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Neutrons put the brakes on stress |
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Don't you hate it when you're driving along, put your foot on the brake, and feel that juddering feeling through the pedal? It happens when the disc brake rotors become distorted through normal use of the brakes.
To the car manufacturing industry it's called 'runout' and is a multimillion dollar warranty problem each year. Not to mention a pain for drivers!
Dr Maurice Ripley and Dr Oliver Kirstein from ANSTO (the Australian Nuclear Science and Technology Organisation) wanted to figure out whether runout is caused by residual stresses from the manufacturing process, or by normal use of the brake. So they decided to test and compare a used and new brake disc.
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Oliver and Maurice hope to spark the car industry's imagination with the findings of their stress testing
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"To picture what metal looks like at the atomic level, imagine spheres stacked evenly around each other in all three dimensions, explained Oliver. "The spheres represent atoms in the metal and the structure is called a metallic lattice."
We're all down with the idea that metal expands when it gets hot - the atoms get excited with the heat and have the energy to move further away from each other, so spaces between the atoms in the lattice get larger.
"When parts of the metal are heated up and cool down at different rates, you may end up with a distorted lattice with some parts expanded and others not," explained Oliver. "This unevenness in the lattice creates residual stress."
While a bunch of methods were available to test the discs, Oliver and Maurice picked neutrons from ANSTO's HIFAR (High Flux Australian Reactor) as their tool of choice.
"Neutrons allow us to look at the inside of the metal without damaging it. They can penetrate through the iron, so we were able to take measurements at a series of points at different depths through the brake disc," said Oliver.
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The old brake disc tested
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Word around the car industry is that when residual stresses are relaxed through heating of the brake disc during use, the discs could potentially distort, causing the runout and that juddering feeling. But everyone was clueless as to what extent the stresses are from the manufacturing process.
"The only previous study of residual stress in a brake disc we could find showed these stresses in a new brake disc to be relatively small," said Maurice. "So we started to think about how much of the runout was caused by residual stress, as had previously been assumed, and how much of this distortion was due to other factors from usage."
And the early results will have the car industry rethinking their ideas.
Like the previous study, Oliver and Maurice found there were not large residual stresses in the new disc brakes. But the used disc brake was another story, with large stresses found.
"Our results suggest that perhaps runout is not caused by relaxation of residual stresses that were in the brake discs from manufacture," Maurice stated. "It looks more likely that runout is due to uneven cooling of the hot disc through use - from severe braking, for example - because we found such large stresses in the old worn out brake compared to the new one."
With their results showcasing the power of neutrons to measure residual stress in important commercial components, Oliver and Maurice hope to spark the car industry's interest. "With the insight of neutrons, the industry may find in combination with improved modelling techniques new ways of manufacturing components to overcome these problems," Oliver said.
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