Operating Manual

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If too close to the X-ray source the geometry of the object can hamper full rotation as illustrated
in this figure.
The subject of CT is more elaborately described in the booklets (German) “Die Röntgenprüfung”
and (English) “The X-ray Inspection”, see literature reference[3].
17.4 CT for defect detection and sizing
Effect of defect orientation
Traditional radiography almost exclusively uses one single exposure from a fix position,
thus one direction of the X-ray beam. This can result in distortion of the defect image on the
film, see section 12.1, or even missing a defect. This single shot practice also applies for weld
inspection. Welds and their adjacent heat affected zones might contain planar (2D) defects,
possibly unfavourably oriented for detection. The probability of detection (POD) of
planar defects is strongly dependent on the angle O/ between the centre line of the
beam (radiation angle) and the orientation of the defect, as shown in figure 13-17.
Only transmission under an angle equal to or close to the orientation of the 2D defect
will provide sufficient contrast. Figure 14-17 illustrates this.
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Each individual detector element measures, during a short exposure period, the total
absorption across a certain angular position of the object. This information including the
coordinates is used to create a numerical reconstruction of
the volumetric data.
This process produces a huge data stream to be stored and simultaneously processed, in par-
ticular when an image of high resolution is required. A three dimensional representation
(3D CT) of the radiographic image requires vast computing capacity. With present day com-
puters, depending on resolution required, the total acquisition and reconstruction time nee-
ded for a 3D image is between a few seconds and 20 minutes.
Reverse engineering
CT offers an effective method of mapping the internal structure of components in three
dimensions. With this technique, any internal anomaly, often a defect, that results in a dif-
ference of density can be visualized and the image interpreted. These properties allow the
use of CT as an NDT tool, permitting examination of samples for internal porosity, cracks,
(de)laminations, inclusions and mechanical fit. It shows the exact location of the anomaly
in the sample providing information on size, volume and density. Due to the fact that CT
images are rich in contrast even small defects become detectable.
CT widely expands the spectrum of X-ray detectable defects in process control and failure
analysis, increasing reliability and safety of components for, e.g., automotive, electronics,
aerospace, and military applications. It opens a new dimension for quality assurance and
can even partially replace destructive methods like cross-sectioning: saving costs and time.
CT is increasingly used as a reverse engineering tool to optimise products and for failure
analysis which otherwise would require destructive examination.
CT metrology
CT systems can also be used for so-called 3D metrology. CT metrology systems replace con-
ventional physical or optical measuring devices for components with complex geometries or
measure dimensions at places with no access at all. These systems include the software to
transfer the part to be measured visible on the CT image into actual dimensions with accu-
racies of ±1 micron.
High resolution and defect sizing
In CT, absorption values are determined with a very high degree of accuracy, which
means that the contrast of an image can be varied over an extraordinaraly wide range.
Absorption/density variations of 0.02 % can be displayed in a range of density 6 and
over. This offers great possibilities for image processing.
For the most challenging X-ray inspections the best results are obtained by high
resolution CT using microfocus or nanofocus X-ray sources. The achievable resolution or
image sharpness is primarily influenced by the focal-spot size of the X-ray tube.
Defect detectability down to 250 nm (0.25 microns) is possible.
Increasingly, 3D CT is used on high-quality castings often in combination with automatic
object and defect identification.
Sometimes the magnification factor is not sufficient. In that case the factor can be
increased by scanning only the region of interest. To achieve maximum magnification, the
region of interest should be within the X-ray beam (cone) as illustrated in figure 12-17.
Fig. 12-17. CT scan of object detail
Fig. 13-17. Effect of position of X-ray source
versus defect orientation
X-ray
source
Object
Limited rotation
Dedector
Focus-object
distance
Highest resolution/
magnification
Focus- detector
distance
Region of
interest
Region of
interest
Fig. 14-17. Image formation versus relative radiation angle
X-ray beam
Positions of X-ray source
Relative radiation angle
Good image
Poor image
No image
Weld
Film
Defect
Height
Defect
Width
Wall
thickness
Angle O/