Using X-ray Computed Tomography for Dimensional Metrology

X-ray computed tomography (CT) has successfully entered the field of coordinate metrology as an innovative and flexible non-contact measurement technology for performing dimensional measurements on industrial parts. It provides unique advantages compared to conventional tactile and optical coordinate measuring machines (CMMs), giving the ability to perform non-destructive measurement tasks that are often not possible with any other measurement technology. These include for example the inspection of complex and high-value Additive Manufacturing products with high density of information and without any need to cut or destroy the components.

In the aerospace market CT can be used to inspect smaller to medium sized components as e.g. turbine blades, aluminum castings and tube welds.     With CT quantitative analyses can be performed at several stages of the different products cycles enabling the optimization of products and manufacturing processes and the evaluation of conformity to product specifications.

The three main components of an X-ray CT system are the X-ray source, rotary table and detector. Different CT system configurations exist: for example, flat panel detectors (DDA) or linear diode array detectors (LDA) can be used. With LDAs the phenomenon of X-rays scattering, relevant while scanning high density materials as in aerospace applications,  does not impact the scan however longer scanning times are required. The X-ray source-to-detector distance and X-ray source-to-object distance define the geometrical magnification of the CT scan and the voxel size of the 3D CT model of the part.  The use of variable X-ray source-to-detector distances, as offered in NSI system portfolio, is also fundamental for aerospace applications to achieve the best signal possible. In fact, being the CT technology  based on X-rays attenuation principles,  the size and the thickness of the part and the material density play a fundamental role. The bigger the component and the denser the material the more power is required for the X-rays penetration.

The output of a CT scan is the 3D model of the part on which it is possible to perform high accuracy holistic measurements of the entire workpiece without any form of contact and need to cut or destroy the part. CT also allows for material inspection and identification of internal defects such as voids, cracks, etc. For example, when inspecting composite materials, CT can also be used to identify delaminations.

Figure below shows an example of wall thickness analysis and measurement of multiple dimensional features on a turbine blade.

Figure 1 a) represents the 3D model of the blade which can be entirely navigated through user defined clipping planes. Figure 1 b) shows how it is possible to measure internal features as well as check the conformity of the airfoil profile to the specifications. In Figure 1 c) an example of wall thickness analysis is reported.

Adjacent figure instead reports an example of porosity analysis on a tube weld.

In this case the color bar represents the different pore sizes which are also visible on the 3D CT model. CT in fact offers the unique capabilities to locate the porosities in the 3D model of the part and to provide information on the different porosities volumes. The size of the porosities or defects that can be detected depends on the scan resolution which is also a function of the part size, geometry and material. Advanced scanning techniques such as NSI Subpix allow to achieve an improved resolution and therefore a larger field of view for a given resolution.

Other CT applications include nominal/actual comparisons in which the volumetric model of the actual part is registered and compared to its nominal model (e.g. the CAD model), and fiber- analysis for composite materials.

Compared to conventional measurement technologies therefore CT provides a wide series of advantages, which include the ability to perform holistic measurements of the component on complex and/or non-accessible features in a non-contact and non-destructive way, and with high density of information. In aerospace applications this is fundamental since many times the high cost of the parts does not allow for destructive testing. CT also enables to evaluate in a relatively short amount of time the conformance of the parts before e.g. high-cost machining processes. When measuring for example the freeform surfaces of turbine blades CT can provide a high density of points in a shorter amount of time than conventional tactile CMMs, and being a non-contact technique there is no need for probe tip compensation when inspecting free form surfaces.

Fundamental factors to be considered when using CT include the achievable geometrical magnification which depends on the part size and geometry, the part material and thickness.