Radiographic testing is an NDE technique, which involves the usage of either gamma rays or x-rays as radiation sources in order to examine the internal structure of a test component. These sources use shorter wavelength and hence higher energy level versions of electromagnetic waves. RT has been proved to have more advantages compared to various other non-destructive examination techniques. It can be used on a wide variety of materials, highly reproducible and the gathered data can be stored and reused for analysis later on. It is widely used in petrochemical industry to inspect machinery, inspect weld repairs and to detect flaws.
Types In Radiography
RT techniques are broadly classified as conventional radiography and various forms of digital radiographic techniques. Each one has their own benefits and limitations.
A sensitive film is used in conventional radiography that reacts to the radiation emitted for capturing an image of the test piece. This image is used to identify the flaws or damages. The greatest disadvantage of this technique is that these films cannot be reused and consumes a lot of time to interpret the results.
Digital radiography does not require any usage of films unlike conventional radiography. It instantaneously displays the radiographic images using digital detector on a computer screen. The exposure time involved is much shorter and hence the images can be more quickly interpreted. In addition, digital images offer much higher quality which enables it to be employed in flaws identification in a material, existence of any foreign objects, weld repairs and in inspection for corrosion under insulation.
Computed radiography, real-time radiography, direct radiography, and computed tomography are the four most commonly used digital radiography techniques in the chemical processing, oil and gas industries. Computed radiography employs a phosphor imaging plate in the place of the film in conventional radiography technique. This technique is comparatively quicker than conventional radiography but slower compared to direct radiography. Direct Radiography (DR) is very similar to computed radiography but differs only in the method of image capturing. This technique offers higher quality images besides being fast. However, it is much expensive compared to computed radiography. Real-time radiography (RTR) works by emitting radiation through the test object. The resultant digital image can be viewed and analyzed in real time. These digital images are easier to be stored, transferred and archived for further reference. But RTR has limited image resolution and a lower contrast sensitivity which impacts the image quality. Computed tomography (CT) collects hundreds to thousands of 2D radiographic scans, which are later super imposed to create a 3D radiographic image. CT images readily provide the characteristics of the internal structure of the test object such as dimensions, internal defects, shape and density.
Benefits and Limitations
Radiographic testing is extensively used on welding and castings. It is well suitable for testing broken wires, cracks, foreign material, unsoldered connections and misplaced components in semiconductor devices. Non-metallic materials, ferrous alloys and composites as well can be radiographed.
Radiography is comparatively expensive than the other NDT methods. Radiographic laboratories require relatively large space allocations and capital investments. Operating personnel should be heavily shielded for protection from high activity sources. Even with proper orientation, tight cracks in thick objects are undetectable. Similar is the case with minute discontinuities such as micro-fissures and micro-porosity. Laminations cannot be detected at allude to their unfavourable orientation using radiography techniques. X-rays and gamma rays as well known to cause severe health problems like blindness, sterility, blood cells and skin damage and even disability or death. Thus the protection of the operating personnel as well as those in the vicinity becomes utmost important. Such high standard safety needs restrict the usage of radiographic testing.