Engineering
The Corbin Bridge in Pennsylvania was the first to have an aluminum deck replacement (in 1996). Aluminum welds for such decks are being characterized by neutron scattering.
Neutron scattering has played an important role behind the scenes to protect public safety and the environment. It has guided improvements to ensure that trains don’t veer off tracks, wings don't fall off airplanes, and pipelines don’t corrode enough to leak oil. By measuring how much distances between planes of atoms have stretched or shrunk (lattice strains), neutrons can probe for residual stresses in the bulk of materials. These internal stresses, which develop in a part during the manufacturing process, can predispose it to cracking, wear, accelerated chemical attack, and even failure brought on by stresses externally imposed on the part during use. Engineers want to know when failure is likely to occur in a part and whether use of different materials and manufacturing processes (e.g., heat treatment) would produce a part that will last longer. Neutron scattering results, combined with computer models, is providing these answers.
Engineers want to know when a part is likely to fail and how use of different materials and processes will affect its performance.
To prevent the unexpected failure of recovery boilers in paper mills (in which boiler explosions often kill or injure workers and cost a paper mill $1 million a day in down time), an effort is under way to identify alternative materials and manufacturing processes for boiler steam tubes. The goal is to prevent cracking in the steel tubes, which allows water to come in contact with hot, corrosive chemicals used to separate wood fibers for making paper. Using neutron scattering to measure residual stresses in boiler steam tubes made in different ways to protect them from corrosion, researchers hope to identify an inexpensive manufacturing process that minimizes the production of residual stresses, increasing tube life and saving human lives.
The extremes of compressive (blue) and tensile (red) strains in a steel plate carrying a weld overlay are shown by neutron diffraction mapping. Undesired welding process variations cause irregularities in the strain distribution.
The aluminum industry is interested in building aluminum bridge decks that are as strong and corrosion resistant as steel decks. Because aluminum decks are lighter than steel ones, less costly support structures could be built. Also, the modular construction of aluminum decks will make it easy to replace bridge parts. Scientists have developed ways to use neutrons to study residual stresses in aluminum welds in aluminum bridge decks.
Neutron scattering allows engineers to measure more subtle structural details in a smaller sample size or in huge engineering samples, such as earth-moving equipment. It enables effective measurements of residual stresses in composites, which are being used increasingly to make cutting tools, aircraft structures, and engine parts because they are stronger and lighter than other materials. A composite consists of a metal matrix in which ceramic fibers are embedded to strengthen it. Because aluminum and silicon nitride, for example, expand and contract differently during the manufacturing process to combine them, residual microstresses are created. Neutron characterizations of microstresses in samples made different ways will help identify the manufacturing processes that produce the strongest composites.
Neutron scattering has been used to determine how to best manufacture and weld piping materials for use in oil pipelines to reduce residual stresses and prevent cracking and oil leaks.
Small-angle neutron scattering (SANS) can map at a nanometer scale the defects that contribute to material failure better than other methods such as electron microscopy. SANS can detect 50-atom clusters, or precipitates, that form in steel in reactor pressure vessels after years of exposure to neutron radiation from the reactor core. The irradiated steel becomes brittle, making it more vulnerable to cracking. Neutrons can be used to verify whether heat treatment of a vessel can remove defects, making it less brittle. To build a large engineering marvel such as a safe, efficient turbine jet engine, it helps to understand how metallic alloys in turbine blades behave under extreme stress at a microscopic level.
Engineers think big, but the success of their projects often depends on insights into the small provided by neutrons.
