Real-time Radiography
Real-time radiography (RTR), or real-time radioscopy, is a nondestructive test (NDT) method whereby an image is produced electronically, rather than on film, so that very little lag time occurs between the item being exposed to radiation and the resulting image. In most instances, the electronic image that is viewed results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs. The fluorescent elements of the screen form the image much as the grains of silver form the image in film radiography. The image formed is a "positive image" since brighter areas on the image indicate where higher levels of transmitted radiation reached the screen. This image is the opposite of the negative image produced in film radiography. In other words, with RTR, the lighter, brighter areas represent thinner sections or less dense sections of the test object.
Real-time radiography is a well-established method of NDT having applications in automotive, aerospace, pressure vessel, electronic, and munition industries, among others. The use of RTR is increasing due to a reduction in the cost of the equipment and resolution of issues such as the protecting and storing digital images. Since RTR is being used increasingly more, these educational materials were developed by the North Central Collaboration for NDT Education (NCCE) to introduce RTR to NDT technician students.
Computed Tomography
Computed (CT) is a powerful nondestructive evaluation (NDE) technique for producing 2-D and 3-D cross-sectional images of an object from flat X-ray images. Characteristics of the internal structure of an object such as dimensions, shape, internal defects, and density are readily available from CT images. Shown below is a schematic of a CT system.
The test component is placed on a turntable stage that is between a radiation source and an imaging system. The turntable and the imaging system are connected to a computer so that x-ray images collected can be correlated to the position of the test component. The imaging system produces a 2-dimensional shadowgraph image of the specimen just like a film radiograph. Specialized computer software makes it possible to produce cross-sectional images of the test component as if it was being sliced.
How a CT System Works
The imaging system provides a shadowgraph of an object, with the 3-D structure compressed onto a 2-D plane. The density data along one horizontal line of the image is uncompressed and stretched out over an area. This information by itself is not very useful, but when the test component is rotated and similar data for the same linear slice is collected and overlaid, an image of the cross-sectional density of the component begins to develop. To help comprehend how this works, look at the animation below.
In the animation, a single line of density data was collected when a component was at the starting position and then when it was rotated 90 degrees. Use the pull-ring to stretch out the density data in the vertical direction. It can be seen that the lighter area is stretched across the whole region. This lighter area would indicate an area of less density in the component because imaging systems typically glow brighter when they are struck with an increased amount of radiation. When the information from the second line of data is stretched across and averaged with the first set of stretched data, it becomes apparent that there is a less dense area in the upper right quadrant of the component's cross-section. Data collected at more angles of rotation and merged together will further define this feature. In the movie below, a CT image of a casting is produced. It can be seen that the cross-section of the casting becomes more defined as the casting is rotated, X-rayed and the stretched density information is added to the image.
In the image below left is a set of cast aluminum tensile specimens. A radiographic image of several of these specimens is shown below right.
CT slices through several locations of a specimen are shown in the set of images below.
A number of slices through the object can be reconstructed to provide a 3-D view of internal and external structural details. As shown below, the 3-D image can then be manipulated and sliced in various ways to provide thorough understanding of the structure.
X-Ray Inspection Simulation
One of the most significant recent advances in NDT has been the development and use of computer modeling that allows inspection variables to be scientifically and mathematically evaluated. In a few cases, these models have been combined with a graphical user interface to produce inspection simulation programs that allow engineers and technicians to evaluate the inspectability of a component in a virtual computer environment. One such program, XRSIM, was designed and developed at Iowa State University's Center for Nondestructive Evaluation. The program simulates radiographic inspections using a computer aided design (CAD) model of a part to produce physically accurate simulated radiographic images. XRSIM allows the operator to select a part, input the material properties, input the size, location, and properties of a defect. The operator then selects the size and type of film and adjusts the part location and orientation in relationship to the x-ray source. The x-ray generator settings are then specified to generate a desired radiographic film exposure. Exposure variables are quickly and easily revised allowing the operator to make and see results of defect size, material, and part or defect orientation.
The almost instantaneous results produced by simulation programs make them especially valuable in education and training settings. Successful radiography depends on numerous variables that affect the outcome and quality of an image. Many of these variables have a substantial effect on image quality and others have little effect. Using inspection simulation programs, inspections can be modified and the resulting images viewed and evaluated to assess the impact these variables have on the image. Many inspection scenarios can be rapidly modeled since the shot setup and exposure can be quickly accomplished and the film-developing step is eliminated. Not only can a greater number and variety of problems be explored, but also the effects of variables can be learned and self-discovered through experimentation, which is one of the most effective modes of learning. Results are not complicated by unnecessary variables such as film processing variables and artifacts. Distractions unrelated to the primary learning exercise are eliminated. Through the use of simulation programs a more effective understanding of the scientific concepts associated with radiography will be developed.
Another important aspect of the program is that it does not require a real part for the inspections. Inspections can be simulated that would otherwise be impossible or too costly to perform outside the computer environment. Flaws of various shapes, sizes, and materials can be easily introduced into the CAD model to produce a sample set for probability of detection exercises.
It should be noted that densities produced in the simulated images may not match exactly the images produced in the laboratory using similar equipment settings. The difference between the actual and simulated radiographs are due to variations in the X-ray spectrum of various tubes and approximations made in the scattering model used to keep the computation times reasonable. As scattering effects become more dominant, the predicted density will agree less with the actual density on the radiograph. For example, when a one-inch steel sample is radiographed at 250 keV, over half of the total flux reaching the detector is due to scattering.
For more information on how the XRSIM program operates, the users manual is available here for downloading. The educational version of the program is available commercially.
Ten X-ray inspection exercises have been developed by the Collaboration for NDT Education that make use of XRSIM program. Educators can download these lessons from this site.
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