RELIABILITY OF ELASTO-PLASTIC STRUCTURE USING FINITE ELEMENT METHOD

A solution of probabilistic FEM for elastic-plastic materials is presented based on the incremental theory of plasticity and a modified initial stress method. The formulations are deduced through a direct differentiation scheme. Partial differentiation of displacement, stress and the performance function can be iteratively performed with the computation of the mean values of displacement and stress. The presented method enjoys the efficiency of both the perturbation method and the finite difference method, but avoids the approximation during the partial differentiation calculation. In order to improve the efficiency, the adjoint vector method is introduced to calculate the differentiation of stress and displacement with respect to random variables. In addition, a time-saving computational method for reliability index of elastic-plastic materials is suggested based upon the advanced First Order Second Moment (FOSM) and by the usage of Taylor expansion for displacement. The suggested method is also applicable to 3-D cases.

An improved pseudo-static method for seismic resistant design of underground structures

This paper describes a commonly used pseudo-static method in seismic resistant design of the cross section of underground structures. Based on dynamic theory and the vibration characteristics of underground structures, the sources of errors when using this method are analyzed. The traditional seismic motion loading approach is replaced by a method in which a one-dimensional soil layer response stress is differentiated and then converted into seismic live loads. To validate the improved method, a comparison of analytical results is conducted for internal forces under earthquake shaking of a typical shallow embedded box-shaped subway station structure using four methods： the response displacement method, finite element response acceleration method, the finite element dynamic analysis method and the improved pseudo-static calculation method. It is shown that the improved finite element pseudo-static method proposed in this paper provides an effective tool for the seismic design of underground structures. The evaluation yields results close to those obtained by the finite element dynamic analysis method, and shows that the improved finite element pseudo-static method provides a higher degree of precision.

A NEW NUMERICAL SCHEME FOR ELASTIC-PLASTIC SIMULATION OF EXCAVATION IN GEO-ENGINEERING

For the path dependency and nonlinearity introduced by incremental construction, numerical method has been widely used in deformation analysis of geo engineering.In the numerical simulation scheme commonly used in the past, the excavating loads are extracted from nodal stresses, which are deduced linearly from the stresses at Gauss point in finite element method.The unneglectable calculation error is contained in this process when elastic plastic constitutive model is employed.The error mentioned above is analyzed in detail.Based on the analysis of excavation process and the principle of finite element theory, a new simulation scheme for excavation is proposed.At the end of this paper, an application in rock engineering is given out.

Structural characteristics of cement-stabilized soil bases with 3D finite element method

Cement-stabilized soil bases have been widely used in expressways due to its high strength,appropriate stiffness,good water resistance,and frost resistance.So far,the structural characteristics and mechanical behaviors of cement-stabilized soil bases were not investigated so much.In this paper,the 3D elastic-plastic finite element method(FEM)was used to analyze the mechanical behaviors and structural characteristics of cementstabilized soil bases from construction to operation.The pavement filling and the traffic loading processes were simulated,and a contact model was used to simulate the contact behavior between each layer of the pavement.Considering the construction process,the structural characteristics and mechanical behaviors of cementstabilized soil bases were studied under asphalt-concrete pavement conditions.Furthermore,the general rules of deformations and stresses in cement-stabilized soil bases under different conditions were discussed,and some suggestions were put forward for the design and construction of cement-stabilized soil bases.

Nanomechanics of Mg-Gd-Y-Nd-Zn alloy with LPSO and MgRE phases

The mechanical properties of two main precipitating phases(LPSO and MgRE)and matrix in Mg-Gd-Y-Nd-Zn bioalloy were examined using nanoindentation method.A new is suggested for characterizing the elastic-plastic behavior,fracture toughness and strain rate sensitivity(SRS)of materials within micro/nanoscale.Firstly,a nanomechanical model was developed for extracting hardness(H),young’s modulus(E)and yield stress(σY)from the characteristic load points which were subsequently analyzed by atomic force microscope(AFM)images.The elasticity data and AFM data were then utilized for determination of plastic deformation in constituent phases.The displacement of the indentation gets the highest value for Mg matrix and between precipitates,depth is more in LPSO rather than that of MgRE.The serrated flow or the behavior of shear bands may originate from the side effect of the interface region in Mg alloys with precipitates.It can be deduced that the KIC produced by both L method and energy-based calculation are both reliable for KIC approximation.The maximum load in simulation withμ=0.2 friction is marginally lesser than that of the frictionless(μ=0)one while elastic recovery of indentation withμ=0.2 is higher to some extent.

Temperature field analysis and its application in hot continuous rolling of Inconel 718 superalloy

A coupled thermo-mechanical model containing metal flow and temperature field for calculating temperature variation has been developed on fourteen-pass hot continuous rolling of round rod for Inconel 718 alloy using 3D elastic-plastic finite element method （FEM）. The temperature of characteristic analysis points in the intermediate cross-section of the workpiece has been simulated at initial temperature ranging from 960 to 1000 ℃ and initial velocity in range of 0.15-0.55 m·s^-1. Based on finite element analysis and microstructural observation in cylindrical hot compression experiments, the appropriate hot continuous rolling technologies have been designed for rod products with different diameters. For a real rolling practice, the simulated surface temperature was examined and is in good agreement with the measured one.

NUMERICAL SIMULATION OF HOT SAWING PROCESS

The present paper gives a numerical simulation of hot sawing process by using elastic- plastic finite element method. The simulation is carried out for the moment from the beginning of deformation to the local yield in work piece. In order to treat the work hardening on deformation resistance, a correction of stress based on deformation rate is taken into consideration. The calculation results shows, there are two stress peaks both on the lower and upper sides ahead of the tooth tip during the elastic press stage, in which the displacement of tooth is below 0.735*10-6 cm. Following the push of saw tooth and rise of saw load, metal at the corner of filing reaches the yield point and a local plastic area appears,where metal shapes new filing part towards the free surface under the extrusion forces. Meanwhile the stress peaks ahead of tooth tip initial also yield regions. The displacement of saw tooth is between 0.735*10-6cm and 1.274*10-6cm in this stage. With the further push of the saw tooth, metal bounded to tooth tip is torn under the combination of tensile and sheer stress. The slide of metal and its local plastic flow form the extension of filing. Then the saw load rises no longer and static sawing procedure continues.

New formula for predicting the plastic buckling pressure of steel torispherical heads under internal pressure

Thin-walled torispherical heads under internal pressure can fail by plastic buckling because of compressive circumferential stresses in the head knuckle.However,existing formulas still have limitations,such as complicated expressions and low accuracy,in determining buckling pressure.In this paper,we propose a new formula for calculating the buckling pressure of torispherical heads based on elastic-plastic analysis and experimental results.First,a finite element(FE)method based on the arc-length method is established to calculate the plastic buckling pressure of torispherical heads,considering the effects of material strain hardening and geometrical nonlinearity.The buckling pressure results calculated by the FE method in this paper have good consistency with those of BOSOR5,which is a program for calculating the elastic-plastic bifurcation buckling pressure based on the finite difference energy method.Second,the effects of geometric parameters,material parameters,and restraint form of head edge on buckling pressure are investigated.Third,a new formula for calculating plastic buckling pressure is developed by fitting the curve of FE results and introducing a reduction factor determined from experimental data.Finally,based on the experimental results,we compare the predictions of the new formula with those of existing formulas.It is shown that the new formula has a higher accuracy than the existing ones.