Global Finite Element Analysis Research Articles

Analytical Assessment of Composite L Angle Strength Under Internal Fuel Pressure in Composite Wing

The Interspar (IS) box stores the turbine fuel in the composite wing of an Aircraft. The filled fuel is under pressure that induces out-of-plane stresses on all the wing’s structural members around the fuel tank’s boundary. The wing is designed using carbon fiber reinforced plastic (CFRP) with the co-curing of the bottom skin and its stringers along with front and rear spars. A suitable number of cut-outs are made on the top and bottom sides of the IS ribs for the stringer to pass through them. The global finite element analysis showed that the stringer cut-out regions in the IS ribs are highly stressed, as they are subjected to out-of-plane load due to internal fuel pressure, which induces transverse tensile stress in the cut-out region. Resin is the weakest material in the composite laminate, but it has to resist the high-stress component due to out-of-plane loads. The top and bottom flanges of the ‘C’ section IS ribs geometrically form an L angle section with a stringer cut-out made in it. At the L angle location, the IS ribs are critical from both the strength and stiffness point of view. Strength and stiffness at the interface of the fastener joint are essential factors in maintaining the composite wing’s overall structural integrity under internal fuel pressure. The authors adopted the same standalone finite element (FE) modelling approach developed for the Bond Energy Method and carried out appropriate analytical studies to ascertain the safety and structural integrity of the composite wing under internal fuel pressure. This article presents the analytical study carried out on the standalone FE model of an L angle with the stringer cut-out introduced at its appropriate position. The static stress analysis is carried out by enforcing the global displacement on this standalone FE model and extracting three force components in CBAR beam elements representing the resin property in the transverse direction of the composite laminate. The stresses are calculated outside the analysis deck using Excel® spreadsheets. The maximum Normal Stress (σn) in the L angle flange is found to be 13.25 MPa, which is well within the resin or transverse tensile strength of the composite laminate 53 MPa, and the shear stress is found to be 11.10 MPa, which is less than the resin shear strength property of 23 MPa. Hence the study showed that the L angle is safe for carrying internal fuel pressure of 12.5 PSI. The study also showed that the strain and displacement values distribution in the standalone FE model are comparable with that of the global wing box model within the range of (8-10) % and (4-5) % difference, respectively.

Determination of overall factor for the application of the General Method based on plastic and strain‐limited N‐M interaction for composite compression members

AbstractThe design of composite compression members can be realized based on the General Method in accordance with Eurocode 4. This method represents a more sophisticated design based on advanced numerical analysis. It may be applied for any type of composite column. The General Method contains two steps; the first step is defined as the global nonlinear finite element analysis (GMNIA) accounting for all geometrical and physical nonlinear effect and considering geometrical and structural imperfection; while the second step represents the analysis on the cross‐section level, by N‐M interaction diagram to determine the overall safety factor γ0. EN1994‐1‐1 requires the N‐M‐Interaction based on the plastic resistance. However, often the General Method is provided for those section, where the plastic resistance is not representing the real behavior. Therefore, the question is discussed why the N‐M‐Interaction should not be developed based on the strain‐limited resistance considering the approaches for concrete stress‐strain curve in accordance with EN 1992‐1‐1 and for structural steel in accordance with prEN1993‐1‐14.In this paper, the application of general method is investigated for different type of composite columns with N‐M interaction diagram evaluated with plastic and strain‐limited resistance. Results are compared and discussed in terms of overall safety factor γ0. Conclusions are provided for the further development of the General Method in Eurocode 4.

Numerical and Experimental Evaluation of a CFRP Fatigue Strengthening for Stringer-Floor Beam Connections in a 19th Century Riveted Railway Bridge

A local and global finite element analysis of the stringer-floor beam connection of a 19th century riveted railway bridge in Spain made of puddle iron were performed to obtain the maximum principal strains in the riveted connecting angles corresponding to bending moments from train loading on the bridge. Due to the anisotropic nature of puddle iron, the connecting angles were modelled using Hill anisotropic plasticity potential and a parametric study in the local FE model of the connection was performed. A laboratory specimen fabricated with original stringers dismantled from the railway bridge was tested to calibrate the numerical models, so the yield stress ratio that best fitted experimental results was obtained. Based on the method of constant fatigue-life diagram and modified Goodman fatigue failure criterion, it was detected that the connecting angles were prone to fatigue crack initiation, as the combination of mean stress and alternating stress amplitude at the toe of the angle fillet remained outside the infinite fatigue-life region. An innovative strengthening system based on adhesively-bonded carbon-fiber reinforced polymer (CFRP) angles was designed to prevent fatigue crack initiation in the connecting angles of the stringer-floor beam connection. Different CFRP laminate layouts were numerically evaluated and a proper configuration was obtained that reduced both the mean stress and the alternating stress amplitude in the connecting angle to shift from finite fatigue-life region to infinite fatigue-life region in the constant fatigue-life diagram. To validate the effectiveness of the proposed CFRP strengthening method, its application on a second laboratory specimen fabricated with original stringers was evaluated experimentally and compared with numerical results. The research study conducted showed that the use of adhesively-bonded CFRP angles was an effective strengthening system in reducing the stress level in the fillet region of the puddle iron connecting angles (where fatigue cracks are prone to initiate) and consequently could increase fatigue life of the stringer-floor beam connection.

Feasible Stator/Rotor Pole Combinations of Variable Flux Reluctance Machines With Second Harmonic Current Injection Method

The second harmonic current injection method is recently developed to enhance the torque density of variable flux reluctance machines (VFRMs). This article investigates the feasibility and effectiveness of the second harmonic current injection method in VFRMs with different stator/rotor pole combinations. By using magnetic gearing effect, it is found that the second harmonic injection method can only be applied to the 6 j /(6 i ± 2) j ( i , j = 0, 1, 2…) stator/rotor pole VFRMs for the torque density enhancement. For the other stator/rotor pole combinations, the second harmonic current has no contribution in average torque production but only leads to excessive torque ripple. Further, by using global optimization method and finite element analysis, 6/8 stator/rotor pole combination is proved to be the optimal stator/rotor pole combination for 6-stator-slots VFRMs with the second harmonic current injection method, being able to exceed the maximal torque density of conventional VFRMs by 12% at a fixed copper loss.

Verification of Submodeling for the Finite Element Analysis of Stress Concentrations

Abstract Submodeling enables finite element engineers to focus analysis on the subregion containing the stress concentrator of interest with consequent computational savings. Such benefits are only really gained if the boundary conditions on the edges of the subregion that are drawn from an initial global finite element analysis (FEA) are verified to have been captured sufficiently accurately. Here, we offer a two-pronged approach aimed at realizing such solution verification. The first element of this approach is an improved means of assessing the error induced by submodel boundary conditions. The second element is a systematic sizing of the submodel region so that boundary-condition errors become acceptable. The resulting submodel procedure is demonstrated on a series of two-dimensional (2D) configurations with significant stress concentrations: four test problems and one application. For the test problems, the assessment means are uniformly successful in determining when submodel boundary conditions are accurate and when they are not. When, at first, they are not, the sizing approach is also consistently successful in enlarging submodel regions until submodel boundary conditions do become sufficiently accurate.

Analysis of the Fatigue Crack Evolution of Corrugated Web Girders

Based on linear elastic fracture mechanics (LEFM), the fatigue crack evolution process and behavior of corrugated web girders were studied. The global finite element analysis (FEA) model of corrugated web girders was first developed and the equivalent structural stress method was used to reveal the dangerous locations along the weld under the bending load. The weld toe between the tension flange and the web weld, which is near the intersection of the inclined fold and the parallel fold, was determined as the fatigue crack easy-initiating location. Then a small region containing the crack-prone site was extracted as the sub-model for a crack propagating simulation. A semi-circle initial crack with 0.1 mm radius was inserted at the crack easy-initiating location. The stress intensity factors (SIFs; KI, KII, and KIII) of some discrete points along the crack front were calculated by the M-integral method. Based on Nasgro law, the geometry of the new crack front with a given extension length was obtained. Finally, the complete evolution process of the crack propagation was simulated. Results showed that the dominant crack propagating mode is open type (Mode I) and KI is the most important propagating driving force. According to the crack front shape evolution, the whole propagating process was divided into 6 stages. An obvious kink of the crack was found in stage 1, which covered only a very short time. The stages 3, 4 and 5 accounted for the majority of life, among which the stage 3 accounted for as high as 81% of the total life. Therefore, the cycles of the weld toe crack propagating from 0.1 mm to the thickness of the flange can be defined as the prediction life of this kind of structures.

Fatigue Strength Assessment of Trimaran Cross-Deck Structure Based on Spectral and Simplified Fatigue Method

In order to investigate the fatigue behaviour of trimaran cross-deck structural details, the spectral and simplified fatigue analysis approaches are proposed. In spectral fatigue approach, three-dimensional (3D) linear potential flow theory and global FE analysis are used for wave loads and stress transfer functions calculation; the stochastic spectral fatigue analysis is carried out considering the weighted wave headings factors. In simplified fatigue approach, based on the direct calculation procedure of LR rules, the evaluation of simplified fatigue loads and loading conditions are presented, and the stress ranges are obtained by global finite element (FE) analysis. Then the fatigue lives of a few hot spots are computed to demonstrate the application of the proposed method. The result shows that the method given in this paper has a good applicability. This study offers methodology for the fatigue analysis of trimaran cross-deck structure, which may be regarded as helpful references for structural design of these types of ships.

Numerical and Experimental Study on Trimaran Cross-Deck Structure’s Fatigue Characteristics Based on the Spectral Fatigue Method

Spectral fatigue analysis is performed on the trimaran cross-deck structural detail. A global finite element (FE) model with local fine meshes at hot spot locations is built for the calculation of structural response. Three-dimensional linear potential flow theory and global FE analysis are used for wave load and stress transfer function calculations. Then, the hot spots’ fatigue damage is calculated, considering various factors influencing spectral analysis, which includes cycle counting correction factors, wave scatter diagram, and the distribution coefficient of wave headings. A full-scale model fatigue test is used to investigate the fatigue behavior of the cross-deck structural detail. Using the fatigue test data, the curve characteristic of the cross-deck structural detail is produced. Finally, the trimaran cross-deck structural detail’s fatigue characteristics are summarized and the effects of the factors influencing spectral fatigue analysis are reported.

Mechanical analysis of the European DEMO central solenoid pre-load structure and coils

The pre-conceptual design of the European DEMO fusion tokamak is currently being developed under the coordination of the EUROfusion Consortium. This paper reports the mechanical analysis of the Central Solenoid (CS), which comes right after the phase of definition of the winding pack proposed by the CEA. The first step consists in a draft-sizing of the tie-plates pre-compression structure. Then a global Finite element Analysis (FEA) of the CS, considering a sector-symmetry model and smeared winding pack properties is built-up in order to check this pre-load structure, and to identify the most critical loading scenario for the conductor. Finally, an axisymmetric local FEA of the CS winding pack is performed in order to study in detail the stress level in the conductor jacket and in the insulations layers. This pre-conceptual analysis procedure allowed to assess the acceptability of stress for the CEA winding pack proposal, and will be operated during the future magnetic system design iterations.

Stress analysis of Al2014-T6 riveted splice joint fuselage structure using finite element analysis

This work describes about the stress of a splice joint in a medium transport aircraft. Typical splice joint panel forming part of a fuselage, consisting of skin plates are riveted is considered for the study. The calculations have been performed by considering the cabin pressurization load on the panel. Aluminium alloy 2014 - T6 material considered for structural elements of the panel. Two dimensional finite element analyses have been carried out on the splice joint panel. The distribution of fasteners loads and local stress field at rivet locations read using element analysis. The global finite element analysis linear static analysis first segment of typical fuselage will be carried out. The Global finite element analysis results will be benchmark for comparing the result from the splice joint panel analysis. Repeated finite element analysis will be carried out to get the response of the parent structure fuselage find location the maximum stress location will be identified from the analysis.

A semi‐analytical curved element for linear elasticity based on the scaled boundary finite element method

SummaryThis work introduces a semi‐analytical formulation for the simulation and modeling of curved structures based on the scaled boundary finite element method (SBFEM). This approach adapts the fundamental idea of the SBFEM concept to scale a boundary to describe a geometry. Until now, scaling in SBFEM has exclusively been performed along a straight coordinate that enlarges, shrinks, or shifts a given boundary. In this novel approach, scaling is based on a polar or cylindrical coordinate system such that a boundary is shifted along a curved scaling direction. The derived formulations are used to compute the static and dynamic stiffness matrices of homogeneous curved structures. The resulting elements can be coupled to general SBFEM or FEM domains. For elastodynamic problems, computations are performed in the frequency domain. Results of this work are validated using the global matrix method and standard finite element analysis. Copyright © 2016 John Wiley & Sons, Ltd.

Bending stress analysis of flexible touch panel

Flexible touch panel is required to undertake extensive bending operations during service; thus, bending testing of flexible touch panel for mechanical behaviors and even reliability is crucial for realizing the technology. This study aims at exploring the bending behaviors of a flexible touch panel under a four-point bending test using finite element analysis. The touch panel is a laminated structure composed of seven thin film layers, namely a polyester layer, an adhesive layer, a polyimide layer, two organic layers and two indium tin oxide conductive layers. Because the touch panel structure is symmetry, only a quarter of the flexible touch panel is established in the analysis model. The mechanical properties of the materials are first obtained using both nanoindentation and uniaxial tensile/compressive testing. Furthermore, a modeling technique based on global and local finite element analysis is applied to evaluate the bending stress at a specified radius of curvature. Special emphasis of the calculation is put on the critical region with respect to stress, which is identified as the most susceptible cracking site.

Mechanical characterization of polyamide cellular structures fabricated using selective laser sintering technologies

The significantly growing use of Additive Manufacturing (AM) enables the fabrication of innovative parts, characterized by lightness and good mechanical properties. The biomedical field takes great advantage of these capabilities: in particular, the ability of producing porous or lattice structure-based parts allows to obtain prostheses with human bone like stiffness, with a positive influence in patient’s lifestyle. The knowledge of the mechanical behavior of materials used in AM and producible geometries is an essential requirement to profit and improve this characteristic: in particular, recent studies focus on the correlation between strength parameters and relative porosity of the part. In a previous work a set of tensile tests have been performed on different types of specimens, reproducing a set of corresponding emptying strategies, with different resulting porosity rates, and a linear predictive model has been proposed. Aim of this work is to integrate the already acquired data, providing an interpretation on previous results by numerical simulations: the influence of porosity rate on mechanical properties was investigated by performing both global and local Finite Element Analyses, finding out an explanation on inverse proportionality between material strength properties and porosity rate. The methodology proved to be a profitable way in the optimization of lattice structures for Additive Manufacturing.

Probabilistic High-Cycle Fretting Fatigue Assessment of Gas Turbine Engine Components

High-cycle fatigue (HCF) is arguably one of the costliest sources of in-service damage in military aircraft engines. HCF of turbine blades and disks can pose a significant engine risk because fatigue failure can result from resonant vibratory stresses sustained over a relatively short time. A common approach to mitigate HCF risk is to avoid dangerous resonant vibration modes (first bending and torsion modes, etc.) and instabilities (flutter and rotating stall) in the operating range. However, it might be impossible to avoid all the resonance for all flight conditions. In this paper, a methodology is presented to assess the influences of HCF loading on the fracture risk of gas turbine engine components subjected to fretting fatigue. The methodology is based on an integration of a global finite element analysis of the disk-blade assembly, numerical solution of the singular integral equations using the CAPRI (Contact Analysis for Profiles of Random Indenters) and Worst Case Fret methods, and risk assessment using the DARWIN (Design Assessment of Reliability with Inspection) probabilistic fracture mechanics code. The methodology is illustrated for an actual military engine disk under real life loading conditions.

Fatigue analysis of ship structures with hinged deck design by finite element method. A case study: Fatigue analysis of the primary supporting members of 4900 PCTC

In the design phase of a 4900 PCTC (Pure Car/Truck Carrier) with hinged deck design, fatigue behavior and fatigue life is investigated for various structural details. The global finite element analysis shows high local stress concentrations at the connections of various details. In this article, the detailed analysis of the investigation of fatigue analysis and fatigue life of the connection between the vertical side web and the main deck (deck 5) of the entire vessel is performed according to Det Norske Veritas (DNV) Rules. In order to achieve more accurate results, three types of finite element analysis (global finite element model, fine-mesh finite element model and stress concentration models) are hierarchically used. The investigation for finite element analysis for fatigue analysis is based on the approach using hot spot stresses.

Fragility Functions for Bridges in Liquefaction-Induced Lateral Spreads

Fragility functions are generated for bridges in liquefied and laterally spreading ground using equivalent static global nonlinear finite element analyses. Bridges are classified based on structural configurations and vintage. Probability density functions are assigned to both structural and geotechnical properties of bridges. Nonlinear equivalent static analyses are conducted with inputs sampled randomly using the Monte Carlo simulation method. Cumulative distribution functions are fitted to the simulated data, and define the probability of exceeding various engineering demand parameters (pier column curvature ductility, pile cap displacement, abutment displacement, etc.) conditioned on the maximum free-field lateral spreading ground surface displacement. Correlations among EDPs are presented to facilitate risk assessment based on a vector of EDPs. The derived fragility functions, combined with seismic hazard analysis, liquefaction potential, and lateral spreading estimation, are useful in the context of performance-based earthquake engineering and risk assessment of current bridge inventory in California.

Frequency domain structural synthesis for quasi-static crack propagation: Global–local analysis

Computational efficiencies of a general formulation for frequency domain structural synthesis are exploited in its development as a flexible framework for global-local static and dynamic analysis. Functioning either as a substructure coupling or structural modification procedure, a quasi-static crack propagation analysis is performed demonstrating both computational efficiency and ease in coupling a local analysis to a global finite element analysis. The two alternative algorithms, based on substructure coupling and structural modification, are compared, and are also compared to a traditional finite element solution, and experimental data. The synthesis-based algorithms are shown to significantly outperform the traditional finite element solution.

Residual stress profiles for mitigating fretting fatigue in gas turbine engine disks

The driving force for fretting fatigue in engine disks is the contact stresses generated by fretting of the blade and the disk surfaces in the attachment region. This paper examines the use of different residual compressive stress profiles to counteract the undesirable effects of contact stresses and to mitigate fretting fatigue. A global finite-element analysis of the disk blade assembly is first performed. The contact pressure and shear traction at the attachment region are extracted from the FEM results and used to compute the contact stress distribution. The contact stresses are then combined with the residual stresses and the bulk stresses. The overall stress distribution is then utilized in a probabilistic crack growth model to predict the risk of disk failure for a military engine under simulated loading conditions. The results are used to identify the minimum residual stress profile for mitigating fretting fatigue in engine disks.

Comparison of an inhomogeneous orthotropic and isotropic material models used for FE analyses

Finite element (FE) analysis has been widely used to study the behaviour of bone or implants in many clinical applications. One of the main factors in analyses is the realistic behaviour of the bone model, because the behaviour of the bone is strongly dependent on a realistic bone material property assignment. The objective of this study was to compare isotropic and orthotropic inhomogeneous material models used for FE analyses of the “global” proximal femur and “small” specimens of the bone (cancellous and cortical). Our hypothesis was that realistic material property assignment (orthotropy) is very important for the FE analyses of small bone specimens, whereas in global FE analyses of the proximal femur, this assignment can be omitted, if the inhomogeneous material model was used. The three-dimensional geometry of the “global” proximal femur was reconstructed using CT scans of a cadaveric femur. This model was implemented into an FE simulation tool and various bone material properties, dependant on bone density, were assigned to each element in the models. The “small” specimens of cortical and cancellous bone were created in the same way as the model of the proximal femur. The results obtained from FE analyses support our above described hypothesis.

Damage Initiation and Propagation in Voided Joints: Modeling and Experiment

This study examines damage initiation and propagation in solder joints with voids, under thermomechanical cyclic loading. An accelerated thermal cycling test is conducted on printed wiring assemblies (PWAs) containing 256 input/output (I/O) plastic ball grid arrays (PBGAs) with voided solder joints. Destructive and nondestructive failure analyses of the solder balls are used to detect the presence of voids and to relate the extent of damage propagation to the number of thermal cycles. Particular cases of voided and damaged joints are selected from these tests, to guide the development of a strategy for modeling damage propagation, using a three dimensional global-local finite element analysis (FEA). The displacement results of the global FEA at the top and bottom of the selected solder balls are used as the boundary conditions in a local FEA model, which focuses on the details of damage initiation and propagation in the individual solder ball. The local model is error seeded with voids based on cases selected in experiment. The damage propagation rate is monitored for all the cases. The technique used to quantify cyclic creep-fatigue damage is a continuum model based on energy partitioning. A method of successive initiation is used to model the growth and propagation of damage in the selected case studies. The modeling approach is qualitatively verified using the results of the accelerated thermal cycling test. The verified modeling technique described above is then used for parametric study of the durability of voided solder balls in a ChipArray Thin Core BGA with 132 I/O (CTBGA132) assemblies, under thermal cycling. The critical solder ball in the package is selected and is error seeded with voids with different sizes and various distances from damage initiation site. The results show that voids in general are not detrimental to thermal cycling durability of the CTBGA132 assembly, except when a large portion of the damage propagation path is covered with voids. Small voids can arrest the damage propagation, but generally do not provide a significant increase in durability because the damage zone deflects around the void and also continues to propagate from other critical regions in the solder ball.