Radiographic testing
Selecting and interpreting industrial X-ray and gamma-ray examinations, with film and digital image-quality limits kept separate from product acceptance.
Tags: inspection, nondestructive-testing, radiography
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Radiographic testing (RT) uses X-rays or gamma rays to image variations in attenuation through a part. Detectability depends on the technique and part geometry. TWI radiography principles
For metallic fusion welds, ISO 17636-1:2022 specifies film radiographic technique. It leaves acceptance levels to the applicable evaluation criteria and points to the ISO 10675 series. A readable image still needs the specified image-quality checks and contractual acceptance rule. ISO 17636-1:2022
What the examination can establish
Illustrative disposition: a weld image shows no reportable indication, but the required IQI feature is not visible. Record an inadequate examination and resolve the technique problem before issuing acceptance. Conversely, a technically satisfactory image containing an indication still requires classification and comparison with the contractual criteria. These are separate decisions; neither should be replaced with “RT passed.” Image-quality basis; acceptance-scope distinction
A conventional radiograph is a projection through the material along a beam path. Overlapping features contribute to the same image, so their apparent position does not by itself establish depth. Porosity and inclusions may produce useful contrast; tight planar discontinuities can be difficult to detect when their orientation produces little change in the radiation path. TWI’s July 1999 research on thick-section welds found that planar-flaw detectability deteriorated with increasing steel thickness and increasing misorientation between the flaw and beam. That study is evidence of technique dependence, not a universal detection threshold. TWI thick-section weld research
A clear image therefore cannot support an unrestricted “crack-free” conclusion. Define the target discontinuities, expected orientations, examination volume, and inaccessible regions. Where a lamination makes little difference to the through-path thickness, another method or an additional direction may be necessary. Surface appearance and radiographic appearance answer different questions. TWI principles and orientation limitations
Sources and image acquisition
X-ray generators produce radiation electrically; source energy and exposure are selected for the examination. Gamma-radiography devices contain radioactive material that continues emitting when not in use, requiring shielding and controlled handling. Turning off electrical equipment and securing a radioactive source are different operations. Neither source family has a universal material-thickness range or exposure recipe. US EPA industrial radiography overview
| Acquisition route | How the image is obtained | Distinction to preserve |
|---|---|---|
| Film radiography | Exposed film is chemically processed and viewed | Processing and viewing affect usable image quality |
| Computed radiography (CR) | A reusable storage-phosphor plate is exposed and then scanned | Scanner, plate, and acquisition settings form a system |
| Digital detector array (DDA/DR) | A detector converts radiation into an electronic image | Detector corrections and performance stability matter |
| Computed tomography (CT) | Multiple projections are reconstructed into sections or a volume | A reconstruction is a different examination from a single projection |
These acquisition routes do not remove the need for suitable source positioning, coverage, and interpretation. ASNT describes CR as laser readout of a latent phosphor-plate image, direct digital acquisition through a detector array, and CT as reconstruction from multiple viewing angles. The word “digital” alone does not identify the technique used. ASNT radiographic testing methods
Image quality is a system property
Film quality includes optical density, contrast, definition, and sensitivity. Optical density describes transmitted viewing light; it is not material density. An image quality indicator (IQI), such as a wire or hole-type device, provides a known feature against which image quality can be assessed. Select its material, size, location, and required visibility under the applicable procedure. An IQI result is evidence about the image, not a direct measurement of an unknown crack. TWI radiograph quality guidance
Digital quality involves more than pixel spacing. ASTM E2737-23 identifies basic spatial resolution, geometric unsharpness, scatter, signal-to-noise ratio, contrast-to-noise ratio, image lag, and possible burn-in, along with integration time, software, radiation quality, and detector corrections. Its scope concerns DDA baseline and periodic performance monitoring; it is not an acceptance test of a DDA and is not intended for isotope use. Apply the correct performance practice to the actual system. ASTM E2737-23, scope and significance
Bounded equipment example: Waygate lists 75-micrometre pixel resolution for its DXR75P-HR and 100 micrometres for its DXR100P-HP. These are manufacturer detector descriptions, not guaranteed minimum detectable flaw dimensions. A 75-micrometre pixel specification cannot independently establish that a 75-micrometre discontinuity will be resolved in a production component. Geometry, contrast, noise, and the required technique remain relevant. Waygate portable DXR portfolio
Technique qualification and acceptance
Choose the governing product/construction specification and required edition before setting up the examination. ISO 16371-2:2017 describes general CR techniques for metallic materials using storage-phosphor plates and explicitly does not provide imperfection acceptance criteria. It refers examination geometry to the relevant basic and product standards. An image meeting a radiographic technique standard still needs evaluation against the specified product criteria. ISO 16371-2:2017, scope
For planning, identify the material and penetrated thickness, source/detector arrangement, views and coverage, image-quality checks, and interpretation requirements. For digital work, establish the detector baseline and periodic checks before relying on production images; retain settings that make repeated assessments comparable. Reprocessing or changing the displayed contrast does not repair missing coverage or recover information that was never acquired. ASTM E2737-23, system monitoring
Execution and usable records
RT needs trained personnel and radiation protection arrangements appropriate to its jurisdiction and equipment. The IAEA’s Radiation Safety in Industrial Radiography, SSG-11, published in 2011, covers both shielded facilities and mobile site work. Plan the work with the authorized radiography organization; this reference is not a source-deployment or exclusion-distance procedure. IAEA SSG-11 publication and scope
A useful examination record connects the component and location map to images, technique/procedure revision, acquisition method, source and detector identification, coverage, quality checks, limitations, evaluation criteria, and disposition. Preserve the examination record rather than an isolated screenshot or an untraceable “acceptable” statement. ASNT emphasizes lasting image records and the evidence of sensitivity provided by IQIs. State unexamined areas explicitly so later users can distinguish acceptable evaluated regions from regions that were never examined. ASNT RT records and personnel