Radiographic Testing overview
Radiographic testing and inspection has been utilized for industrial applications for many years. Earlier, radiography was referred to as x-ray. In order to meet the growing industry needs, different and technologically advanced methods of testing were introduced and classified under radiographic testing techniques. Users requiring radiographic testing for part analysis usually seek out nondestructive testing labs to outsource radiographic testing projects for quick and accurate results.
What is radiographic testing?
Radiographic testing & inspection involves exposing a test subject to varying doses of radiation which is captured by a recording medium such as a detector panel or a radiographic film. Radiographic testing is a method of inspection categorized as Nondestructive Testing (NDT). This means that the process of inspecting a part or test subject will cause no harm or destruction to the part in its entirety.
Industrial radiography utilizes radiation to test subjects for failure detection and reliability assessment. In doing so, there are two distinct ways of applying industrial x-ray radiation for analysis purposes. For thin material, electrically generated x-radiation (x-rays) are commonly used, whereas gamma radiation (gamma rays) are used for thicker and denser materials. Gamma radiation is released by decaying radioactive materials, commonly Iridium-192 and Coablt-60. Co-60 are regularly used for higher density materials (anything with a density higher than 3 inches of steel).
Interpretation of Radiographic images
Generally, regardless of the radiographic technique, the resulting image have similar interpretation. Radiation penetrates through a test subject and reaches the detector or radiographic film. After capturing the x-rays, the resulting image is viewed in grayscale. Higher density areas absorb radiation, whereas areas with lower density or defects such as porosity or cracks, do not absorb radiation. Radiation passes through defects and is depicted in a darker greyscale value (black) on the resulting image. Areas with higher density which absorb the radiation are shown in lighter greyscale value (white).
History of Radiographic testing
Professor Wilhelm Rontgen had discovered x-ray technology in 1895. The “X” signifies an unknown form of radiation, which Rontgen assumed would eventually change once the form of radiation is further understood. X-rays quickly became a popular method of inspection for medical applications to diagnose and analyze the human body. Soon after, higher doses of x-rays were utilized for industrial applications, primarily aerospace part evaluation. Today, x-ray technology is used across all industries for quality control purposes. Over the years, advancements in the technology allowed users to conduct multiple different types of radiographic testing techniques including computed tomography, computed radiography and digital radiography.
Types of radiographic testing
There are three distinct types of radiographic testing and inspection techniques, as highlighted by the American Society of Nondestructive Testing (ASNT):
- Film Radiography
- Computed Radiography (CR)
- Digital Radiography (DR)
How different radiographic testing techniques work
According to the American Society of Non-destructive Testing (ASNT), there are four distinct techniques in regards to industrial radiography. Below is a brief description of how these technique work:
Film radiography – Film radiography uses conventional x-ray film, which is detained in an envelope. The radiation passes through the film, which captures a 2D X-ray. The resulting image is processed with the assistance of special chemicals – this is referred to as the “wet-process.”
Computed radiography – Computed radiography is a combination of film radiography and digital radiography. The x-ray film is substituted by a reusable photo-stimulated phosphor (PSP) plate. The radiation passes through to the PSP plate and captures a 2D X-ray image. The image is processed with the assistance of a laser reader which translate the resulting image in digital form.
Digital radiography – Digital radiography uses a digital detector panel. A test subject is sandwiched by the x-ray source and the detector panel. Radiation shoots through the part and the detector panel captures a 2D x-ray. This is immediately transferred to a computer for review and analysis.
Benefits of radiographic testing
Radiographic testing and inspection serve many benefits for industrial applications, regardless of the industry. A few major benefits include:
- Ensures safety and reliability of part
- Identify, locate and measure defects
- Density variations can be analyzed for structural evaluation
- Options of different approached for industrial radiography
- Quick, cost effective and accurate
- Ability to use part after testing and analysis: NDT
- Qualify and approval of part components
- Allows users to make informative and qualified decisions
Uses & applications of radiographic testing
Radiography and x-ray testing is a nondestructive method of inspection that has become an indispensable quality control tool for inspection and analysis of industrial parts. Regularly applied to check fatigue, failures and defects internally in polymer based material or metal based materials, radiography has provided industry with invaluable insight. Some industries that find extensive use and applications for radiographic testing and inspection include:
- Military & Defense – ex. Ballistics
- Aerospace – ex. Castings
- Packaging – ex. Structural integrity/leak or failure analysis or package
- Automotive – ex. Piston head
- Medical Devices – ex. Stints
- Manufacturing – ex. Pre-production qualification of part
Quality of Industrial Radiographs
Although radiographic testing and inspection may seem like a relatively simple method of testing, it requires attention to detail. There are many factors that come into consideration when developing a resulting image with the highest quality possible:
Material – The higher the density of the material of the test subject, the higher the dose of radiations tends to be in order to penetrate through the material. Adversely, the lower the density of the material, the dose of radiation must be adjusted accordingly, in order to avoid resulting images with scatter. Moreover, density of a particular test subject tends to absorb radiation, with lower levels of radiation reaching the detector panel through dense material. With area that consist of defects such as cracks or porosity, radiation tends to pass right through the defect and on to the detector panel. In a resulting image, cracks and defects appear darker on the greyscale value, since no radiation was absorbed. Areas with high density material will absorb radiation, therefore, appearing lighter on the greyscale value.
Size/ thickness of material – Size and thickness of the material play a significant role in the quality of the resulting image. If the size is too large or thickness of the material is too dense, radiation may not be able to penetrate through it effectively, resulting in a low quality image. Adversely, if the part is relatively small or consist of very thin walls, the amount of radiation may be too high, which may result in a scattered image.
Contrast/sharpness/graininess – A minimum contrast is required in order to retrieve high quality images to analyze failures, such as porosity. Image contrast is the density variations between an area and the background density on the radiograph – if the contrast is clear, it will aid in realizing failures more accurately. Similarly, sharpness of the imaging results can help look into detail of the part deviations from initial design and certain defects. The graininess of an image can also determine whether the results are of quality. Aspects like the orientation, and radiation exposure may need to be adjusted in order to access better quality images.
Orientation – Depending on the purpose of inspection – whether it is to locate internal failures or evaluate wall thickness for example – the orientation of the part plays a significant role. If users know that a specific area needs to be inspected, the orientation of the part has to be placed in accordance with the x-ray or gamma ray source. If the entire part has to be scanned, the orientation of the part must be placed closer or further from the x-ray source, depending on the project needs, the dose of radiation exposure and the size of the detector panel.
Certification to operate radiographic testing equipment
In order to operate radiographic testing and inspection equipment, users must undergo a process of certification. A form of certification, governed by the Nuclear Regulatory Commission (NRC) and recognized by the American Society of Nondestructive Testing (ASNT), includes a standard of certification courses, minimum number of training and on the job hours and a written exam, to earn a certification. There are three levels of NDT certification:
Level 1 – Individuals certified with level 1 are restricted to perform only certain calibrations or tests under supervisions by a higher level individual. They are able to report results, and are expected to follow instructions regarding procedure.
Level 2 – Individuals certified with level 2 are open to set up and calibrate inspection equipment, perform inspection according to standards and codes. These individuals are able to provide instruction to level 1 technicians, as well as report, interpret and evaluate results. Have knowledge of standards and codes.
Level 3 – Individuals certified with level 3 are specialized, experienced and qualified engineers or technicians. These technicians are able to direct NDT labs, and have extensive knowledge about testing, service and manufacturing processes as well as codes and standards.