been developed for making three-dimensional flaw measurements in materials using a real-time X-ray imaging laboratory. This environment affords precise control over all positional variables, offers multiple degrees of freedom for sample movement, and the continuous nature of the real-time image eliminates ambiguity in determining correspondence points among multiple views of the sample. This system is based on film stereography in which two stereo projections are obtained of a sample either by translating the X-ray source or by translating the sample. The three-dimensional coordinates of features of interest such as crack endpoints and centroids of void-like flaws are determined by measuring the disparity between corresponding points in the stereo pair and triangulating to find the depth of the point within the sample. This new system generalizes the sample motion for arbitrary shifts and rotations, and easily accommodates more than two views to yield a least-squares estimate of the three-dimensional point locations. The system is implemented by a set of software modules which augments an existing real-time laboratory. All sub-systems to manipulate the sample position, process the image, select feature points from the display screen, and compute three-dimensional feature coordinates were seamlessly integrated. Calibration routines were implemented to accurately determine the X-ray source, sample, and detector geometry. The spatial distortion and blurring effects of the X-ray detector were characterized and modelled. An image warp was applied to correct spatial nonlinearities, and image restoration was used to increase the resolution of the detector. A high-speed digital signal processing board was used to implement on-line image processing routines for detector corrections and contrast enhancement. The performance of the complete system was determined by measuring fabricated samples and industrial samples containing crack-like defects. The three-dimensional measurements were accurate to ±0.02 cm. This system delivers much of the information found in a computed tomography image at much lower cost, and is faster and more accurate than film-based stereography.