Interpretation of Weld Radiographs

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Introduction

This training CD has been developed to introduce the basic requirements for successful interpretation of weld radiographs and to provide examples of weld defect images as generated by X-ray and gamma radiography.

The CD consists of several sections, which may be accessed from a menu in any order and at any time allowing the student to move freely around the programme contents. In addition to photographic images, both audio commentary and text is used; audio to introduce sections and describe animations and text for more detailed descriptions within the sections.

The radiographic techniques section provides descriptions and animations to show how the technique used affects the appearance of the weld image on the radiographic film.

Assessment of film quality describes the checks necessary to ensure that the radiograph complies with various requirements and is suitable for assessment of the weld quality.

Weld quality is explained by means of images and descriptions of both external and internal weld features, which can appear on the radiographic image.

There is a tutorial section presented as an interactive question and answer session.

Finally the student may undertake a self-assessment test of ten radiographs which gives a percentage success score and then permits review of the correct solution for each radiograph on completion of the test.

Radiographic Techniques Section

Access to the weld and the geometry of the component dictate which radiographic technique should be used. This section describes the common techniques for plate and pipe butt welds and animations are used to show how the appearance of the weld image on the film changes with the position of the adiation source.

Animation is a particularly effective method of transferring this knowledge as it can show both the movement of the radiographic source and the effect on the radiographic image simultaneously.

Single Wall Single ImageWith the single wall single image (SWSI) technique, radiation from the source passes through the weld and is recorded on the film. This technique is invariably applied for the radiography of plate butt welds and for the examination of pipe or vessel butt welds where access to inner and outer surfaces is available.

PanoramicThe panoramic technique is a version of SWSI where the source of radiation is positioned at the center of a cylindrical component such as a pipe or vessel with the film wrapped around the outer surface of the weld. In this way the entire length of weld can be examined with one exposure. A single piece of film or a series of overlapping films may be used to cover the entire weld length.

Figure 1. Panoramic radiographic technique animation

Double Wall Double ImageThe double wall double image (DWDI) technique used for small diameter pipe welds, typically less than 90 mm outer diameter is described. The technique allows a weld to be examined in two or three exposures according to source placement.

Figure 2. Double wall double image radiographic technique animation

The animation shows how moving the radiation source allows both the upper and lower weld regions to be evaluated. Double Wall Single ImageIn the double wall single image (DWSI) technique radiation from the source passes through both walls of the component, but only the image of the weld region closest to the film is suitable for evaluation since the weld section nearest to the source appears blurred and distorted in the image.

Figure 3. Double wall single image radiographic technique animation

On larger diameter pipes, or if the source can be moved closer to the pipe surface the upper weld image can be moved completely off the film leaving the area of interest clear for evaluation, as seen in figure 3 frame 3. Imparting this complex information without the use of animation would be both difficult and time consuming. Alignment of Radiation The detection of planar defects such as cracks is sensitive to the radiation beam direction. This animation shows how alignment of the radiation beam changes the appearance of the defect in the image.

Figure 4. Inclined crack animation

Figure 4 shows an inclined crack that appears as a faint broad shadow in the radiograph. When the radiation is directed parallel to the plane of the crack, its image becomes darker and more sharply defined. However, when the radiation is directed obliquely to the plane of the crack the image becomes faint and eventually disappears as the angle of incidence increases.

Requirements for Viewing

There are several requirements which must be met when carrying out the viewing and interpretation of weld radiographs. These are described with the aid of photographs and audio commentary. A pre-requisite for satisfactory interpretation is that the interpreter must have adequate eyesight, whether corrected or uncorrected, and be able to recognise features in the image caused by various conditions.

The ability to recognise the features on a radiograph comes largely with experience. To assist in the interpretation of a radiograph the interpreter should be aware of the radiographic technique used and should have some knowledge of the weld configuration and welding procedure used.

Viewing radiographs should be carried out using a film viewer in a darkened room. Care must be taken to avoid marking or damaging the film.

Film Quality Section

The interpretation process requires that film quality be of an acceptable standard so that weld quality can properly be assessed. In order to satisfy relevant codes and standards it is necessary that the stated requirements for radiograph identification, density and image quality sensitivity be achieved. This section discusses the monitoring of these parameters. Radiograph identification The labels required for identification of a typical weld radiograph are described. As a minimum the interpreter should be looking for job or component identity, location markers in the form of radiographic images and the date upon which the radiograph was taken (figure 5). If all three elements are present the identification is deemed acceptable for the purpose of this exercise.

Figure 5. Radiograph identification screen with pop-up text annotation

Film density The density or blackness of a radiograph affects the contrast of the image produced, contrast increasing with increasing density. For this reason minimum density requirements are specified in codes and standards. The influence of density on image quality is examined. The section includes an interactive task where the student is asked to simulate the measurement of radiograph density using the mouse by pointing and clicking at selected points on the image. The student is expected to evaluate the acceptability of the densities displayed against prescribed criteria. Radiographic sensitivity This section examines how contrast and definition influence radiographic sensitivity and how the quality of the image can be evaluated through the use of image quality indicators. It highlights the importance of ensuring acceptable image quality. Different types of image quality indicators are described and an interactive presentation shows the effect of contrast and definition on the sensitivity of the radiographic image.

Figure 6. Static electricity film artefact

Film artefacts Radiographs can sometimes be misinterpreted due to images appearing on the radiograph that are not associated with the weld. These indications, referred to as 'artefacts', can be due to handling damage or film processing faults. Those due to film damage may sometimes be identified by viewing under reflected light. This section presents some of the more commonly encountered artefacts. Figure 6 shows a radiograph having an artefact caused by the presence of static electricity.

Weld Quality Section

Having established that film quality is acceptable, the radiograph can then be examined for the presence of defects in the weld. Both surface and internal weld features appear in the image and can be assessed. Weld Surface FeaturesCommon weld surface conditions that can appear in the radiograph are described and shown as both photographic and radiographic images.

Figure 7. Excessive root penetration screen design showing a photograph of the condition and two radiographic examples

When a condition is selected from the weld surface features list, a detailed description is presented together with a photograph or diagram and thumbnails of radiographic examples (Figure 7). Clicking on a thumbnail image displays the full screen radiograph including detailed information relating to the weld itself (Figure 8).

Figure 8. Excessive root penetration

Weld DefectsThis section shows a few of the many possible radiographic images produced by internal weld defects. Examples are described using diagrams, photographs and radiographic images. As in the weld surface features section, selection of an item from the list displays a detailed description (Figure 9) and clicking on a thumbnail (Figure 10) shows the full screen view of the radiograph (Figure 11).

Figure 9. Weld cracking screen design 1

Figure 10. Weld cracking screen design 2

Figure 11. Weld cracking radiograph

Tutorial Chapter

The tutorial presents a series of interactive questions that require the student to demonstrate the extent of knowledge gained from the CD. Answers are revealed or solutions provided on completion of each question. Figure 12 shows a graphical multi-choice question in which the student must select the schematic that most closely applies to the weld radiograph. The interactive nature of the question and answer session reinforces the information presented earlier in the course and aids memory retention.

Figure 12. Tutorial multi-choice question

Assessment Chapter

The self assessment section consists of ten radiographs each of which contains one or more examples of weld defects which have been described elsewhere in the CD. The radiographs are presented as shown in Figure 13.

Figure 13. Assessment menu

The student can select a radiograph to examine by clicking on a thumbnail. This presents the image together with a list of possible weld defect conditions (Figure 14).

Figure 14. Assessment question

The radiograph can be examined in more detail by clicking on the magnify icon which presents a full screen image including details of the weld (Figure 15).

Figure 15. Full screen weld radiograph

A further magnification stage of about double size is available and because the image is larger than full screen, the image can be 'dragged' around the screen to allow close up examination of any particular area.

By selecting the appropriate defects from the defect list (Figure 14) and clicking on the 'submit' button, the ten thumbnails are again displayed. Radiographs already submitted appear faint to indicate that they have been answered. It is possible to reselect an answered radiograph and change the submission. Another useful feature of the programme allows the student to review any other section of the CD during the assessment and then return to complete the assessment.

On completion of the ten radiographs the student is presented with a percentage score and success or otherwise for each radiograph is indicated. It is now possible to re-examine each of the radiographs and to review the correct solutions for each of the images by clicking on the 'view correct answers' button that has become available on the defect list screen on completion of the assessment.

The assessment process has been designed as a formal examination with the correct answers revealed to the user only upon completion of all ten questions. Then the student is prompted to review their performance and then revisit the areas that require further study. This automatic feedback tailored for each student provides a personalised 'to-do' list' that enables effective study focused on the topics requiring further work.

Conclusions

This training programme is considered suitable as an introduction to the subject of interpreting weld radiographs and is presented in a form that allows the student to interact with the material and study at their own pace. By presenting the information in an interactive form the student gains experience of the appearance of welds and weld features as they explore the programme in addition to learning the importance of viewing procedure and film quality.