Engineering Mechanics Institute Conference 2013

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Fracture Toughness Prediction at The Microscopic Scale

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Ange-Therese Akono
Massachusetts Institute of Technology
United States

Pedro Miguel Reis
Massachusetts Institute of Technology
United States

Franz-Josef Ulm
Massachusetts Institute of Technology
United States

The prediction of the fracture resistance of heterogeneous at the microscopic scale remains a challenging problem. In the case of conventional fracture testing methods such as the three-point bending test on single edge notched specimen, the presence of a fracture process zone around the crack tip makes it necessary to work on macroscopic specimen to obtain a linear elastic fracture behavior. To overcome this limitation we focus on micro scratch tests as an alternative means to characterize the fracture toughness. Scratch tests consist in pushing down a Rockwell diamond stylus across the surface of a weaker material and are widely used to characterize the wear and adhesion of ceramics, thin films and coatings. Although acoustic emissions provide evidence of fracture processes at work during the test, we used scanning electron microscopy to investigate the physical failure mechanisms. A Linear Elastic Fracture Mechanics model was then developed to relate the measured forces and depth of penetration to the geometry of the scratching probe and the fracture toughness. This model was turned into an experimental technique that involved the calibration of the scratching probe using a reference material. Application of the experimental technique to steel and aluminum alloys and with scratch depths less than 50 micrometers yielded estimates of the fracture toughness that are in excellent agreement with measurements obtained from conventional fracture testing methods (three-point bending test on single edge notched specimen, compact-tension test,…) performed on macroscopic specimen. This new experimental technique therefore opens new venues for the study of brittle failure at the microscopic scale.


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