What is Radiography?
Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography (“diagnostic” and “therapeutic”) and industrial radiography.
Industrial radiography is a non-destructive testing method that allows many types of manufactured components to be examined to verify the internal structure and integrity of the sample. Industrial radiography can be done with either X-rays or gamma rays.
Both are forms of electromagnetic radiation. The difference between different forms of electromagnetic energy is related to wavelength. X-rays and gamma rays have the shortest wavelength and this property allows various materials such as carbon steel and other metals to penetrate, pass through, and exit. Specific methods include industrial computed tomography.
What is Radiography Testing?
Radiographic Testing (RT) is a non-destructive testing (NDT) method that uses either x-rays or gamma rays to examine the internal structure of manufactured components identifying any flaws or defects.
In the radiography test, the test part is placed between the radiation source and the film (or detector). The material density and thickness differences of the test part attenuate (i.e. reduce) the penetrating radiation through interaction processes that include scattering and/or absorption. The differences in absorption are then recorded on film(s) or electronically.
Various imaging methods are available in industrial radiography, techniques for displaying the final image; i.e. Film radiography, real-time radiography (RTR), computed tomography (CT), digital radiography (DR) and computed radiography (CR).
There are two different radioactive sources available for industrial use; X-ray and Gamma-ray. These radiation sources use higher energy level, i.e. shorter wavelength, versions of the electromagnetic waves. Because of the radioactivity involved in radiography testing, it is of paramount importance to ensure that the Local Rules is strictly adhered during operation.
Computed Tomography (CT) is one of the lab based advanced NDT methods that TWI offers to industry. CT is a radiographic based technique that provides both cross-sectional and 3D volume images of the object under inspection.
With these images, the internal structure of the test object can be examined without the overlay associated with 2D radiography. This function enables a detailed analysis of the internal structure of a large number of components.
Why is a radiography test required?
Radiographic testing provides a permanent record in the form of an X-ray image and provides a highly sensitive image of the internal structure of the material. The amount of energy absorbed by the object depends on its thickness and density. The energy that is not absorbed by the object causes exposure to the radiographic film.
Types of Radiography
There are numerous types of RT techniques, including conventional radiography and digital radiography tests of multiple forms. Each one works slightly differently and has its own advantages and disadvantages.
Conventional radiography uses a sensitive film that reacts to the emitted radiation to capture an image of the part to be tested. This image can then be examined for signs of damage or defects. The main limitation of this technique is that movies can only be used once and it takes a long time to process and interpret.
In contrast to conventional radiography, digital radiography does not require a film. Instead, a digital detector is used to display radiographic images almost instantly on a computer screen. This allows for a much shorter exposure time so that the images can be interpreted more quickly. In addition, the digital images are of a much higher quality compared to conventional radiographic images.
With the ability to capture high-quality images, the technology can be used to identify material defects and foreign objects in a system, inspect weld repairs, and inspect insulation for corrosion.
The four most common digital radiography techniques used in the oil and gas and chemical processing industries are computed radiography, direct radiography, real-time radiography, and computed tomography.
1) Computed Radiography
Computed radiography (CR) uses a phosphor imaging plate to replace film in conventional radiography techniques. This technique is much faster than film radiography, but slower than direct radiography. CR requires several additional steps compared to direct radiography.
First, the image of a component is indirectly captured on a phosphor plate and then converted into a digital signal that can be displayed on a computer monitor. Image quality is fair but can be improved with appropriate tools and techniques (e.g. adjusting contrast, brightness, etc. without compromising integrity). It is important to understand how tools like adjusting contrast affect the image. Care should also be taken to ensure that minor defects are not hidden after improvements.
2) Direct Radiography
Direct radiography (DR) is also a form of digital radiography and computed radiography very similar. The main difference is in the way the picture is taken. In DR, a flat panel detector is used to take a picture directly and display that picture on a computer screen. Although this technique is fast and produces higher quality images, it is more expensive than computed radiography.
3) Real-Time Radiography
Real-time radiography (RTR) is, as the name suggests, a form of digital radiography that takes place in real time. RTR emits radiation through an object. These beams then interact with either a special phosphor screen or a flat panel detector that contains microelectronic sensors. The interaction between the panel and radiation creates a digital image that can be viewed and analyzed in real time.
The brighter areas in the picture are the result of more radiation touching the screen. This corresponds to the thinner or less dense section of the component. Conversely, darker areas are the result of less radiation interacting with the screen and indicate where the component is thicker.
In addition to the possibility of making the images available more quickly and analyzing them in real time, RTR offers several other advantages. For one thing, digital images do not require physical storage space and are therefore easier to store, transfer and archive than films.
On the other hand, this method also has several disadvantages. Compared to conventional radiography, RTR has a lower contrast sensitivity and limited image resolution. Images created via RTR often suffer from uneven lighting, limited resolution, poor sharpness and noise. These factors have a major impact on image quality.
4) Computed Tomography
Computed tomography (CT) is a technique that takes hundreds to thousands (depending on the size of the component) of 2D radiographic scans and superimposes them to create a 3D X-ray image.
In an industrial setting, CT can be achieved in two ways. In one method, the component to be inspected remains stationary while the radiation source and the X-ray detector rotate around the component. This technique is more likely to be used for large components. The second method is to have the radiation source and X-ray detector remain stationary while the component is rotated 360 degrees. This second technique is more useful when the component is small or has complex geometry.
Although this technology is contemporary, expensive and requires a large amount of data storage, CT provides highly accurate images, is repeatable and reproducible, and minimizes human error.
Benefits of Radiography Testing
- Can inspect assembled components
- Minimum surface preparation required
- Detects both surface and subsurface defects
- Provides a permanent record of the inspection
- Verify internal flaws on complex structures
- Isolate and inspect internal components
- Automatically detect and measure internal flaws
- Measure dimensions and angles within the sample without sectioning
- Sensitive to changes in thickness, corrosion, flaws, and material density changes
Applications Of Radiography Testing
Radiographic Testing is widely used in the;
- Aerospace industries
- Military defense
- Offshore industries
- Marine industries
- Power-gen industries
- Petrochem industries
- Waste Management
- Automotive industries
- Manufacturing industries
- Transport industries