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Unloading is a quite common phenomenon in the excavation process of large underground caverns, but the formed unloading region in the surrounding rock will bring an extremely adverse effect on the system stability. At present, correlation studies mainly focus on the distribution of the stress field and the displacement field after the excavation completed. However, studies that specifically test the fracturing process with the rock properties weakening during the excavation process are rare. Considering the above reasons and based on the damage mechanics theory, this paper established a numerical model used to analyse the damage-fracturing process of surrounding rock mass, with the nonuniformity properties of rock materials explicitly considered. Taking the underground caverns of Jinping Hydropower Station as the study object, the evolution of the unloading region was simulated dynamically during the excavation process, and good simulating results were presented. In sum, the study findings point out that ...

Structural failure process and failure mode are two important issues that a lot of attentions has been paid in structural failure analysis. It has been revealed that the structure failure is mostly started from local material level, and then developed to global destruction. The failure process is inherent with the multiscale effects, and there could be varies failure modes in the process. A characteristic failure curve could reflect the failure process and the singular point of the curve indicates the change of system status. To obtain the local information of the characteristic failure curve, the wavelet multiscale method is adopted for analyzing. With the local extreme value of wavelet coefficients in each scale, the singularity point of the curve can be figured out and correspondingly the structural critical load is determined. Furthermore, by defining an index called the Failure Extent Index (FEI) F m , the incremental ratio of plastic energy and elastic energy, two mainly failure modes, namely the progressive and catastrophic, are studied. Analysis results show that the proposed method, with strict mathematic theory basis, could capture the local feature of failure curve, and could obtain the critical load accurately. Besides, by analyzing the variation tendency of the index F m , the failure mode can be determined. The proposed method for determining the critical load and failure mode for solid structure can provide some valuable information for the structure and the obtained results are available for further analysis and practical operation.

Damage in the structures alters the structural dynamic behavior. Damage generally occurs locally in the components of structures, and its effect often exhibits in the changes of both local and global characteristics of structures. Better understanding of this relationship helps to monitor and assess the condition of structures and develop dynamics-based structural health monitoring techniques. In this study, the relationship between the local damage and structural dynamic behavior is investigated. To validate the concept, an experiment of a three-dimensional (3-D) steel frame structure with different magnitudes of local damage is illustrated. The experimental results indicate that the change of global resonant response between the intact and damage structure is not obvious. The change of local resonant responses measured near the location of saw-cut crack damage is quite significant. The experimental study conducted indicates that the local resonant responses at the low order resonance can be used as an effective damage identification method to detect and quantify the local damage in the 3D frame structures.

It is a common method to strengthen the damaged RC structures with bonded steel plates. At present the ultimate bearing capacity of RC structures strengthened with bonded steel plates is calculated mostly using the theory based on the test. Four beams, including one reference beam, two strengthened concrete beams in primary force and secondary force respectively, and one strengthened concrete beam which was not anchored enough, were tested under four-point bending (4PB) in order to get the data of strain of longitudinal bars, bonded bottom steel plate in tension and deflection of beams in the middle span. The experimental program was supported by a three-dimensioned finite analysis using ABAQUS. At the end of experiments and finite analysis, it is concluded that the investing strengthening technique can significantly improve the load-carrying capacity and the phenomenon of stress concentration at the end of interface, as well as the damage at interface, can be well simulated with cohesive element provided by ABAQUS.

Understanding of cracking behavior of cover concrete is important for assessment of the remaining service life of corrosion-affected concrete structures. Predicting initiation of concrete cracking is commonly based on the evolution of stresses caused by the volume expansion of corrosion products on reinforcing bars. This paper presents an analytical method that is capable of providing the solution to two-dimensional (2D) elastic stress field caused by nonuniform volume expansion around the corroding reinforcing bars. The developed method features the formulation of a displacement model, which is applied as a boundary condition to simulate the effect of nonuniform corrosion on the deformation of the surrounding concrete. A closed-form solution to the stress field is obtained using the complex variable method of Muskhelishvili. Validation of the solution is supported by a comparison with finite-element-based simulation results. The solution reveals the evolution of stresses surrounding two typical locations...

The global stability of high arch dam is one of the key problems in the safety study of arch dams, but no feasible method with theoretical basis is available. In this paper, based on the stability theory of mechanical system, it is demonstrated that the global failure of high arch dams belongs to a physical instability starting from local strength failure, which is the extreme point instability according to the characteristics of load-displacement curve obtained from the failure process of dam-foundation system. So the global failure of dam-foundation system should be studied with the stability theory of mechanical system. It is also pointed out that the current stability analysis methods used in engineering are consistent with the stability theory, but not established according to the mechanical system stability theory directly. A perfect method can be obtained through the study of physical disturbance equations.

A general initial water penetration (seepage) fracture criterion for concrete is proposed to predict whether or not harmful water penetration (hydraulic fracturing), other than microcracking, will occur in concrete structures in a severe high water pressure environment. The final regression, of the different macroscopic failure types in concrete to microscopic Mode I cracking, allows the use of only one universal criterion to indicate the damage. Thus, a general initial water penetration fracture criterion is approximately defined as a strain magnitude of 1000×10−6, based on the concept of tensile strain derived from experimental results in the relevant literature. Then, the locations of harmful water penetration fracture (hydraulic fracture) in the high arch dam mass of the Jinping first class hydropower project are analyzed using the nonlinear finite element method (FEM) according to the proposed criterion. The proposed criterion also holds promise for other concrete structures in high water pressure environments.

This paper studies the influence of a stress relaxation zone caused by foundation excavation on the performance of Jinping-I arch dam, using the orthogonal experiment method. Firstly, by adopting the characteristic stress of the dam as an index and coupling it with range analysis, the most significant factors and the degree of relaxation influence on the arch dam performance are found. This is then combined with monitoring data from the dam, and the extent of influence on the arch dam performance caused by the relaxation is evaluated through numerical analysis. The results show that the relaxation of foundation excavation has a negative impact on the performance of the arch dam in general, the extent of elastic modulus degradation in the relaxation zone being the main factor affecting stresses in the dam. The relaxation of the rock mass leads to an increase in dam stress in local areas near the relaxation zone, but has little effect on more distant areas.

In this paper, the problem of modeling crack in 2D viscoelastic media is studied using the extended finite element method. The paper focuses on the definition of enrichment functions suitable for cracks assessment in viscoelastic media and the generalized domain integrals used in the determination of crack tip parameters. The opening mode and mixed mode solutions of crack tip fracture problems in viscoelastic media are also undertaken. The results obtained by the proposed method show good agreement with the analytical methods and provide reasonable background information to enhance the modeling of crack growth in viscoelastic media.

To get the actual ultimate bearing capacity of concrete dam, the effect of geometric nonlinearity and strain softening on it, which appears in the failure process of concrete dam, is studied. Overload method is adopted to obtain the bearing capacity of a concrete dam by taking into consideration strain softening in the material constitutive law, geometric nonlinearity in geometric equation and equilibrium differential equation. Arc-length method is used to find the extreme point and descending branch of the load-displacement curve of the dam. The results present that the effect cannot be ignored. And geometric nonlinearity of structure and strain softening of materials should be considered for numerical analysis of ultimate bearing capacity of a concrete dam.

Gravity dam is a typical structure that has been frequently used in the fields of water conservancy engineering, and the safety of the structure has received widespread attention recently. Due to earthquakes or other reasons, gravity dams normally have damage such as cracks in practical service. Damage in the structures can alter the structural dynamic behavior and seriously affect structural performance. Maintaining safety and integrity of the gravity dam structures requires a better understanding of dynamic response of structure with damage and associated damage detection method. In order to study thoroughly the dynamic behavior of gravity dam with damage, the sweep vibration responses of the gravity dam with and without damage are investigated. The experimental results show that the peak-peak acceleration responses all increase for the structure is with crack. At the same time, a structural damage detection method, i.e., the local damage factor (LDF) method, is considered in the study of gravity dam damage detection when the dam is subjected to the base excitation. It is shown that the LDF method can be used as a damage index and is capable of evaluating both the presence and relative severity of structural damage, and it can be used as a viable condition assessment and damage identification technique to detect and quantify the damage in the gravity dam.

The paper is devoted to proposing a constitutive model based on micromechanics. The joints in rock masses are treated as penny-shaped inclusion in solid but not through structural planes by considering joint density, closure effect, joint geometry. The mechanical behavior of the joints is represented by an elasto-plastic constitutive law. Mori-Tanaka method is used to derive the relationship between the joint deformations and macroscopic strains. The incremental stress-strain relationship of rock masses is formulated by taking the volume average of the representative volume element. Meanwhile, the behavior of joints is obtained. By using implicit integration algorithms, the consistent tangent moduli are proposed and the method of updating stresses and joint displacements is presented. Some examples are calculated by ABAQUS user defined material subroutine based on this model.

The dynamic model of the hydraulic control system of the shift clutch is established through the flow equation of the valve port, the flow continuity equation and the balance equation of the valve core force, the piston motion of clutch cylinder is divided into three stages. And simulate the dynamic curves of the hydraulic pressure of the hydraulic inlet of the hydraulic cylinder during different stages.

An improved neuro-wavelet modeling (NWM) methodology is presented, and it aims at improving prediction precision of time-varying behavior of engineering structures. The proposed methodology distinguishes from the existing NWM methodology by featuring the distinctive capabilities of constructing optimally uncoupled dynamic subsystems in light of the redundant Haar wavelet transform (RHWT) and optimizing neural network. In particular, two techniques of imitating wavelet packet transform of RHWT and reconstructing the major crests of power spectrum of analyzed data are developed with the aim of constructing the optimally uncoupled dynamic subsystems from time-varying data. The resulting uncoupled dynamic subsystems make the underlying dynamic law of time-varying behavior more tractable than the raw scale subwaves arose from the RHWT, and they provide a platform for multiscale modeling via individual modeling at the uncoupled dynamic subsystem level. Furthermore, on each uncoupled dynamic subsystem, the technique of optimal brain surgeon in conjunction with a new dynamic mechanism refreshing is employed to optimize the neural network, and the recombination of the modeling outcomes on every subsystem constitutes the overall modeling of time-varying behavior. The improved NMW methodology offers a feasible framework of multiscale modeling due to its flexibility, adaptability and rationality, and it is particularly useful for prediction applications of time-varying behavior of engineering structures. As an illustrative example, the improved NWM methodology is applied to model and forecast dam deformation, and the results show that the methodology possesses positive advantages over the existing multiscale and single scale modeling techniques. The improved NMW methodology is promising and valuable for the safety monitoring and extreme event warning of engineering structures.

The extended finite element method (XFEM) is a new numerical method for modeling discontinuity. Research about numerical modeling for concrete hydraulic fracturing by XFEM is explored. By building the virtual work principle of the fracture problem considering water pressure on the crack surface, the governing equations of XFEM for hydraulic fracture modeling are derived. Implementation of the XFEM for hydraulic fracturing is presented. Finally, the method is verified by two examples and the advantages of the XFEM for hydraulic fracturing analysis are displayed.

In rock-fill dams of 200 to 300 m high, maximum compressive stress can exceed 6 MPa. At such a high stress level, rock-fill materials may experience particle crushing during shear. The material behavior is highly sophisticated exhibiting pressure and density dependency as well as particle crushing effects. Through scrutinizing the relevant experimental data on rock-fill materials and sands published in the literatures, this paper is focused on understanding basic mechanical characteristics of this sort of materials in the light of constitutive modelling.

The low efficiency of feedback analysis is one of the main shortcomings in the construction of underground cavern engineering. With this in mind, a method of intelligent autofeedback and safety early-warning for underground cavern is proposed based on BP neural network and FEM. The training sample points are chosen by using uniform test design method, and the autogeneration of FEM calculation file for ABAQUS is realized by using the technique of file partition, information grouping, and orderly numbering. Then, intelligent autoinversion of mechanics parameters is realized, and the automatic connection of parameter inversion, subsequent prediction, and safety early-warning is achieved. The software of intelligent autofeedback and safety early-warning for underground cavern engineering during construction is developed. Finally, the applicability of the proposed method and the developed software is verified through an application example of underground cavern of a pumped-storage power station located in Southwest China.

Damage in a structure alters its dynamic characteristics. Significant research has been conducted in damage detection and structural health monitoring using dynamics-based techniques. But simultaneously determining the presence, severity, and location of damage using the existing damage detection methods can still prove challenging. In this study, a new practical method of structural damage detection called Local Damage Factor (LDF) is presented, which is capable of determining the presence, severity, and location of structural damage at the same time. By including the dynamic characteristics of the intact local structure in the LDF method, the influence of structural nonlinearity, imperfections, and system noise is considered, so that the accuracy of damage detection is improved. Furthermore, a modified LDF (MLDF) method is proposed, which can detect damage without requiring benchmark data for the intact local structure. As a demonstration, the proposed LDF and MLDF methods are applied to damage detection in a 3-D steel frame structure, and the experimental results indicate that both methods can effectively determine the presence, severity, and location of a crack cut into one of the pillars in the frame with a saw. The LDF method is effective in a way that can eliminate both the nonlinear severity effect of structure itself and the ambient noise inherent in the intact structure, whereas the MLDF method is advantageous in that it does not require information about the intact local structure, which is often unavailable for damage detection. The proposed LDF method of in situ damage detection is illustrated using the concrete columns in a wharf structure. The successful detection of damage in the 3-D steel frame, as well as the wharf, demonstrates that the proposed local damage factor technique can be effectively and efficiently used in damage detection and structural health monitoring of structures.

A compact explicit expression for the tangent stiffness tensor is presented. Throughout the analysis, the formulation holds for general isotropic elastic materials and does not require solving eigenvector problems. On the theoretical side, a very simple solution of a tensor equation is obtained. Then the expressions for the derivatives of general symmetric isotropic tensor functions of a symmetric tensor are developed. On the computational side, particular attention is given to the consideration of the special case, Green elastic materials, in which the strain energy does not admit a closed-form expression in terms of principal invariants. Finally, a simple formulation of the tangent stiffness tensor for Ogden material model is supplied. Copyright © 2006 John Wiley & Sons, Ltd.

After two decades of rapid growth, multiscale research is currently undergoing a transition from an initial surge of excitement to a more rational stage. The major hindrance in multiscale modeling is continuous use of classical scale-separation theories in conflict with scale-coupling phenomena, and often a price to pay is we end up with some physically inconsistent models or parameters, e.g. non-local theory in damage mechanics, negative mass or moduli in elastodynamics. In this note we propose development of new scale-coupling theories as the most important direction of multiscale research. In Sect. 2, a conceptual distinction is made between scale-separation and scale-coupling strategies. In Sect. 3, a scale-coupling mechanics theory is introduced in a format that leads to derivation of non-local and gradient theories. Conclusion is made at the end.

The numerical evaluation of high-order modes of a uniform Euler–Bernoulli beam has been studied by reformatting the classical expression of mode shapes. That method, however, is inapplicable to a stepped beam due to the nonuniform expressions of the mode shape for each beam segment. Given that concern, this study develops an alternative method for the numerical evaluation of high-order modes for stepped beams. This method effectively expands the space of high-order modal solutions by introducing local coordinate systems to replace the conventional global coordinate system. This set of local coordinate systems can significantly simplify the frequency determinant of vibration equations of a stepped beam, in turn, largely eliminating numerical round-off errors and conducive to high-order mode evaluation. The efficacy of the proposed scheme is validated using various models of Euler–Bernoulli stepped beams. The principle of the method has the potential for extension to other types of Euler–Bernoulli beams with discontinuities in material and geometry. (The Matlab code for the numerical evaluation of high-order modes for stepped beams can be provided by the corresponding author upon request.)

The trap, in the translating formulation of the shear strength parameters(cohesion c, friction angle ϕ) and uniaxial strength parameters (tensile strength σ t compressive strength σ c ) of rocks as Crude Kuroiwa, sandstone and etc., is discussed by comparing with the relative parameters measured from experiments. A statistical formula between the parameters of the deduced and measured uniaxial strength is represented, and a theoretical formula is derived and verified in the transmission process between the deduced and measured shear strength parameters.

As it is not enough to assess the integral reliability of the complex 3D high arch dam-foundation system just from local damage in the deterministic analysis, the improved response surface method is firstly introduced to get the failure possibility of individual uncertain factor, and then PNET method to fix the integral reliability considering several uncertain factors. Then the main works are presented as following: (1) find out the main individual failure modes; (2) establish the limit state functions of the main individual failure modes respectively; (3) fix the relevance among the main failure mode and calculate the integral reliability. Finally the results are used to assess the failure possibility and integral reliability of Jinping high arch dam-foundation system.

The hydraulic fracturing is one of common failure in hydraulic high water pressure tunnels. The process of hydraulic fracturing of high water pressure tunnel with different initial fracture length in tunnel inner wall is simulated using nonlinear FEM. The couple effect of fracture inner water pressure with surrounding rock mass deformation is considered in the model. The fracture propagation process with increasing of tunnel inner water pressure is analyzed, its effect on stress, displacement and plastic zone of tunnel surrounding rock mass is obtained. The results are compared with that obtained without considering existence of fracture water pressure. There are different results from two analysis method. The results show that only couple analysis can obtain relative authentic stress, displacement and plastic zone of tunnel surrounding rock mass, providing base data for correct stability assessment of surrounding rock mass.

Since the anisotropic characteristic of deformation and strength of layered rock masses, giving the reasonable constitutive model is of great significance for numerical analysis of the rock engineering concerned of layered rock masses. Based on the McLamore & Gray strength criterion, the isotropic Drucker-Prager strength criterion which is widely used in rock engineering is extended to be an anisotropic elastoplastic constitutive model for layered rock masses by making use of the conversion relationship between the Mohr-Coulomb strength criterion and the Drucker-Prager strength criterion. Then the presented anisotropic constitutive model is embedded in ABAQUS for its numerical implementation based on the derivation of the implicit integration algorithm and consistent tangent modulus. Numerical example is given for demonstration of the validity of the proposed constitutive model and its numerical implementation.

Currently, in order to adequately exploit the abundant hydropower energy resources in China, a number of high concrete dams are under construction. It is quite prevalent that dams' cracking caused by inappropriate handling of temperature during the construction process of concrete dams endangers the dams' safety. Therefore, it is absolutely significant to study the variations of the thermal stress field during the construction process of high concrete dams. Based on Finite Element Method (FEM) software ANSYS 10.0, a newly developed program is edited and applied to study and calculate the non-steady temperature field and the thermal stresses through ANSYS Parametric Design Language (APDL), and verified to be reliable. The thermal stress field during the construction process of Longtan Roller Compacted Concrete (RCC) gravity dam is numerically simulated by the above developed program. The distributions of the thermal stress field and the water-thermal coupling stress field under reservoir normal storage are simulated and analyzed. Some meaningful conclusions are obtained from numerical simulations.

According to the characteristic structural features of jointed rock masses, a natural element method (NEM) model is proposed for the mechanics analysis of jointed rock masses based on the Sibsonian interpolation or Laplace interpolation. In the model, the jointed rock mass is regarded as a system of intact rock blocks connected by discontinuous joints modeled by interfaces. The displacement field of each block is constructed by the natural neighbor interpolation. To deal with the discontinuities of rock masses, the displacement fields are constructed to be discontinuous between blocks. The displacement fields and their gradients are continuous in each block. Numerical simulations illustrate NEM has advantages of both element free method and finite element method, and does not have their disadvantages. The present method is developed for two-dimensional linear elastic analysis for jointed rock structures, and can be extended to three-dimensional and non-linear analysis.

To develop national economy and use the water resources and hydropower resources sufficiently, a lot of high arch dams, with the height of more than 200 m, have been and will be built in China. Although arch dams have good mechanical behavior, there is still failure possibility due to the huge water pressure and high stress level in dam, complex topographic and geologic conditions, formidable environment and high intensity earthquake. As one of the three main aspects concerning the safety of high arch dam, the study on global destruction, has been elaborated in the literatures, and research advance in the other two aspects, namely the failure risk and local damage of high arch dams, will be reviewed in this paper. In recent years, the failure risk of high arch dams has been investigated, and the model for identifying dam failure risk factors has been established. It is shown that the foundation deterioration and strong earthquake are the major risk sources for high dam failure. With the fault tree method, the failure mode and failure probability of high arch dams are studied, and the principle for determining failure mode and the method of calculating failure probability are proposed. Meanwhile, the determination principle of acceptable risk standard for high arch dam was proposed, and the acceptable risk rate and the acceptable standard value of various risk losses were given. For the local damage of arch dam, it is pointed out that the local damage belongs to the strength failure at material level. The study on local failure mechanism of arch dam is reviewed, based on the theories that from traditional strength theory to damage mechanics and meso-mechanics theory. Aiming at the cracking, the main pattern of local failure of high concrete dam, the research advances in the analysis methods and cracking criteria for smeared crack model and discrete crack model are summarized, and the research findings of preventive measures for local failure are shown.