·Table of Contents
·Methods and Instrumentation
FFreshex: A combined System for Ultrasonic and X-Ray Inspection of weldsOlivier DUPUIS, Valérie KAFTANDJIAN
Laboratoire CNDRI, INSA, Bat 303, 69 621 Villeurbanne Cedex
OIS Engineering Ltd., Grist Mill, Crittens Road, Cobholm, GT Yarmouth, NR31 0AG, UK
ROBIT As., P.O. Box 100, N-1361 Billingstad, Norway
THOMSON TUBES ELECTRONIQUES, Z.I. Centr'Alp, 38430 Moirans, France
* FFRESHEX : Fast Film REplacement System for High Resolution X-ray weld inspection with ultrasonic data fusion, BRITE/EURAM BE 3681 France
The idea of the Ffreshex project was first to design a new X-ray detector based on Time Delay Integration, that is, several lines of the same part of the object are integrated in order to increase the signal to noise ratio. This allows the use of sensitive elements of small size (54mm) which ensures a good spatial resolution. The detector lines integrate the X-ray signal because the object is moving continuously during acquisition in synchronism with the detector line readout clock
The second idea of the Ffreshex system was to improve the reliability of weld inspection through the use of both X-rays and ultrasound. These techniques are complementary and are already often used in combination to confirm the presence of defects. The aim was thus to combine both techniques automatically.
|Fig 1: Scheme of the acquisition system|
|Fig 2: Picture of the Ffreshex orbiting mechanism with the cable handling system.||Fig 3: View of the 8 ultrasonic probes and a pipe section installed in the orbiting mechanism.|
a) Ultrasonic data
8 probes are used in pulse-echo mode in order to gather information of the whole weld volume. Each probe gives an image such as that shown in the following figure.
|Fig 4: Ultrasonic image given by one of the probes. The zone corresponding to the weld is between the two dark lines. This is a B-scan image, that is, the abscissa along the weld is the horizontal direction, and the time of flight is the vertical one.|
The signals from the 8 probes are analysed and processed in order that echoes coming from the same position inside the weld are detected by several probes gathering information from the same indication.
The amplitude of each echo is compared to that of a reference echo from a known hole and thresholds were defined in order to classify the echoes with respect to the reference.
The result of the ultrasonic stage before combination is thus a list of detected indications with their respective positions inside the weld volume, which results in the creation of a bounding box around each position, depending on the defect size, and amplitude.
The image given by the new detector is derived from the integration of 204 lines, each line being constituted by 1024 pixels of 54mm in size. Because of this integration, the resulting image shows a good contrast and sensitivity performance (1.14% with wire type IQI on 35mm of steel at 3 mm/s, 200 kV, 15 mA). The following figures show two images.
|Fig 5: Radioscopic image (TDI detector) on a welded carbon steel pipe. 3 wires of the IQI (10 FE EN) are visible|
|Fig 6: Part of a weld with an artificial slot of 0.5 mm in width and 25 mm in length|
The radioscopic image is processed automatically in order to detect the defects, based on adaptive thresholds. The development of any image processing algorithm is in fact a compromise between a high percentage of detected defects, and a limited number of false indications. Weld images are particularly difficult to process due to the weld cap, which is non-uniform, and irregularities of the weld, which can be detected at the same time as real defects. Thus, once the objects are detected, some features are computed for each object, namely the area and the contrast with respect to the noise. These features will be used in the data combination stage.
The information obtained from each method is a list of objects that have been detected, associated with their parameters (amplitude, contrast ...). From these parameters, it is necessary to compute a confidence level associated to each NDT method (which is called a mass function in the Dempster-Shafer or evidence theory). This confidence level can be compared to the confidence of the radiographer when he looks at a film and says «I am more or less sure that there is a defect . This «more or less» has to become a precise value in the fusion algorithm. This stage is a crucial point that determines the performance of the fusion algorithm.
In our study, for each testing method, the computation of the confidence levels is done after an apprenticeship stage where the knowledge of the NDT experts is used. This part is explained in .
Then the X-ray and ultrasonic confidence levels are combined by the Dempster rule of combination. If both methods are in agreement concerning one object, the confidence after fusion is higher. If both methods are in contradiction, the fusion approach gives a warning that indicates a conflict. When an object is detected only by one method, the confidence after fusion remains the same. In the end, all the fusion results are then given to the operator for final decision (objects and their parameters and confidence levels).
After image processing, all the objects detected on the radioscopic image were compared to the defects identified by an expert on the radiographic film (in order to assess the quality of both the radioscopic image and the image processing). This film was made at the spool yard with a radioactive source located inside the pipe (panoramic single wall image with a Selenium 75 source).
In the following figures, all the true defects are surrounded by an ellipse (true means that the objects correspond to a defect detected by the expert on the film), and the others are said false. The X-ray image contains 6 false defects corresponding to high local variations of the weld thickness. Most of the porosities are detected and the three artificial slots are also detected but the lack of fusion cannot be detected because it is very small and has a very low contrast. We can also distinguish some undercut in the lower right part of the image detected as many small objects.
The following images represent the defects detected by both methods, where the image width is the external circumference of the pipe and the two horizontal lines define the external weld cap limits. Both images are in the same reference system for the defect position and each square of the grid is 5 mm x 5 mm.
|Fig 7: Objects detected with X-ray inspection (after image processing and registration in a common reference system)|
|Fig 8: Objects detected with US inspection (after processing, probe merging and registration in a common reference system)|
Concerning the ultrasonic defects, all slots are detected and also 3 undercuts and one lack of fusion. None of the porosities can be detected due to their very small sizes. This image contains only one false defect. We must say here that the object is said false because it is not visible on the X-ray film, because it is the only reference we have. But it might also be a lack of fusion, missed on the X-ray film.
After the fusion stage, the results are displayed for the operator, as detailed in the figure 9. The final decision is left to him, but the confidence level is given by a colour.
|Fig 9: Operator display interface : top left window : section view of the weld volume; top right window : view of the pipe circumference; bottom window : weld top view. Two cursors allow selecting a part of the weld to be displayed. A frame indicates the decision on the selected defect as a colour (red for surely a defect, orange for probably a defect and green for surely not a defect)|
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