The Gold Standard of Cracking Tests

cracking tests
High magnification images of the surface of a crack – referred to as the fracture surface – provide information on how cracks are formed and progress through a material as it breaks. For some gold alloys (models for more common materials), corrosion can cause a large amount of internal pores to form, representing the type of structure that might develop in a material after a long time in a nuclear reactor. This type of corrosion-induced structure can result in unpredictable and premature failure of components. Below 1 millimeter, the fracture surface shown does not have any sharp, abrupt edges, demonstrating that the break was not brittle (i.e., not like glass shattering). At this scale, the fracture results in blunt edges similar to fracture from stretching apart taffy (arrow). Image courtesy of Karl Sieradzki, Arizona State University

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 lengthscales (below one micrometer) the fractures are like modeling clay that can stretch before tearing apart.

The Impact

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.

Summary

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.