A new type of semi-rigid thin-walled steel-concrete composite beam-to-column joint has been proposed in this paper.Five semi-rigid composite beam-to-column joint specimens subjected to hogging moments under monotonic ...A new type of semi-rigid thin-walled steel-concrete composite beam-to-column joint has been proposed in this paper.Five semi-rigid composite beam-to-column joint specimens subjected to hogging moments under monotonic loading were tested to study the static behavior of this new type of joint.The main variable parameters for the five joint specimens were the longitudinal reinforcement ratio and the joint type.The experimental results designated that the magnitude of extension of the longitudinal reinforcement is the most important factor that influenced the moment-rotation characteristic of the new type of joint.The concrete slabs could resist 3.8%-19.1% of the total shear load applied to the cross-sections near the beam-to-column connection.The edge stiffened elements,such as the flange of the lipped I-section thin-walled steel beam,were capable of having considerable inelastic deformation capacity although they had comparatively large width-to-thickness ratios.The shear failure of the concrete cantilever edge strip must be taken into account in practical design because it has significant influence on the anchorage of the longitudinal reinforcement in the new type of external joints.展开更多
Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the sta...Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the static oxidation behavior of(Hf,Ti)C and the effect of Ti substitutions were investigated in air at 2500℃ under an oxygen partial pressure of 4.2 kPa.After oxidation for 2000 s,the thickness of the oxide layer on the surface of(Hf,Ti)C bulk ceramic is reduced by 62.29%compared with that on the HfC monocarbide surface.The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides,HfO_(2),and carbon.The Ti-rich oxycarbide((Ti,Hf)C_(x)O_(y))dispersed within HfO_(2) formed the major structure of the oxide layer.A coherent boundary with lattice distortion existed at the HfO2/(Ti,Hf)C_(x)O_(y) interface along the(111)crystal plane direction,which served as an effective oxygen diffusion barrier.The Hfrich oxycarbide((Hf,Ti)CxOy)together with(Ti,Hf)C_(x)O_(y),HfO_(2),and precipitated carbon constituted a dense transition layer,ensuring favorable bonding between the oxide layer and the matrix.The Ti content affects the oxidation resistance of(Hf,Ti)C by determining the oxide layer's phase distribution and integrity.展开更多
In this study, AA2519 alloy was initially processed by multi axial forging (MAF) at room and cryogenic temperatures. Subsequently, the microstructure and the mechanical behavior of the processed samples under quasi-st...In this study, AA2519 alloy was initially processed by multi axial forging (MAF) at room and cryogenic temperatures. Subsequently, the microstructure and the mechanical behavior of the processed samples under quasi-static loading were investigated to determine the influence of cryogenic forging on alloys’ subgrains dimensions, grain boundaries interactions, strength, ductility and toughness. In addition, the failure mechanisms at the tensile rupture surfaces were characterized using scanning electron micro-scope (SEM). The results show significant improvements in the strength, ductility and toughness of the alloy as a result of the cryogenic MAF process. The formation of nanoscale crystallite microstructure, heavily deformed grains with high density of grain boundaries and second phase breakage to finer particles were characterized as the main reasons for the increase in the mechanical properties of the cryogenic forged samples. The cryogenic processing of the alloy resulted in the formation of an ultrafine grained material with tensile strength and toughness that are ~41% and ~80% higher respectively after 2 cycles MAF when compared with the materials processed at ambient temperature. The fractography analysis on the tested materials shows a substantial ductility improvement in the cryoforged (CF) samples when compared to the room temperature forged (RTF) samples which is in alignment with their stress-strain profiles. However, extended forging at higher cycles than 2 cycles led only to increase in strength at the expense of ductility for both the CF and RTF samples.展开更多
Unloading failure of rocks,especially highly stressed rocks,is one of the key issues in construction of underground structures.Based on this,analytical models for rocks under quasi-static and intensive unloading condi...Unloading failure of rocks,especially highly stressed rocks,is one of the key issues in construction of underground structures.Based on this,analytical models for rocks under quasi-static and intensive unloading conditions are established to study the failure behavior of highly stressed rocks.In case of rock failure under quasi-static unloading,the rock mass ahead of working face is regarded as an elasto-brittle material,and the stress-displacement curves are used to characterize the tensile fracture of peak-stress area.It is observed that,when intensive unloading happens,there is an elastic unloading wave（perturbation wave） propagating in the rock mass.If the initial stress exceeds the critical stress,there will be a fracture wave,following the elastic unloading wave.To study the propagation feature of fracture wave,the conservation laws of mass,momentum and energy are employed.Results show that the post-peak deformation,strength and energy dissipation are essential to the failure process of highly stressed rocks.展开更多
The static and dynamic precipitation behavior of solution-treated binary Al-20 wt.% Zn alloy is investigated via artificial aging, cold rolling and artificial aging combined with cold rolling. The solution-treated Al-...The static and dynamic precipitation behavior of solution-treated binary Al-20 wt.% Zn alloy is investigated via artificial aging, cold rolling and artificial aging combined with cold rolling. The solution-treated Al-Zn alloy exhibits high thermal stability during aging, and low densities of nano-sized Zn particles are precipitated along with AI grain boundaries after aging at 200℃ for 13 h. Compared with static precipitation, dynamic precipitation occurs more easily in the Al-Zn alloy. Zn clusters are obtained after cold rolling at an equivalent plastic strain of 0.6, and the size of the Zn phase reaches hundreds of nanometers when the strain is increased to 12.1. The results show that the speed of static precipitation can be significantly enhanced after the application of 2.9 rolling strain. Grain refinement and defects induced by cold rolling are considered to promote Zn precipitation. The hardness of Al-Zn alloy is also affected by static and dynamic precipitations.展开更多
Crack initiation and subcritical crack growth in glass sheet and SIC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime ...Crack initiation and subcritical crack growth in glass sheet and SIC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime process of brittle materials involves three possible forms of crack growth: subcritical crack growth, partly subcritical crack growth and instantaneous fracture without subcritical crack growth. Curves of ν-K obtained in step-by-step static fatigue tests and in constant loading rate tests showed different trends for borosilicate glass sheet. α-SiC that is generally considered immune to mechanical fatigue effect and environmental attack was also tested under static loading and the lifetime was measured. The results showed that the threshold load to damage effect was over 80% of the initial strength for the SiC.展开更多
This paper presents a study of the quasistatic and dynamic deformation behaviors of conventional and microalloyed medium-carbon steels in a wide temperature range. As strain rate increased, the flow stress increased a...This paper presents a study of the quasistatic and dynamic deformation behaviors of conventional and microalloyed medium-carbon steels in a wide temperature range. As strain rate increased, the flow stress increased at room temperature, but occasionally did not at elevated temperatures. The flow stress of the microalloyed steel containing precipitates was less sensitive to strain rate at room temperature than that of the conventional steel due to a relatively larger activation length. Microstructural observation of the steels deformed after compression test indicated that inhomogeneous deformation became more serious with increasing strain rate and temperature without fracturing in the highly localized region.展开更多
基金Sponsored by the National Natural Science Foundation of China (Grant No.50478027)
文摘A new type of semi-rigid thin-walled steel-concrete composite beam-to-column joint has been proposed in this paper.Five semi-rigid composite beam-to-column joint specimens subjected to hogging moments under monotonic loading were tested to study the static behavior of this new type of joint.The main variable parameters for the five joint specimens were the longitudinal reinforcement ratio and the joint type.The experimental results designated that the magnitude of extension of the longitudinal reinforcement is the most important factor that influenced the moment-rotation characteristic of the new type of joint.The concrete slabs could resist 3.8%-19.1% of the total shear load applied to the cross-sections near the beam-to-column connection.The edge stiffened elements,such as the flange of the lipped I-section thin-walled steel beam,were capable of having considerable inelastic deformation capacity although they had comparatively large width-to-thickness ratios.The shear failure of the concrete cantilever edge strip must be taken into account in practical design because it has significant influence on the anchorage of the longitudinal reinforcement in the new type of external joints.
基金This work was supported by the National Natural Science Foundation of China grant numbers[52072410].
文摘Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the static oxidation behavior of(Hf,Ti)C and the effect of Ti substitutions were investigated in air at 2500℃ under an oxygen partial pressure of 4.2 kPa.After oxidation for 2000 s,the thickness of the oxide layer on the surface of(Hf,Ti)C bulk ceramic is reduced by 62.29%compared with that on the HfC monocarbide surface.The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides,HfO_(2),and carbon.The Ti-rich oxycarbide((Ti,Hf)C_(x)O_(y))dispersed within HfO_(2) formed the major structure of the oxide layer.A coherent boundary with lattice distortion existed at the HfO2/(Ti,Hf)C_(x)O_(y) interface along the(111)crystal plane direction,which served as an effective oxygen diffusion barrier.The Hfrich oxycarbide((Hf,Ti)CxOy)together with(Ti,Hf)C_(x)O_(y),HfO_(2),and precipitated carbon constituted a dense transition layer,ensuring favorable bonding between the oxide layer and the matrix.The Ti content affects the oxidation resistance of(Hf,Ti)C by determining the oxide layer's phase distribution and integrity.
文摘In this study, AA2519 alloy was initially processed by multi axial forging (MAF) at room and cryogenic temperatures. Subsequently, the microstructure and the mechanical behavior of the processed samples under quasi-static loading were investigated to determine the influence of cryogenic forging on alloys’ subgrains dimensions, grain boundaries interactions, strength, ductility and toughness. In addition, the failure mechanisms at the tensile rupture surfaces were characterized using scanning electron micro-scope (SEM). The results show significant improvements in the strength, ductility and toughness of the alloy as a result of the cryogenic MAF process. The formation of nanoscale crystallite microstructure, heavily deformed grains with high density of grain boundaries and second phase breakage to finer particles were characterized as the main reasons for the increase in the mechanical properties of the cryogenic forged samples. The cryogenic processing of the alloy resulted in the formation of an ultrafine grained material with tensile strength and toughness that are ~41% and ~80% higher respectively after 2 cycles MAF when compared with the materials processed at ambient temperature. The fractography analysis on the tested materials shows a substantial ductility improvement in the cryoforged (CF) samples when compared to the room temperature forged (RTF) samples which is in alignment with their stress-strain profiles. However, extended forging at higher cycles than 2 cycles led only to increase in strength at the expense of ductility for both the CF and RTF samples.
基金sponsored by the National Science Fund for Distinguished Young Scholars(50825403)the National Key Basic Research Program of China(2010CB732003,2013CB036005)the Science Fund for Creative Research Group of the National Natural Science Foundation of China(51021001)
文摘Unloading failure of rocks,especially highly stressed rocks,is one of the key issues in construction of underground structures.Based on this,analytical models for rocks under quasi-static and intensive unloading conditions are established to study the failure behavior of highly stressed rocks.In case of rock failure under quasi-static unloading,the rock mass ahead of working face is regarded as an elasto-brittle material,and the stress-displacement curves are used to characterize the tensile fracture of peak-stress area.It is observed that,when intensive unloading happens,there is an elastic unloading wave（perturbation wave） propagating in the rock mass.If the initial stress exceeds the critical stress,there will be a fracture wave,following the elastic unloading wave.To study the propagation feature of fracture wave,the conservation laws of mass,momentum and energy are employed.Results show that the post-peak deformation,strength and energy dissipation are essential to the failure process of highly stressed rocks.
基金Supported by the National Basic Research Program of China under Grant No 2013CB733000the Natural Science Foundation of Guangxi Province under Grant No 2015GXNSFBA139238the Guangxi'Bagui'Teams for Innovation and Research
文摘The static and dynamic precipitation behavior of solution-treated binary Al-20 wt.% Zn alloy is investigated via artificial aging, cold rolling and artificial aging combined with cold rolling. The solution-treated Al-Zn alloy exhibits high thermal stability during aging, and low densities of nano-sized Zn particles are precipitated along with AI grain boundaries after aging at 200℃ for 13 h. Compared with static precipitation, dynamic precipitation occurs more easily in the Al-Zn alloy. Zn clusters are obtained after cold rolling at an equivalent plastic strain of 0.6, and the size of the Zn phase reaches hundreds of nanometers when the strain is increased to 12.1. The results show that the speed of static precipitation can be significantly enhanced after the application of 2.9 rolling strain. Grain refinement and defects induced by cold rolling are considered to promote Zn precipitation. The hardness of Al-Zn alloy is also affected by static and dynamic precipitations.
基金This work was supported by the National Natural Science Foundation of China(No.59872035 and No.59902227)the Science Foundation of Beijing(2002019).
文摘Crack initiation and subcritical crack growth in glass sheet and SIC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime process of brittle materials involves three possible forms of crack growth: subcritical crack growth, partly subcritical crack growth and instantaneous fracture without subcritical crack growth. Curves of ν-K obtained in step-by-step static fatigue tests and in constant loading rate tests showed different trends for borosilicate glass sheet. α-SiC that is generally considered immune to mechanical fatigue effect and environmental attack was also tested under static loading and the lifetime was measured. The results showed that the threshold load to damage effect was over 80% of the initial strength for the SiC.
文摘This paper presents a study of the quasistatic and dynamic deformation behaviors of conventional and microalloyed medium-carbon steels in a wide temperature range. As strain rate increased, the flow stress increased at room temperature, but occasionally did not at elevated temperatures. The flow stress of the microalloyed steel containing precipitates was less sensitive to strain rate at room temperature than that of the conventional steel due to a relatively larger activation length. Microstructural observation of the steels deformed after compression test indicated that inhomogeneous deformation became more serious with increasing strain rate and temperature without fracturing in the highly localized region.
