This study presents a coupled thermo-hydro-mechanical-fatigue(THM-F)model,developed based on variational phase-field fatigue theory,to simulate the freeze-thaw(F-T)damage process in concrete.The fracture phasefield mo...This study presents a coupled thermo-hydro-mechanical-fatigue(THM-F)model,developed based on variational phase-field fatigue theory,to simulate the freeze-thaw(F-T)damage process in concrete.The fracture phasefield model incorporates the F-T fatigue mechanism driven by energy dissipation during the free energy growth stage.Using microscopic inclusion theory,we derive an evolution model of pore size distribution(PSD)for concrete under F-T cycles by treating pore water as columnar inclusions.Drawing upon pore ice crystal theory,calculation models that account for concrete PSD characteristics are established to determine ice saturation,permeability coefficient,and pore pressure.To enhance computational accuracy,a segmented Gaussian integration strategy based on aperture levels is employed.The pore pressure estimation model is applied to assess the frost resistance of concrete with varying air-entraining agent contents,confirming that optimal air-entrainment significantly improves pore structure and lowers the overall freezing point of pore ice.The derived permeability coefficient and pore pressure estimation models are integrated into the THM-F coupled framework,which employs a staggered iterative solution scheme for efficient simulation.Mesoscale numerical examples of concrete demonstrate that the proposed THM-F model effectively captures structural degradation and accurately tracks the procession of F-T-induced fatigue cracks.Validations against experimental measurements,including temperature variations,stress-strain curves,and strain history,shows excellent agreement,underscoring the model’s accuracy and applicability.This study provides a robust theoretical and computational framework for quantitative analysis of coupled F-T-fatigue damage in concrete.展开更多
A series of Co/TiO2 catalysts were prepared by incipient wetness impregnation. The pH value of the impregnation solution was adjusted by using nitric acid and urea. The XRD, TPR and XPS characterization results indica...A series of Co/TiO2 catalysts were prepared by incipient wetness impregnation. The pH value of the impregnation solution was adjusted by using nitric acid and urea. The XRD, TPR and XPS characterization results indicate that the change of the pH value of the impregnation solution is correlated with the crystal particle size of cobalt oxide species （Co3O）, dispersion behavior and metal-support interaction. When the pH value of the solution is higher than the IEP （ion equipotential point） of titania, the larger Co3O4 particle size could be obtained on the surface of the catalyst. High pH value favores the adsorption and dispersion of active cobalt species over the titanium dioxide, and facilitates the interaction between cobalt and titania, and depresses the reduction degree of the catalyst. In the F-T reaction, catalysts prepared at high pH value have low activities and high methane selectivities. When the pH value of the solution is lower than the IEP of titania, the Co3O4 particle size is small, the interactions between cobalt species and support are weak and the reduction degrees are high, which further enhance the F-T reaction activity and suppress methane selectivity.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.11932006 and 12172121).
文摘This study presents a coupled thermo-hydro-mechanical-fatigue(THM-F)model,developed based on variational phase-field fatigue theory,to simulate the freeze-thaw(F-T)damage process in concrete.The fracture phasefield model incorporates the F-T fatigue mechanism driven by energy dissipation during the free energy growth stage.Using microscopic inclusion theory,we derive an evolution model of pore size distribution(PSD)for concrete under F-T cycles by treating pore water as columnar inclusions.Drawing upon pore ice crystal theory,calculation models that account for concrete PSD characteristics are established to determine ice saturation,permeability coefficient,and pore pressure.To enhance computational accuracy,a segmented Gaussian integration strategy based on aperture levels is employed.The pore pressure estimation model is applied to assess the frost resistance of concrete with varying air-entraining agent contents,confirming that optimal air-entrainment significantly improves pore structure and lowers the overall freezing point of pore ice.The derived permeability coefficient and pore pressure estimation models are integrated into the THM-F coupled framework,which employs a staggered iterative solution scheme for efficient simulation.Mesoscale numerical examples of concrete demonstrate that the proposed THM-F model effectively captures structural degradation and accurately tracks the procession of F-T-induced fatigue cracks.Validations against experimental measurements,including temperature variations,stress-strain curves,and strain history,shows excellent agreement,underscoring the model’s accuracy and applicability.This study provides a robust theoretical and computational framework for quantitative analysis of coupled F-T-fatigue damage in concrete.
文摘A series of Co/TiO2 catalysts were prepared by incipient wetness impregnation. The pH value of the impregnation solution was adjusted by using nitric acid and urea. The XRD, TPR and XPS characterization results indicate that the change of the pH value of the impregnation solution is correlated with the crystal particle size of cobalt oxide species （Co3O）, dispersion behavior and metal-support interaction. When the pH value of the solution is higher than the IEP （ion equipotential point） of titania, the larger Co3O4 particle size could be obtained on the surface of the catalyst. High pH value favores the adsorption and dispersion of active cobalt species over the titanium dioxide, and facilitates the interaction between cobalt and titania, and depresses the reduction degree of the catalyst. In the F-T reaction, catalysts prepared at high pH value have low activities and high methane selectivities. When the pH value of the solution is lower than the IEP of titania, the Co3O4 particle size is small, the interactions between cobalt species and support are weak and the reduction degrees are high, which further enhance the F-T reaction activity and suppress methane selectivity.
