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Microindentation offers hope for high-throughput testing
Researchers at the Georgia Institute of Technology have developed a new set of protocols to determine tensile yield strength from microindentation tests. Micro indentation offers all the advantages of a high-throughput measurement technique. It requires little in the way of sample preparation, is extremely fast, and lends itself to automation. However, when it comes to bulk properties, indentation tests are not as informative as traditional tensile tests.
Extracting tensile strength from microindentation measurements has been the focus of several researchers in Georgia Tech’s MINED (Materials INformatics for Engineering Design) Group. Applying multi-scale models and finite-element simulation, they have determined theoretically that the ratio of indentation to tensile yield strength is 2.2 for an isotropic, perfectly plastic material. They then verified it experimentally, using data from several samples of Al-6061 and titanium alloys with diverse microstructures. In the process, they also varied the size of the indenter, using one with a radius of 0.5 mm and another with a radius of 6.35 mm.
The larger indenter performed better on the titanium alloys than the smaller one. The differences were less noticeable and relatively constant when testing the aluminum. Based on the results, as long as the primary zone of indentation is large enough compared to feature length scales (e.g. grain size), the 2.2 ratio holds true, making indentation a viable substitute or precursor to tensile testing.