A conventional rigid plastic finite element method based on mixed formulation sometimes shows false collapse mechanisms containing hourglass modes and locking modes. These false mechanisms are mainly due to the spacial discretization of stresses within an element; a constant stress state is assumed within an element. To overcome these difficulties, a new spacial discretization of stresses is proposed in this paper; two additional variables per element are introduced to express stress distribution in an element. The validity of a newly proposed method is numerically scrutinized by solving the following four typical examples whose analytical solutions have been available; 1) Limit load of a rigid flat punching, 2) Limit load of a hollow cylinder subject to inner pressure, 3) Limit bending of a cantilever beam and 4) Limit load of a plate with a hole subject to biaxial stress. Numerical solutions show good agreement to analytical solutions for all the examples. It can be concluded that the proposed method is able to control both hourglass and locking modes successfully. It is also able to obtain rational plastic solutions with respect to both limit loads and collapse mechanisms.
The simplest structural design such as single column dominates the majority of the viaducts for the railways and the highways. For these viaducts with simple design, i) a method for detection of plastic response of structures under seismic load based on restoring force characteristics, ii) a method for synthesis of restoring force characteristics solely based on acceleration measurement and iii) a method for detection of plastic response of structures using sensor network are proposed in this paper. A pair of time synchronized acceleration data sets obtained from accelerometers installed on the top and the bottom the existing viaduct and the SDOF model for viaducts with relatively simple design plays the significant role in the synthesis of the restoring force characteristics. This paper presents the overview of the measument and analysis method of restoring force characteristics based on acceleration measurement. Then, the applicability and the limitation of the proposed method are discussed based on the results from shaking table experiment.
This paper presents the displacement monitoring system named "GPS wireless sensor network". This system employs a low cost L1 GPS (Global Positioning System) receiver and wireless sensor network. Low cost sensor node and cable-less system enable dense deployment of observation points. In this research, it is required to solve the trade-off between energy consumption and accuracy of displacement measurements. Measuring continuous data and applying a trend analysis or Kalman filter might yield high accuracy displacement estimation but also needs much energy dissipation. To solve the trade-off relationship, a data compression method and a static GPS positioning analysis method are newly developed and implemented in the system. In these methods, assumption of quasi-static displacement is taken into account. A prototype system is developed and experiments are conducted in a construction site and an ideal condition using the developed prototype. The experimental results show that the prototype works properly and is able to measure displacements in sub-centimeters accuracy.
The present paper addresses an optimization strategy of textile fiber reinforced concrete (FRC) with emphasis on its special failure behavior. Since both concrete and fiber are brittle materials, a prominent objective for FRC structures is concerned with the improvement of structural ductility, which may be defined as energy absorption capacity. Despite above unfavorable characteristics, the interface between fiber and matrix plays a substantial role in the structural response. This favorable 'composite effect' is related to material parameters involved in the interface and the material layout on the small scale level. Therefore the purpose of the present paper is to improve the structural ductility of FRC at the macroscopic level applying an optimization method with respect to significant material parameters at the small scale level. The method discussed is based on multiphase material optimization. This methodology is extended to a damage formulation. The performance of the proposed method is demonstrated in a series of numerical examples; it is verified that the structural ductility can be considerably improved.
The present contribution deals with an optimization strategy of fiber reinforced composites. Although the methodical concept is very general we concentrate on Fiber Reinforced Concrete with a complex failure mechanism resulting from material brittleness of both constituents matrix and fibers. The purpose of the present paper is to improve the structural ductility of the fiber reinforced composites applying an optimization method with respect to the geometrical layout of continuous long textile fibers. The method proposed is achieved by applying a so-called embedded reinforcement formulation. This methodology is extended to a damage formulation in order to represent a realistic structural behavior. For the optimization problem a gradient-based optimization scheme is assumed. An optimality criteria method is applied because of its numerically high efficiency and robustness. The performance of the method is demonstrated by a series of numerical examples; it is verified that the ductility can be substantially improved.
This paper focuses on the explicit dynamic analysis of wave propagation in heterogeneous materials using the extended finite element method (X-FEM). Since the enriched approximation in the X-FEM enables us to introduce the material discontinuities independently of finite element mesh, mesh-free and image-based analyses of heterogeneous materials can be realized. To apply the X-FEM to wave propagation analyses, we discuss the lumping techniques of mass matrix in enriched elements, and show that the mass matrix components in enriched elements does not tend to zero even if the material interface reaches the boundaries of enriched elements. In this paper, Chapter 2 shows the X-FE approximation of displacement in enriched elements and the formulation of dynamic explicit solution method with the X-FEM. By taking 1-D dynamic X-FE analyses as an example, we examine the lumping techniques of mass matrix in enriched elements and the influence on numerical accuracy and critical time step in Chapter 3. To examine the applicability and validity of the image-based X-FEM, numerical examples of wave propagation in heterogeneous materials are presented in Chapter 4.
In this paper, considering a straight crack in an infinitely extended anisotropic linear elastic material, we obtain a closed form solution of the stress and the displacement. The given far field stresses are such that the crack faces are in frictional contact and may slide against each other. We note that the final stress and displacement in the body do not uniquely determined by the final far field stresses but are dependent on the loading history since the crack faces are in frictional contact. Azhdari, A., et al. obtained the same stress intensity factors as ours for the above mentioned problem without considering the stress boundary conditions. We examine their method in detail in the appendix.
MPS method (Moving Particle Semi-implicit or Simulation) is expected as an earthquake response calculation method which can deal with overall responses from elastic wave propagation to failure phenomenon consistently. MPS method solves a wave equation numerically with using particles. Its formulation is very close to that of DEM (Descrete Element Method). It is known that the initial particle configulation affects the failure behavior when DEM is used. Because the influence by the initial configulation is similiarly recognized in MPS analysis, a method which constructs a random initial model is proposed. A formulation for shear and tensile failure, and new contact of particles due to large deformation is also proposed. Failure or collapse analyses caused by fall, own weight or compressive force are conducted. The proposed method is examined through these numerical simulations.
A simple model for the Discrete Element Method (DEM) with fewer parameters is proposed so that boundary value problems in geotechnical engineering can be studied. The interparticle contact bond model, which reproduces the behavior of cohesive geomaterials, and the rolling friction model, which expresses the interlocking effect of grain shapes with circular particles, are introduced into the conventional contact model for the two-dimensional DEM. Both of these models require only one parameter. Parametric analyses, based on several direct shear tests, are performed to validate the proposed model and to obtain relationships between the DEM parameters and the failure criteria for geomaterials by means of c and ϕ
This paper proposes an application of the distinct element method on daming up performance of woody debris capturing structure subjected to woody debris. In order to simulate the performance of woody debris, a column shape element model is developed. The debris flow experiments, with various combinations of the length of woody debris and the interval of vertical colu mns of the structure, are carried out in advance. Those results are simulated by the proposed method with considering a flow velocity model in this study. The flow velocity mode l considered the turbulence of water flow occurred in this experiment. The simulation results by using flow velocity model that ignore the turbulence effect cannot express the experimental results adequately. The simulation results by using flow velocity model that include the turbulence mode l show good agreement from the view point of debris capturing mechanism and debris flow capturing probability.
In order to investigate the deformation and failure behaviors of ground with the interaction among tsunami, coastal structure and ground (seabed and rubble-mound over it), both of dram type-centrifuge model test and numerical simulation with the Smooth Particle Hydrodynamics (SPH) method were performed. In the model test, the tsunami fluid force with large momentum applied to a caisson-type breakwater was reproduced, and deformation behaviors of the ground were observed by PIV image analysis; subsequently, the pore water pressure in the ground could be measured. When we placed a highly permeable rubble mound over the seabed ground, the hydraulic gradient increased due to seepage flow from the tsunami, and this led to the occurrence of a local seepage failure. In the numerical analysis, we compared the resulting data with the experimental results related to dam break and the results obtained by finite difference calculation (using a numerical wave experimental channel called CADMAS-SURF which is widely used in coastal engineering field), and this confirmed the reproducibility of tsunami fluid behaviors. We could then calculate the tsunami flow field, including coastal structures, rubble mounds consisting of non-deformable porous materials, and the seabed ground. An overflow phenomenon and the pressures of the tsunami's wave distribution could be quantitatively reproduced. This clarified how the tsunami would act on coastal structures and showed how conditions likely to cause seepage failures in rubble mounds and the seabed ground with different hydraulic gradients and permeability could be identified.
This paper proposes re-formulation of the non-linear concrete constitutive relation that is proposed by Maekawa's group, so that a fast solver for large scale non-linear finite element method is applicable. The reformulation is made in a rigorously mathematical manner. It succeeds to eliminate tensor inversion in computing an elasto-plasticity tensor and to use a positive-definite and symmetric elasticity tensor in computing stress rate in terms of strain rate. Numerical experiments are carried out to examine the convergence and equivalence of the re-formulated constitutive relation and to demonstrate the positive-definiteness of the elasticity tensor used in computing the stress rate.