Digital Radiographic Testing

Digital Radiographic Testing (DR/CR) is a non-destructive testing method based on the principle that X-rays or gamma rays passing through a material are absorbed to differing degrees at discontinuities, and it produces the result as a digital image rather than a film. As in conventional radiography, a radiation source (an X-ray tube or an isotope such as Ir-192, Se-75 or Co-60) is placed on one side and a detector on the other. The radiation is absorbed more strongly in thick or dense regions of the material, while it passes with less absorption through low-density regions such as voids, porosity, cracks or thinned sections. These differences in radiation intensity reaching the detector are converted into a grey-scale image that reveals the internal structure of the material. Because the method images internal discontinuities in their true geometric projection, it provides direct visual evidence of the location, size and type of the flaw.

The distinguishing feature of the method is the use of reusable imaging plates or direct digital detector arrays (DDA) as the detector. The phosphor-based imaging plate used in computed radiography (CR) stores the energy as a latent image when exposed to radiation; the plate is then stimulated by laser scanning in a reader and the released light is converted into digital data. Since the erased plate can be reused, consumable costs and the need for processing chemicals are eliminated. In direct detector systems the image appears on screen within seconds of the exposure. In both approaches the resulting image is stored across a far wider range of grey levels than the narrow dynamic range of film, which requires tightly controlled exposure.

The most important technical advantage of the digital image is that, being numerical, it can be post-processed. From a single exposure the operator can change contrast and brightness, apply edge sharpening, magnify regions for closer examination and measure wall thickness directly on the image. Image quality is verified with wire-type or hole-type image quality indicators (IQI/penetrameter); the spatial resolution of the system (basic spatial resolution, SRb) and the signal-to-noise ratio must meet the quality class required by the relevant standards (for example the normal or enhanced technique). To limit geometric unsharpness, the source-to-detector distance, the beam geometry and the exposure parameters are selected according to the source-material combination.

In terms of application depth, the method is particularly effective in detecting volumetric and planar discontinuities in weld seams, such as porosity, slag inclusions, lack of penetration and cracks. On insulated pipelines, corrosion and wall thickness loss can be monitored using the double-wall technique (profile radiography) without shutting the line down or removing the insulation. The fact that digital data is archivable and repeatable makes it possible to compare the same area over time (corrosion monitoring). Requiring generally shorter exposure times than film also provides an advantage in terms of radiation safety and site efficiency.

During evaluation the images are interpreted by qualified Level II/III personnel according to the acceptance criteria of the relevant product or manufacturing standard (for example the quality level limits for welds). Every indication detected is reported together with its type, size, location and accept/reject status, supported by digital measurement evidence. The inspection is carried out within the framework of general radiation safety and dose limitation rules, in designated and controlled areas.

TÜV AUSTRIA SILA KALİTE performs digital radiographic testing within the scope of its TS EN ISO/IEC 17020 accreditation, with the impartiality of an independent inspection body and in accordance with the relevant application standards. The findings obtained therefore constitute valid, traceable and reliable evidence in third-party audits and manufacturing acceptance processes.