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UBC Theses and Dissertations

Estimating the yield stress in aluminum alloys using instrumented spherical indentation Inoue, Yuri


Instrumented indentation tests using a spherical indenter are a potential source to obtain mechanical properties from small samples and also to obtain localized mechanical properties within a sample. Thus, the data obtained from indentation tests would provide valuable insights into the mechanical properties of materials which are not feasible with conventional tensile testing. Despite the potential advantages of indentation tests, there are still questions which need to be considered when conducting indentation test and analyzing the data. In particular, a robust procedure is required to determine the true contact point between the indenter tip and the sample. In the present work, instrumented indentation tests were used to extract stress-strain curves from the load, displacement and the continuous stiffness measurements. Indentation tests were conducted on an Al-Mg-Si precipitation hardened alloy with two different heat treatments (underaged and peak aged). The various parameters that must be considered when conducting indentation tests and analyzing the measured load, displacement, and continuous stiffness data have been examined. A new approach is proposed to determine the true contact point of the indenter to the sample using a combination of Kalidindi’s method and the deviation from Hertzian elastic contact mechanics. The critically resolved shear stress (CRSS) for the onset of plasticity was determined for indentation tests using indenter tip radii of 5, 10, 50, and 250𝜇m and then compared to the CRSS estimated from polycrystal plasticity simulations fit to experimental tensile stress-strain results. A significant indenter size effect was observed and as such a conversion factor is proposed to relate the CRSS measured for different indenter tip radii to the bulk response.

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