As the news headlines show, cracks in pipelines carrying natural gas, fossil fuels, and water can cause serious damage. Further, cracks and corrosion are a serious concern at nuclear power stations. Scientists used high-speed photography and digital image analysis to observe both the events that cause cracks and the speed with which the cracks travel.
Using a silver-gold alloy, a model for more common materials, they found that corrosion cracks begin with the formation of a porous layer. Cracks form in the porous layer and can travel up to 200 meters, or roughly a tenth of a mile, in one second. The cracks cause the metal to shatter like glass on the micrometer scale, even though at smaller length scales (below one micrometer) the fractures are like modeling clay that can stretch before tearing apart.
Preventing failures at nuclear power generating systems, water treatment plants, and other infrastructure sites is vital for both secure energy production and public safety. Understanding the causes and nature of high-speed fractures in materials can lead to materials and processes that prevent future failures.
Corrosive environments, such as those found inside nuclear power plants and other sites, can cause stainless steel, brass, or other metallic alloys to fail. The materials develop porous layers that can lead to the failure of the material and the corresponding engineered structures. One explanation for the damage is that a crack begins in the porous layer and then injects -- at high speed -- into the non-porous parent phase of the material. Using ultrafast photography and digital image correlation, scientists at Arizona State University studied the static and dynamic fracture properties of freestanding monolithic nanoporous gold as a function electrochemical potential.
The experiments reveal that at electrochemical potentials typical of porosity formation, these structures can support a fracture at crack velocities of 200 m/s. The results identify the important role of high-speed fractures in the failure of vital materials and could help in designing crack-resistant materials.
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