High-fidelity Finite Element Model Research Articles

High-Fidelity Finite Element Modeling and Analysis of Adaptive Gas Turbine Stator-Rotor Flow Interaction at Off-Design Conditions

ABSTRACTThe objective of this work is to computationally investigate the impact of an incidence-tolerant rotor blade concept on gas turbine engine performance under off-design conditions. When a gas turbine operates at an off-design condition such as hover flight or takeoff, large-scale flow separation can occur around turbine blades, which causes performance degradation, excessive noise, and critical loss of operability. To alleviate this shortcoming, a novel concept which articulates the rotating turbine blades simultaneous with the stator vanes is explored. We use a finite-element-based moving-domain computational fluid dynamics (CFD) framework to model a single high-pressure turbine stage. The rotor speeds investigated range from 100% down to 50% of the designed condition of 44,700 rpm. This study explores the limits of rotor blade articulation angles and determines the maximal performance benefits in terms of turbine output power and adiabatic efficiency. The results show articulating rotor blades can achieve an efficiency gain of 10% at off-design conditions thereby providing critical leap-ahead design capabilities for the U.S. Army Future Vertical Lift (FVL) program.

Structural design and optimization of a novel shimmy damper for nose landing gears

Passive vibration suppression devices known as shimmy dampers are vital in maintaining stability and safety of certain landing gears. Yet, systematic design, performance analysis, and optimization of such devices are rarely discussed in the literature. This paper presents structural design optimization of a novel shimmy damper for nose landing gears. This new design is a multifunctional mechanism integrating the shimmy damper into the torque link system of the nose landing gear. It features symmetric load distribution, and it can be tailored to existing nose landing gears. Here, the damper design concept is developed in the structural sense and optimized for the nose landing gear of a Piper Cheyenne aircraft. Dynamic loads from a representative shimmy scenario are employed in the analysis and design procedures. Utilizing equivalent static load method, topology optimization with transient loads is performed to obtain optimal material distribution satisfying the objective function and constraints. Flexible multibody dynamics analysis based on a high-fidelity finite element model is utilized in the analysis of design candidates and for validating the final design. To ensure adequate strength under the dynamic torque loads and to offer sufficient damping needed for stabilizing the nose landing gear, a three-piece torque link mechanism emerged through multiple design iterations guided by the topology optimization. Using numerical simulations, the final design is shown to satisfy the strength requirement while providing sufficient damper stroke. The results from the present study signal a vast potential in improving shimmy mitigation strategies by eliminating the need for costly redesigns of landing gears susceptible to shimmy.

Seismic control of a single-tower extradosed railway bridge using the E-Shaped steel damping bearing

An extradosed bridge is a hybrid structure that combines the main elements of the classic cable-stayed bridge and continuous girder bridge. Almost all extradosed bridges have the main girder transversely restrained at the pier locations, which may cause extremely large transverse shear force and bending moment on bridge substructures during strong earthquakes. This study thoroughly investigates the efficacy of using the E-shaped steel damping bearing (ESSDB) for lateral seismic control of a prototype extradosed railway bridge. Firstly, high-fidelity finite element models are developed for nonlinear time history analyses for both the ESSDB isolated bridge and two conventional non-isolated configurations (i.e., rigid frame system and continuous system). Parametric sensitivity study is then carried out, which indicates a tendency of decreasing transverse deck displacements and increasing pier base transverse forces with the increase of the ESSDB yield force. The optimal ESSDB yield force is thus selected mainly based on a tradeoff between maximizing the structural force reduction and limiting the deformation demand of the ESSDB. Finally, through detailed comparisons of bridge seismic responses with the two non-isolated cases, it shows that the ESSDB can significantly reduce the transverse seismic force demands in bridge substructures, at a cost of a moderate increase in superstructure displacements and formation of permanent inelastic deformation in the ESSDB. It is concluded that the ESSDB can provide a simple and efficient solution to the transverse seismic control of the extradosed railway bridge.

A Bayesian probabilistic approach for damage identification in plate structures using responses at vibration nodes

Structural health monitoring for plate structures is important since they are essential structural components in many applications. One interesting topic for structural health monitoring methods is to achieve accurate damage detection with a small number of sensors and without the requirement of a high-fidelity finite element model. Traditional damage detection techniques for plates need many sensors to be distributed on the surface of the plate. This paper adopts dynamic responses at a few vibration nodal points combined with a Bayesian probabilistic approach for damage identification in plate structures. Vibrational amplitudes at nodal points, also referred as node displacement or NODIS, have the potential to achieve real-time damage assessment with a relatively small number of sensors. Thus, they can serve as efficient structural damage indicators. Despite these advantages, this method has not been applied to plate-type structures. This paper proposes a vibration-based SHM method for plates that is suitable for real-time monitoring, requires a small number of industrial sensors, does not rely on a high-fidelity FE model and can be applied for damage assessment of location and severity. In the Bayesian framework, an efficient perturbation-based surrogate model is derived for plate structures to replace the expensive FE model. The accuracy of the perturbation-based surrogate model is investigated and compared with FE results. Then, this paper evaluates the performance of the NODIS-based Bayesian framework with the perturbation method by comparing it with FE results. At last, the proposed method is applied to a carbon fiber reinforced polymer sandwich structure with different grinding depths.

Bayesian Model-Updating Using Features of Modal Data: Application to the Metsovo Bridge

A Bayesian framework is presented for finite element model-updating using experimental modal data. A novel likelihood formulation is proposed regarding the inclusion of the mode shapes, based on a probabilistic treatment of the MAC value between the model predicted and experimental mode shapes. The framework is demonstrated by performing model-updating for the Metsovo bridge using a reduced high-fidelity finite element model. Experimental modal identification methods are used in order to extract the modal characteristics of the bridge from ambient acceleration time histories obtained from field measurements exploiting a network of reference and roving sensors. The Transitional Markov Chain Monte Carlo algorithm is used to perform the model updating by drawing samples from the posterior distribution of the model parameters. The proposed framework yields reasonable uncertainty bounds for the model parameters, insensitive to the redundant information contained in the measured data due to closely spaced sensors. In contrast, conventional Bayesian formulations which use probabilistic models to characterize the components of the discrepancy vector between the measured and model-predicted mode shapes result in unrealistically thin uncertainty bounds for the model parameters for a large number of sensors.

Efficient Virtual Tribomechadynamics by Means of Joint Modes for Detailed Investigation of Complex Local Stick and Slip Behavior Inside a Joint

Abstract In the last years, the numerical and experimental research effort on joint nonlinearities and tribomechadynamics has increased. Thereby, local sticking and slipping effects as well as the influence of friction caused damping on the global dynamics are of interest. Conventional computational approaches like model order reduction techniques or the finite element method lead either to insufficient result quality or a high computational burden. For the efficient numerical consideration of jointed structures in combination with model order reduction, joint modes based on trial vector derivatives have been presented. These joint modes enable accurate computation of local nonlinear contact and friction forces together with efficient time integration even for high fidelity finite element models. This article describes the application of joint modes for efficient virtual tribomechadynamics. Therefore, a generic structure including a bolted joint is used. It is investigated if these joint modes reproduce local friction stress, and sticking/slipping areas comparable to the nonlinear finite element method within reasonable computational times. Moreover, global damping effects are studied at different preload levels and related to local sticking/slipping behavior. The numerical studies confirm that joint modes lead to accurate results with low computation effort and hence allow an efficient and detailed virtual investigation of complex joints. In addition, this publication shows that the consideration of tangential stiffness for the computation of joint modes remarkably increases the local result quality.

Contact Stress and Linearized Modal Predictions of As-Built Preloaded Assembly

Abstract The member stiffness and pressure distribution in a bolted joint is significantly influenced by the contact area of the mechanical interface under a prescribed preload force. This research explores the influence of as-built surface profiles for nominally flat interfaces of a C-Beam assembly with two well-defined contact regions. A high-fidelity finite element model is created such that the model uncertainty is minimized by updating and calibrating the piece parts prior to the preload assembly procedure. The model is then assembled and preloaded to evaluate the contact stresses and contact area for both nominally flat and perturbed non-flat surfaces based on three-dimensional surface topography measurements. The predicted pressures are validated with digitized pressure-sensitive film measurements. The high-fidelity modeling reveals how the compliance and thickness of the pressure-sensitive film alter the measured pressures, leading to incorrect evaluations of the stresses and contact area in the joint. The resulting low-level dynamic behavior of the preloaded assembly is shown to be sensitive to the true contact area by linearizing the nonlinear finite element model about the preloaded equilibrium and performing a computational modal analysis. The resonant frequencies are validated with experimental measurements to demonstrate the effect of the contact area on the modal characteristics of the bolted assembly. Vibration modes and loading patterns exhibit varying levels of sensitivity to the contact area in the joint, leading to an improved physical understanding of the influence of contact mechanics on the low-level linear vibration modes of jointed assemblies.

Simple equations for predicting the rotational ductility of fiber-reinforced-polymer strengthened reinforced concrete joints

Achieving a certain limit of rotational ductility in retrofitted reinforced concrete (R.C) joints is very important in the design of earthquake-resistant structures. Strengthening of R.C joints using wraps of fiber-reinforced polymers (FRP) is a common attractive technique and has an effect on the ductility of such joints. This study focuses on developing simple conceptual equations to predict the ductility of exterior reinforced concrete (R.C) beam-column joints as a function of the applied FRP. The equations are derived based on statistical regression through parametric study using results from a high-fidelity finite element model created using ABAQUS. The validated model includes material and geometric nonlinearities, in addition to the use of realistic nonlinear contact behavior between FRP and concrete. The proposed simple equations can be used as an initial conceptual design step for checking the adequacy of R.C beam-column joints in seismic design of R.C buildings. The proposed equations consider FRP, relative column-to-beam inertia, and transverse reinforcement in the beam and joint as the main parameter. This study defines the types of failure based on the ductility, and it develops the equations for ductile and brittle failures for both CFRP-strengthened joints and non-strengthened joints. This research confirms quantitatively the effectiveness of using CFRP to increase the ductility in most cases of the R.C beam-column joints. However, contribution of the CFRP is limited for some cases.

Formation of Plastic Creases in Thin Polyimide Films

Abstract We present a combined experimental and analytical approach to study the formation of creases in tightly folded Kapton polyimide films. In the experiments, we have developed a robust procedure to create creases with repeatable residual fold angle by compressing initially bent coupons. We then use it to explore the influence of different control parameters, such as the force applied, and the time the film is being pressed. The experimental results are compared with a simplified one-dimensional elastica model, as well as a high fidelity finite element model; both models take into account the elasto-plastic behavior of the film. The models are able to predict the force required to create the crease, as well as the trend in the residual angle of the fold once the force is removed. We non-dimensionalize our results to rationalize the effect of plasticity, and we find robust scalings that extend our findings to other geometries and material properties.

A variational Bayesian neural network for structural health monitoring and cost-informed decision-making in miter gates

Many physics-based and surrogate models used in structural health monitoring are affected by different sources of uncertainty such as model approximations and simplified assumptions. Optimal structural health monitoring and prognostics are only possible with uncertainty quantification that leads to an informed course of action. In this article, a Bayesian neural network using variational inference is applied to learn a damage feature from a high-fidelity finite element model. Bayesian neural networks can learn from small and noisy data sets and are more robust to overfitting than artificial neural networks, which make it very suitable for applications such as structural health monitoring. Also, uncertainty estimates obtained from a trained Bayesian neural network model are used to build a cost-informed decision-making process. To demonstrate the applicability of Bayesian neural networks, an example of this approach applied to miter gates is presented. In this example, a degradation model based on real inspection data is used to simulate the damage evolution.

Mode shape transformation for model error localization with modal strain energy

A modeling error location method based on modal strain energy is presented in this paper. Errors in the design model with shell elements are located by an error indicator which is based on changes between the equivalent modal strain energy and the modal strain energy of the design model. The equivalent modal strain energy is defined as a quadratic form using the stiffness matrix of the design model and the mode shape of the reference coming from the sophisticated and high fidelity finite-element model, called the supermodel, or the full-field measurement. The major obstacle to obtain the equivalent modal strain energy is how to match the mode shapes of a solid element and those of a shell element since each node of the solid element contains only three translation degrees of freedom (dofs) while each node of the shell element has six dofs, including three translation and three rotation components. In order to solve this problem, a mode shape transformation method from the solid element to the shell element is proposed using the shape functions or linear approximation. Using this approach, the errors in the design model can be determined and the updating parameters can be selected so that the updated model has physical meaning and can represent the dynamic characteristics of the real structure. The simulation of a simple plate is used initially to illustrate the effectiveness of the proposed method. Then, a rotor test rig casing is taken as an example for further investigation. A comparison of the updating parameters selected by the proposed method and the traditional sensitivity analysis technique is then undertaken. It is verified that the updating parameters selected based on error location have physical sense and represent the true errors in the design model through the updating results. The advantage of this technique is that only detailed mode shapes from the reference is required. The approach shows potential for further industrial engineering applications.

Reduced order modeling of hysteretic structural response and applications to seismic risk assessment

A reduced order modeling approach is presented to alleviate the computational burden associated with using high-fidelity finite element models (FEMs) to describe hysteretic structural response for earthquake engineering time-history analysis. The reduced order model (ROM) is developed using data from the original high-fidelity FEM. Static condensation is first used to obtain the condensed stiffness matrix and the linear equations of motion for the dynamic degrees of freedom (DoFs). The restoring forces prescribed by the linear stiffness matrix are then substituted with hysteretic ones by replacing the linear springs connecting each of the DoFs with hysteretic ones. Different hysteretic models are considered, including peak-oriented, Masing and Bouc-Wen type of hysteresis, whereas more complex relationships obtained by combining multiple simpler hysteretic models are also discussed. The hysteretic spring parameters are calibrated by comparing the reduced order model time-history response to the time-history response of the initial FEM for a range of different excitations. The characteristics for each of the considered springs are separately selected and an efficient solution of the associated calibration problem is facilitated through a sequential, hierarchical approach. The excitations utilized for the reduced order model calibration are carefully selected so that nonlinear characteristics of the FEM are appropriately excited to support the tuning of all the important hysteretic spring features. The accuracy and the computational savings of the calibrated reduced order model are evaluated for risk assessment applications, separately examining different levels of intensity. Comparison extends to three structures, with the high-fidelity FEMs developed in OpenSees.

Structure behavior of reinforced concrete beam-slab assemblies subjected to perimeter middle column removal scenario

The perimeter columns of framed structures are more vulnerable to terrorist attacks due to accessibility. The beams and slabs above the damaged columns are the primary structural members to redistribute gravity load to avoid progressive collapse. Therefore, to investigate the structural behavior of reinforced concrete (RC) beam-slab assemblies against progressive collapse introduced by a perimeter middle column removal scenario, an experimental program and numerical analyses were carried out in this paper. Three 3/10 scaled specimens, each of which consisted of two square slab panels and seven beams, were tested with equivalent uniformly distributed loading (UDL) achieved by a 12-loading apparatus. High-fidelity finite element models were used to conduct parametric studies after the validation, recheck the validity of the loading apparatus and highlight the effect of loading positions. The concerned parameters include the geometric parameters of beams caused by different seismic design intensity, the slab thickness and reinforcement ratio, and the type of reinforcing bars (i.e. deformed and plain). The results indicate that progressive collapse is resisted by compressive arch action and flexural action of the beams and slabs connecting the stub above the removed column at small deformation stage, whereas catenary action of longitudinal beams and tensile membrane action of slabs are more prevailing at large deformation stage. Moreover, higher seismic design intensity results in a larger resistance at small deformation stage, and plain bars cause larger deformation capacity. Finally, loading positions approaching the stub above the removed column tends to show smaller structural resistance.

A digital twin feasibility study (Part II): Non-deterministic predictions of fatigue life using in-situ diagnostics and prognostics

The Digital Twin (DT) concept has the potential to revolutionize the way systems and their components are designed, managed, maintained, and operated across a vast number of fields from engineering to healthcare. The focus of this work is the implementation of DT for the health management of fatigue critical structures. This paper is the second part of a two-part series. The first of the series demonstrated the use of multi-scale, initiation-to-failure crack growth modeling to form non-deterministic predictions of fatigue life. In this second part, a general method for reducing uncertainty in fatigue life predictions is presented that couples in-situ diagnostics and prognostics in a probabilistic framework. Monte Carlo methods and high-fidelity finite element models are used to (i) generate probabilistic estimates of crack state throughout the life of a geometrically-complex test specimen and (ii) predict fatigue life with decreasing uncertainty as more of these diagnoses are obtained. The ability to predict accurately and in the presence of uncertainty is demonstrated, suggesting that the proposed DT method is feasible for fatigue life prognosis and should be pursued further with a focus on increasing application realism.

Application of extremum response surface method-based improved substructure component modal synthesis in mistuned turbine bladed disk

To improve the computational efficiency of vibration characteristics and reliability analysis for a detailed numerical model of the mistuned turbine bladed disk, a new methodology called extremum response surface method-based improved substructural component modal synthesis (ERSM-ISCMS) is proposed by combining the ISCMS and ERSM. First, the degrees of freedom of the detailed finite element model for the numerical mistuned turbine bladed disk are decreased by ISCMS, which is called as reduced-order model. Compared with high fidelity finite element model, the time saving ratio and the computational accuracy of the first 40 order frequencies are, respectively, 36.37% and 99.99%~99.86% obtained by ISCMS under the same working environment, which can satisfy the engineering requirements. Then, the ERSM is applied to analyze the dynamic probability of the maximum vibration response for the numerical mistuned turbine bladed disk. The investigation indicates that the computational efficiency of ERSM is higher 38.92% than that of traditional RSM in the same computer and the same reduced-order model. Thus, the ERSM-ISCMS is a more effective method to investigate dynamic probabilistic analysis of the mistuned turbine bladed disk, it benefits for the complex structures and develops the theory method for the mechanical reliability design.

Multi-fidelity analysis and uncertainty quantification of beam vibration using correction response surfaces

A multi-fidelity model for beam vibration is developed by coupling a low-fidelity Euler-Bernoulli beam finite element model with a high-fidelity Timoshenko beam finite element model. Natural frequencies are used as the response measure of the physical system. A second order response surface is created for the low-fidelity Euler-Bernoulli model using the face centered design. Correction response surfaces for multi-fidelity analysis are created by utilizing the high-fidelity finite element predictions and the low-fidelity finite element predictions. It is shown that the multi-fidelity model gives accurate results with high computational efficiency when compared to the high-fidelity finite element model.

YIELD OPTIMIZATION BASED ON ADAPTIVE NEWTON-MONTE CARLO AND POLYNOMIAL SURROGATES

In this paper we present an algorithm for yield estimation and optimization exploiting Hessian based optimization methods, an adaptive Monte Carlo (MC) strategy, polynomial surrogates and several error indicators. Yield estimation is used to quantify the impact of uncertainty in a manufacturing process. Since computational efficiency is one main issue in uncertainty quantification, we propose a hybrid method, where a large part of a MC sample is evaluated with a surrogate model, and only a small subset of the sample is re-evaluated with a high fidelity finite element model. In order to determine this critical fraction of the sample, an adjoint error indicator is used for both the surrogate error and the finite element error. For yield optimization we propose an adaptive Newton-MC method. We reduce computational effort and control the MC error by adaptively increasing the sample size. The proposed method minimizes the impact of uncertainty by optimizing the yield. It allows to control the finite element error, surrogate error and MC error. At the same time it is much more efficient than standard MC approaches combined with standard Newton algorithms.

Voltage Stress Modeling and Measurement for Random-Wound Machine Windings Driven by Inverters

This article studies the voltage stress in random-wound machine windings under steep-fronted voltage excitations from inverter drives. The voltage stress in this article includes overshooting voltages between any two conductors in the windings, and overshooting voltages between individual conductors and the stator ground wall. This stress may accelerate insulation degradation in inverter-fed machines compared to traditional AC machines. A high-fidelity finite element (FE) model was used to analyze individual conductors of random-wound windings and predict the voltage stress during switching transients. Experimental validation of the model was conducted using a purpose-made random-wound three-phase winding, designed to allow access to multiple coils and parallel strands. Firstly, the voltage propagation and the voltage stress for both single-phase and three-phase windings were simulated and tested under Pulse Width Modulation (PWM) voltage excitations without a rotor. Both voltage propagations among all the coils and individual turns were measured and compared with simulated results. The voltage stress on each part of the machine stator winding insulation system was tested and studied. Then, similar simulations and tests were conducted with a demagnetized rotor installed to study how the rotor saliency affects the voltage propagation and voltage stress. The measured winding voltage propagations for all scenarios generally agree with the FE simulated results.

Parametric investigation on the sealant behavior of tunnel segmental joints under water pressurization

Water leakage through segmental joints is a clutch issue for shield tunnels during both the construction and operation stages. However, very limited knowledge has been obtained in this area. To address this deficit, this paper investigates the sealant behavior of gasketed joints in shield tunnels using a high-fidelity finite element (FE) model. In the developed FE models, the material nonlinearity of the rubber-fabricated sealing gaskets is simulated by the Mooney-Rivlin hyperelastic constitutive model; the complex contact behaviors (e.g., amongst gaskets and segments and gaskets themselves) are properly captured. The model is capable of realistically reproducing detailed water-leakage behavior and computing the water-leakage pressure through application of the built-in fluid pressure penetration (FPP) module in ABAQUS. The FE model is validated against available experimental data of gasket-in-groove mechanical tests and joint waterproof tests. It is demonstrated that the modelling approach developed in the paper can replicate adequately the load-deformation response and reasonably predict water-leakage pressures. The validated FE model is then used to undertake a comprehensive set of parametric studies that elucidate the influence of nine key design parameters on the sealant performance of gasketed joints. It is found that controlling joint opening and/or rotation has a fundamental effect on ensuring satisfactory waterproof behavior, even if a relatively large joint offset exists. The sealant capacity of the joint is positively related to the hardness of the gasket, while it is not highly sensitive to the geometry of the groove and properties of contact behavior. The findings contribute to an improved understanding of sealant behavior of gasketed joints under water pressurization and permit the potential development of codified design frameworks.

Numerical investigation on load redistribution capacity of flat slab substructures to resist progressive collapse

To study the load redistribution capacity of reinforced concrete (RC) flat slab structures subjected to a middle column loss scenario, high fidelity finite element (FE) models were built using commercial software LS-DYNA. The numerical models were validated by experimental results. It is found that the continuous surface cap model (CSCM) with an erosion criterion considering both the maximum principal and shear strain could effectively predict the punching shear failure at slab-column connections. The validated FE models were employed to investigate the effect of boundary conditions, amount of integrity reinforcement, and slab thickness on the load redistribution capacity of flat slab structures. Furthermore, multi-story RC flat slab substructures were built to capture the load redistribution behavior of different floors. Parametric studies indicate that ignoring the constraints from surrounding slabs may underestimate the load redistribution capacity of the flat slab substructures. Therefore, it is suggested that in future numerical or experimental studies, rigid horizontal constraints should be applied at the slab edge of the substructure to well represent the constraints from surrounding slabs. In addition, it is also found that the amount of integrity reinforcement would significantly affect the post-punching performance of flat slab structures. It is suggested that the minimum integrity reinforcement ratio should be 0.63%.