Thursday, October 25, 2012
Mechanical and Civil Engineering Seminar
How do you Scrape (Cold) Butter? Scratch Test: A New Approach for Multiscale Characterization of Fracture Properties
Ange-Therese Akono, Ph.D. Student, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology
The scratch test consists in plowing and cutting with a scratch device the surface of a weaker material and is likely the most elemental conceptualization of a mechanics-of-materials test ever conceived by mankind. Ancient Greeks and Roman writers mention the test in their works. However, the first rationalization of the test is due to the German scientist Friedrich Mohs who in 1824 defined a quantitative metric, the Mohs scale of hardness, for the classification of various materials according to their scratch resistance. Yet despite its long history, the exact physics of the test had not been defined. The challenge lies in quantitatively predicting the scratch resistance in function of the mechanical properties of the material, the scratch depth and the geometry of the scratch device. By means of a hybrid experimental and theoretical investigation of the fracture scaling in scratch tests, we will show that scratching is a fracture dominated process. An analytical model is developed that relates the fracture toughness to the geometry of the scratch device and to the loading parameters of the test. This model is turned into an inverse application technique that can be used at multiple scales to accurately predict the fracture properties of materials as it will be demonstrated for geo-materials, polymers, and metals. Moreover, the scalability of scratching for different probes and depths makes it possible to examine the influence of microstructure on the global fracture properties. Finally the model is extended to account for rate-dependent materials, in which case a method is developed to characterize the rate-independent fracture toughness. The field of applications is vast and will be illustrated through several investigations including fracture resistance of hardened cement paste and shale, fracture behavior of gas shale, fracture resistance of thin films and so on.
(*) with Pedro M. Reis, Nicholas X. Randall, and Franz-Josef Ulm.
Refreshments are at 10:45 a.m. in 210 Thomas