Finite Element Preprocessor Research Articles

Constitutive model of high-temperature rapid fracture of Si3N4 ceramics for engine turbine discs and their pore insensitivity parameters

ABSTRACT The metallic materials used to manufacture turbine components can potentially be replaced with ceramics such as Si3N4, but the stress on turbine discs is difficult to determine experimentally, which hinders the development and optimization of ceramics. The finite element method solves this problem, but it requires an accurate constitutive model and parameters. Existing methods for determining model parameters lack versatility and do not consider the internal pores of ceramics. This work proposes a method for determining the parameters of the high-temperature rapid fracture constitutive model of ceramics with internal pores based on the results of industrial computed tomography (CT) and high-temperature fracture toughness experiments. The constitutive model for a ceramic with internal pores was selected based on the typical service conditions of turbine discs. Industrial CT was used to measure pore information. A finite element preprocessor was developed to create simulated specimens with pores. The high-temperature fracture toughness of the ceramics was measured. Pore insensitivity was the parameter obtained from the high-temperature constitutive model of ceramics with pores using this method. The maximum relative error between the predicted and experimental flexural strengths was 6.8%. It provides a new idea for determining the parameters of constitutive models for materials with pores.

Deformable mapping technique to correlate lesions in digital breast tomosynthesis and automated breast ultrasound images.

To develop a deformable mapping technique to match corresponding lesions between digital breast tomosynthesis (DBT) and automated breast ultrasound (ABUS) images. External fiducial markers were attached to the surface of two CIRS multi-modality compressible breast phantoms (A and B) containing multiple simulated lesions. Both phantoms were imaged with DBT (upright positioning with cranial-caudal compression) and ABUS (supine positioning with anterior-to-chest wall compression). The lesions and markers were manually segmented by three different readers. Reader segmentation similarity and reader reproducibility were assessed using Dice similarity coefficients (DSC) and distances between centers of mass (dCOM ). For deformable mapping between the modalities each reader's segmented dataset was processed with an automated deformable mapping algorithm as follows: First, Morfeus, a finite element (FE) based multi-organ deformable image registration platform, converted segmentations into triangular surface meshes. Second, Altair HyperMesh, a FE pre-processor, created base FE models for the ABUS and DBT data sets. All deformation is performed on the DBT image data; the ABUS image sets remain fixed throughout the process. Deformation was performed on the external skin contour (DBT image set) to match the external skin contour on the ABUS set, and the locations of the external markers were used to morph the skin contours to be within a user-defined distance. Third, the base DBT-FE model was deformed with the FE analysis solver, Optistruct. Deformed DBT lesions were correlated with matching lesions in the base ABUS FE model. Performance (lesion correlation) was assessed with dCOM for all corresponding lesions and lesion overlap. Analysis was performed to determine the minimum number of external fiducial markers needed to create the desired correlation and the improvement of correlation with the use of external markers. Average DSC for reader similarity ranged from 0.88 to 0.91 (ABUS) and 0.57 to 0.83 (DBT). Corresponding dCOM ranged from 0.20 to 0.36 mm (ABUS) and 0.11 to 1.16 mm (DBT). Lesion correlation is maximized when all corresponding markers are within a maximum distance of 5 mm. For deformable mapping of phantom A, without the use of external markers, only two of six correlated lesions showed overlap with an average lesion dCOM of 6.8 ± 2.8 mm. With use of three external fiducial markers, five of six lesions overlapped and average dCOM improved to 4.9 ± 2.4 mm. For deformable mapping of Phantom B without external markers analysis, four lesions were correlated of seven with overlap between only one of seven lesions, and an average lesion dCOM of 9.7 ± 3.5 mm. With three external markers, all seven possible lesions were correlated with overlap between four of seven lesions. The average dCOM was 8.5 ± 4.0 mm. This work demonstrates the potential for a deformable mapping technique to relate corresponding lesions in DBT and ABUS images by showing improved lesion correspondence and reduced lesion registration errors with the use of external fiducial markers. The technique should improve radiologists' characterization of breast lesions which can reduce patient callbacks, misdiagnoses and unnecessary biopsies.

Response analysis of longitudinal vibration of sucker rod string considering rod buckling

Considering the limitation of simulation model of the deep well and ultra-deep well, an improved simulation model of sucker rod pumping system (SRPS) is presented. The main innovations of the simulation model are nonlinear longitudinal vibration model of sucker rod string (SRS) and numerical integral model of nonlinear system with pump time varying parameters. In the detail, the relationship of rod buckling and plunger load is analyzed using a finite element preprocessor module of ANSYS. Based on analysis results and experimental results, the change rule of equivalent stiffness is studied. Using parameters of oil well, the response of longitudinal vibration of SRS is analyzed. The new model is verified by comparing it with the available analytical solution, and good agreement is found. The simulation results show that the instantaneous equivalent stiffness of helical buckling section of SRS follows exponential distribution. The equivalent stiffness is affected by some parameters, among them, the values of water cut, density of crude oil and pump depth have a significant effect. With new model presented in this paper, the subharmonic resonance is found, which is characteristic of nonlinear vibration. Comparing with the steel rod, carbon fiber rod and fiberglass rod, the resonant phenomenon of single rod is more likely to happen for the wire rope sucker rod. However, when sucker rod is double stage rod, the resonant phenomenon of fiberglass rod is more likely to happen than other rods. The above results are significant to the optimization of SRS.

Estimation of Stress Corrosion Cracking Growth Behavior Under Weld Residual Stress in the Bottom of a Reactor Pressure Vessel by Finite Element Analysis

A method for evaluating crack growth by repeatedly modeling and analyzing the transitional crack shapes is developed for a general computing environment in which a commercial finite element preprocessor and analysis code are used. The proposed method calculates stress intensity factors (SIFs) by finite element analysis (FEA) by directly distributing estimated weld residual stress obtained from noncracked components on the crack surface on the basis of the superposition principle. In present case, to specify a nonuniform residual stress distribution, a subroutine for a commercial FEA code (ABAQUS) was developed. Arbitrary crack shapes during the crack propagation were expressed by applying the submodeling technique which allowed arbitrary crack shapes to be meshed. The sequence of steps in the proposed method was designed to make it possible to consider complicated stress distributions, such as weld residual stress, and to express arbitrary crack shapes. The applicability of the proposed FEA based method was verified by comparing the result of a stress corrosion cracking (SCC) growth analysis results of a flat plate obtained with the proposed method and with the ASME code procedure. As an application example, the SCC growth behavior of a crack at the bottom of a nuclear reactor pressure vessel (RPV) involving a dissimilar metal weld and a unique geometry was evaluated by the proposed method. The evaluation results were compared with results obtained using a conventional method, i.e., the influence function method (IFM). Since both sets of results were in reasonable agreement, it was concluded that IFM can be applied to this case. Previously, it was difficult to assess the applicability of conventional methods, such as the code procedure and IFM, to a complicated problem because of the existence of complicated residual stress fields, dissimilar metals, and the complicated crack shapes involved. The proposed method using FEA allows the applicability of conventional methods to complicated crack growth evaluations to be assessed.

Design and Optimization of Four Wheeler Connecting Rod Using Finite Element Analysis

Every vehicle that uses an internal combustion engine requires at least one connecting rod depending upon the number of cylinders in the engine. It undergoes high cyclic loads of the order of 108 to109cycles, which range from high compressive loads due to combustion, to high tensile loads due to inertia. Therefore durability of the component is of critical importance. Due to these factors the connecting rod has been the topic of research for different aspects such as production, materials, performance simulation, etc.The main objective of research was to explore weight reduction opportunities for the production of forged steel, aluminium and titanium connecting rods. This has entailed performing a detailed load analysis. Therefore, this study has dealt with two subjects, first, static load stress analysis of the connecting rod for three materials, and second, optimization for weight of forged steel connecting rod.In this research, firstly a proper geometrical model was developed using CATIA. Then the model is imported to the HYPERMESH which is a finite element pre-processor that provides a highly interactive and visual environment to analyse product design performance and the Finite Element model was developed. The stresses were found in the existing connecting rod for the given loading conditions using Finite Element Analysis software ANSYS 11.0.The topology optimization technique is used to achieve the objectives of optimization which is to reduce the weight of the connecting rod.

Modelling and behaviour of cylindrical shell structures with helical features

Spiral welding or bonding is a particularly efficient and cost-effective method of constructing continuous tubes. However the understanding of the mechanics of such structures is not yet well developed. This is in no small part due to the difficulties involved in their computational analysis.Cylindrical shells are traditionally modelled using rectangular finite elements oriented parallel to the meridional and circumferential axes. However, spiral features are particularly challenging to model because such features are not orthogonal to the axes of the cylinder. Commercial finite element pre-processors often struggle to mesh these with anything other than a free triangulation. A superior mesh would consist of well-conditioned rectangular elements oriented orthogonally with respect to the axes of the helix, termed a ‘helical mesh’, but this requires significant programming effort. A helical mesh is particularly important if features of the shell such as geometric imperfections, residual stresses, weld material and heat affected zones, and bonding in systems using adhesives are required to follow the helical form. Helically wound structural forms are widely used in different applications that demand continuous cylindrical forms. The most common uses in structural engineering are in spiral wound tubes, piles, chimneys and pipelines.This paper describes a powerful computational procedure developed by the authors to generate high quality helical meshes. Special emphasis is placed on the modelling of geometric deviations defined relative to the helix, appropriate for the analysis of spiral welded and stiffened tubes. The effect of helical meshing is illustrated using benchmark examples of perfect and imperfect cylinders under axial compression.

Finite element models of the left ventricle based on 3D speckle tracking echocardiography

Introduction: Integration of cardiovascular imaging and modelling techniques is an emerging approach for the investigation of cardiac mechanics. Quantitative and accurate estimation of cardiac motion and deformation are important for evaluating physiological and pathological cardiac mechanical function. Three-dimensional (3D) speckle tracking echocardiography enables determination of motion and deformation pattern of the left ventricle (LV) in vivo. In this study, we constructed 3D finite element (FE) models of the LV from 3D speckle tracking echocardiography data in order to measure regional LV wall motion and deformation. Methods: Imaging of the LV was performed on 8 healthy volunteers and 2 patients with cardiomyopathy (1 dilated and 1 hypertrophic cardiomyopathy) with a commercial 3D echocardiography system (Artida, Toshiba Medical Systems) by means of a 3D transthoracic probe. 3D speckle tracking data were then post-processed with a custom written FE preprocessor and a commercial FE solver (Abaqus 6.10 Simulia, Providence, RI, USA) in order to compute 3D finite element models of the LV and to visualize local strains and displacements. Results: Computed 3D FE models of the LV are depicted in figure 1. FE modelling enabled detailed strain analysis for up to 1260 segments of the LV wall in contrast to the standard 16-segment analysis by the echocardiography-workstation. ![Figure][1] Conclusion: Three-dimensional FE models derived from in vivo speckle tracking echocardiography data allow detailed quantitative evaluation of cardiac motion and deformation with higher resolution than standard echocardiographic speckle tracking analysis. This integrated modeling of the LV may provide more insights in the interpretation of cardiac mechanics under normal and abnormal conditions on an individualized basis. [1]: pending:yes

Modelling complex geometry using solid finite element meshes with correct composite material orientations

This article addresses the requirement to generate solid hexahedral finite element meshes for modelling composite components with complex geometric shapes. Standard commercial finite element pre-processors are not capable of unambiguously defining the material orientations in such situations. This has necessitated the development of a purpose written pre-processor, which extrudes solid elements with element orientations that accurately match complex doubly curved geometries. An efficient methodology for the creation of the mesh has been outlined through the combined use of the dedicated software and MSC PATRAN. An example presented shows that this technology has been successfully used to model a representative vane from a gas turbine engine.

A Method for Automatically Optimizing Medical Devices for Treating Heart Failure: Designing Polymeric Injection Patterns

Heart failure continues to present a significant medical and economic burden throughout the developed world. Novel treatments involving the injection of polymeric materials into the myocardium of the failing left ventricle (LV) are currently being developed, which may reduce elevated myofiber stresses during the cardiac cycle and act to retard the progression of heart failure. A finite element (FE) simulation-based method was developed in this study that can automatically optimize the injection pattern of the polymeric "inclusions" according to a specific objective function, using commercially available software tools. The FE preprocessor TRUEGRID((R)) was used to create a parametric axisymmetric LV mesh matched to experimentally measured end-diastole and end-systole metrics from dogs with coronary microembolization-induced heart failure. Passive and active myocardial material properties were defined by a pseudo-elastic-strain energy function and a time-varying elastance model of active contraction, respectively, that were implemented in the FE software LS-DYNA. The companion optimization software LS-OPT was used to communicate directly with TRUEGRID((R)) to determine FE model parameters, such as defining the injection pattern and inclusion characteristics. The optimization resulted in an intuitive optimal injection pattern (i.e., the one with the greatest number of inclusions) when the objective function was weighted to minimize mean end-diastolic and end-systolic myofiber stress and ignore LV stroke volume. In contrast, the optimization resulted in a nonintuitive optimal pattern (i.e., 3 inclusions longitudinallyx6 inclusions circumferentially) when both myofiber stress and stroke volume were incorporated into the objective function with different weights.

A multi-body system approach for finite-element modelling of rail flexibility in railroad vehicle applications

In this investigation, a non-linear finite-element formulation for modelling the rail structural flexibility in multi-body railroad vehicle systems is presented. Two different types of interpolations are used in the kinematic equations developed in this study; the geometry interpolation and the deformation interpolation. In the proposed formulation, the rails can have arbitrary geometry, which is described using the isoparametric geometric interpolation. The coordinates of the polynomials used in this interpolation represent constant position and gradient coordinates, which can be used to describe accurately the rail geometry. On the other hand, the rail deflections are described using the deformation interpolation and the non-linear finite-element floating frame of reference formulation. In the formulation proposed in this investigation, the rail tangent and normal vectors as well as other geometric parameters such as the curvature and torsion at the wheel/rail contact points are expressed in terms of the rail deformation coordinates. The non-linear dynamic coupling between the rail geometry and the vehicle dynamics is also considered in the formulation proposed in this paper. In particular, the coupling between the rail deformation and geometry, contact coordinates, and the non-linear vehicle dynamics is taken into consideration. Furthermore, the longitudinal, lateral, and spin creepages are expressed in terms of the rail deformations, which are the result of the wheel/rail contact forces. This non-linear coupled analysis allows for more accurate prediction of the railroad vehicle dynamics. The main outcome of this study is the development of a new procedure that allows building a complex track model that includes significant details using a finite-element preprocessor computer program. The detailed track model can be used as an input to a general purpose flexible multi-body computer program for a non-linear analysis that accounts for the dynamic coupling between the track flexibility and the vehicle coordinates. Numerical results are presented in order to demonstrate the use of the formulation proposed in this study.

Automating node and element assignments on conforming four-sided sections defining a domain for mapping quadrilateral elements

Many commercial finite element pre-processors use mapping methods which generate quadrilateral elements in four-sided sections which define an analysis domain. Sections share a side (or edge) with at most one additional section. This defines a conforming block-structured grid. The manual effort required to assign nodes to each edge makes this a time-consuming, error-prone process. This paper describes a technique for automating the assignment of nodes on the conforming boundaries of four-sided sections which define a domain. The input to the method includes a grid request and the topology of the domain as defined by the sections and edges. The grid specifies the desired number of elements to fill the domain and the type of mapping rule to be applied to the topology. Examples are shown in two dimensions, but the technique could be extended to three dimensions.

Added mass of high-altitude balloons

Acoustic theory is used to find the added mass for several rigid, immersed bodies. The classical cases of a thin circular disk and a sphere are used to determine the mesh fineness required for engineering accuracy. A family of five Smalley-shaped balloons (zero circumferential stress in the film) is then considered, at different inflation ratios. Acoustical boundary elements are used. The fluid is assumed incompressible and, therefore, the added masses are identical in spirit with those from hydrodynamics. Although not important for this study, compressibility effects can be included for other bodies, if needed. Both vertical and horizontal accelerations are considered. Results show that the pear-shaped balloons behave in an intermediate way between spheres and cylinders, as expected. Such accurate values for added mass will allow better simulation of balloon flight, particularly for dynamic motion resulting from ballast or pay load drop. A major feature of this article is to demonstrate the feasibility of calculating added masses for arbitrarily shaped bodies using acoustics. The authors feel this approach will become a standard working tool for studies of immersed bodies such as balloons, parachutes, and submarines because of the ease of computation. The method uses commercial finite element preprocessors for building the model, calculation of enclosed volumes, and transferring rigid body information to the acoustic computer program.

Model reduction methods for dynamic analyses of large structures

User-friendly finite element pre-processors permit detailed modelling of engineering structures with complicated topologies, often resulting in a much larger number of degrees of freedom than that motivated by the expected complexity of the structural response. Dynamic analyses of these large-scale models might require a prohibitive amount of CPU time. In particular, the uncoupling of the equations of motion for linear systems with non-proportional damping by means of a complex eigenvector basis is computationally very demanding. We present general model reduction procedures, applicable to whole structures or substructures, that produce a series of reduced models each spanning appropriate subspaces (general Krylov spaces) of the solution space. Crucial steps in such procedures are the selection of basis vectors for the (sub)structures and the control of reduction errors. The set of basis vectors is generated by using a simple physical approach for successively reducing the residual error in the governing equilibrium equations. A frequency window method (shifting) is used to capture the relevant frequency content. The residuals of the equilibrium equations, computed on fully expanded level, are used as reliable measures of the reduction errors. The efficiency of the techniques is demonstrated in some numerical experiments and on a large engineering problem, using a program based on SAP IV.

Impacting the design process using solid modelling and automated finite element mesh generation

Finite element analysis is commonly used to obtain information about structural behaviour without building prototypes. Today, it is being used primarily as an after-the-fact verification tool. Using finite element analysis in earlier stages of design can, in addition to verifying that the design meets certain specifications, better impact the actual design concept. Historically, considerable time has been needed to create the finite element model, particularly when using solid elements. The model building time has been large enough to prevent analysis results from impacting the design. The turnaround time for model building and analysis needs to be reduced to increase the impact on the design process. Model building time has been reduced by ever-improving computer graphic model building tools. It is important that finite element preprocessors are both fast and easy to use. The faster they are, the more impact they can have on the early design by decreasing analysis turnaround time. Ease of use increases the potential number of users by allowing casual users and experts to be productive. The combination of a good solid modeller for geometry creation with automated mesh generators shows promise in decreasing analysis turnaround time. The solid modeller offers a variety of ways to create geometry quickly. The mesh generators keep the amount of necessary user interaction and intervention to a minimum. The overall result is a reduction in the time necessary to build many models. This paper discusses aspects of using a solid modelling system in conjunction with mesh generators with a focus on productivity improvements and accuracy of results. Examples generated with the SDRC I-DEAS software system are used to illustrate a range of techniques. The solid modeller (Geomod) is capable of creating objects using a variety of high-level construction techniques, including Boolean operations, profile extrusion, and surface skinning. This allows for geometry creation that is not only fast but easy to use. A free mesh generator (Triquamesh) uses this geometry to generate finite element meshes automatically. It allows meshes to be generated on n-edged surfaces and n-sided volumes with a minimum of user input. By using a combination of these tools, it is possible to reduce model building time significantly. This allows proposed design changes to ve evaluated earlier in the design cycle and have greater impact on the final design.

3-dimensional, adaptive FE-preprocessing using a programmable CAD-system

A brief overview of the graphic features of a useful CAD system is presented to demonstrate the possibilities in automatic finite-element (FE)-mesh generation. The features required of an FE preprocessor are stated. An algorithm and family of 3-D elements for adaptive mesh refinement are given. They can be used to refine small parts of an FE mesh without any effect on the remaining regions. >

An Efficient Method to Predict the Effect of Design Modifications

Static and dynamic analyses are important tools for the improvement of structural systems. Consider a system where a number of different changes may be synthesized to improve static and dynamic behavior. The analysis depends on numerical optimization schemes for systems with a large number of degrees of freedom. These analyses become computationally burdensome for redesign analysis. This paper presents an efficient method to predict the effect of design modifications on mechanical and structural systems with many degrees of freedom. The method minimizes a cost function which includes natural frequencies, size of design change and static deflections. It uses a finite-element preprocessor to find derivatives of mass and stiffness matrices and computationally efficient techniques to find eigen-value and eigenvector derivatives for use in the optimization.

The synthesis of near‐optimum finite element meshes with interactive computer graphics

AbstractA new approach to the generation of near‐optimum finite element meshes is presented. Like other current methods, the procedure presented uses the results of a previous analysis to improve the idealization, but unlike other procedures a new element mesh is generated in each cycle with the aid of interactive computer graphics. The procedure generates near‐optimum two‐dimensional meshes on one cycle employing a grid optimization criterion based on variations in the strain energy density and using a software system which operates like an interactive finite element preprocessor. The example problems presented demonstrate the efficiency and flexibility of the approach.