ARCHIVED - Neutrons reveal unseen secrets
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December 07, 2009— Ottawa, Ontario
Scientists at the NRC Canadian Neutron Beam Centre (NRC-CNBC) in Chalk River have handled metal ranging from pieces of the ill-fated Challenger and Colombia Space Shuttles to train rails implicated in ecological disasters in Alberta. In those cases and others, their goal was to find out what went wrong.
But knowing what goes on inside metals even before such failures occur can help the designers and engineers that use them. This is why Dr. Ron Rogge and his colleagues are helping Defence Research and Development Canada (DRDC) Atlantic — one of seven DRDC establishments operated by the Department of National Defence — make Canadian submarines safer.
Dr Rogge says X-rays are a well-known way to measure surface stress in metals non-destructively. But X-ray devices that can be made portable see less than a millimetre into steel.
By contrast, a technique called neutron diffraction residual stress measurement can see far deeper into metals to map their internal stresses at the atomic level. Since surface and internal stresses may differ greatly, it can give a more complete picture of the strains created as submarines are welded together.
"The challenge is that neutron scattering requires a huge neutron source, such as at Chalk River," Dr. Rogge says. It's not a portable technique, so the equipment cannot be taken to submarines, which are thousands of miles away at coastal naval bases. However, the testing of relatively small pieces of metal, typically between 2 cm and 70 cm square, can reveal a great deal.
When shipbuilders weld curved metal plates together to form boat hulls, this leaves residual tensions internally because the liquefied metal in a weld cools at a different rate than the solid parts it joins. And internal tensions pre-load a hull with stresses that reduce the amount of external stress the hull can take before failing.
"Wherever you have a weld, there will be stresses, unless there was a subsequent stress-relieving treatment," says Dr. Rogge. "There is only one non-destructive technique for looking at stresses deep inside material and that is with neutrons."
Submarine captains who may need to sail and dive their boats close to their limits require a clear picture of the basic strengths and potential weak points that can be found even in newly-built vessels. To help obtain a clear picture, DRDC Atlantic has often asked Dr. Rogge's team to help it assess the as-built condition and different kinds of repairs to Canadian submarines.
Over time, corrosion thins and weakens the high-tensile strength steel hull plates of submarines. To keep boats working at their original rated depths, shipyards use a technique called weave welding to overlay new material and increase a submarine hull's thickness. Since any welding adds new strains to a hull, the team's research seeks to find what techniques work best. This project remains underway, and no results have been published yet.
But it was, in fact, a particular submarine refit that led to one of Dr. Rogge's more unusual naval jobs.
In 2000, Canada took delivery of the first of four British Navy Upholder-class submarines and renamed them the Victoria class. During a recent half-life refit of the former HMS Unseen, renamed HMCS Victoria, naval technicians removed small sections of plate and welded in new pieces as part of a platform upgrade. Since the replacement occurred at an unusual place in the boat's structure, at a point crossed by one of the submarine's critical main circumferential welds, DRDC Atlantic scientists took the old piece to NRC-CNBC for analysis. This was a rare chance to test a part of the original submarine in ways that couldn't be done on the full hull, even in dry dock.
"The old piece happened to have some interesting metallurgical details," says Rod McGregor, the engineer at DRDC in Victoria. "One of them was that circ-seam weld, for which NRC-CNBC is perfectly suited to give us a full, through-thickness characterization."
"There is an ongoing effort to re-evaluate the ultimate submarine operational limits using the latest computational methods, which requires explicit understanding of the physical character of the pressure-hull," adds McGregor. "This was an exclusive opportunity to assess the strength and stresses of both the hull metal and the original manufacturer's weld to help validate the models we're using to predict fatigue life and operational limits."
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