Finite Element Method Predictions Research Articles

Correlation between Surface Integrity and Low Cycle Fatigue Life of Machined Inconel 718

This work aims to improve the surface integrity and LCF life of machined Inconel 718. The correlation between the LCF life of Inconel 718 and various states of machined surface integrity is explored. In this paper, the surface integrity of Inconel 718 specimens is enhanced by low-plasticity burnishing (LPB). The LCF life of specimens with different surface integrity is predicted using a micro–macro finite element method (FEM). Firstly, the LCF specimens with different surface integrity are machined by turning (turned specimen), polishing (matrix specimen), and LPB process (LPBed specimen). Secondly, the LCF experiment is carried out to reveal the effect of surface integrity on LCF life. Finally, the LCF micro–macro FEM model is proposed to predict the LCF of machined Inconel 718 specimens. The representative volume element (RVE) model is established based on the measured surface integrity and microstructure of Inconel 718 specimens. The effect of surface integrity on LCF life is transformed to equivalent load. The micro–macro FEM model combined with Tanaka–Mura dislocation crack initiation theory and extended finite element method (XFEM) is applied to predict the LCF life of the machined specimens. The study results show that the LCF life of LPBed specimens can be improved by 90.5% and 36.1% compared with that of turned specimens and polished matrix specimens, respectively. The errors between FEM prediction results and experimental results are 13.1%, 9.2%, and 12.2%, respectively. The proposed micro–macro FEM model could be utilized to predict the LCF life of Inconel 718 with different surface integrities, and to apply the LCF life prediction further in industry.

A Review of Finite Element Methods for Room Acoustics

Accurate predictions of the wave-dominated region of an acoustic field in a room can be generated using wave-based computational methods. One such method is the finite element method (FEM). With presently available computing power and advanced numerical techniques, it is possible to obtain FEM predictions of sound fields in rooms with complicated geometries and complex boundary conditions in realistic time frames. The FEM has been continuously developed since its inception and attempts to provide solutions in real time using finite element-based methods are beginning to appear in the literature; these developments are especially interesting for auralization and virtual acoustics applications. To support these efforts, and provide a resource for neophytes, the use of the FEM for room acoustics is reviewed in this article. A history is presented alongside examples of the method’s derivation, implementation, and solutions. The current challenges and state-of-the-art are also presented, and it is found that the most recent contributions to the field make use of one or a mixture of the following: the finite element-based discontinuous Galerkin method, extended reaction boundary conditions written in the frequency domain but solved in the time domain, and the solution of large-scale models using parallel processing and graphics processing units.

Subsurface damage detection via noncontact laser based surface level strain sensing smart skin with carbon nanotubes

Subsurface damage may remain undetected for long periods and accumulate until it leads to structural failure. Non-destructive measurements of surface strain can provide critical data for detecting early symptoms of hidden damage and helping to prevent serious consequences. This study investigates the capabilities of the developed method called strain-sensing smart skin (S4) to find internal damage. The S4 method uses single-wall carbon nanotubes in a thin polymer coating as optically interrogated fluorescent strain sensors to measure the strain map. A set of aluminum specimens with internal cavities representing damage were coated both with S4 films and with digital image correlation (DIC) speckle patterns. Maps of surface strain from the two methods were then compared (after the application of tensile stress to the specimens) to compute finite element method (FEM) strain maps. The strain maps obtained from S4 point-wise laser scanning agreed well with FEM predictions, while DIC strain maps were not very precise. S4 maps revealed strain map features with superior accuracy. It is concluded that S4 strain mapping can be used as a non-destructive evaluation method for detecting and visualizing subsurface damage in structural elements for structural health monitoring.

Analysis of Electric Transmission Failure Induced by Electrostatic Migration and Deposition of Abrasive Dusts in SADA

The electric transmission failure induced by the electrostatic migration and deposition of the friction-pair-produced abrasive dusts in the Solar Array Drive Assembly (SADA) is studied for the first time by integrating experiments and Finite Element Method (FEM) simulation. Particle morphology and composition are characterized by SEM and X-ray EDS, respectively. The size, distribution and average charge of the abrasive dusts are characterized by PSDA as well as the home-made particle charge measurement device. The results show that the dusts’ sizes are in the range of 10–40 μm, and the dusts are primarily composed of Ag and a small amount of Cu and S. Sliced abrasive dusts with an average diameter of 30 μm and a charge of 85 e are used for the FEM prediction of particle trajectory, which is consistent with the theoretical calculation results. The electromagnetic field distribution in the SADA and the electrostatic migration and deposition of abrasive dusts are predicted and analyzed by adopting the FEM method. The experimentally observed dusts’ trajectories in a vacuum chamber with 0.02 Pa vacuum degree are consistent with the simulation results, which qualitatively verifies the accuracy of the FEM model. The predicted results show that the irradiation-induced conductivity increase in polyimide material has little influence on the electric field distribution and the migration and deposition of the abrasive dusts but has great influence on the local current density. Both the potential differences between the two adjacent conducting rings and the loose contact between the electric brush and the conducting rings have significant influence on the migration and deposition of the abrasive dusts, which may greatly increase the surface discharge risk and the electrical transmission instability in the SADA. This study is conducive to the safe and stable operation of the on-orbit spacecrafts.

Stress analysis of circular membrane-type MEMS microphones with piezoelectric read-out

In this study, the impact of residual tensile stress on a membrane-type piezoelectric MEMS (Micro Electromechanical Systems) microphone is investigated both theoretically by FEM (finite element method) simulations and experimentally by stress measurements, by manufacturing devices and by recording the acoustic performance. Based on these findings, a design is proposed composing of segmented electrodes which are arranged close to the silicon frame as well as on the inner part of the membrane. The MEMS microphone features an aluminum nitride layer which allows a piezoelectric read-out, and multiple metal bottom and top electrodes distributed radially and azimuthally. FEM simulations were done over a large range of different layer thicknesses, membrane radii, and tensile stress levels, which indicate the existence of a sweet-spot when targeting a maximum output signal. The data showed, that thicker membranes perform better under tensile stress than thinner membranes. Furthermore, a relation between output signal and residual stress level was established, which showed that the FEM predictions agree well with the measurement results of piezoelectric microphones. • Thicker MEMS membranes preferred over thinner counterparts when tensely pre-stressed • Finite element analyses revealed sweet spot for maximizing piezoelectric output • Design propositions for stressed closed membranes without stress relaxing measures • Microphone measurements and finite element methods results agree well

Galvanic Corrosion Between Coated Al Alloy Plate and Stainless Steel Fasteners, Part 2: Application of Finite Element Method and Machine Learning to Study Galvanic Current Distributions

Aluminum alloy panels joined with stainless steel fasteners have been known to occur in aerospace structures, due to their respective optimized mechanical properties. When connected via a conductive solution, a high-driving force for galvanic corrosion is present. The combination of the dissimilar materials, indicating galvanic corrosion, and complex geometry of the occluded fastener hole, indicating crevice corrosion, leads to the detrimental combined effect of galvanic-induced crevice corrosion, as investigated previously in Part I. The present work extends the validated finite element method (FEM) model to predict the current distribution and magnitude in a variety of geometric and environmental conditions, with the goal of preventing corrosion damage within the highly susceptible fastener hole. Specifically, water layer thicknesses ranging from bulk full-immersion (800 μm) to atmospheric (89 μm) conditions were investigated, as well as the impact of external scribe dimensions. Two avenues for mitigation were determined: (1) to force the majority of current away from the fastener hole and onto the bulk surface of the panel and (2) to lower the overall galvanic coupling current. A random forest machine learning algorithm was developed to generalize the FEM predictions and create an open-source applicable prediction tool.

Reducing Thermal Crack Risks Caused by Restraint in Young Concrete - A Case Study on Walls of Water Tanks

Abstract Concrete cracks in structures such as water tanks and nuclear power stations cause anxiety to owners, contractors and engineers. These cracks may significantly increase the structure’s permeability and thus increase leakage, reduce durability, and eventually lead to loss of structural functionality. Therefore it is important to minimize their occurrence and size. To identify effective ways of minimizing cracking in young concrete segments, a parametric study was conducted using the finite element method (FEM). Parameters considered include casting sequence, joint position, wall height, and cooling. The study examined continuous and jumped casting approaches to the casting of a cylindrical reinforced concrete tank for a sewage-treatment plant, with and without the application of the ‘kicker’ technique in which the lower part of the wall is cast with the slab. The main cause of cracking is thermal change and restraint imposed by adjacent older structures, and the FEM predictions agree well with experimental observations. Continuous casting is most effective at minimizing cracking because it creates only two contact edges between newly cast and existing structures producing the lowest level of restraint. The kicker technique is shown to be very effective for reducing restraint and consider rephasing.

Computing the bond strength of 3D printed polylactic acid scaffolds in mode I and II using experimental tests, finite element method and cohesive zone modeling

The advent of the Three-Dimensional (3D) printing technique, as an Additive Manufacturing technology, made the manufacture of complex porous scaffolds plausible in the tissue engineering field. In Fused Deposition Modeling based 3D printing, layer upon layer deposition of filaments produces voids and gaps, leading to a crack generation and loose bonding. Cohesive zone model (CZM), a fracture mechanics concept, is a promising theory to study the layers bond behavior. In this paper, a combination of experimental and computational investigations was proposed to obtain bond parameters and evaluate the effect of porosity and microstructure on these parameters. First, we considered two different designs for scaffolds beside a non-porous Bulk design. Then, we performed Double cantilever beam and Singe Lap Shear tests on the 3D printed samples for Modes I and II, respectively. Afterward, we developed the numerical simulations of these tests using the Finite element method (FEM) to obtain CZM bond parameters. Results demonstrate that the initial stiffness and cohesive strength were pretty similar for all designs in Mode I. However, the cohesive energy for the Bulk sample was approximately four times of porous samples. Furthermore, for Mode II, the initial stiffness and cohesive energy of the Bulk model were five and four times of porous designs while their cohesive strengths were almost the same. Also, using cohesive parameters was significantly enhanced the accuracy of FEM predictions in comparison with fully bonded assumption. It can be concluded that for the numerical analysis of 3D printed parts mechanical behavior, it is necessary to obtain and suppose the cohesive parameters. The present work illustrates the effectiveness of CZM and FEM combination to obtain the layer adhesive parameters of the 3D printed scaffold.

Simulating Current Distribution of Oxygen Evolution Reaction in Microcells Using Finite Element Method

A three-dimensional finite element method (FEM) model simulating the electrochemical behaviour of Pt coplanar thin-film electrodes used for liquid cell transmission electron microscopy measurements was developed. The model included reaction kinetics and it was applied for the oxygen evolution reaction (OER). Kinetic parameters of OER in the liquid cell were experimentally acquired and applied to the FEM model. Comparison between the experimental and simulated polarization curves demonstrated the reliability of the FEM predictions. The simulations were used to produce maps of the potential and current density distributions of the working and counter electrodes as well as for calculating the distribution of the current density in the liquid electrolyte. Two distinctive electrode geometries were evaluated with the FEM model. It was predicted that non-symmetrical electrode designs can cause unexpected electrochemical behaviour with respect to the electrolyte current density between working and counter electrodes accompanied by the presence of hot spots. The findings suggest that FEM simulations could be key to designing well-performing electrochemical microcells for liquid phase electron microscopy experiments.

Adhesion study between micron-scale graphite particles and rough walls using the finite element method

The characteristics of how graphite particles adhere to the walls of high-temperature gas-cooled reactors are very important for analyzing reactor source terms. In the present study, atomic force microscopy (AFM) is used to measure the particle adhesion force and the wall morphology, then Lennard-Jones potential theory and the finite element method (FEM) are coupled to calculate the adhesion force between different particles and rough walls. The obvious deviations between the AFM measurements and the theoretical models are due to the contact hypothesis of a sphere with a smooth flat plane in the latter. The FEM results reveal the formation of maximum adhesion at the pull-off point of the particle and the corresponding stress distribution. For aspherical particles, the local curvature of the particle contact interface is the main factor affecting the adhesion force. In addition, for rough walls, different contact regimes correspond to different adhesion trends. When discrete wall roughness with hemispheres and truncated cones is used, the FEM predictions are closer to the AFM measurements, indicating that roughness dominates the adhesion weakening.

Comparison of Test Methodologies for Computing Bending and Shear Stiffness of Cross-Laminated Timber

Abstract Current testing methods for evaluating the out-of-plane bending and shear stiffness terms of cross-laminated timber (CLT) materials use a relatively long span panel. When new CLT layups and composites are proposed, especially when manufactured from pilot-scale facilities, the length requirement may be a barrier for material property evaluation. The purpose of this paper was to compare current bending and shear stiffness test methods of CLT with alternative forms, including a simultaneous calculation method and the five-point bending test, which uses a shorter beam than current testing methods. Both experimental evaluation and finite element modeling of the mechanical properties were used to compare the proposed test methods. In general, numerical and experimental results demonstrated that effective bending stiffness (EIeff) values of CLT panels evaluated by different test methods were similar (maximum 12.7 % difference). Effective bending and shear stiffness values from the finite element method (FEM) agreed with experimental results. FEM demonstrated that both the simultaneous solution and the five-point bending test can be used as alternative test methods for evaluation of the bending and shear stiffness for CLTs. However, the effective shear stiffness values of CLTs from current industry standards were at least 450 % less than both the experimental evaluation and FEM prediction.

Advanced machining of miniature unmanned aircraft vehicle components using nanostructured cutting tools

The advanced machining of components used in miniature unmanned aircraft vehicles is the focus of this study. The finite element method (FEM) is used to predict forces and temperatures using cutting tool inserts with a thin nanostructured film of high integrity. Similarity models are used to validate the finite element results and to understand the influence of micromachining parameters on cutting temperatures generated when machining Al 380-0 alloy. The predicted results are compared to experimental forces and temperatures using a three-dimensional piezoelectric function dynamometer and a short-range infra-red wavelength thermal camera. Nanostructured thin layer coatings lower machining forces and temperatures, which are validated through FEM predictions and experimental observations. The experimental results suggest that increasing the cutting tool’s rake angle at higher depths of cut will reduce cutting temperatures, which are predicted using the similarity models for micromachining.

Development of 3D boundary element method for the simulation of acoustic metamaterials/metasurfaces in mean flow for aerospace applications

Low-cost airlines have significantly increased air transport, thus an increase in aviation noise. Therefore, predicting aircraft noise is an important component for designing an aircraft to reduce its impact on environmental noise along with the cost of testing and certification. The aim of this work is to develop a three-dimensional Boundary Element Method (BEM), which can predict the sound propagation and scattering over metamaterials and metasurfaces in mean flow. A methodology for the implementation of metamaterials and metasurfaces in BEM as an impedance patch is presented here. A three-dimensional BEM named as BEM3D has been developed to solve the aero-acoustics problems, which incorporates the Fast Multipole Method to solve large scale acoustics problems, Taylor’s transformation to account for uniform and non-uniform mean flow, impedance and non-local boundary conditions for the implementation of metamaterials. To validate BEM3D, the predictions have been benchmarked against the Finite Element Method (FEM) simulations and experimental data. It has been concluded that for no flow case BEM3D gives identical acoustics potential values against benchmarked FEM (COMSOL) predictions. For Mach number of 0.1, the BEM3D and FEM (COMSOL) predictions show small differences. The difference between BEM3D and FEM (COMSOL) predictions increases further for higher Mach number of 0.2 and 0.3. The increase in difference with Mach number is because Taylor’s Transformation gives an approximate solution for the boundary integral equation. Nevertheless, it has been concluded that Taylor’s transformation gives reasonable predictions for low Mach number of up to 0.3. BEM3D predictions have been validated against experimental data on a flat plate and a duct. Very good agreement has been found between the measured data and BEM3D predictions for sound propagation without and with the mean flow at low Mach number.

Investigating fatigue failure of core motion mechanism for a small oil-free wobble-plate compressor

Abstract The study is driven by a recent experimental test for a small-size high-speed oil-free wobble-plate compressor, in which abrupt local fatigue failure of the wobble plate is encountered. To explore the causes, a fatigue prediction method is proposed, which involves the dynamic force calculation, stress prediction, and fatigue analysis. The dynamic forces is obtained by solving the coupled dynamic, thermodynamic and kinematic models. The dynamic stress with thermal effect is predicted, where the temperature field is calculated by solving the thermal conduction equation with the experimentally measured solid-surface temperature acting as boundary conditions, and the obtained temperature distribution is incorporated into the FEM (finite element method) prediction for thermal-dynamic stress. It is found that intensive friction heating causes the wobble plate significant local temperature rise and it is largely responsible for the structural failure. The Goodman mean stress correction method and wobble plate material S–N (Stress amplitude versus cycle Number) curves are incorporated to predict the fatigue life of wobble plate. The predicted fatigue life is comparable to the experimentally measured, which has verified the developed method. The wobble plates of 7075-T6 aluminum alloy and H62 brass alloy are compared, and the former produces a smaller dynamic stress for its smaller elasticity modulus and larger specific heat but a shorter fatigue life due to its S–N curve smaller stress amplitude.

Exploring the relationship between applied fabric strain and resultant local yarn strain within the elastic fabric based on finite element method

As kinds of unique textiles with high extensibility and elasticity, elastic fabrics are widely used for sportswear, medical textiles and electronic textiles. The tensile behavior of fabric is very important for post-processing and usage of fabric. However, there are very few studies on the resultant yarn strain subjected to the applied fabric extension, which is crucial for understanding the tensile behavior of elastic fabric and expanding its potential applications. In order to address the tensile behavior of elastic fabric, a mechanical model of fabric was built for the analysis of local yarn strain within the fabric through finite element method (FEM). The FEM models with full consideration of the tensile property of elastic yarns were built to predict the relationship between the applied fabric strain and the resultant yarn strain. An experimental study was conducted to characterize the yarn deformation behavior during the fabric stretching and compare with the FEM prediction. The FEM simulations show a good agreement with the experimental results. Based on the FEM models, the effects of fabric structural parameters on such a relationship were investigated for understanding the deformation mechanism and thus optimizing the tensile properties of fabrics. A yarn strain of 27.5% could be achieved under 60% fabric strain by adopting an optimized warp yarn spacing of 0.8 mm. The results not only shed light on the origin of high extensibility and elasticity of fabrics but are of great value to the design and development of elastic materials for various applications such as textile-based electronics.

Quantitative comparison of experimental and Mohr-Coulomb finite element method simulation flow characteristics from quasi two-dimensional flat-bottomed bins

Four different flow characteristics obtained from Eulerian finite element method (FEM) simulations using a Mohr-Coulomb material model are quantitatively compared to experiments performed on laboratory-scale concentric and eccentric quasi two-dimensional flat-bottomed bins. Particle image velocimetry (PIV) experiments are performed on Ottawa 20–30 sand discharging through the bins. The experimental steady state velocity profiles, mass discharge rate (MDR), duration of steady MDR (TSS), and free surface profiles are compared to the FEM predictions. The significance of material dilation and associated softening (DS) on FEM flow characteristics is examined through comparisons to simulations without dilation or softening (NDNS). FEM DS simulations using a Mohr-Coulomb material model that includes material dilation and softening are shown to be capable of predicting the four flow characteristics in both discharging bins with reasonable accuracy.

Prediction of mechanical behavior of 3D bioprinted tissue-engineered scaffolds using finite element method (FEM) analysis

Three-dimensional (3D) printing can be a promising tool in tissue engineering applications for generating tissue-specific 3D architecture. The 3D printing process, including computer-aided design (CAD), can be combined with the finite element method (FEM) to design and fabricate 3D tissue architecture with designated mechanical properties. In this study, we generated four types of 3D CAD models to print tissue-engineered scaffolds with different inner geometries (lattice, wavy, hexagonal, and shifted microstructures) and analyzed them by FEM to predict their mechanical behaviors. For the validity of computational simulations by FEM, we measured the mechanical properties of the 3D printed scaffolds. Results showed that the theoretical compressive elastic moduli of the designed constructs were 23.3, 56.5, 67.5, and 1.8 MPa, and the experimental compressive elastic moduli were 23.6 ± 0.6, 45.1 ± 1.4, 56.7 ± 1.7, and 1.6 ± 0.2 MPa for lattice, wavy, hexagonal, and shifted microstructures, respectively, while maintaining the same construct dimension and porosity. In addition, van der Waals hyperelastic material model was successfully utilized to predict the nonlinear mechanical behavior of the printed scaffolds with different inner geometries. These findings indicated that the CAD-based FEM prediction could be used for designing tissue-specific constructs to mimic the mechanical properties of targeted tissues or organs.

A novel 3D re-entrant unit cell structure with negative Poisson’s ratio and tunable stiffness

:In this paper, a novel three-dimensional (3D) unit cell structure with butterfly-like perforations was designed, and negative Poisson’s ratio and tunable stiffness were achieved in such a geometry. The Poisson’s ratio and strain-stress relationship of structures with different geometric parameters were determined using the finite element method (FEM). Samples with identical geometric variables to those of finite element models were fabricated via 3D printing technique, and their Poisson’s ratios and stress–strain relationships were experimentally determined and compared with the FEM results. Results showed that the proposed 3D cellular structures exhibit negative Poisson’s ratios, and a minimal value of −0.7091 could be reached. The stress–strain curve of each structure exhibited three distinct stages (elastic response, rib buckling and self-contact), with different elastic moduli being observed at each stage, and demonstrated a tunable range of the compressive stiffness ratio between stages varying from 0.1866 to 1.4006(tunable stiffness), as determined by FEM analysis. Good agreement was found between the experimental results and FEM predictions. The design concept can be implemented and optimized for specific applications via geometric parameters manipulation.

Finite Element Method for Prediction of Rock Breaking Performance of the Tipped Hob With Different Tooth Profiles

The raise boring machine, which plays a crucial role in wellbore construction, has been widely applied in subway tunnels and mine construction projects. High levels of quality and safety, make it recognized and promoted by an increasing number of users. The tipped hob as a rock-breaking cutter of the raise boring machine poses a direct influence on the efficiency of the well-bore construction. In this paper, the finite element software LS-DYNA is employed to establish the model of the crushed rock with the tipped hob, and the accuracy of the model is examined by experiments. On this basis, the variation laws and correlations of feed force, lateral force and positive force under different penetration depths are studied. The results show that as the penetration depth increases, the feed force, lateral force and positive force all grow exponentially. In the geological environment with high rock hardness, spherical tooth can effectively reduce cutter wear and conical tooth can achieve higher crushing ability. The research results provide a theoretical basis for the selection of a suitable tipped hob to improve rock breaking efficiency.

Mechanical property modeling of aluminium nanocomposite reinforced with carbon nanotubes

In this study, the mechanical properties of an aluminium matrix nanocomposite reinforced with carbon nanotubes (CNTs) are modeled by using the finite-element method (FEM). The mechanical properties include the Young’s modulus, yield strength and rupture strength of the nanocomposites. The effects of the volume fraction of the CNTs, interfacial type and loading direction on the mechanical properties are studied. The FEM results for the mechanical properties of the nanocomposite are compared with the results of experimental data and the micromechanical models. The FEM predictions for the mechanical properties for different volume fractions of the CNTs provide better results than the micromechanical models. When the volume fraction of the CNTs increases from 0 to 2 vol%, the Young’s modulus, the yield strength and the rupture strength of the nanocomposite increase by 18, 18 and 29%, respectively. Due to random dispersion of the CNTs in the aluminium matrix, the mechanical properties are approximately the same in different directions. The results show that the friction interface is weaker than the adhesion interface and then the friction interface causes the mechanical properties of the nanocomposite to weaken.