|NDT.net - April 2000, Vol. 5 No. 04|
|TABLE OF CONTENTS|
|Fig 1: View of a) the complete line scanner and b) of the working principle of the unit.|
The applied principle of line scanning yields several advantages in comparison to film radiography. The radiation direction corresponds to the central projection technique which always enables radial penetration. All radial flaws are detected with maximum contrast. The special slit collimator of the camera reduces the intensity of the scattered radiation significantly. The properly aligned system (fig. 3) yields therefore better image quality than film radiography for pipes with thick walls and especially for water filled pipes. The developed system provides radiometric images with high image quality for a penetrated steel thickness of 4 - 46 mm (2 - 23 mm wall thickness) in a comparable time to manual radiography. Due to the limited resolution of the applied camera of about 160 µm (measured with double wire IQI by EN 462-5) theoptimum application range covers wall thicknesses of 10-25 mm from the present point of view. The testing of higher wall thicknesses is also possible but requires modification of the line camera.
The description of the line camera design is outlined in [2-4]. After a few modifications the camera could be operated near the physical sensitivity limit. The spectra of different x-ray tubes and g-sources were calculated to simulate the detector properties and the expected sensitivities as a function of the wall thickness of steel and the measurement time [4, 5]. Only x-ray tubes enable the application of the line scan technique in a reasonable time which is comparable with the manual radiography technique and its image quality. The line camera yields a sensitivity of about 90% (~ 81% DQE) of the calculated physical limit up to about 35 mm penetrated steel thickness at 225 keV.
The high scatter ratio of water filled pipes reduces the image contrast considerably. Therefore film radiography is performed for empty pipes usually. The absorption of water cannot be reduced. But in comparison to film radiography the scatter ratio k can be diminished by the collimation technique to about zero. This collimation improves the specific contrast csp for water (or other media) filled pipes as well as for thick walled pipes, where csp = /(1+k).
|Fig 2: Comparison of a digitized radiograph (left image) and a line camera image (right image) of a water filled pipe weld.|
Fig. 2 shows the comparison of a radiograph taken from a water filled steel pipe (diameter 160 mm and wall thickness 12 mm) and the image made with the line camera. For better comparison the radiograph was digitized and printed under comparable conditions. The radiograph is characterized by the typical effect of changing optical density from the edges (left and right) to the centre. The contrast is relatively low and the scanned image is easier to interpret due to the typical magnification. The contrast gain of the scanned image can be visually recognized.
|Fig 3: Principle of laminographic inspection of circumferential welds. Camera and tube are turned around the pipe with a phase shift of 180°. The tube is shifted step by step in direction of the pipe axis for each orbital scan. Therefore, different projections are measured as basis for a 3D-presentation|
The principle of the application is based on the multi-angle technique. The x-ray tube is shifted in the direction to the pipe axis step by step. For each step a full 360o scan is taken. In contrast to the x-ray tube the line camera is not shifted.
This algorithm corresponds to the coplanar translational laminography  transformed to the inspection of a pipe. If the x-ray tube is shifted parallel to the pipe axis and in addition, each step is combined with an angular shift, the principle of coplanar rotational laminography can be applied . Due to this analogy between the configuration of multi-angle technique and laminography, the three-dimensional (3D) reconstruction of the weld is possible. Then, either all measured radiometric scans can be measured or (and) the weld can be inspected plane by plane from the reconstructed 3D-data set [fig. 3].
The tomosynthesis was performed on the basis of different linear and non-linear backprojection estimates [6-8]. In this case all estimates give similar results. However, estimates with regulated additive accumulation of beamsums are slightly better than strictly multiplicative procedures, such as the minimum projection method. The tomosynthesis which can usually be performed from a few projections only (for crack detection) needs a higher SNR and contrast per projection than the classical computed tomography. SNR improvement was achieved using non-linear averaging (e.g. median) and contrast enhancement by an additional threshold operation of the filtered projections corresponding to the maximum and minimum of the histogram analysis of all relevant projections in a preselected region of interest.
|Fig 4: Profiles of a welded wall of an austenitic pipe after stress crack corrosion, The crack (black) is located near the weld (light area). A and B are reconstructed profiles at different positions.|
Fig. 4 shows the reconstruction of a crack which was found in the weldment of a separated austenitic pipe of a nuclear power station. This pipe had a diameter of 160 mm and a nominal wall thickness of 8.8 mm. The reconstruction was performed with the average method [4, 6-8].
For high accuracy applications the principle of planar tomography is applied. The X-ray source is shifted corresponding to fig. 5 parallel to the pipe axis only and the camera is located opposite to the X-ray tube below the weld to inspect. During the movement of the X-ray tube 200 - 400 projections are acquired in a radiation angle range of ± 20°. The camera is not shifted for each set of projections. In principle a higher angle range is preferable. The ± 20° were selected due to practical restrictions.
|Fig 5: Principle of planar tomography. The X-ray tube is shifted alog the surface parallel to the pipe axis and several hundred projections are taken for reconstruction.||Fig 6: Reconstructed profile of a circumferential weld of an austenitic pipe after oscillation|
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