In-house Finite Element Program Research Articles

Numerical prediction of austenite grain size in a bar rolling process using an evolution model based on a hot compression test

A microstructure evolution model was formulated by characterizing the kinetics of static (SRX) and metadynamic (MDRX) recrystallization in consideration of the experimental data using single and double compression tests of AISI 4135 at various temperatures and strain rates. The evolution model consisted of equations for SRX, MDRX, and grain growth was implemented into an in-house finite element program to simulate the process. Numerical prediction of austenite grain size (AGS) evolution during a hot bar rolling of AISI 4135 was conducted and presented. The predicted results were compared with the experimental data obtained from the hot bar rolling and the numerical results based on other AGS models available in the literature which were derived based on torsion tests. The present model determined in the current investigation based on compression tests shows better agreement with the experimental data than the earlier works. The critical strains determined from compression tests were relatively smaller than those from the torsion tests, which influenced the overall recrystallization and grain growth behaviors. Also, the current model was beneficial to understand the effect of recrystallization behavior and control the microstructure evolution during hot bar rolling.

Finite element analysis of piezoelectric underwater transducers for acoustic characteristics

This paper presents a simulation technique for analyzing acoustic characteristics of piezoelectric underwater transducers. A finite element method is adopted for modeling piezoelectric coupled problems including material damping and fluid-structure interaction problems by taking system matrices in complex form. For the finite element modeling of unbounded acoustic fluid, infinite wave envelope element (IWEE) is adopted to take into account the infinite domain. An in-house finite element program is developed and technical issues for implementing the program are explained. Using the simulation program, acoustic characteristics of tonpilz transducer are analyzed in terms of modal analysis, radiated pressure distribution, pressure spectrum, transmitting-voltage response and impedance analysis along with experimental comparison. The developed simulation technique can be used for designing ultrasonic transducers in the areas of nondestructive evaluation, underwater acoustics and bioengineering.

Comparison between Experimental and 3D Finite Element Analysis of Reinforced and Partially Pre-Stressed Concrete Solid Beams Subjected to Combined Load of Bending, Torsion and Shear

This paper presents a non-linear analysis of three reinforced and two partially prestressed concrete solid beams based on a 20 node isoparametric element using an in-house 3D finite element program. Anon linear elastic isotropic model, proposed by Kotsovos, was used to model concrete behaviour, while steel was modelled as an embedded element exhibiting elastic-perfectly plastic response. Allowance was made for shear retention and for tension stiffening in concrete after cracking. Only in a fixed direction, smeared cracking modelling was adopted. The beams dimensions were 300x300 mm cross section, 3800 mm length and were subjected to combined bending, torsion and shear. Experimental results were compared with the non-linear predictions. The comparison was judged by load displacement relationship, steel strain, angle of twist, failure load, crack pattern and mode of failure. Good agreement was observed between the predicted ultimate load and the experimentally measured loads. It was concluded that the present program can confidently be used to predict the behaviour and failure load of reinforced and partially prestressed concrete solid beams subjected to a combined load of bending, torsion and shear.

Transient hydrogen diffusion analyses coupled with crack-tip plasticity under cyclic loading

The effect of hydrogen on the material strengths of metals is known as the hydrogen embrittlement, which affects the structural integrity of a hydrogen energy system. In the present paper, we developed a computer program for a transient hydrogen diffusion–elastoplastic coupling analysis by combining an in-house finite element program with a general purpose finite element computer program to analyze hydrogen diffusion. In this program, we use a hypothesis that the hydrogen absorbed in the metal affects the yield stress of the metal. In the present paper, we discuss the effects of the cyclic loading on the hydrogen concentration near the crack tip. An important finding we obtained here is the fact that the hydrogen concentration near the crack tip greatly depends on the loading frequency. This result indicates that the fatigue lives of the components in a hydrogen system depend not only on the number of loading cycles but also on the loading frequency.

Transient Hydrogen Diffusion-Elastoplastic Coupling Analysis near a Blunting Crack Tip

The effect of hydrogen on the metals is known as hydrogen embrittlement, which affects the structural integrity of a hydrogen energy system. Hydrogen atoms near a crack tip play an important role in the hydrogen embrittlement. In the present paper, we developed a computer program for an transient hydrogen diffusion-elastoplastic coupling analysis by combining an in-house finite element program for hydrogen diffusion analysis with a general purpose finite element computer program for stress analysis. In the hydrogen diffusion equation, we consider both the hydrogen concentration at the normal interstitial lattice sites and that at the trap sites, and also take account of hydrogen diffusion flux due to hydrostatic stress. We use a hypothesis that the hydrogen absorbed in the metal affects the yield stress of the metal. Therefore, the hydrogen diffusion problem is coupled with the elastoplastic stress problem. Using the computer program developed in the present study, we performed the transient hydrogen diffusion-elastoplastic coupling analysis of a cracked plate made of a bcc-metal, and obtained the hydrogen concentration, hydrostatic stress and plastic strain near the crack tip. In the present paper, we discuss the effects of the initial hydrogen concentration, the boundary condition of hydrogen on the free surface and the loading frequency. An important finding obtained from the present study is the fact that the hydrogen concentration near the crack tip depends greatly on the loading frequency. The fact indicates that the fatigue lives of the components in a hydrogen system depend not only on the number of loading cycles but also on the loading time.

A finite element parametric study on the mechanical properties of J-shaped steel hysteresis devices

The present paper discusses the mechanical properties of J-shaped steel hysteresis devices, abbreviated to “J-dampers”, interposed between the upper structure of a spatial structure and its substructure. The mechanical properties of a J-damper, which are characterized by its key geometrical and material parameters, are evaluated through parametric analyses using an in-house Finite Element program. The hysteresis behavior of a J-damper is also modeled by a bi-linear elasto-plastic hysteresis model with the strain-hardening effect. Its elastic stiffness, yield strength and dimensionless hardening ratio, which are expressed as functions of the parameters, are formulated on the basis of mathematical models and the results of parametric studies. Finally, some remarks relevant to the application of the proposed hysteresis model are made for the design of the J-damper.

2D Idealisation of Hollow Reinforced Concrete Beams Subjected to Combined Torsion, Bending and Shear

This paper presents a finite element model for idealisation of reinforced concrete hollow beams using 2D plane elements. The method of ensuring compatibility between the plates using two-dimensional model to analyze this type of structures is discussed. Cross-sectional distortion was minimised by incorporating end diaphragms in the FE model. Experimental results from eight reinforced concrete hollow beams are compared with the non-linear predictions produced by a 2D in-house FE program. The beam dimensions were 300x300 mm cross section with 200x200 mm hollow core and 3800 mm long. The beam ends were filled with concrete to form solid end diaphragms to prevent local distortion. The beams were subjected to combined bending, torsion and shear. It was found that the two-dimensional idealisation of hollow beams is adequate provided that compatibility of displacements between adjoining plates along the line of intersection is maintained and the cross-sectional distortion is reduced to minimum. The results from the 2D in-house finite element program showed a good agreement with experimental results.

Analysis of Nonlinear Coupled Diffusion of Oxygen and Lactic Acid in Intervertebral Discs

The transport of oxygen and lactate (i.e., lactic acid) in the human intervertebral disc was investigated accounting for the measured coupling between species via the pH level in the tissue. Uncoupled cases were also analyzed to identify the extent of the effect of such coupling on the solute gradients across the disc. Moreover, nonlinear lactic production rate versus lactic concentration and oxygen consumption rate versus oxygen concentration were considered. The nonlinear coupled diffusion equations were solved using an in-house finite element program and an axisymmetric model of the disc with distinct nucleus and anulus regions. A pseudotransient approach with a backward integration scheme was employed to improve convergence. Coupled simulations influenced the oxygen concentration and lactic acid concentration throughout the disc, in particular the gradient of concentrations along the disc mid-height to the nucleus-anulus boundary where the solutes reached their most critical values; minimum for the oxygen tension and maximum for the lactate. Results suggest that for realistic estimates of nutrient and metabolite gradients across the disc, it could be important to take into account the coupling between the rates of synthesis and overall local metabolite/nutrient concentration.

A SIMULATION OF THE TURBULENCE RESPONSE OF HEAT EXCHANGER TUBES IN LATTICE-BAR SUPPORTS

Tube failures due to excessive flow-induced vibrations are a major concern with regards to the operation of heat exchangers in nuclear power and chemical process plants. Among the possible excitation mechanisms, turbulence-induced forces have a persistent effect and thus are an important factor in determining the long-term reliability of heat exchangers. Impact forces which occur when tube/support collisions take place play a vital role in determining tube wear. To address this issue, a tube/support interaction model was implemented in an in-house finite element program and validated against three published examples. Pseudo-forces in conjunction with modal superposition were utilized in solving the nonlinear equations of motion of loose tubes in lattice-bar supports. Time-domain simulations of the nonlinear response of the tube are presented to determine the effect of various tube and support parameters on the vibratory characteristics of the systems. Special attention was paid to the effect of increased clearance on the response of tubes in lattice-bar supports. The tube response, impact force and contact ratio were analysed and represented in dimensionless form. The dimensionless parameters developed proved effective in collapsing the data pertaining to different flow velocities over single curves. These are useful in identifying the roles of several key variables in altering tube dynamics. Moreover, these parameters may also be used to scale the results to account for differences in geometrical and material properties.

Prediction of Impact Contact Forces of Composite Plates Using Fiber Optic Sensors and Neural Networks

Real-time determination of contact forces due to impact on composite plates is necessary for on-line impact damage detection and identification. We demonstrate the use of fiber optic strain sensor data as inputs to a neural network to obtain contact force history. An experimental and theoretical study is conducted to determine the in-plane strains of a clamped graphite/epoxy composite plate upon low-velocity impacts using surface-mounted extrinsic Fabry-Perot interferometric (EFPI) strain sensors. The plate is impacted with a semispherical impactor with various impact energies using the drop-weight technique. The significant features of the strain and contact force response are contact duration, peak strain, and strain rise time. We have designed and built an instrumented drop-weight impact tower to facilitate the measurement of contact force during an impact event. The impact head assembly incorporates a load cell to measure the contact forces experimentally. An in-house finite-element program is used to establish the validity of the EFPI fiber optic sensor contact force response. The finite-element model is based on a higher-order shear deformation theory and accounts for geometric nonlinearity. Experimental load cell data and finite-element impact-induced contact force responses are in close agreement. The load cell data is used to train a three-layer feed-forward neural network which utilizes the Delta Bar Delta back-propagation algorithm. The output of the neural network simulation is the contact force history and the inputs are fiber optic sensor data in two different locations and time in 10-ms intervals. The efficiency and accuracy of the neural network method is discussed. The neural network scheme recovers the impact contact forces without using any complex signal processing techniques.

A Finite Element Model for the Rolling Loss Prediction and Fracture Analysis of Radial Tires

Abstract With the development of tire mechanics and computer technology, tire deformation, rolling resistance, and temperature distribution under rolling conditions may be predicted accurately through finite element analysis (FEA). Deep knowledge of tire fracture and failure behavior may also be obtained by FEA. During the past years, an in-house finite element program has been developed in our research laboratory which can analyze the tire deformation, stress, and strain under the static inflation and footprint load conditions and can predict the tire rolling resistance and temperature distribution as well. This paper gives a brief description of the mathematical and mechanical foundations of the developed FEA code and the computing procedures, emphasizing the tire material loss model and the calculation procedure of strain energy release rate in tire fracture analysis. Two characteristics of the presented model compared with the published literature are the three-dimensional anisotropic properties included in the loss model of cord-rubber materials and a new VCCT (Virtual Crack Closure Technique), which is simple and physically direct, saves on the amount of computation, and is developed to compute the fields of strain energy release rates (Serrs) in the crack front to analyze tire fracture behavior.

Measurement and analysis of impact-induced strain using extrinsic Fabry-Pérot fiber optic sensors

In-plane strain responses of surface-mounted extrinsic Fabry-Pérot interferometric (EFPI) fiber optic strain sensors are investigated. EFPI fiber optic strain sensors are mounted on three graphite/epoxy laminated composite plates. The plates are impacted with various size steel balls using a drop-weight technique. The impacts did not cause apparent damage. The first impact-induced strain peak was characterized by the rise time, peak value, full width at half maximum and decay time. The transient low-velocity impact response of EFPI fiber optic strain sensors is compared to the response from conventional electrical resistance strain gages and polyvinylidene fluoride (PVDF) piezoelectric film sensors. Orientation dependence and other characteristics of the EFPI fiber optic sensing technique are discussed. An in-house finite element program incorporates geometric nonlinearity and transverse shear deformation for the impact events. The finite element results closely match the experimental strain data for the first peak strain response.

A CO-ROTATIONAL FORMULATION FOR GEOMETRICALLY NONLINEAR FINITE ELEMENT ANALYSIS OF SPATIAL BEAMS

A co-rotational procedure is presented in this paper for handling arbitrarily large three-dimensional rotations associated with geometrically nonlinear analysis of spatial beam structures. This procedure has been incorporated into two commonly used 3-D beam elements, the 2-node cubic beam element and the 3-node superparametric beam element, in our in-house general purpose finite element program, VAST. In the present procedure, the element tangent stiffness matrices are generated by using the standard updated Lagrangian formulation, while a co-rotational formulation is employed to update the internal force vectors during the Newton-Raphson iterations, A number of example problems have been analyzed and the result are in good agreement with analytical or published numerical solutions.

Large deformation finite element treatment of changes in the volume of fluid-filled cavities enclosed in a structure

A large deformation elasto-static finite element formulation for the stress analysis of structures with fluid-filled cavities is presented. The formulation accounts for any arbitrarily induced changes in the volume or pressure of the fluid-filled cavities thus allowing for the response analysis of the structure while the externally applied loads may remain unchanged. The fluid regions do not need to be discretized and contribute both to the tangent stiffness matrix and incremental load vector of the structure. The analysis at each increment requires the solution to two distinct sets of simultaneous equations. Initially, some incremental displacement vectors due to as many load vectors as the number of cavities plus one are computed. The incremental cavity pressures are then calculated by solving another set of simultaneous equations. Having incorporated this formulation in an in-house three-dimensional nonlinear finite element program, two sample problems are analysed.

Nonlinear finite element analysis of wrapping uniaxial elements

This paper presents a finite element technique to analyse the structures having uniaxial elements wrapping around general solid body edges. These edges may be fixed or allowed to deform during the course of loading. After presenting the contact criteria, the incremental tangent stiffness matrices of these elements are developed. Each element is formulated by a single stiffness matrix accounting for all the comprising sub-elements. The formulated matrix is full with coupling between degrees of freedom of all the nodes on the element. This formulation accounts for both large displacements and large strains. A proper transformation between the experimentally obtained material properties and those needed in the formulation is also presented. Finally, having incorporated this element in an in-house nonlinear finite element program, three simple structural problems are analysed.