Papers by Author: Zheng Yang

Abstract: The elastic stress and strain fields in an elasto-plastic circular cross-section bar with semicircular groove subjected to uniaxial tension are systematically investigated using the finite element method. It is found that the stress and strain concentrations are different, especially after the partial region near the groove root yielding. The coupled influences of the loading levels and Poisson’s ratios on the stress and strain concentrations are examined. The maximum of strain concentration factor is always at the groove root, but the maximum of stress concentration factor is at the groove root only while the loading levels are lower.

Abstract: Damping evaluation is of great importance in predicting the dynamic response of systems. To get the accurate damping ratios of a system, many identification methods have been proposed and developed. But only few of them achieved accurate results for in-situ buildings due to the fact that the responses are significantly influenced by noise. This paper proposes a new method to accurately identify the damping ratios of in-situ buildings. The method is based on ambient excitation technique which requires no artificial excitation applied to SSI system and to measure output-only. The damping ratio identification is then performed by combining the improved random decrement method and Ibrahim time domain method. To demonstrate the validity of the proposed approach, a case study is performed and the results are compared with the conventional peak-peaking method results. The results show the proposed method can effectively identify the modal parameter of either frequencies or damping ratios of in-situ buildings subjected to ambient excitation.

Abstract: A system is developed for simulating the collapse behavior of reinforced concrete frame structures from seismic loading. The purpose for developing this system is that the whole process from elastic-plasticity to collapse is modeled in the analysis. Consequently, the collapse mode, the collapse duration, the distribution of the debris after collapse and other information can be obtained. The numerical analysis is based on the distinct element method. The element shape is assumed to be rectangular solid, which is based on the cross section of the structural members. The elements are connected by dummy springs. The impact action among concrete blocks after the connecting springs fail is taken into account by the impulse models of impact, which are derived from the results of the experiments and the numerical tests. A comparison between a shaking-table test and the analysis is presented, which shows that the results of the simulation are in good agreement with the results of the experiment.

Abstract: The elastic stress and strain fields of a finite-thickness plate containing two interacting holes are systematically investigated using the finite element method. The maximum stress and strain concentrations occur on the mid plane only in the thin plates. They do not occur on the mid plane and their locations are different in the same plate for the thick plates. The values of the maximum stress and the strain concentration factor and their locations depend on the distance between the two holes and the plate thickness. The stress and the strain concentration factor are different, even if in an elastic state. The stress concentration factor and the strain concentration factor on the plate surface decrease rapidly with increasing thickness and becomes lower than the values corresponding to the plane stress state and the values of the mid plane. They are too low to reflect the overall stress concentrations as the plate thickness increases. The differences between the maximum value and the surface value of the stress concentration factor, the strain concentration factor increase rapidly and tend to their respective constant values with increasing plate thickness. These constant values depend on the distance between the two holes and the difference of the stress concentration factor is larger than that of the strain concentration factor in the same plate.

Abstract: The elastic stress and strain fields in a circular cross-section bar with semielliptic groove subjected to uniaxial tension are systematically investigated using the finite element method. It is found that the stress and strain concentrations are different even if the material is in the elastic state. The relation between the stress and the strain concentration factors depends on Poisson’s ratio and the groove’s geometric configuration. The coupled influences of the shape factor of the semielliptic groove and the relative depth of the groove on the stress and strain concentrations are examined. The equations that describe the stress and strain concentration factors for different groove’s geometric configurations and Poisson’s ratios are obtained based on finite element results.

Abstract: In this paper, the elastic stress and strain fields in curved beams of finite thickness with end moments are systematically investigated using 3D finite element analysis. For a curved beam of finite thickness, the through-thickness distributions of the stress and strain concentration factors are not uniform and their distributions are different. The radius of curvature and the thickness of the beam have significant effect on the stress and stain concentrations. The relations of the maximum value and the value at the corner root of surface of the stress and strain concentration factors with different thickness for various radius of curvature are respectively obtained.

Abstract: Single or multiple of delaminations have been found frequently on the fracture surface of X70 pipeline steel. In this study, the delamination cracks and their influence on the fracture of pipeline are investigated by both experiment and three-dimensional fracture analyses. It is shown that the three-dimensional stress state is prerequisite for delamination crack and the strength distribution of material influences the form and direction of delamination crack. The delamination cracks are produced on the weak interfaces among the material by the tensile stress perpendicular to them before the fracture passes. The direction of delamination crack depends on the three-dimensional stress fields and strength distribution of material near the crack tip or notch root. The delamination cracks of the fracture through thickness of pipe wall make the effective thickness decrease and the delamination cracks of surface crack are perpendicular to the direction of fracture propagation direction. The delamination cracks reduce the stress triaxiality near crack tip and in turn, improve the fracture toughness of X70 pipeline steel.

Abstract: Based on detailed three-dimensional finite element analyses, elastic stress and strain field of ellipse major axis end in plates with different thickness and ellipse configurations subjected to uniaxial tension have been investigated. The plate thickness and ellipse configuration have obvious effects on the stress concentration factor, which is higher in finite thickness plates than in plane stress and plane strain cases. The out-of-plane stress constraint factor tends the maximum on the mid-plane and approaches to zero on the free plane. Stress concentration factors distribute ununiformly through the plate thickness, the value and location of maximum stress concentration factor depend on the plate thickness and the ellipse configurations. Both stress concentration factor in the middle plane and the maximum stress concentration factor are greater than that under plane stress or plane strain states, so it is unsafe to suppose a tensioned plate with finite thickness as one undergone plane stress or plane strain. For the sharper notch, the influence of three-dimensional stress state on the SCF must be considered.

Abstract: The delamination cracks and its effects on the fracture of pipeline steel are investigated experimentally by using of Drop-Weight Tear Test (DWTT). The delamination cracks are produced by the stress perpendicular to the weak interfaces before main crack beginning or accelerating, no new delamination crack is produced during the stabile propagation of fracture. The quantity, splay degree of delamination crack and the space between two delamination cracks are influenced by the stress state of the crack tip at beginning or accelerating point of main crack and the length of delamination crack is influenced by the stress state of the crack tip during the propagation of fracture. The surface of delamination crack is cleavage fracture appearance with large cleavage facet. There is no delamination crack on the brittle fracture surface below the brittle-to-ductile temperature or on the brittle fracture region of mix-mode fracture surface with ductile and brittle region. The part of fracture surfaces with delamination crack ought to be evaluated as the shear area because the delamination cracks are produced only on the ductile fracture surface or on the ductile part of fracture surface.