Crystallographic plasticity was applied to study the initiation of micro cracks on the smooth surface of polycrystalline under uniform applied stress. Even under the uniform external stress, due to the different cryst...Crystallographic plasticity was applied to study the initiation of micro cracks on the smooth surface of polycrystalline under uniform applied stress. Even under the uniform external stress, due to the different crystallographic orientations of the grains in the polycrystalline, there is un-uniform stress distribution and the deformation is also not uniform. Under the fatigue loading, the roughness increases with the number of fatigue, and deformation will localize in some places, where micro cracks form.展开更多
The internal micro cracks with the critical length about 30?μm and thickness less than 1?μm were introduced into the pure titanium samples by uniaxial tension compression low cycle fatigue method. The experimental r...The internal micro cracks with the critical length about 30?μm and thickness less than 1?μm were introduced into the pure titanium samples by uniaxial tension compression low cycle fatigue method. The experimental results indicate that the internal fatigue micro crack clearly evolves from the original penny shaped crack into a string of spherical voids in the longitudinal section plane of the fatigue sample after the vacuum diffusive healing at the high temperature. The quantitative relationship between the radius and the spacing of spherical voids depends on the crack position (within grains, on grain boundaries or transgranular sites) and its orientations within the grain. The diffusive healing, the related thermodynamics and mechanism, and the effect of the surface tension anisotropy on the relationship between void diameter and void spacing are also discussed.展开更多
To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,...To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the thermodynamic analysis results are consistent with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al_(2)O_(3)and TiN-Al_(2)TiO_(5)-Al_(2)O_(3)to Ce2O_(3),Ce_(2)O_(2)S,CeAlO_(3),TiN-Al_(2)TiO_(5)-Ce_(2)O_(3)and TiN-Al_(2)TiO_(5)-Ce_(2)O_(2)S composite inclusions,which can be confirmed by metallographic examination.The elastic constants were calculated,and the bulk modulus,Young's modulus,shear modulus and Poisson's ratio were evaluated by the Voigt-Reuss-Hill(VRH)approximation.All inclusions except Ce_(2)O_(3)show apparent brittleness.TiN,Al_(2)O_(3),Al_(2)TiO_(5)and CeAlO_(3)present much higher hardness than iron matrix,while the hardness of Ce2O_(3)or Ce_(2)O_(2)S is close to that of iron matrix.The thermal expansion coefficients of Ce_(2)O_(3)and CeAlO_(3)are close to that of iron matrix,whereas,Ce_(2)O_(2)S inclusion has largely different thermal expansion coefficient from iron matrix and may deteriorate the steel performance at higher temperatures.The relatively small differences between Ce inclusions and iron matrix in terms of hardness,toughness,brittleness,and thermal expansion coefficient can explain the improvement of the mechanical properties of the tested steel.展开更多
True triaxial rockburst experiments with four different unloading rates were performed on four prism specimens of granite sampled from Beishan, China. The damage evolution in the rockburst test was investigated from t...True triaxial rockburst experiments with four different unloading rates were performed on four prism specimens of granite sampled from Beishan, China. The damage evolution in the rockburst test was investigated from two aspects including fracture surface crack and fragment characteristics. The scanning electron microscopy was used to observe the micro crack information on fragment surface. Combing binarization and box counting dimensions, the fractal dimensions of cracks were obtained. Meanwhile,the fragments were collected and a sieving experiment was conducted. We weighed the fragments qualities, counted the amount of fragments and measured the fragments length, width and thickness.Utilizing four methods to calculate damage fractal dimensions of fragments, the trend of fractal value changing with unloading rates can be roughly described. It can be concluded from these experiments that the fractal dimension either for crack or for fragment holds a decreasing trend with the decreasing unloading rate, indicating a reduction of damage level.展开更多
When two contacting solid surfaces are tightly closed and invisible to the naked eye, the discontinuity is said to be microscopic regardless of whether its length is short or long. By this definition, it is not suffic...When two contacting solid surfaces are tightly closed and invisible to the naked eye, the discontinuity is said to be microscopic regardless of whether its length is short or long. By this definition, it is not sufficient to distinguish the difference between a micro- and macro-crack by using the length parameter. Microcracks in high strength metal alloys have been known to be sev- eral centimeters or longer. Considered in this work is a dual scale fatigue crack growth model where the main crack can be mi- cro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the micro- structure. This region is referred to as the ＂micro-tip＂ and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions. The combination is sufficient to model microscopic entities in the form of voids, inclusions, precipitations, interfaces, in addition to subgrain imperfections, or cluster of dislocations. This is accomplished by using the method of ＂singularity representation＂ such that closed form asymptotic solutions can be obtained for the development of fa- tigue crack growth rate relations with three parameters. They include： （1） the crack surface tightness o-＊ represented by Cro/Cr~ = 0.3-0.5 for short cracks in region I, and 0.1-0.2 for long cracks in region II, （2） the micro/macro material properties reflected by the shear modulus ratio/1＂ （=,L/micro/]-/macro varying between 2 and 5） and （3） the most sensitive parameter d＊ being the micro-tip characteristic length d＊ （=d/do） whose magnitude decreases in the direction of region I ---~II. The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C（AK）n relation where AK has now been re-derived for a microcrack with surfaces tightly in contact. The contact force will depend on the mean stress ~m or mean stress ratio R as the primary parameter and on the stress amplitude era as the secondary parameter.展开更多
文摘Crystallographic plasticity was applied to study the initiation of micro cracks on the smooth surface of polycrystalline under uniform applied stress. Even under the uniform external stress, due to the different crystallographic orientations of the grains in the polycrystalline, there is un-uniform stress distribution and the deformation is also not uniform. Under the fatigue loading, the roughness increases with the number of fatigue, and deformation will localize in some places, where micro cracks form.
文摘The internal micro cracks with the critical length about 30?μm and thickness less than 1?μm were introduced into the pure titanium samples by uniaxial tension compression low cycle fatigue method. The experimental results indicate that the internal fatigue micro crack clearly evolves from the original penny shaped crack into a string of spherical voids in the longitudinal section plane of the fatigue sample after the vacuum diffusive healing at the high temperature. The quantitative relationship between the radius and the spacing of spherical voids depends on the crack position (within grains, on grain boundaries or transgranular sites) and its orientations within the grain. The diffusive healing, the related thermodynamics and mechanism, and the effect of the surface tension anisotropy on the relationship between void diameter and void spacing are also discussed.
基金Project supported by National Natural Science Foundation of China(51774190,51664044)Natural Science Foundation of Inner Mongolia,China(2018LH05005).
文摘To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the thermodynamic analysis results are consistent with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al_(2)O_(3)and TiN-Al_(2)TiO_(5)-Al_(2)O_(3)to Ce2O_(3),Ce_(2)O_(2)S,CeAlO_(3),TiN-Al_(2)TiO_(5)-Ce_(2)O_(3)and TiN-Al_(2)TiO_(5)-Ce_(2)O_(2)S composite inclusions,which can be confirmed by metallographic examination.The elastic constants were calculated,and the bulk modulus,Young's modulus,shear modulus and Poisson's ratio were evaluated by the Voigt-Reuss-Hill(VRH)approximation.All inclusions except Ce_(2)O_(3)show apparent brittleness.TiN,Al_(2)O_(3),Al_(2)TiO_(5)and CeAlO_(3)present much higher hardness than iron matrix,while the hardness of Ce2O_(3)or Ce_(2)O_(2)S is close to that of iron matrix.The thermal expansion coefficients of Ce_(2)O_(3)and CeAlO_(3)are close to that of iron matrix,whereas,Ce_(2)O_(2)S inclusion has largely different thermal expansion coefficient from iron matrix and may deteriorate the steel performance at higher temperatures.The relatively small differences between Ce inclusions and iron matrix in terms of hardness,toughness,brittleness,and thermal expansion coefficient can explain the improvement of the mechanical properties of the tested steel.
基金supported by the National Key Basic Research Program (No. 2010CB226800)the Innovation Team Development Program of the Ministry of Education (No. IRT0656)the Fundamental Research Funds for the Central Universities (No. 2010YL14)
文摘True triaxial rockburst experiments with four different unloading rates were performed on four prism specimens of granite sampled from Beishan, China. The damage evolution in the rockburst test was investigated from two aspects including fracture surface crack and fragment characteristics. The scanning electron microscopy was used to observe the micro crack information on fragment surface. Combing binarization and box counting dimensions, the fractal dimensions of cracks were obtained. Meanwhile,the fragments were collected and a sieving experiment was conducted. We weighed the fragments qualities, counted the amount of fragments and measured the fragments length, width and thickness.Utilizing four methods to calculate damage fractal dimensions of fragments, the trend of fractal value changing with unloading rates can be roughly described. It can be concluded from these experiments that the fractal dimension either for crack or for fragment holds a decreasing trend with the decreasing unloading rate, indicating a reduction of damage level.
文摘When two contacting solid surfaces are tightly closed and invisible to the naked eye, the discontinuity is said to be microscopic regardless of whether its length is short or long. By this definition, it is not sufficient to distinguish the difference between a micro- and macro-crack by using the length parameter. Microcracks in high strength metal alloys have been known to be sev- eral centimeters or longer. Considered in this work is a dual scale fatigue crack growth model where the main crack can be mi- cro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the micro- structure. This region is referred to as the ＂micro-tip＂ and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions. The combination is sufficient to model microscopic entities in the form of voids, inclusions, precipitations, interfaces, in addition to subgrain imperfections, or cluster of dislocations. This is accomplished by using the method of ＂singularity representation＂ such that closed form asymptotic solutions can be obtained for the development of fa- tigue crack growth rate relations with three parameters. They include： （1） the crack surface tightness o-＊ represented by Cro/Cr~ = 0.3-0.5 for short cracks in region I, and 0.1-0.2 for long cracks in region II, （2） the micro/macro material properties reflected by the shear modulus ratio/1＂ （=,L/micro/]-/macro varying between 2 and 5） and （3） the most sensitive parameter d＊ being the micro-tip characteristic length d＊ （=d/do） whose magnitude decreases in the direction of region I ---~II. The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C（AK）n relation where AK has now been re-derived for a microcrack with surfaces tightly in contact. The contact force will depend on the mean stress ~m or mean stress ratio R as the primary parameter and on the stress amplitude era as the secondary parameter.
