QMT Features: September 2010
Pushing the boundaries
University of Manchester achieves breakthroughs in science and engineering with 3D X-ray computed tomography.

Palaeontologists rub shoulders with aircraft designers at the Henry Moseley X-ray Imaging Facility located at the University of Manchester. The Facility is openly available to academic and industrial users, who gather instant scientific proof regarding otherwise hidden information using radiography and computed tomography (CT).

Projects run on Nikon Metrology systems have shed new light on Velociraptor behaviour and 3D vascular networks in animal organs. They also provide unique insights into power plant metal corrosion and damage propagation in lightweight composite aerospace structures.

Located at the School of Materials at the University of Manchester, the Henry Moseley X ray Imaging Facility houses a suite of six computed tomography (CT) systems. “Worldwide academic and industry researchers have access to top-class equipment that offers full resolution and length scale capabilities for samples ranging from heavy engineering items to micron-sized biological specimens,” says Professor Phil Withers, founder and Director of the X-ray imaging facility. “Evaluating stunning 3D models reconstructed from a series of X-ray images revolutionises many research fields, including materials science, biology, mineralogy, palaeontology, entomology, medicine and life science.”

About dinosaurs
The Henry Moseley X-ray Imaging Facility is shedding light on a diverse range of natural samples. Recently, fossilised portions of ungual claws of a Velociraptor dinosaur were inspected to generate an accurate 3D finite element (FE) microstructurally faithful model. Analogue material from a similar dinosaur as well as the pedal digit and claw of an eagle owl, were analysed to provide input data for the Velociraptor claw FE model. Strength and strain simulations confirmed that the claws were resistant to extreme forces in the longitudinal direction and therefore well adapted for climbing.

“Medicine is undoubtedly a growth area in high resolution X-ray micro tomography,” says Chris Martin, senior experimental officer managing the operation of the Henry Moseley Imaging Facility. “A nice example is a research project for investigating the action of cancer treatments. It encompasses ex-vivo studies of the vascular system of blood vessels in animal brains, livers, kidneys and lungs.”

When evaluating these remarkable 3D images on powerful workstations, the software determines the volumetric fraction of the blood vessels. This project illustrates that ex-vivo CT inspection offers much higher resolution than in-vivo magnetic resonance imaging (MRI), which typically falls short on detailed visualisation of the smaller vessels.

Composites failure mechanisms
According to Chris Martin, another growing research activity concerns new lightweight materials gaining popularity, particularly for aerospace applications. He mentions current research projects to develop and exploit in-situ rigs to enable multi-mode stressing of composite samples – a keen interest of the international aerospace companies the School of Materials collaborates with. “Identifying failure mechanisms in composites is a tricky business, knowing that the damage often remains largely invisible externally until late in the testing process. X-ray and CT technology help gain a better understanding of the failure mechanisms and develop mathematical formulae describing the degrading performance characteristics.”

At various stages throughout the fatigue process, the composite samples are investigated in the walk-in radiation bay of the unique Nikon Metrology 320kV microfocus X-ray system. Such voluminous parts easily fit in the large cabinet bay, which is equipped with a fully programmable 5-axis manipulator designed for samples up to 100kg.

The X-ray source is a proprietary 225/320kV microfocus source with a spot size that provides  image resolution up to 3 microns. A premium 2000 x 2000 pixel Perkin Elmer flat panel detector accurately digitizes cracks and fractions formed in the composite material, within a 400x400mm field of view. “Superior X-ray technology is needed to get sufficient contrast, as composite parts are low-density by nature and absorb different energies in different directions,” Chris Martin comments.

Metal corrosion mechanisms
A similar approach is applied to study metal corrosion mechanisms that occur in nuclear reactors or chemical plants. CT observations provided insight into the development of corrosion pits, stress corrosion cracks  and their geometries, to improve system design and deduce mathematical formulae. When dealing with metal and other dense materials, the system can be equipped with a rotating target source. Such a source generates an X-ray flux that is up to 5 times higher without risking permanent source damage, providing faster data acquisition and/or higher image accuracy.

Dynamic investigations
Chris Martin says that the Nikon Metrology 225/320kV inspection system also supports dynamic investigations. Its walk-in radiation bay provides sufficient space to install instrumentation to study how specimens evolve over time, either naturally or under a range of loads, temperatures or other stimuli. A triaxial loading cell, for example, can be used to monitor the evolution of voids, inclusions, fractions and disturbances in large rock and soil samples. For the inspection of smaller parts, researchers at the imaging facility use a similar, yet more compact 225kV CT inspection system from Nikon Metrology.

“To maximize CT infrastructure availability for fundamental research and commercial projects, we decided to operate the facility 24/7,” says Professor Phil Withers. “To free up CT equipment after data capture, the X-ray data is automatically transferred to a central cluster of computers, which handles the reconstruction of the 3D models from a series of X-ray images. This guarantees maximum productivity, while local reconstruction resources remain available in case of failure.”

“Equally important is that we try to push the limits of CT technology by focusing on the optimisation of the reconstruction software. We benefit from strong programming expertise present at the University of Manchester, and have active academic links with Nikon Metrology and other CT specialists. Detailed insight into our own reconstruction software allows us to optimise data acquisition and precisely figure out how to interpret 3D computer tomography models.

Both our micro-CT systems from Nikon Metrology respond to a broad range of academic and industrial applications. The systems’ high accuracy, large field of view and fast image acquisition are well appreciated. Our experience with these systems is that parties applying CT for a specific purpose generally discover more purposes for this enabling technology.  We are currently expanding the imaging facility to further explore the nano length scale, in order to provide an even wider range of possibilities.”
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