서강대학교

초록/요약

For various applications, it is important to understand tribological and mechanical properties of brittle materials with considering size effect. Here, we investigated characteristic of brittle materials by using scratch / indentation test. In Part I, we investigate scratch-tip-size effect and change of friction coefficient in nano / micro scratch tests using XFEM. Compared to the experimental studies, numerical studies on scratch test have been limited due to the difficulty of numerical modeling. Here, we experimentally investigate the scratch-tip-size effect (STSE) based on coefficient of friction (COF) change to make extended finite element (XFE) models considering STSE, COF change, failure modes, yielding strength, shear strength, elastic recovery and pile-up. XFE models are then validated through nano / micro scratch tests on soda-lime glass. Finally, components of COF are extracted and then effects of tensile cracks and parameters of Drucker-Prager model on COF are investigated. We expect that proposed XFE models contribute to the understanding of damage characteristics during the scratch test. In Part II, we determine true mechanical properties of brittle materials through nanoindentation. Due to cracking, true mechanical properties of brittle materials have not been obtained by experiments. Here, a novel method based on nanoindentation is developed to determine true mechanical properties (elastic modulus, zero pressure yield strength and friction angle in a linear Drucker-Prager model) of brittle materials which show pressure-dependent yield behavior, without the interference of cracking. Nanoindentation experiments on glass, rock and ceramic materials were conducted, and then a finite element model was established by applying the linear Drucker-Prager model. Based on experimental data and numerical results, true mechanical properties are determined and the method validated. Furthermore, the validity of determined mechanical properties is confirmed by comparing values with those obtained in previous studies. We expect that the proposed method contributes to describing pressure-dependent plastic behavior of brittle materials. In Part III, we present numerical implementation technique of piecewise Drucker-Prager yield model. Piecewise Drucker-Prager (PDP) yield model has been developed to overcome the limitation that existing pressure-dependent yield models cannot be flexibly modified. Despite the usefulness of PDP model, constitutive model and numerical algorithm have not been established systematically. Here, we develop constitutive model, integration algorithm and consistent tangent operators for PDP model. The constitutive model is derived considering strain hardening with yield flows. The integration algorithm is developed based on return mapping (to smooth portion, apex and corner). The consistent tangent operators with developed integration algorithm are formulated for each return mapping case. With the present formulation, a user material subroutine (UMAT) of proposed PDP model is developed and then verified by simulating triaxial and nanoindentation tests by using Abaqus / Standard.