Papers by Keyword: Stress Ratio

Abstract: The fatigue crack growth experiments of 2024-T4 aluminum alloy were carried out to study the thickness effects on the fatigue crack growth rate. Round compact specimens with two different thickness, 3.8mm and 12.5mm, were subjected to Mode I loading with four R-ratios (0.05, 0.1, 0.5 and 0.75) and loading amplitudes. An optical reading microscope with a magnification of 40 was used to measure the crack length. Stable crack growth is characterized by the standard form of the Paris law, material constants of the Paris law corresponding to each R-ratio were obtained by fitting the experimental data. The fatigue crack growth rate of specimens with a thickness of 12.5mm is apparently higher than that of specimens with a thickness of 3.8mm when R-ratio is equal to 0.1, 0.5 and 0.75. While the effect of thickness is relatively less significant for the case of . It can be concluded that the fatigue crack growth rate increases with R-ratio or thickness when one of them is identical.

Abstract: Effects of stiffness, carrying efficiency and fatigue of the end anchor cable on the mechanical behavior of long span cable-stayed bridge, were discussed respectively. Firstly, the concept of the effective stiffness and the stress ratio were introduced to discuss the effects of the dead load stress level and the stress ratio on the effective stiffness of the end anchor cable. Secondly, the effect of the cable material on the vertical carrying efficiency of the structure was analyzed. Finally, the main influential factors on the fatigue performance of the end anchor cable were analyzed in detail. It is shown that improving the dead load stress level and keeping the low stress ratio would increase the effective stiffness of the end anchor cable. The section of the end anchor cable effects on stiffness of the structure under high stress level. It can be drawn that as a novel material, The CFRP end anchor cable will increase the load carrying level of the long span cable-stayed bridges. It can also be concluded that the length of side span and main span, the height of pylon, the area of the cable section and the live load collection degree are all the main effective factors to the stress amplitude of the end anchor cable. It is suggested that in the practical design of large span cable-stayed bridge, all the effective factors together with the global state of the structure should be taken into account comprehensively.

Abstract: Using the static and dynamic triaxial experimental machine, experimental study of the strength of wet-screened aggregate concrete are carried out under biaxial compression stress ratios, 0.25, 0.5, 0.75 and 1 after 0, 50, 100, 150, 200, 250, 300 cycles of freeze-thaw. The change of surface of wet-screened aggregate concrete specimens after different cycles of freeze-thaw are observed and described. The failure characteristic of specimens and the direction of the cracks are also observed. Based on the test data, the influence of freeze-thaw cycles and compressive stress ratio on the ultimate compressive strength and corresponding stress-strain relationship is analyzed respectively. The relationships between the ultimate compressive strength and freeze-thaw cycles, the stress ratios are given, respectively. On this basis, the unified failure criterion with consideration of the influence of freeze-thaw cycles and stress ratio is proposed. It can serve as a reference for the maintenance, design and the life prediction of ocean structures, hydraulic structures, marine structures and offshore platform in cold regions.

Abstract: The new CFG pile-board structure composite foundation is a ground treatment technique based on CFG pile foundation and pile-board structure composite foundation. It can make full use of the load distributing function of board, the bearing capacity and the deformation compatibility of soil between piles, by taking advantage of the pile-platform-soil interaction. A part of soft ground in a high-speed railway was taken as the engineering background and study object. The settlement controlling effect of common CFG pile ground and new CFG pile-board structure composite foundation were analyzed by finite element numerical method, and various factors to the effect on settlement-controlling were discussed. Pile-soil stress ratio of CFG pile and reinforced concrete pile were studied. Some useful conclusions of the numerical simulation of the new CFG pile-board structure composite foundation were obtained.

Abstract: Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause fracture. In order to understand the strength of a steel component, it is important to locate these cracks. We developed a scanning Hall probe microscope (SHPM) equipped with GaAs films sensors and observed fatigue cracks at room temperature in air while they were growing. In our previous works, we determined the correlation between crack growth and magnetic field in high carbon tool steels (JIS SKS93 and JIS SUJ2). We also reported the sensitivity of the SHPM equipped with a three-dimensional line-probe that was developed to decrease the sensor gaps. By using the line-probe sensor we succeeded to measure the magnetic flux density distributions in very close proximity to the specimen’s surface. However, in order to further understand the relation between magnetic flux density and crack growth, other materials, microstructures and fatigue test conditions should be evaluated. In the present work, we focus on the effect of stress ratios on the changes of the magnetic flux density in annealed carbon tool steel.

Abstract: With the aircraft structure design criterion from traditional static strength design to damage tolerance design and with the independent research and development of new-type civil turbofan regional aircraft and the implement of the plan to develop the country's own large passenger jets in China, it is essential to do some researches on casting TC4 alloys for the lack of the data of fatigue properties. The detail fatigue rating cut-off (DFR_cutoff) values of casting TC4 alloys are measured and calculated by double dots method, the thresholds in fatigue crack propagation and the fatigue crack growth rates at different stress ratios are studied and the fatigue fracture at different stress ratios are observed by scanning electron microscopy. The results show that DFR_cutoff value by double-dot method is 375.83 Mpa. The thresholds of fatigue crack propagation decrease with the increase of the stress ratio, whereas the fatigue crack growth rates increase with the increment of the stress ratio and the relationship curves between fatigue crack propagation rates and the stress intensity factor range have been obtained. Moreover, SEM observations indicate that the fatigue trips become wide with the increasing of the stress ratio.

Abstract: An investigation into the fatigue crack growth behavior of N80 steel was carried out for casing-drilling technology. The results show that the effect of stress ratio on the △K corresponding to access of the tensile overload region is remarkable, and increasing the stress ratio is found to increase the FCGR at the same △K and reduce △K of access of the tensile overload region; the transgranular failure machanism is dominant for the N80 steel in the all crack growth stage; the fracture surface roughness decreases as the cracks propagate through threshold region and into Paris region, whereas it increases as the cracks propagate through Paris region and into the tensile overload region; the typical ductile-dimpled fracture mechanism observed on the tensile overload region is similar to the tensile fracture mode, although the orientation of the dimples significantly inclines to fatigue crack growth direction.

Abstract: As a kind of flexible fabric reinforced composites, the woven membrane materials suffer from the bi-axial tensile loads in the application of the lightweight structures. Therefore, the experiments under bi-axial tensile loads are essential for the acquaintance of the mechanical characters of the membrane materials. This paper has summarized thoroughly the details of the bi-axial tensile tests, especially the in-plane cruciform tests. The aspects include the shape and configuration of the cruciform sample, the practice of the applied stresses and the testing of the strains in the two axial directions. It could be expected to be the reference for the establishment of the testing standard of the bi-axial tensile experiment for the membrane materials.

Abstract: Fatigue crack growth was simulated by using a newly developed unified model on the fatigue initiation and crack growth based on an incremental multiaxial fatigue criterion. The cyclic elastic-plastic stress-strain field was analyzed using the general-purpose finite element software (ABAQUS) with the implementation of a robust cyclic plasticity theory. The fatigue crack growth rates with respect to three different stress ratios were selected as the benchmark to check the unified model. The predicted results agreed with the experimental data very well. The insensitivity of the crack growth rate to the stress ratio is due to the fast mean stress relaxation.

Abstract: The aim of this research is to find a feasible method for the estimation of the tensile behaviour of woven membrane construction materials under bi-axial loading. During the model development, the woven membrane materials are treated as orthotropic, elastic and linear. Based on the constitutive relationships for orthotropic and elastic materials, two equations sets are deduced for the estimation of modulus in the warp and the fill direction respectively when suffering from the bi-axial loading. To checkout the validity of the estimation, seven groups of biaxial tensile tests with stress ratios of 1:0, 1:1, 2:1, 5:1, 1:2, 1:5 and 0:1 have been carried out on two kinds of PVCcoated woven membrane materials. From the comparison of the experiments and the prediction, it can be noticed that the model can achieve ideal results when it comes to the estimation of modulus in the leading direction, which is the direction with a higher stress than the other one. Therefore, based on the hypothesis of orthotropic, elastic and linear materials, there is an easy way to estimate the bi-axial tensile properties of woven membrane materials in the leading direction only through the uni-axial tensile tests.