Reference standards for XCT measurements of additively manufactured parts

Abstract

An increasing number of industrial sectors are considering the potential of additive manufacturing as an asset to improve their production. Indeed, additive manufacturing enables the fabrication of very complex geometries and inner cavities that cannot be manufactured with conventional techniques. However, in critical sectors such as aerospace, defence and medical, the parts need to be certified, which requires parts to be non-destructively characterised in terms of flaws, geometry and dimensional accuracy. X-ray computed tomography is the only current 3D volumetric technique, which is suited for the non-destructive analysis of internal flaws, geometry and measurements of internal dimensions and roughness. However, regardless of its huge potential, X-ray computed tomography is not as mature a technology for dimensional metrology as compared to conventional tactile coordinate measuring machines. In most cases there is no traceability to SI units in the dimensional domain. Recently, numerous reference standards (i.e. physical artefacts) addressing X-ray computed tomography dimensional accuracy have been published, but they do not necessarily address the calibration of XCT system in connection with AM parts. In this work, a new and improved standard in three different materials has been designed with a dual purpose: Fully calibrating X-ray computed tomography for dimensional measurements while being representative of additively manufactured parts in terms of flaws and material, meeting the needs of the industry. These standards will be used to metrologically validate X-ray computed tomography for the inspection of additively manufactured parts.

Full text

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Reference standards for XCT measurements of additively manufactured parts
Anne-Françoise Obaton1, Christopher Gottlieb Klingaa2, Clément Rivet1, Kamran Mohaghegh3, Sina Baier4, Jan Lasson
Andreasen5, Lorenzo Carli5, Leonardo De Chiffre2
1LNE, 1 rue Gaston boissier 75015 Paris, France, e-mail: anne-francoise.obaton@lne.fr, clement.rivet@lne.fr
2DTU Mekanik, Produktionstorvet 425, 2800 Kgs. Lyngby, Denmark, e-mail: cgkli@mek.dtu.dk, ldch@mek.dtu.dk
3Metrologic ApS, Håndværkersvinget 10, 2970 Hørsholm, Denmark, e-mail: kam@metrologic.dk
4DTU Fysik, Fysikvej 311, 2800 Kgs. Lyngby, Denmark, e-mail: sbaier@fysik.dtu.dk
5Novo Nordisk A/S, Brennum Park 20K, 3400 Hillerød, Denmark, e-mail: jlsa@novonordisk.com, lzcl@novonordisk.com
Abstract
An increasing number of industrial sectors are considering the potential of additive manufacturing as an asset to improve their
production. Indeed, additive manufacturing enables the fabrication of very complex geometries and inner cavities that cannot be
manufactured with conventional techniques. However, in critical sectors such as aerospace, defence and medical, the parts need
to be certified, which requires parts to be non-destructively characterised in terms of flaws, geometry and dimensional accuracy.
X-ray computed tomography is the only current 3D volumetric technique, which is suited for the non-destructive analysis of
internal flaws, geometry and measurements of internal dimensions and roughness. However, regardless of its huge potential, X-
ray computed tomography is not as mature a technology for dimensional metrology as compared to conventional tactile
coordinate measuring machines. In most cases there is no traceability to SI units in the dimensional domain. Recently, numerous
reference standards (i.e. physical artefacts) addressing X-ray computed tomography dimensional accuracy have been published,
but they do not necessarily address the calibration of XCT system in connection with AM parts. In this work, a new and improved
standard in three different materials has been designed with a dual purpose: Fully calibrating X-ray computed tomography for
dimensional measurements while being representative of additively manufactured parts in terms of flaws and material, meeting
the needs of the industry. These standards will be used to metrologically validate X-ray computed tomography for the inspection
of additively manufactured parts.
Keywords: X-ray computed tomography (XCT), dimensional metrology, reference standards, additive manufacturing
1 Introduction
Additive manufacturing (AM) is a promising manufacturing method, which enables the production of very complex parts with
inner cavities. This advantage as well as several others such as on-demand mass production of customized parts, are very
attractive for the aerospace, defense and medical sectors. However, in such critical sectors, the integrity of the fabricated AM
parts needs to be ensured in order for these parts to be certified. This requires quality control methods, including non-destructive
testing (NDT), to be implemented and particularly volumetric NDT to inspect both internal and external features of the parts. At
the present time, the most powerful volumetric method in term of inspection capability is X-ray computed tomography (XCT).
Indeed, it enables a volumetric visualization giving indications of flaws in the part, but also enables geometrical deviations of
the part from its nominal geometry to be determined (comparison between nominal geometry and model obtained with XCT).
Furthermore, dimensional measurements can also be performed on the 3D XCT volume. However, XCT lacks traceability and
the uncertainties on dimensional measurements using XCT have to be evaluated.
In order to characterise XCT to perform dimensional measurements, several standards have been manufactured by different
institutions all over the world. However, a lot of these standards aim at calibrating XCT regarding only one of its specificities
and are not representative of AM parts.
In this paper, a new reference standard has been designed and manufactured in three different materials taking into account the
typical AM characteristics (types of flaws and material) and attempts to calibrate several XCT specificities simultaneously. Thus,
it combines several measurable features in a single standard dedicated to XCT dimensional calibration, as well as XCT scanning
of AM parts.
First, a list of XCT existing standards are presented. Second, the design (shape, dimensions, and materials), aim, fabrication and
metrological characterisation conducted using a coordinate measuring machine (CMM) of the standards are presented, as well
as some preliminary XCT scans using commercial tomographs.
More info about this article: http://www.ndt.net/?id=25111
Copyright 2020 - by the Authors. License to iCT Conference 2020 and NDT.net.

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2 Existing standards
In order to design an appropriate multi-functional standard for XCT metrological calibration, dedicated to AM, first a list of
existing standards for XCT measurements was compiled (Table 1).
Table 1: Existing XCT standards for dimensional measurements [1]–[23].
Standard
Manufacturer
Material
Dimension
(mm)
Standard
Manufacturer
Material
Dimension
(mm)
Step cylinder
Empa,
Switzerland
Aluminium
40, 60, 80,
100, 120, 160,
200, 220
H: 160
Hole: 20
Multi-wave
standard
Florianópolis
, Brazil
Structural
aluminum
league
(ASTM 2024-
T3).
Ext40
Int22
H30
Step cylinder
gauge with a
central bore hole
NMIJ, Japan
Lead-free MgSi
aluminium
alloys
max50
central hole 8
Mini cylinder head
BAM,
Germany
-
-
Step cylinder
DTU,
Denmark
POM
largest outer
Ø17.5 inner Ø3
Mutli-sphere
standard
METAS,
Switzerland
Zerodur (Z) or
Al or CFRP
cylinders+17
steel (S)
spheres
Al 25H20.1
CFRP
Ø26.8H21
ZØ28,H23.2
14 S spheres
Ø1,
3 S spheres
Ø1.5
Step pyramide
Empa,
Switzerland
Aluminium
160×160×40
Miniaturized
single cylinder
head
PTB,
Germany
Aluminium
90×90×90
Step wedge
DTU,
Denmark
Aluminium
11 steps H6
Multi-material
ring
PTB and
BAM,
Germany
Titanium,
Aluminium,
Steel, Brass,
Polymer
(Trovidur)
max25
Multi-material
hole cube
PTB,
Germany
Aluminium and
titanium or
aluminium and
cesic or cesic
and titanium
30×30×30
17 holes 4
Cylindrical multi-
material assembly
DTU,
Denmark
PEHD 500 and
PP-H
7.5
H10
Step gauge
DTU,
Denmark
Aluminium
2011 or PPS or
PEEK or bis-
acryl or bi-
material
PEEK/PPS
L60
11 grooves
l3.50
Hollow cylinder
PTB,
Germany
Aluminium
30
Cylindrical step
gauge in a tube
DTU,
Denmark
Aluminium
inside a glass
tube
Tube L60
gauge L56
6 grooves l3.50
QFM Cylinder
University of
Erlangen,
Germany
Titanium
H80
out 50
in 40
CT Tube
DTU,
Denmark
Ruby spheres
on carbon fiber
tube
Spheres Ø8
Pan flute standard
University of
Padova,
Italy
Glass tubes on a
carbon fibre
frame
2.5 to 12.5

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Standard
Manufacturer
Material
Dimension
(mm)
Standard
Manufacturer
Material
Dimension
(mm)
Micro sphere
tetrahedron
BAM,
Germany
Ruby balls on a
pyramidal
polystyrene
holder
Ø14.29
Ball-bar
PTB,
Germany
Ceramic balls
on a carbon
fibre
L300
Micro sphere
tetrahedron
PTB,
Germany
Ruby ball on an
amorphous
carbon shaft
Ø0.5 - 3
Sphere disk
Nikon
CFRP+ruby
sphere
Larger disk 160
Micro sphere
tetrahedron
PTB,
Germany
Ruby
0.5 or 3
Ball plate
DTU,
Denmark
Ruby spheres
on carbon fibre
plate
Plate 55×55
sphere 5
pitch 10
Mini star probe
PTB,
Germany
Carbon fiber
reinforced
polymer
(CFRP) and
ruby spheres
Horizontal
distance
between the
spheres 10
Ball Plate
METAS,
Switzerland
Aluminium
substrate+121
steel spheres
400×400
spheres Ø10
CT tree
DTU,
Denmark
Carbon fiber
reinforced
polymer
(CFRP) + ruby
balls
Fibers 16 to 40
balls 3
Gauge
Yxlon,
Germany
Carbon fiber
plates or boron
nitrite+ruby
spheres
The spheres
form a square
of 16 nominal
edge length
CT Tetrahedron
University of
Padova,
Italy
Ruby and
carbon fiber
frame
Spheres  5, 4,
3
carbon fiber 2
Multi matérial
standard
Yxlon,
Germany
Ruby spheres
-
Probe forest
VTT,
Finland
Steel+carbon
fiber+ruby
spheres
Distance A-E
33
Sphere  6
Hole plate
NMIJ, Japan
and PTB,
Germany
Aluminium or
steel
Plate 6×6×1 or
48×48×8
28 holes 4
Forest Gauge
NMIJ, Japan
-
-
Hole plate
Empa,
Switzerland
Steel
144×144×24
Multi sphere
standard
Zeiss
Ruby spheres+
ceramic or
CFRP shafts
Several sizes
Hole plate
VTT,
Finland
Aluminium or
steel
4 sizes 6, 10,
20, 50 mm
Hole Ø0.6, 1, 2,
5
Multi-material
sphere
PTB,
Germany
Al2O3
(white)/Si3N4
(black)
10
Printed circuit
board
PTB,
Germany
Invar foilswith
hole grid
Thickness 50
µm, 7.5 ×7.5
15 ×15
30 ×30
Spheres of
different
diameters
Technologica
l Center
AIMEN,
Spain
Ruby
L20
10, 9, 8
Fibre gauge
University of
Padova,
Italy
Glass Fibres
12 fibres
125
L350 to 700 µm

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Standard
Manufacturer
Material
Dimension
(mm)
Standard
Manufacturer
Material
Dimension
(mm)
Spherical calotte
plate
PTB,
Germany
Zerodur
20×20×4.5
Hyperbolic
paraboloid
CMI and
CTU,
Czech
republic
Titanium or
polymer
30×30×15
Spherical calotte
cube
PTB,
Germany
Titanium
cube 10×10×10
5×5 calottes
0.8
Spatial hyperbolic
paraboloid
CMI,
Czech
republic
Certal
(AlZn5MG3Cu)
100×100×100
Cactus step Gauge
standard
KU Leuven,
Belgium
Aluminium
45×45×45
Threaded tube
DTU,
Denmark
Brass and nickel
L46.4
4.1
Gap standard
PTB,
Germany and
University of
Padova, Italy
Aluminium and
titanium or
aluminium and
cesic
Gap 500 µm to
0 µm, step 10
µm to 1000 µ m
Reference object
with artificial
porosities
University of
Padova, Italy
Aluminium
Ø15
Body
H15 or H23
4 removable
cylindrical pins
Ø5
Corner cube
standard
METAS,
Switzerland
Quartz
glass+ruby
spheres
or
silicon-nitride
(SiN) spheres
Ruby
14.7
H 10.2, sphere
Ø 1
SiN Ø 8, height
6.1, sphere Ø
0.4
The aims of the standards, considering their shape and features, are summarized in Table 2.
Table 2: Aims of the standards according to their shape and features.
Standard shape and features
Aims
Step cylinders and step wedges
 External geometrical measurement error
 Scale factor correction
 Maximal penetration thickness (contrast)
 Beam hardening correction
 Optimization of a threshold value
Step cylinders with a central or stepped bore hole inside
 Internal geometrical measurement error
 Scale factor correction
 Optimization of a threshold value
Hollow cylinders
 Scale factor correction
 Form error
External spheres, calottes, cylinders
 Scale factor correction
 Length measurement error
 Form error
 CT machine geometry determination
Ball plate
 Flat-panel detector distortion correction
Corner cubes with spheres
 Measurement – simulation comparisons and
Simulation validations
Free form
 Free form capability measurement
External groove
 Spatial resolution
Internal geometry
 Defect detection

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3 Design of the standards
3.1 Shape of the standards
Considering the existing standards (Table 1), to reach our purpose to design a standard covering several specificities of XCT for
its calibration while being dedicated to AM parts, it was decided that the three standards in different materials would have the
same nominal shape (see Figure 1): A monoblock A consisting of five stacked cylinders of different diameters (step cylinder)
with a through central hole. Thirty three sphere calottes B, with identical diameters, are evenly distributed on the five steps.
Around the central hole, four holes of different depths for removable cylindrical plugs C containing inner counterbores are drilled
and four calottes D of different diameters are placed on the top of the plugs. Furthermore, four removable inserts E with two
external grooves are part of the standard.
Figure 1: Geometrical shape of the new standard.
3.2 Dimensions of the standard
The dimensions of the standard are given in Figure 2.
Figure 2: Dimensions of the new standard. Top: side and top views of the global shape. Bottom left: side view of the internal plug. Bottom
center: top view of the internal plug. Bottom right: side view of the external insert.
3.3 Materials of the standards
There are several materials that are used in AM such as polymers, ceramics and metals. Considering our interactions with the
industry, polymers and metals were prioritized, more specifically, acrylonitrile butadiene styrene (ABS) which has proven to be
stable [24], stainless steel and aluminium, three materials commonly used in AM.

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As the shape and size of the standard in the three different materials are identical, the internal plugs and external inserts are
switchwable from one global shape to another in a different material. This is not of particularly interest for AM at the present
time, but it is meant being relevant for the metrological characterisation of XCT systems with regards to multimaterial parts.
4 Aim of the standards
Compared to the already available XCT standards (Table 1), the proposed standard is multi-functional and specifically dedicated
to XCT performing scans on AM parts. It allows for the calibration of XCT regarding several specificities simultaneously and
to detect internal metrological features down to 200 to 600 µ m. The fact that it combines several different measurable features
in a single standard allows a considerable reduction of the scanning time to qualify a XCT. Indeed, instead of scanning several
standards with different purposes, this one is a multi-purpose standard. The features of the standard and their metrological
purposes are listed below:
 The step cylinders are suitable to detect the maximum possible material thickness which can be penetrated by a given
XCT system, including multi-material evaluation, thus to evaluate the contrast resolution of the XCT for different
thicknesses.
 The internal plugs with sphere calottes, of different diameters, on their top allow for evaluating the capability of XCT
to detect internal features in a mono- or multi-material part such as porosities, which are common flaws in AM [9].
These plugs can be examined at different material thicknesses, which enables the evaluation of the ability of the XCT
to detect tiny features for different thicknesses.
 The inner counterbores allow for diameter and form error of internal holes to be evaluated.
 The external grooves enable the structural resolution of the XCT to be evaluated.
 Finally, the sphere calottes evenly distributed on each step allow the determination of the scale factor as well as the
length measurement error.
5 Fabrication of the standards
Removable plugs with inner counterbores and inserts with external grooves allow easier manufacturing of the standards.
The global shapes of the standards have been machined at DTU Mekanik on a Mikron UCP 600, and the plugs and inserts on a
Mikron HSM 400 U LP, both from AgieCharmilles (Figure 3).
Figure 3: Stainless steel standard (left), plug with inner counterbore and sphere calottes on the top (center), insert with external grooves
(right).
6 Metrological qualification of the standards
The standards have been designed in such a way that they can be fully qualified with a CMM, as well as with XCT. Indeed, the
plugs with inner counterbores and inserts with external grooves are removable to allow metrological calibration with CMM.
Thus, when assembled, the plugs allow measurements of inner metrological calibrated features.
6.1 Measurand selection
The following measurands have been selected:
1. The position of the center of the thirty-three spheres, which fit the calottes, evenly distributed on each step, to enable
dimensional measurements between calottes (Figure 4a).
2. The width of each groove and the distance between the two grooves measured at the surface and at the middle length
of the grooves (Figure 4b).
3. The diameter of the sphere fitting the porosities on the top of the plug (Figure 4c).

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4. The diameter, roundness and centre of the circles fitting the 0.6 mm inner counterbore in the plug at different heights
from the surface: 0.1, 0.4, 0.7 and 1.3 mm (Figure 4d).
Figure 4: Schematic representation of the measurands indicated by the red arrows. a) Top view of the standard with the thirty-three spheres
calottes. b) Side view of the insert with two external grooves. c) Top view of the plug with the four sphere calottes. d) Side view of the plug
with the inner counterbores.
6.2 Metrological characterisation of the standards
A Zeiss Prismo CMM (Figure 5 left) was used for the metrological characterisation of the global shape, the internal plugs and
the external inserts at DTU Mekanik, and then a Zeiss Accura II CMM (Figure 5 right) will be used at LNE for comparison.
Figure 5: Zeiss Prismo CMM (left) at DTU Mekanik and Zeiss Accura II (right) CMM at LNE.
7 XCT characterisation of the standards
A comparison campaign of XCT machines with these reference standards will be organized in the frame of the European project
“AdvanCT” (Computed Tomography for dimensional and surface measurements in industry) which has received funding from
the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and
innovation programme. The purpose of the interlaboratory comparison is to investigate the performance of industrial CT with
respect to dimensional measurements for traceability, more specifically for quality control of AM parts.
The first scans have been performed at DTU Fysik on the stainless steel (SS) global shape using a Nikon XT H 225 ST, while a
Zeiss Xradia Versa 410 was used to scan the ABS global shape. Scans of the ABS global shape will also be performed using a
Werth Tomoscope XS XCT scanner at DTU Mekanik. Finally, the standards will be sent to other countries for measurements.
The Nikon XT H 225 ST microfocus XCT is composed of a source with a maximal voltage of 225 kV, a maximal power of 225
W and focal spot sizes from ca. 3 µm to 225 µW, dependent on the used target geometry and power. A tungsten target in reflection

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mode was used for the scans. The Zeiss Xradia Versa 410 is composed of a source with a voltage ranging between 40 and 150
kV and with a maximal power of 10 W. The system has a set of different objectives of which the Large Field of View (LFOV)
objective was used for the scans.
The set up of the standards inside the XCT are presented in Figure 6 whereas the scanning settings are provided in Table 3 and
Table 4. The XCT images of the standards shown in Figures 7 and 8 enable to see the internal inserts in ABS in ABS global
shape at the higher thickness of the standards, but also in the SS global shape, in other words in the case of multi-material
standard. However, in the case of multi-material standard, the image resolution might not be sufficient to perform dimensional
measurements. Nevertheless, it is high enough to perform dimensional measurements of all the defined measurands in the case
of the mono-material standards.
Figure 6: Set up of the stainless steel global shape in the Nikon XT H 225 ST (left) and of the ABS global shape in the Zeiss Xradia Versa
410 (right).
Table 3: Scanning settings used for the stainless steel (SS) global shape with ABS plugs and either ABS, SS or aluminium inserts with the
Nikon XT H 225 ST.
Voltage
(kV)
Power
(W)
Exposure
time
(s)
Filter
Number of
projections
Number of
frames per
projection
Binning
Scan
duration
Reconstructed
voxel size
(µm3)
220
20
2.8
1 mm Sn
1571
8
2x2
12 h 21 min
36.0×36.0×36.0
Figure 7: Nikon XT H 225 ST images of the SS global shape with SS external inserts and ABS internal plugs.
Table 4: Scanning settings used for the ABS global shape with ABS plugs and either ABS or SS inserts with the Zeiss Xradia Versa 410.
Insert
material
Voltage
(kV)
Power
(W)
Exposure
time
(s)
Filter
Number of
projections
Binning
Scan
duration
Reconstructed voxel
size
(µm3)
ABS
40
10
9
LE1
3201
1x1
9h 26min
19.36×19.36×19.36
SS
120
10
5
LE1
3201
1x1
6h 42min
19.36×19.36×19.36
SS zoom
140
10
52
LE1
3201
1x1
49h 26min
5.65×5.65×5.65

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Figure 8: Zeiss Xradia Versa 410 images of the ABS global shape with ABS external inserts and ABS internal plugs.
8 Conclusions and future work
A compilation of existing X-ray computed tomography (XCT) standards for dimensional measurements was presented as well
as their aims correlated to their shape and features. Considering this compilation, the design of new standards was defined (shape,
dimensions and materials) to reach the goal we set out to achieve: Fully calibrating XCT for dimensional measurements being
representative of additively manufactured (AM) parts in terms of flaws and material used in AM in critical industrial sectors. It
was decided to fabricate standards with the same shape and dimensions in three different materials: ABS, stainless steel and
aluminium. Furthermore, these standards have removable internal plugs and external inserts enabling multi-material
combinations. The measurands were selected before the metrological qualification of the manufactured standards with a
coordinate measuring machine (CMM). Preliminary XCT scans of the standards were performed, which are highly satisfactory.
The following steps will be to realize measurements on the XCT images and to start the comparison campaign of XCT machines.
Acknowledgements
The authors would like to thank Claus Bremer and Søren Hansen from DTU Mekanik for the fabrication of the standards. The
authors are also very grateful to Klaus Liltorp from DTU Mekanik and to Arthur De Soete from LNE for their contribution in
CMM calibration and in providing the images respectively. Sina Baier is grateful for financial support from Innovation Fund
Denmark [grant no. 5152-00005B] and the 3D Imaging Center at The Technical University of Denmark is gratefully
acknowledged for their support on X-ray computed tomography acquisition and evaluation. This work was supported by the
EMPIR programme, co-financed by the Participating States and from the European Union’s Horizon2020 research and
innovation programme.
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