By Olivia Grubisich
Researchers have established a new protocol for deducing the tensile strength of a material in the commonly used Brazilian fracture test. Their work has broad implications, including a current research project to improve the treatment of kidney stones. The findings bring to light misconceptions in previous analyses and provide new, standardized guidelines for how Brazilian test results should be interpreted.
The team led by CEE professor Oscar Lopez-Pamies and Duke University professor John Dolbow, in collaboration with CEE alumnus and Georgia Tech professor Aditya Kumar (MS 16, PhD 20), details their findings in a paper recently published in the Journal of Mechanics and Physics of Solids.
Brazilian engineer Lobo Carneiro first introduced the Brazilian test in 1943, when he proposed using concrete pillars to relocate a church in Rio de Janeiro. The church was never moved, but in the process of trying, Carneiro inadvertently developed what would become the Brazilian test: an indirect measure of tensile strength that involves compressing a disk of concrete between two stiff platens until it fractures.
Since then, the test has become a standard for practitioners evaluating tensile strength of not only concrete but other brittle materials like rock and ceramics. The easy specimen preparation and loading procedure make it a convenient evaluation for practitioners to perform. However, its application of non-uniform triaxial stress in lieu of uniform axial stress creates discrepancies in how results for the test get interpreted, hindering its overall reliability.
“We are able to bring resolution to this decades-old problem by deploying the fracture theory of Kumar, Bourdin, Francfort, and Lopez-Pamies (2020), which has been recently established as a complete theory of fracture capable of accurately describing the nucleation and propagation of cracks in linear elastic brittle materials under arbitrary quasistatic loading conditions,” Lopez-Pamies writes.
Their results uncovered two main misconceptions held in earlier Brazilian test analyses. They first disproved the idea that a material’s strength is a singular, scalar quantity within the stress space. Instead, they found that strength is most accurately understood as an entire 3D surface in that same space. Researchers also found that the nucleation of cracks often begins long after the materials’ strength has been violated, while earlier studies believed these two events happened simultaneously.
From this information, Lopez-Pamies and his team were able to determine a new protocol for interpreting Brazilian test results. The uniform method they established improves upon current standards from the International Society of Rock Mechanics and American Society of Testing Materials and will allow practitioners who use the Brazilian test to operate with enhanced efficiency.
This work is a major finding in an NSF CMMI collaborative project between the University of Illinois Urbana-Champaign and Duke University. The project, “A Unified Theory of Crack Nucleation and Growth of Materials Subjected to Repetitive Surface Acoustic Waves and Dynamic Patterns,” seeks to improve techniques for lithotripsy, a kidney stone treatment that uses shock waves to break stones into smaller parts.
“Kidney stones are similar to other brittle materials such as concrete, rocks, and glass in that they deform elastically up to a limit and then they fracture,” Lopez-Pamies writes. “The key to find better lithotripsy treatments is to understand how fracture nucleates in kidney stones when they are subjected to impact. This problem is intimately related to the Brazilian test since both are problems that are governed by how fracture nucleates under a boundary condition of localized contact.”