Key Engineering Materials Vols. 348-349

Abstract: The damage constitutive relationship is a key problem to nonlinear analysis of structural members. It is much more necessary to study systematically related damage constitutive relationship of steel reinforced concrete (SRC) members because of their peculiarity. Based on the experimental phenomena analysis and damage theory, the mesoscopic failure mechanism of concrete as well as the interface of shaped steel and concrete is analyzed. At the same time, their mesoscopic damage models and corresponding assumptions of the models are established by considering material failure characteristics. Subsequently, the mechanism of the models is analyzed, which indicates that they fit to simulate failure process of the materials. According to the models, the related damage constitutive relationship for SRC members is deduced by principle of energy and force balance. Combined with corresponding experimental study, it is shown that the constitutive relationship presented is reasonable and practicable, so it facilitates SRC member nonlinear analysis.

Abstract: Based on damage mechanics and failure model, a seismic damage model applying to steel reinforced high strength and high performance concrete frame joints is put forward, which takes deformation and cumulative damage into account. The damage performance of five tentative frame joints is compared and analyzed according to deformation and dissipated hysteretic energy under different loading levels, and the main influence factors on damage performance are established. The damage indexes of tentative frame joints are calculated, and the influence of strength grade of concrete and axial compression ratio on damage performance is discussed. The results indicate that the general damage is controlled by the damage of concrete in the earlier stage of loading, and by the damage of shape steel and stirrup in the later stage of loading; joints with lower axial compression ratio and lower concrete strength grade possess better hysteretic energy performance. Damage analysis provides an available means for the research of steel reinforced high strength and high performance concrete frame joints under seismic load. The damage model presented can be a reference for seismic damage analysis of the structure, prediction of earthquake damage in future, estimation of economic loss and repair after earthquake.

Abstract: Steel-concrete composite structures are widely used in high buildings for its excellent seismic behaviors, whereas faults or cracks, which have great influence on interfacial mechanical behaviors of structural members, inevitably form near the interface between steel and concrete during the process of molding. Therefore, it is necessary to study the mechanical characteristics of the crack tip near the interface. In this paper, the application scope of the path-independence of J-integral in steel-concrete composite structure with a crack is discussed. According to the conservation law of J-integral for the steel-concrete composite structure with a crack parallel to the interface, a hypothesis that the value of strain energy release rate (SERR) of the mode-$fracture is independent of the crack location when the crack is parallel and close to the interface is put forwarded. And this hypothesis is verified through finite element method (FEM). A schematic model for a skew crack near the steel-concrete interface is provided. The variation law of SERR with the Dundur’s parameters and the angle between crack direction and interface are calculated by FEM. At last, calculating method of the stress intensity factor as well as the SERR for a skew crack near the interface is suggested. All these may contribute to further investigation on interfacial mechanical behaviors for steel-concrete composite structure.

Abstract: The latest experimental study on steel reinforced high strength and high performance concrete (SRHSHPC) specimens shows that there exists interfacial bond softening phenomenon between embedded steel and high strength and high performance concrete (HSHPC), and it makes the shear transfer capacity between shaped steel and HSHPC be progressively reduced. To predict failure load in design, a theoretical model for interfacial bond softening behavior is required. As interfacial bond softening behavior is a nonlinear process involving material properties, it can be analyzed once the relation of interfacial bond stress (τ ) and slippage ( s ) is known. In this paper, the mechanism of interfacial bond-slip is studied, thus a simplified τ − s relation including ascending and descending parts is proposed and employed to analyze the interfacial nonlinear bond-slip process. Based on the interfacial equilibrium between steel and HSHPC as well as the τ − s relation, the basic governing equations in both softened region and elastic region are established and solved for steel strain or stress. At last, the application of the model is verified through comparison with experimental results. The calculating results of the model are found to be in good agreement with experimental results, showing that the model can describe the bond-slip process in real material systems.

Abstract: Damage probability is a new concept in modern engineering design. If the cumulative damage probability of member or material is known, then the rational design can be realized. Based on study of high-strength and high-performance (HSHP) concrete applied to steel reinforced concrete (SRC) structures, the equation of the cumulative damage probability of HSHP concrete is derived by Weibull theory. The calculation and analysis indicate that the theoretical equation presented give a suitably mathematical limit for damage probability of specimens. At the same time, the new method for Weibull parameters is established, which depends on the theory of the damage probability and computer graphics technology. The comparison of experimental curves with theoretical curves shows that the established equation can properly simulate the damage evolution of HSHP concrete. Them value, one of Weibull parameters, is studied in detail, which reveals that it is a key parameter to quality control. At last, the effect of specimen volume upon damage probability is also discussed. The research provides methods and ideas to improve performance of concrete material and simulate damage probability of HSHP concrete.

Abstract: Steel reinforced high strength and high performance concrete (SRHSHPC) specimens were experimented to study the mechanical behaviors between steel and concrete interface. In experiment, interfacial bond softening process was observed, which can be explained in terms of damage along the interface, leading to progressive reduction of shear transfer capability between steel and high strength and high performance concrete (HSHPC). In this paper, bond softening process along the interface is considered in the analysis of crack-induced debonding. Interfacial bond-slip mechanism between steel and HSHPC is studied in detail based on fracture mechanics. With the help of acoustic emissions technology, the crack propagation in the interlayer was observed, thus the interfacial crack propagation and fracture model is set up. Under the assumption that the interlayer is weak concrete compared with concrete matrix, the stress field as well as displacement field around the crack tip is deduced. The characteristics of interfacial fracture process are discussed and a model for interfacial fracture process zone is built up. With this model, the size of fracture process zone can be derived. At last, the influence of the fracture process zone on interfacial fracture toughness is determined using critical fracture toughness. All these may contribute to improvement of theory for SRHSHPC composite structure.

Abstract: The paper presents numerical and experimental results of continued studies of curved fatigue crack growth in arbitrarily pre-cracked isotropic sheets under biaxial proportional plane loading. The predictor-corrector method (PCM) was extended in order to analyse the growth of multiple crack systems. As a result, the program PCCS-2D was written to run within ANSYS without any user interaction. In order to check the accuracy and efficiency of the method biaxial crack growth simulations were carried out for a fracture mechanics cruciform specimen. The results are compared with experimental findings obtained by specimens made of 6061 aluminium alloy in T651 condition using a 250kN biaxial servohydraulic testing machine. From the numerical and experimental results, we conclude, that the proposed predictor-corrector method can be used in curved crack growth simulation also under biaxial proportional loading conditions.

Abstract: Scattering of SH wave by an elastic half space with a circular cavity and a crack in any position and direction is studied with Green’s function, complex function and multi-polar coordinate method. First, a suitable Green’s function is constructed, which is the fundamental solution of the displacement field for a half space with a circular cavity impacted by an out-plane harmonic line source loading at an arbitrary point in half space. Then a crack in any position and direction is constructed by means of crack-division in half space. Finally the displacement field and stress field are established in the case of coexistence of circular cavity and crack, and the expression of dynamic stress intensity factor (DSIF) at the tip of crack is given. According to numerical examples, the influences of different parameters on DSIF are discussed.

Abstract: The effectiveness of the application of shot peening after nitirding for improving the fatigue strength of high-performance steel components has not been adequately treated in literature, and it is not clear if shot peening can really and drastically increase mechanical performances of nitirded elements. In this paper an approach to assess the influence of shot peening on nitrided components is presented. It is based on fracture mechanics concepts. Smooth specimens with a micro-hole acting as a pre-crack were fatigue tested. The values of micro-hardness and of the residual stresses were measured and related to the experimental results. By elaborating the results it was possible to determine the threshold values of the stress intensity factors of the nitrided and shot peened material. A formula to predict the value of
Kth of shot peened and nitrided steels is proposed.

Abstract: A deep comprehension of the damage mechanisms involved in contact fatigue should optimize material and heat treatment choice for a specific application. In this work rolling disc-on-disc contact fatigue tests have been performed on a hardened and tempered UNI EN 42CrMo4 . The adopted test method creates the best conditions in order to develop micro-pitting on disc surface. Extensive micro-fractographic examinations have been carried out, on the damaged surfaces, through a scanning electronic microscope (SEM). For this steel, loaded with Hertzian pressure of 1000 MPa, the failure mode is always micro-pitting which begins at the surface, and it is not a sub-superficial damaging. If micro-pits develop, they will coalesce in larger craters. By this way, the probability that micropitting will degenerate into sub-superficial destructive pitting is very high.