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April's CSP Research Profile Letter
Scratching a weaker material with a tougher one is no doubt the most elemental conceptualization of a mechanics-of-materials test ever conceived by mankind. In use as a tool to compare the relative hardness of two materials since ancient time, the first abstraction of scratch resistance into a quantitative metric of material classification is due to Carl Friedrich Christian Mohs (1773--1839), who in 1824 put the ability of one mineral sample to scratch another on an ordinal (rank-ordering) scale, the Mohs scale of mineral hardness. Recent research at M.I.T. now shows that the scratch test provides a means to determine the fracture properties of materials. The idea of the scratch test is simple. It suffices to recall the scraping of cold butter with a knife; that is, plowing and cutting with a scratch device (knife) the surface of a weaker material (butter). Provided with an accurate fracture mechanics model, it is possible to employ this technique to determine the fracture toughness of all types of material, from butter to steel, and concrete. Here we employ this technique to determine the fracture toughness of cement-based materials at multiple scales from depth sensing scratch tests.
This microscratch research has been performed using a CSM dual-purpose Micro Combi Tester. (for details about equipment, see www.csm-instruments.com)
One of the most promising techniques that emerged from the implementation of nanotechnology in materials science and engineering to assess mechanical properties at small scales is nanoindentation. The idea is simple: by pushing a needle onto the surface of a material, the surface deforms in a way that reflects the mechanical properties of the indented material. Yet, in contrast to most metals and ceramics, for which this technique was originally developed, concrete materials are highly heterogeneous and multi-phase materials from a scale of a few nanometers to macroscopic scales. This brought about statistical nanoindentation techniques developed at MIT: by carrying out a large array of nanoindentation tests on a surface of a multi-phase material, mechanically active phases are identified by statistical means.
This nanoindentation research has been performed using a CSM Instruments Nanoindentation Tester (NHT). (for details about equipment, see www.csm-instruments.com)
