Static creep and cyclic creep tests were carriedout on a Cr-Mo rotor steel from 0.5 to 0.6 T_m.Cyclic creep retardation occurred under the testconditions.With an increase of stress frequency,cyclic creep strain rate d...Static creep and cyclic creep tests were carriedout on a Cr-Mo rotor steel from 0.5 to 0.6 T_m.Cyclic creep retardation occurred under the testconditions.With an increase of stress frequency,cyclic creep strain rate decreases and rupture timeincreases.The cyclic creep strain rate for the stresswave of a square shape is higher than that of a tri-angle shape.The apparent stress exponent of cycliccreep and the apparent activation energy of cycliccreep are both higher than those of static creep.Theminimum strain rate is inversely proportional torupture time for both static and cyclic creeps at dif-ferent stresses,temperatures,frequencies and waveshapes.The cyclic creep retardation mechanism wasexplored by the observation and analysis of the dis-location structure and fracture surface.展开更多
Vertical section method has been used to measure the fractal dimension of cyclic creep frac-ture surfaces of rotor steel 34CrMoA at 550℃.The relationship between fractal dimensionand cyclic creep properties has been ...Vertical section method has been used to measure the fractal dimension of cyclic creep frac-ture surfaces of rotor steel 34CrMoA at 550℃.The relationship between fractal dimensionand cyclic creep properties has been analyzed.The fractal dimension is dependent upon thepower product of minimum creep rate and rupture time,and satisfies the linear function withrupture strain.展开更多
Cyclic creep tests of Al-Mg alloy have beenconducted at a wide range of stress andtemperature.When Mg content is beyond 0.45%,the strain bursts can be observed in the cyclic creepcurves.There exists a critical stress ...Cyclic creep tests of Al-Mg alloy have beenconducted at a wide range of stress andtemperature.When Mg content is beyond 0.45%,the strain bursts can be observed in the cyclic creepcurves.There exists a critical stress for the occur-rence of the strain burst,which increases with an in-crease of Mg content and is almost 60% of the ul-timate tensile stress.The occurrence of strain burstis proposed to be controlled by the easiness of thecross-slip of dislocations.For Al-Mg alloy,thedynamic strain aging plays an important role in thestrain bursts.展开更多
Concrete creep under both static and cyclic loading conditions was investigated. Four groups of high-strength high-performance concrete (HSHPC) prism specimens were fabricated, and three of these specimens were loaded...Concrete creep under both static and cyclic loading conditions was investigated. Four groups of high-strength high-performance concrete (HSHPC) prism specimens were fabricated, and three of these specimens were loaded periodically by the MTS Landmark Fatigue Testing Machine System. Creep characteristics and creep coefficients of HSHPC under static loading and cyclic loading, respectively, were obtained and compared. The experimental results show that the creep strains under cyclic loading with a mean stress of 0.4fcp and an amplitude of 0.2fcp increase significantly compared with the creep strains under static loading, and the maximum value was 1.2-2.3 times at early stages. In addition, the creep coefficient increases nonlinearly with the number of cyclic loading repetitions. The influence coefficient for cyclic loading 1.088x(N/N0)0.078was introduced based on the previous HSHPC creep model, and then the modified creep model under cyclic loading was established. Finally, the residual method, the CEB coefficient of variation method and the B3 coefficient of variation method were applied to evaluate the modified creep model. The statistical results demonstrate that the modified creep model agrees well with the experimental measurements. Hence, it has important theoretical and practical values for accurately predicting the deflection of concrete bridges under cyclic traffic loading.展开更多
The distribution of saturated soft clay is greatly wide in China. The current main measures adopted to deal with soft soil foundations may lead to environmental pollution, even some engineering accidents may happen on...The distribution of saturated soft clay is greatly wide in China. The current main measures adopted to deal with soft soil foundations may lead to environmental pollution, even some engineering accidents may happen on soft soil foundations. In order to solve engineering problems of saturated soft soil foundations well, researches of mechanical properties of them are necessary. One of the most important mechanical characteristics of saturated soft clay is its cyclic accumulative deformation under cyclic loadings. For saturated soft clay, the cyclic accumulative deformation is similar to the creep behavior under static loadings. Therefore, the cyclic accumulative deformation is equivalent to the creep, the number of loading cycles is seen as the time, and this study develops a practical method for predicting the cyclic accumulative deformation of saturated soft clay with the creep theory. The method is a pseudostatic elasto-plastic finite element method implemented by ABAQUS software. A fitted equation between cyclic accumulative strain and number of loading cycles and the empirical relationship of parameters of fitted equation were established with aseries of cyclic triaxial compression tests. Then with this empirical relationship of parameters, the method developed by this study was employed to predict the cyclic accumulative deformation under cyclic triaxial tension tests. Predicted results were in good agreement with test results, and the effectiveness of this method was thus validated for different stress states. The method was then applied in analyzing the cyclic accumulative deformation for soft soil foundation of a pile-supported wharf structure.展开更多
Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [1 1 2], [0 1 1] and [1 1 4] oriented single crystals of nickel_based superalloy DD3 with different cyclic strain rates at 950℃. The c...Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [1 1 2], [0 1 1] and [1 1 4] oriented single crystals of nickel_based superalloy DD3 with different cyclic strain rates at 950℃. The cyclic strain rates were chosen as 1 0×10 -2 , 1 33×10 -3 and 0 33×10 -3 s -1 . The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy(SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950℃, and an orientation and strain rate modified Lall_Chin_Pope(LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied.展开更多
Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure a...Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.展开更多
文摘Static creep and cyclic creep tests were carriedout on a Cr-Mo rotor steel from 0.5 to 0.6 T_m.Cyclic creep retardation occurred under the testconditions.With an increase of stress frequency,cyclic creep strain rate decreases and rupture timeincreases.The cyclic creep strain rate for the stresswave of a square shape is higher than that of a tri-angle shape.The apparent stress exponent of cycliccreep and the apparent activation energy of cycliccreep are both higher than those of static creep.Theminimum strain rate is inversely proportional torupture time for both static and cyclic creeps at dif-ferent stresses,temperatures,frequencies and waveshapes.The cyclic creep retardation mechanism wasexplored by the observation and analysis of the dis-location structure and fracture surface.
文摘Vertical section method has been used to measure the fractal dimension of cyclic creep frac-ture surfaces of rotor steel 34CrMoA at 550℃.The relationship between fractal dimensionand cyclic creep properties has been analyzed.The fractal dimension is dependent upon thepower product of minimum creep rate and rupture time,and satisfies the linear function withrupture strain.
文摘Cyclic creep tests of Al-Mg alloy have beenconducted at a wide range of stress andtemperature.When Mg content is beyond 0.45%,the strain bursts can be observed in the cyclic creepcurves.There exists a critical stress for the occur-rence of the strain burst,which increases with an in-crease of Mg content and is almost 60% of the ul-timate tensile stress.The occurrence of strain burstis proposed to be controlled by the easiness of thecross-slip of dislocations.For Al-Mg alloy,thedynamic strain aging plays an important role in thestrain bursts.
基金Supported by Key Project of Chinese National Programs for Fundamental Research and Development(No.2017YFB 0310105-03)Key Foundation Projects of Hubei Province(No.2016CFA074)Technology Innovation Major Project of Hubei Province(No.2018AAA001-04)
文摘Concrete creep under both static and cyclic loading conditions was investigated. Four groups of high-strength high-performance concrete (HSHPC) prism specimens were fabricated, and three of these specimens were loaded periodically by the MTS Landmark Fatigue Testing Machine System. Creep characteristics and creep coefficients of HSHPC under static loading and cyclic loading, respectively, were obtained and compared. The experimental results show that the creep strains under cyclic loading with a mean stress of 0.4fcp and an amplitude of 0.2fcp increase significantly compared with the creep strains under static loading, and the maximum value was 1.2-2.3 times at early stages. In addition, the creep coefficient increases nonlinearly with the number of cyclic loading repetitions. The influence coefficient for cyclic loading 1.088x(N/N0)0.078was introduced based on the previous HSHPC creep model, and then the modified creep model under cyclic loading was established. Finally, the residual method, the CEB coefficient of variation method and the B3 coefficient of variation method were applied to evaluate the modified creep model. The statistical results demonstrate that the modified creep model agrees well with the experimental measurements. Hence, it has important theoretical and practical values for accurately predicting the deflection of concrete bridges under cyclic traffic loading.
基金funded by Science Commission Fund of Chongqing(Grant No.cstc2016jcyj A0123)Open Fund of Key Laboratory of regulation technology for inlandwaterway in transportation industry of Chongqing Jiaotong University (Grant No. NHHD-201506)
文摘The distribution of saturated soft clay is greatly wide in China. The current main measures adopted to deal with soft soil foundations may lead to environmental pollution, even some engineering accidents may happen on soft soil foundations. In order to solve engineering problems of saturated soft soil foundations well, researches of mechanical properties of them are necessary. One of the most important mechanical characteristics of saturated soft clay is its cyclic accumulative deformation under cyclic loadings. For saturated soft clay, the cyclic accumulative deformation is similar to the creep behavior under static loadings. Therefore, the cyclic accumulative deformation is equivalent to the creep, the number of loading cycles is seen as the time, and this study develops a practical method for predicting the cyclic accumulative deformation of saturated soft clay with the creep theory. The method is a pseudostatic elasto-plastic finite element method implemented by ABAQUS software. A fitted equation between cyclic accumulative strain and number of loading cycles and the empirical relationship of parameters of fitted equation were established with aseries of cyclic triaxial compression tests. Then with this empirical relationship of parameters, the method developed by this study was employed to predict the cyclic accumulative deformation under cyclic triaxial tension tests. Predicted results were in good agreement with test results, and the effectiveness of this method was thus validated for different stress states. The method was then applied in analyzing the cyclic accumulative deformation for soft soil foundation of a pile-supported wharf structure.
文摘Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [1 1 2], [0 1 1] and [1 1 4] oriented single crystals of nickel_based superalloy DD3 with different cyclic strain rates at 950℃. The cyclic strain rates were chosen as 1 0×10 -2 , 1 33×10 -3 and 0 33×10 -3 s -1 . The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy(SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950℃, and an orientation and strain rate modified Lall_Chin_Pope(LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied.
文摘Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.
