Abstract
This Ph.D thesis is focused on indentation fracture generated by a flat-tipped cylindrical indenter, as most research activities up to now have been confined to Hertzian and Vickers indentation methods using spherical and sharp-point indenters. One key advantage of the flat-tipped cylindrical indentation method over Hertzian and Vickers is that a closed-form analytical solution of the singular stress field at the sharp contact edge has recently been derived. In this thesis, numerical simulations have been used to verify the elastic analytical model, and the surface-contact-induced cracking angle and stress distribution near the contact edge. The flat-tipped indentation method is used to determine the fracture toughness and flexural strength of brittle polymers at various temperatures.
Based on the singular stress field at the sharp contact edge, the concept of indentation stress intensity factor KIND has been introduced and its critical value, or the indentation fracture toughness KIND-C is linked to the critical indentation load at which the contact crack is initiated. Using the recently-developed analytical solution, KIND-C is linked to the common Mode-I fracture toughness KIC.
The elastic singular stress field at the sharp contact edge is evaluated by Finite Element Analysis simulation and compared with the theoretical solution. The cracking angle derived from the FEA simulation, based on the maximum strain energy release rate, is about 65˚, akin to the analytical solution.
The flat-tipped cylindrical indentation is also used to determine the fracture modes of multi-layer PMMA thin plates on a soft rubber substrate. The stress field, obtained from the FEA simulations of the indentation tests, confirms the bending failure from the bottom surfaces of those thin plates, thus related to the strength of PMMA.
In summary, the thesis presents the flat-tipped indentation method, the theoretical analysis, FEA simulations, indentation tests of brittle polymers, and tests of thin plates on a soft rubber substrate. Those indentation tests allow determination of the fracture toughness and flexural strength of brittle materials and provide new alternative methods besides the commonly-used Hertzian and Vickers indentation methods.
Based on the singular stress field at the sharp contact edge, the concept of indentation stress intensity factor KIND has been introduced and its critical value, or the indentation fracture toughness KIND-C is linked to the critical indentation load at which the contact crack is initiated. Using the recently-developed analytical solution, KIND-C is linked to the common Mode-I fracture toughness KIC.
The elastic singular stress field at the sharp contact edge is evaluated by Finite Element Analysis simulation and compared with the theoretical solution. The cracking angle derived from the FEA simulation, based on the maximum strain energy release rate, is about 65˚, akin to the analytical solution.
The flat-tipped cylindrical indentation is also used to determine the fracture modes of multi-layer PMMA thin plates on a soft rubber substrate. The stress field, obtained from the FEA simulations of the indentation tests, confirms the bending failure from the bottom surfaces of those thin plates, thus related to the strength of PMMA.
In summary, the thesis presents the flat-tipped indentation method, the theoretical analysis, FEA simulations, indentation tests of brittle polymers, and tests of thin plates on a soft rubber substrate. Those indentation tests allow determination of the fracture toughness and flexural strength of brittle materials and provide new alternative methods besides the commonly-used Hertzian and Vickers indentation methods.
| Original language | English |
|---|---|
| Qualification | Doctor of Philosophy |
| Supervisors/Advisors |
|
| Publication status | Unpublished - 2016 |