A gathering of international experts in Australia is helping to improve the science that explains why things break. Ensuring a car stops when you apply the brakes, or that trains run on time is the combination of good planning, engineering and some scientific ingenuity.
Neutron diffraction contributes to this ingenuity.
In science speak, neutron diffraction provides information about the structure of the material and how far the atoms are stretched apart. A sample to be examined is placed in a beam of thermal neutrons to obtain a diffraction pattern that provides information of the structure of the material.
For the layperson, this is what’s called a stress experiment using a beam from a nuclear reactor where scientists profile the object at the atomic level to identify how and where it can potentially break.
Objects as small as 0.2mm thick wear-resistant coatings to as large as an aircraft wing stiffener can be translated and mapped-out to explain the strains (and therefore the stresses) within the object.
It’s very accurate and has been recognised by experts round the world for the last 30 years as the most effective way to get at the stresses deep inside an object.
X-ray measurements of surface stresses have been around for about 90 years but the big advantage of neutrons is their penetration devling a number of centimetres inside the object of interest.
Case study: Helping to keep the trains running on time
Australia’s rail industry is benefiting from the use of neutron diffraction to solve the problem of rail squats.
Squats appear on rails as a result of rolling-contact fatigue damage and occur on or near the surface, generally on the crown or ball of the rail head. It’s easily identified visually, as they appear as dark spots or “bruises” on the running surface of the rails.
While typically starting as minor abnormalities they can quickly turn into dangerous vertical cracks and are becoming a major source of concern for rail authorities around the world.
The Australian Nuclear Science and Technology Organisation has been working with Rail Corp Australia and the University of Queensland to try and solve this problem as part of the CRC for Rail Innovation initiative.
Using ANSTO’s residual stress diffractometer Kowari researchers, including Bragg scientist Dr Vladimir Luzin, are modelling the behaviour of cracks on both service and virgin rails under a range of contact conditions to understand what affects squats.
Kowari is a strain scanner used to profile various types of objects from geological materials, like rocks or minerals through to engineering components like car brakes or rails.
“Many attempts have been made in the past to find out how a squat grows under a moving wheel-rail contact load,” says Dr Luzin, but “a squat’s life time from crack initiation to spalling-off has not yet been studied,” he said.
The findings of this study will be used to simulate the effect of the residual stress state on crack initiation and how it spreads and will hopefully one day lead to improvements in its manufacturing and service.
The future of neutron stress diffractometers
Diffraction’s applications stretch further afield and have assisted the likes of the automotive and aerospace industries profiling objects like machine components through to metal coatings.
It's proven itself as a valuable tool for the power sector contributing to the understanding of stress corrosion cracking in extreme environments such as the pipework associated with power stations.
To advance the overall technique, instrument scientists from across the world gathered at ANSTO in January for the Current State and Future of Neutron Stress Diffractometers Workshop.
Experts from major reactor facilities, including major reactor facilities in Canada, Germany, France, Japan, South Africa, and the United States attended to review different aspects of neutron stress diffractometry.
At the top of the agenda was assessing recent important technical developments in the area of neutron scanners/diffractometers based on steady-state reactors, especially techniques related to the radial collimators, neutron focusing devices (monochromators, neutron guides, lenses, etc.) and positioning systems.
Discussion topics at the workshop
While the feedback was mostly from the Australian user community about Kowari, it represented a range of scientific disciplines with users discussing their existing and future needs in advanced neutron diffraction stress analysis with the instrument scientists.
Trevor Finlayson from the University of Melbourne spoke on composite materials and emphasized good capabilities of the reactor based stress scanners to measure stress distribution even in weakly diffracting phases, such as Si in Al with a volume fraction of only 7%.
Andrew Venter from the Nuclear Energy Corporation of South Africa, presented a case where stress was measured in WC coatings as thin as 200 μm and Ryan Cottam from the Swinburne University of Technology introduced some laser based industrial technologies,examples of successful stress measurements and described needs for more research.
At a glance, other presentations included a novel talk by Chris Wilson of Monash University who reported results on the in-situ deformation of ice showing that neutron tests can have a major effect on geology, while ANSTO’s Michael Law anchored the workshop discussing the need for neutron diffraction studies of radioactive materials.
Published: 02/02/2012