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Speaker abstracts

Tracking the evolution of a defect characteristic of AFP layup during cure with in-process micro-XCT scanning.
L. R. Pickard1, K. Smith2, J. Kratz2, K. Potter2
1National Composites Centre/University of Bristol, UK
 2University of Bristol, UK

Micro-XCT scanning is often used for inspection of composite materials to identify defects and damage. A defect which occurs early in the manufacturing process may change as the composite item is cured, by application of heat and pressure or vacuum. An increased understanding of how these defects change during the cure process can inform design and manufacturing process choices. This paper presents the detailed evolution of a deliberate gap in the composite, throughout the cure process, using a novel setup for In-Process Micro- X-ray CT.

A Nikon XTH-320 industrial CT scanner using a 225KeV reflection target was equipped to allow heating- and hence curing- of an uncured sample under vacuum. Short scans at of approximately 7 minutes, with 1600 frames per projection at 250ms exposure, were performed repeatedly at 180KeV, as a batch program during cure. VG Studio Max was used to perform a void analysis on a set region of interest for each scan, tracking the changes over time.

Here we discuss the evolution of a cylindrical sample of carbon fibre sheets pre-impregnated with an epoxy resin, containing a 2 ply thick 2mm gap across the centre. The evolution of the gap is compared to the theoretical resin behaviour.

In-process micro- X-ray CT has the potential to provide far more detailed information on the behaviour of composites during cure than has been available prior to today. This work provides both proof of the principle, as an experimental method, and shows detailed results measuring the evolution of a gap during the cure of carbon fibre prepreg materials.

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XCT metrological applications in aerospace
K. Pickup

BAE Systems

As manufacturing evolves within the aerospace world, the real challenge is often proving to the legislator that the part was made correctly with sufficient integrity to cope with its intended design environment. This applies to the finished product and for every process involved in making the product to ensure repeatable, efficient manufacturing. To assist in this, X-ray Computed Tomography is starting to play a key role. XCT can reach places that traditional tactile systems simply cannot go and often extends the Region of Interest (ROI) with increased data mining providing superior Statistical Process Control (SPC).

As we expect greater performance of our materials and designs in terms, every anomaly must be analysed to ensure the part is fit for flight. XCT allows us to 3D map the location and topology of indications, the data from which can then be used in accurate simulations of the part or simply to form a digital twin – which is becoming the manufacturing standard.

As well as new product assessment, we are using XCT for metrological assessment of failed items to improve understanding of failure modes and provide an undisturbed view of the failure. Finally, with the increased use of Additive Manufacturing (AM) in aerospace, XCT is proving invaluable assisting with design iteration and final product inspection.

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Operator influences on CT scanning: a round robin study

J. M. Warnett1, A. Attridge1, N. Brierly2, R. Van Gelder3, L. Ashton4, M. A. Williams1
1 WMG, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
2 MTC, Ansty Park, Coventry, CV7 9JU, UK
3 National Composites Centre, Bristol and Bath Science Park, Emersons Green, Bristol, BS16 7FS, UK
4 AMRC, Advanced Manufacturing Park, Wallis Way, Catcliffe, Rotherham, S60 5TZ, UK

XCT is widely used in high value manufacturing applications as it can provide information on internal and external geometries, identification of porosity and location of defects. Numerous authors have demonstrated its capability as a dimensional measurement tool, quantifying errors often smaller than the voxel size itself. Aside from source/detector variations and geometric errors, the operator has a significant influence on the results through parameter selection. Current standards available from BSI, ASTM and VDI/VDE offer general guidance on the operation of an XCT scanner and how different parameters can affect the results, but missing is a generic workflow on how one should setup a scan. This inevitably leads to a degree of variability between resultant data that impacts dimensional results.

In this study two work pieces, one polymer and one metallic, were used with several measurands identified that were initially measured using a CMM as a reference measurement. All the centres had a similar CT system (variation of Nikon 225) on which they initially scanned the object, selecting their own parameters. The data was reconstructed using the same settings, evaluated in VG studio using a standard surface determination and dimensioned according to measurements obtained by CMM. Without voxel scaling, the variation due to geometric error dominates resulting in deviations to the order of 10’s of voxel compared to CMM measurements. After voxel scaling, errors are reduced to the order of sub micron, but there is still some relatively large variations between centres. It is difficult to de-couple the impact of individual parameters while it is known they must have an effect. To demonstrate this, a second round robin of the work pieces were complete where operators scanned with prescribed parameters. Here it was observed that the variation between centres was reduced by at least 50% compared to operators own selection. This result in its own right demonstrates the need for a more standardised workflow for operators to follow so uncertainty in XCT measurement can be accurately described.

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A sensitivity analysis for the measurement of internal additively manufactured surfaces by X-ray computed tomography
A. Thompson1, N. Senin1,2, I. Maskery1, L. Körner1, S. Lawes1, R. Leach1
1Manufacturing Metrology Team, University of Nottingham, Nottingham, NG7 2RD, UK
2Department of Engineering, University of Perugia, 06125, Italy

Recent studies have shown that X-ray computed tomography (XCT) can be used for the measurement of internal and otherwise difficult-to-access surfaces that have previously been considered unmeasurable, for example, those commonly present on complex additively manufactured (AM) parts. However, investigations into the sensitivity of XCT instruments to measurement process control parameters, when the measurement is directed towards the acquisition of topographic information at microscopic scales, are yet to be undertaken. In this work, the effects of geometric magnification and reconstruction sampling in XCT surface topography measurement are examined. A hollow Ti6Al4V artefact fabricated by laser powder bed fusion is used as a test case. Topographies obtained by varying XCT measurement parameters are compared through the use of novel statistical topography modelling methods combined with the use of industrially-established surface topography descriptors (areal texture parameters). XCT topographies are also compared to reference results acquired using focus variation microscopy and coherence scanning interferometry instruments.

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Dimensional 3D X-ray Computed Tomography at High Absorbing Samples

T. Mayer
GE Oil & Gas - Digital Solutions

Computed Tomography (CT) becomes the technology of choice for nondestructive testing and more and more for metrology tasks, e.g. for automotive castings, aerospace turbine blades or 3D printed parts. For 3D metrology, a major challenge is the measurement of hidden features. Increased cycle time requirements combined with a high inspection depth require improved methods to deal with imaging artefacts.

Scattering of X-rays is the main factor for such artefacts in conventional cone beam CT. While state of the art scatter reduction simulates scatter based on CAD data or sample’s material properties, the new method is really measuring the scatter portion of that specific sample in the CT scanner and minimizes it from the CT result for every individual voxel. Dimensional metrology of 3D cone beam CT scans becomes possible because the sample edges in the volume are detectable like in CT scans generated with up to hundred times slower fan beam CT.

In the presentation, the influence of advanced scatter correction technology on 3D measurements will be shown.
Considering the fact that 3D metrology with CT always uses automatic surface detection algorithms to determine the surface of the 3D volume to measure, compared to conventional cone beam CT the new method allows more material penetration (up to 30%) at the same scan parameters to still determine the exact surface. At same material penetration length, the new scatter correction method allows a more precise surface detection due to less artifacts negatively affecting the metrology results.

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A study into the use of simple holeplates to measure the apparent distortion in the geometry of reconstructed volumes
H. Corcoran1,2, S. Brown2, S. Robson1, R. Speller1, M. McCarthy1, W. Sun2
1 University College London, Gower Street, London, WC1E 6BT, UK
2 National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK

X-ray computed tomography (XCT) is being used extensively for dimensional measurements in an industrial and manufacturing setting. The use of a complex aluminium holeplate to measure the effects of beam hardening on unidirectional and bidirectional lengths has been studied before by other authors and is central to an emerging ISO standard. Previous work by the authors has highlighted the presence of significant deformations in the surface of reconstructed holeplate cylindrical holes of the order of up to 60 µm. In order to better understand the physical reasons for these deformations the authors have investigated a series of simple holeplates.

This paper presents work focusing on three simple holeplates that have the same 48 mm x 48 mm x 8 mm dimensions and 4 mm hole radius as the original holeplate but have only one hole in them, the position of which varies for each holplate. These holeplates were used to measure the apparent distortion of the geometry of the reconstructed holes. The holeplates were measured at varying angles and magnifications.

Results demonstrate that the orientation of the holeplate on the manipulator affects the geometry of the reconstructed cylindrical holes. In all reconstructions, systematic deviations from a perfect cylinder in the computed hole radius of between -65 µm to 20 µm were found. Results indicate that the perceived distortion of the hole is related to the amount of material the X-rays have travelled through, which in turn is related to the attenuation of the polychromatic X-ray beam and therefore beam hardening. This theory was confirmed with the manufacture and imaging of a circular holeplate, which when imaged horizontally, demonstrated only the random (±10 µm) measurement perturbations expected from the system being tested. This work is important not only in considering the use of a holeplate as an ISO reference but also to the general case of XCT for high accuracy dimensional metrology where voids in dense materials are being imaged.

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Verifying the dimensional performance of XCT systems with metrology capability.

M. McCarthy

Michael McCarthy is an honorary Professor at University College London (UCL) and Lead Consultant at ‘Engineering Metrology Solutions’ (EMS). He chairs BSI’s Technical Product Realization (TPR/1/4) panel for XCT and is a principle member of the ISO TC213 working group for XCT. His presentation will provide an overview of the current draft XCT document ISO 10360 part 11 which was released for review in March 2017.

Many published parts of the ISO 10360 series are well established, covering the acceptance and reverification of coordinated measuring systems employing both tactile and optical probing systems. However, the ISO 10360 part 11 under development is being designed to specifically focus on supporting metrology systems using the principle of computed tomography (CT). The internationally agreed intention of this proposed XCT addition to ISO 10360 series is to achieve comparability with the characteristics of other coordinate measuring systems employing tactile or optical sensors. Some of the comparability issues and the resultant challenges they present will also be discussed.

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A versatile laminographic scanning system as an addition for an existing CT scanner
N. S. O’Brien1, C. E. Wood2, and T. Blumensath1,2
1mu-VIS X-Ray Imaging Centre, University of Southampton
2ISVR, University of Southampton

There is increasing interest in performing 3D scans of specimens whose large aspect ratios mean they are ill-suited to conventional computed tomography (CT) for non-destructive testing, inspection and quality-control purposes. There is increasing demand for methods to identify both defects arising during manufacture, and damage sustained during the life-cycle of the parts. Various techniques may be applied when X-ray scanning such specimens, including variable energy or variable exposure CT scanning, and laminographic scanning using non-standard trajectories. We have designed and built a demonstrator allowing for the implementation of a range of trajectories other than traditional full rotation CT to perform region-of interest laminographic scanning on large planar specimens within an existing, custom bay CT scanner located at the University of Southampton. Here we give an overview of key aspects of the system and present initial results from scanning carbon fibre reinforced polymer (CFRP) panels, as may be applied in a nondestructive testing scenario.

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Automated processing for 3D powder characterization

P. Gajjar, J. Carr, J. Behnsen, P. J. Withers
University of Manchester

The quantitative geometric characterization of powders from sub-micron to micron length scales can be achieved through a wealth of techniques. These include light scattering, diffraction, 2D imaging as well as sieving and sedimentation methods to estimate, for example, particle size and shape distributions. However, all of these techniques prove to be of limited use for complex powders in which 3D features such as shape, coatings and internal porosity are important.

We present a method for utilizing X-ray microtomography (μCT) for obtaining the true 3D geometry of powders at multiple length scales. This information can be used to generate accurate size and shape distributions as well as quantifying multiple phases. An automated workstream was also developed for processing 3D image datasets to produce repeatable and reliable results for quantitative powder characterization.

This scope of μCT for characterisation is demonstrated for a range of different powder types from coarse civil aggregates and metallic powders to common household detergents. In addition, μCT characterisation of fine lactose crystals that are used as excipients in inhaled therapies gives invaluable insights into the different milling and processing techniques used during the formulation manufacturing.

Additive manufacturing, pharmaceuticals and paint processing are just three of many potential applications that could harness the 3D characterisation ability of XCT, with opportunities across multiple industrial fields.

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Detection of unmelted powder in additive manufactured components using computed tomography

A. Tawfik, R. Racasan, P. Bills, L. Blunt

EPSRC Future Advanced Metrology Hub, University of Huddersfield, Huddersfield, United Kingdom

Additive manufacturing (AM) is recognized as a core technology for producing advanced high value components. The possibility of producing complex and individually modified components as well as prototypes gives additive manufacturing a substantial advantage over conventional subtractive machining. One of the current barriers for most industries in implementing AM is the lack of build repeatability and a deficit in quality assurance standards. The mechanical properties of the components depend critically on the density achieved therefore defect/porosity analysis must be carried out to verify the components’ integrity and viability.

This paper presents a methodology for differentiating between unmelted powder and defects/pores in additive manufactured components using computer tomography thus allowing the detection of pores even when they are “filled” with unmelted powder. The powder used was Ti6AL4V with a grain size of 45-100µm, typically employed with Arcam electron beam melting (EBM) machines. The samples consisted of a plastic test tube filled with powder and a known volume small plastic particles that were placed inside acting as pores/defects. A Nikon XTH 225 industrial CT was used to measure the samples to detect the pores/defects locations and volumes.

To reduce the number of process variables, the measurement parameters, such as filament current, acceleration voltage and X-ray filtering material and thickness are kept constant. VgStudio Max 3.0(Volume Graphics, Germany) software package was used for data processing, surface determination and defects/ porosity analysis. The impact of surface determination on the results, repeatability and accuracy are discussed. The main focus of the study is exploring the optimum methods to enhance the detection capability of pores/defects filled with powder using computer tomography.

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Limited view X-ray CT for dimensional analysis

G. A. Jones and P. Huthwaite
NDT Group, Imperial College London, South Kensington, London, UK.

The increasing use of complex and irregularly shaped components for safety-critical applications has led to the increasing adoption of X-ray CT as a NDE inspection tool. X-ray CT enables 2D and 3D images of the internal features of the object to be computed and subsequently subject to dimensional analysis. However, standard X-ray CT methods require thousands of projections, each regularly distributed evenly through 360° to produce an accurate image. The large number of projections and the regularity of sampling can result in lengthy data acquisition times and can lead to bottlenecks in manufacturing throughput. To alleviate these bottlenecks in throughput companies may be forced to purchase additional X-ray CT capability at great cost.

In recent years there has been a drive by the medical industry to reduce patient X-ray exposure by limiting the number of projections whilst maintaining image quality in CT applications. Spurred by the ever increasing power of computers and the advents of graphics card processors a variety of limited view tomographic techniques capable of generating high quality images with less data have been developed. Central to these new algorithms is the principle of compressed sensing whereby an understanding of the signal sparsity is exploited to produce accurate reconstructions of the signal of interest with fewer samples than those required by the Shannon-Nyqist theorem. We present a survey of limited view algorithms for x-ray CT of a turbine blade with the aim of producing accurate internal structure estimates using minimal data.

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Additional problems with CT metrology for large samples
D. Bate

Nikon Metrology

There is a lot of effort underway worldwide to characterise the use of X-ray Computed Tomography for metrology of parts in a way similar to the more traditional tactile CMMs and optical techniques.

Much of this work is happening at X-ray energies < 225 kV and also on low to medium density samples (Plastics, Aluminium) or small samples of higher density. While there are a number of issues that need to be resolved (Beam hardening, cone beam artefacts (with FBP) or similar reconstruction effects) these will be present at all energies.
As we try and expand the technique into larger samples there are two key issues that will be discussed in this paper:

1) Penetration: In order to scan larger parts a combination of higher flux and higher energy are required to give enough detected X-ray flux to give high quality CT data. This paper will discuss the approach of using minifocus sources for greater flux and the use of Novel high energy microfocus sources to overcome this issue (including the new facility being developed here in Warwick)

2) Scattering: As the energy of the X-rays increases and the density of the samples increases the contribution of scattering to the noise increases, this leads to a SNR issue if not handled. This paper will look at techniques to either reject the scatter or to correct for the scatter errors

As can be seen at the < 225 kV energies used traditionally the dominant scatter mechanism is coherent scatter and this introduces little noise to the image, but as the energy increase past about 400 kV the Compton scattering (incoherent) becomes the dominant mechanism and this lead.

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