Table of Content
- What is Radiographic Testing?
- Basic Principles of Radiographic Testing
- Advantages of Radiographic Testing
- Disadvantages of Radiographic Testing
- Key Takeaways
The progress of an intelligent species in Earth's transient yet ever-nurturing environment depends strongly on the technological and structural foundations being reliable and retaining integrity to variable ambient stimuli. Electro-magnetic Radiation is abundant in the universe with some possessing the ability to ionize other atoms by detaching electrons on account of greater energy per Quantum.
Radiography has been employed as a tool for structural imaging since the discovery of X-rays by William Röntgen in 1895, finds its purpose in medical imaging and the usage of Ionizing radiation (X- Rays and Gamma Rays) for industrial imaging Known as Radiography testing. Radiography testing is among the contemporary NDT Techniques employed for the examination of engineering structures without the risk of compromising structural integrity. These NDT Inspections involve the propagation of Ionizing Electromagnetic radiation through the structure under observation while recording the Observations on a Radiographic Film for structural deformities and imperfections. The versatility associated with this Method of NDT Testing finds widespread usage in all sectors of industry enabling remote yet highly specific inspections furthering the real-time capabilities of structural examination in the most hostile and inhospitable environments.
What is Radiographic Testing?
Radiography Test or Radiographic Test (RT) stands as one of the most used finding applications in Engineering, Architecture, medicine, and security among other sectors of Industry and technology. This testing methodology involves the use of Ionizing Electromagnetic Radiation (X-rays or Gamma Rays) propagated through the subject/structure under observation While the recordings are observed on a radiographic plate for further NDT X-ray testing.
The Image thus recorded on the plate is inspected using image darkness as a guide for object thickness and as a composition indicator along with variations in the image indicating deformities and discontinuities in the structure.
Basic Principles of Radiographic Testing
As discussed in the previous sections, X-rays were discovered by Wilhelm Röntgen in 1895, 116 years ago. These electromagnetic waves have short wavelengths with a measure of less than a hundred nanometres (nm). Gamma Rays were discovered in 1900, by a chemist named Paul Villard. The isolation of Radium by Marie and Pierre Curie made this discovery possible. Gamma waves, like X-rays, have a short wavelength, the shortest wavelength in the electromagnetic spectrum.
X-rays and Gamma rays are not visible to the human eye and do not have a charge or mass. Electrically and magnetically charged fields do not affect the travel of these waves, and they move un-wavered in a straight line. They also have short wavelengths. The penetrative power of a wave is inversely proportional to the wavelength. Hence, the shorter the wavelength, the greater the penetrative power. This ability aids in analyzing the entire internal structure of a specimen.
In Radiography testing, a specimen is held or placed in the path of the wave and flanked by the source of the X-ray or Gamma rays and the film used to capture the resulting image. The intensity and clarity of the image formed depend on the quantity of radiation that successfully penetrates and passes through the specimen under study. A lighter resulting image is formed if there is less exposure of the film to the radiation, whereas a darker image is formed when there is more exposure of the film.
The types of radiographic sources available are:
- Conventional Sources
- Micro-focus X-ray equipment
- Nano-focus X-ray equipment
- Linac
- Betatron
- Synchrotron
- Isotropic Sources like Iridium 192, Cobalt 60, Thulium 170, Ytterbium 169, Caesium 137, and Selenium 75.
The types of radiographic detectors available are:
- Radiographic films with grain sizes ranging from D4 to D7
- Radiographic Image intensifiers
- Vidicons that are X-ray sensitive in nature.
- Fluorescent screens and charged coupled devices.
- Imaging plates
- Digital flat panels like Amorphous Selenium Panels and Amorphous Silicon panels.
- Linear diode arrays
A vital part of Radiographic Imaging is the contrast of the subject, film contrast, and image definition. These factors are affected by multiple factors, such as:
- Energy is used in the process.
- Intensity of waves
- Scattered radiation is caused by the interaction of beams with the specimen under study.
- Focal spot size
- Characteristics of the detector used.
When Radiography waves like X-rays and Gamma rays pass an object, they interact with its internal structures and are scattered. Therefore, on impinging X-rays or Gamma rays through a specimen, and using a detecting surface on a detector, the operator may obtain an image of the specimen, with its internal structures and deformities. This image is two-dimensional in nature and can be inferred by an experienced operator who can observe the changes in contrast and composition of the image and gauge if the specimen has defects or deformities.
Due to the two-dimensional nature of a film image, it might be difficult to obtain data on defects that alter the thickness of the object. The obtained image is also highly dependent on the orientations of the flaws concerning the radiation beam applied to the specimen. Radiography quality can be measured using devices called Image Quality Indicators (IQIs). This device measures the level of penetration of waves into a specimen and is also called a penetrometer. This device does not aid in detecting flaws and is instead used to measure the accuracy and sensitivity of the process.
Image Quality Indicators are of two types namely Hole type Image Quality Indicators and Wire Image Quality Indicators. Care should be taken by operators while using penetrative waves of any nature as they alter the chemical nature of the things the waves propagate through. This can be life-threatening to the operator and careful use of protective gear like lead boundary walls, lead suits, and shoes should be compulsory. Care should be taken by operators while using penetrative waves of any nature as they alter the chemical nature of the things the waves propagate through. This can be life-threatening to the operator and careful use of protective gear like lead boundary walls, lead suits, and shoes should be compulsory.
Advantages of Radiographic Testing
Unlike many other NDT, RT techniques overcome the general limitations and inability to test internal structures of materials and complex geometries. The benefits of Radiographic testing are vast and constantly growing in number, some of which are:
- Radiographic testing is not limited to test subjects of specific materials and hence can be used for a variety of materials and many applications.
- Data losses and the need for thorough inferences and meticulous transcription of test data are eliminated. The data obtained from radiographic testing is permanent, visually represented, and can be either digitized directly or developed on a film.
- Radiographic waves like X-rays and Gamma rays are highly penetrative in nature and can infiltrate the material, giving clear imagery of the internal structure of the material under test.
- Fabrication errors that sometimes reflect on a material surface can often be misinterpreted by many other NDT Techniques, as the inability of the technique to interpret sub-surface and internal defects may make the defects appear as minor surface defects.
- Radiographic testing can detect internal defects accurately while providing a clear idea of the nature, size, and depth of the deformity. This makes the process of detecting fabrication errors simple.
- Discontinuities in the structure of the test subject can be analyzed by radiographic testing techniques.
- Surface preparation is skipped in radiographic testing and hence the process is time-effective.
- The accuracy provided by radiographic testing is impeccable and the methodology can detect minute flaws.
Disadvantages of Radiographic Testing
Radiographic testing is a highly sensitive technique and can provide ample benefits to the quality testing phase and intermediate analysis phases of a production process. This methodology, however, like every other scientific technique comes with limitations that may or may not be avoidable or resolvable. Those limitations include the following:
- Radiographic equipment is limited to the direction in which the impeding radiographic waves are applied. This makes it difficult to obtain sufficient testing reach in test subjects with complex geometries.
- Radiography testing specimens can only be tested from two sides as the apparatus used in radiographic testing can only be used in specific orientations.
- Specimens with defects oriented parallel to the surface of the material and angular defects are difficult or unlikely to be picked up by radiographic testing.
- X-rays and Gamma rays are extremely penetrative rays in the electromagnetic spectrum. These rays can interact and pass through clothes, plastic, human tissue, etc. Hence extra measures need to be carried out to protect the operators and workers in the vicinity of the testing machines to ensure their safety. Lack of regard for such measures can lead to long-term damage to the humans in the proximity of the Radiographic testing machine.
- Radiography testing equipment is designed to perform analysis using highly penetrative waves like X-rays and Gamma rays and hence the equipment and peripherals are expensive.
- The depth of indications is hard to infer from the results of a radiographic examination.
- The impinging radiographic waves must be oriented in accordance with the angle of the discontinuities, it will result in the test results not detecting such defects.
Key Takeaways
- Radiography is the preferred method for NDT due to its ability to eliminate limitations based on materials, size, and testing time.
- Newer techniques have been developed to meet evolving needs, allowing for accurate analysis of materials and specimens.
- Radiography has been used in medicine for centuries, but it is crucial to consider theoretical data for industrial evaluation.
- New techniques, such as films, real-time systems, and digital detectors, allow for automation and remote control, eliminating human involvement in the testing process.
References
1. AERB
2. MDPI
3. Rajagiri School of Engineering & Technology
4. Cockcraft
