What is Computed Tomography (CT) Scanning?
Computed Tomography (CT) Scanning, also known as industrial CAT scanning, is based on radiographic technology, which provides an ideal testing technique in order to locate and measure volumetric detail in three dimensions. The ability of x-ray to penetrate through varying densities allows CT inspection results to provide nondestructive physical characterization of internal features and structures of a part or component.
Industrial CT scanning is able to access internal data equally well on metallic and nonmetallic specimens, solid and fibrous materials, and smooth and irregularly surfaced objects.
Benefits of using Industrial CT Scanning
The main advantage for this technology is the fact that it provides highly accurate testing results without applying any pressure, stress or external forces on the subject being scanned. The following is a brief list of all the major benefits this technology offers:
- Nondestructive testing method
- Quantitative density evaluations
- Geometrical representations
- Cross sectional data and 3 dimensional data
- Images are easy to interpret than conventional radiographic data
- Quality control tool for failure investigation and preproduction inspection
- Internal and external part inspection
A potential drawback with CT imaging is the possibility of artifacts in the data. An artifact is anything in the image that does not accurately reflect the true geometry in the part being inspected. Because they are not real, artifacts limit the user’s ability to quantitatively extract density, dimensional, or other data from the image. Therefore, as with any technique, the user must learn to recognize and be able to discount common artifacts. Some image artifacts can be reduced or eliminated with CT scanning by improved scanning practices, and others are inherent in the methodology. The reconstruction problem places a number of severe constraints on the data. Since the presence of random noise corrupts the information, one would expect the minimum sampling requirements to be greater than they are for noise-free data as well as to be sensitive to the algorithm employed.
How CT Scanning works
From being used for medical applications in the 1970’s, CT technology has transformed into a conventional method of testing and inspection for industrial applications. With the use of an X-ray source, detector panel and a rotary table, users are able to access accurate internal and external data set of a testing subject. As a part rotates 360 degrees on a rotary table, a pre-determined number of 2D x-ray cross sectional slices are captured, depending on the amount of data required.
When the first CT scanning instruments were introduced in the 1970’s for medical applications, finite-series expansion algorithms, algebraic reconstruction techniques and simultaneous iterative reconstruction techniques were utilized for reconstruction of the cross-sectional slices and data sets. These methods of reconstruction have become obsolete for commercialized industrial CT scanning instruments, due to their lack of speed. For use of industrial CT systems today, transform methods, a restorative algorithm (based on analytical inversion formulas) are implemented as they as much faster than traditional methods of reconstruction of CT data set. This method of reconstruction also allows for far greater image quality and accuracy.
Once the data set has been re-constructed into a 3D rendering, users are able to access internal and external part features including geometries, failures and explore the interaction of internal structures and assemblies.
Types of Industrial CT Scanning Equipment
Generally, there are two basic types of medical and industrial CT scanners. There are parallel x-ray beam scanners and fan x-ray beam scanners. Parallel x-ray beam scanners can be categorized into first and second generation systems, similarly fan x-ray beam systems are categorized into third and fourth generation systems. These different types of CT scanners have been named according to their method of data collection, as opposed to the shape or structure of the beam itself.
An artifact (anything in the imaging results that does not reflect the true geometry in the part being scanned) can limit users access to retrieve accurate density, dimensions, failures and other data. Hence, qualified and experienced users must be appointed to carefully understand and avoid artifacts in the resulting data. This can be done by enhanced scanning practices and processing CT systems with updated calibration as necessary.
Industrial CT Scanning: Types of Analysis
There are many different routes users are able to access with industrial CT dataset for extensive part analysis. The most common types of analysis include the following:
Porosity Analysis – Industrial CT scan results can locate internal and external part failures, such as porosity and inclusions. These defects can be located, measured and color coordinated according to project requirements and tolerance.
Wall Thickness Analysis – Industrial CT scan results can analyze the distribution of material within a part. This can assist users in identifying areas prone to weakness.
Reverse Engineering – Industrial CT scan results can provide an intermediary CAD file in point cloud or polygon format. The data can be exported in different formats for ease of manipulation and use.
Part to CAD Analysis – Industrial CT scan results can be compared against part CAD file to identify any deviations from initial design.
Part to Part Analysis – Industrial CT scan can be conducted on multiple identical parts to identify any deviations.
Dimensional Analysis – Industrial CT scan results allow users to access internal and external part geometry. These results can be used for multilpe different applications including GD&T programming, coordinate measurement, First Article Inspection reports and PPAP dimensional requirements.
Composite Analysis – Industrial CT scan results assist users in evaluating the internal structure of composite material, non-destructively.
Failure Analysis – Industrial CT scan results can be used to evaluate internal failures, such as structural integrity, cracks, deviations from initial design, foreign material and porosity.
ASTM E1441-11, Standard Guide for Computed Tomography (CT) Imaging, ASTM International, West Conshohocken, PA, 2011, www.astm.org