Material Imperfections Research Articles

Nonlinear Finite Element Analysis of Tubular Steel Wind Turbine Towers near Man Door and Ventilation Openings to Optimize Design against Buckling

The safe and cost-effective design of wind turbine towers is a critical and challenging aspect of the future development of the wind energy sector. This process should consider the continuous growth of towers in height and blades in length. Among potential failure modes of tubular steel towers, shell local buckling due to static axial compressive stresses from the rotor, blades, and tower weight, as well as dynamic flexural compressive stresses from wind actions on the rotating blades and the tower itself, are dominant as thickness is optimized to reduce weight. As man door and ventilation openings are necessary for the towers’ operation, the local weakening of the tower shell in those areas leads to increased buckling danger. This is compensated for by tower manufacturers by the provision of stiffening frames around the openings. However, the cold-forming and welding of these frames are among the most time-consuming aspects of tower fabrication. Working towards the optimization of this design aspect, the buckling response of tubular steel towers near such openings is investigated by means of nonlinear finite element analysis, accounting for geometrical and material nonlinearity and imperfections (GMNIA), and also considering several wind directions with respect to the openings. The alternatives of stiffened and unstiffened openings are investigated, revealing that a thicker shell section around the opening may be sufficient to restore lost stiffness and strength, while the stiffener frame may also be eliminated, offering substantial benefits in terms of manufacturing effort, time and cost.

Determining individual glomerular proteinuria and periglomerular infiltration in a cleared murine kidney by a 3-dimensional fast marching algorithm

Three-dimensional imaging has advanced basic research and clinical medicine. However, limited resolution and imperfections of real-world 3D image material often preclude algorithmic image analysis. Here, we present a methodological framework for such imaging and analysis for functional and spatial relations in experimental nephritis. First, optical tissue clearing protocols were optimized to preserve fluorescence signals for light-sheet-fluorescence-microscopy and compensated attenuation effects using adjustable 3D correction fields. Next, we adapted the Fast — Marching — Algorithm to conduct backtracking in 3D environments and developed a tool to determine local concentrations of extractable objects. As a proof-of-concept application, we used this framework to determine in a glomerulonephritis model the individual proteinuria and periglomerular immune cell infiltration for all glomeruli of half a mouse kidney. A correlation between these parameters surprisingly did not support the intuitional assumption that the most inflamed glomeruli are the most proteinuric. Instead, the spatial density of adjacent glomeruli positively correlated with the proteinuria of a given glomerulus. Since proteinuric glomeruli appear clustered, this suggests that the exact location of a kidney biopsy may affect the observed severity of glomerular damage. Thus, our algorithmic pipeline described here allows analysis of various parameters of various organs composed of functional subunits, such as the kidney, and can theoretically be adapted to processing other image modalities.

Stochastic effects on solution landscapes for nematic liquid crystals

ABSTRACT We study the effects of additive and multiplicative noise on the solution landscape of nematic liquid crystals confined to a square domain within the Landau-de Gennes framework, as well as the impact of additive noise on the symmetric radial hedgehog solution for nematic droplets. The introduction of random noise can be used to capture material uncertainties and imperfections, which are always present in physical systems. We implement random noise in our framework by introducing a Q -Wiener stochastic process to the governing differential equations. On the square, the solution landscape for the deterministic problem is well understood, enabling us to compare and contrast the deterministic predictions and the stochastic predictions, while we demonstrate that the symmetry of the radial hedgehog solution can be violated by noise. This approach of introducing noise to deterministic equations can be used to test the robustness and validity of predictions from deterministic liquid crystal models, which essentially capture idealised situations.

Model-free characterization of topological edge and corner states in mechanical networks

Topological materials can host edge and corner states that are protected from disorder and material imperfections. In particular, the topological edge states of mechanical structures present unmatched opportunities for achieving robust responses in wave guiding, sensing, computation, and filtering. However, determining whether a mechanical structure is topologically nontrivial and features topologically protected modes has hitherto relied on theoretical models. This strong requirement has limited the experimental and practical significance of topological mechanics to laboratory demonstrations. Here, we introduce and validate an experimental method to detect the topologically protected zero modes of mechanical structures without resorting to any modeling step. Our practical method is based on a simple electrostatic analogy: Topological zero modes are akin to electric charges. To detect them, we identify elementary mechanical molecules and measure their chiral polarization, a recently introduced marker of topology in chiral phases. Topological zero modes are then identified as singularities of the polarization field. Our method readily applies to any mechanical structure and effectively detects the edge and corner states of regular and higher-order topological insulators. Our findings extend the reach of chiral topological phases beyond designer materials and allow their direct experimental investigation.

Plane strain problem of flexoelectric cylindrical inhomogeneities

In nanotechnology, flexoelectric solids exhibit notable electrical polarization induced by internal strain gradients, rendering them promising for various applications. However, inherent material imperfections are inevitable. Particularly within flexoelectric solids, substantial strain gradients exist in proximity to internal defects, resulting in localized concentration of electrical polarization and potential structural failure. Among the various defect types, circular-shaped inhomogeneity is prevalent. This paper comprehensively investigates the plane strain problem on cylindrical inhomogeneities within flexoelectric solids. The full-field analytical solution is derived for this problem for the first time. Given that flexoelectric theory encompasses pure strain gradient elasticity theory, the strain gradient elasticity solution for plane strain cylindrical inhomogeneities is also established for the first time. This study reveals that the stiffness and size of the inhomogeneity, along with the loading ratio in two directions, exert a noteworthy influence on the local electromechanical coupling behavior near the inhomogeneity. Finally, the mixed finite element method is utilized to approximate the solution numerically, and the close agreement between the finite element results and the analytical solution demonstrates this study’s reliability and rigor. Therefore, this investigation imparts valuable insights into examining defects in flexoelectric solids and serves as a foundation for studying more intricate defect typologies.

A Comprehensive Review of Electron Microscopy in Materials Science: Technological Advances and Applications

In nanomaterials and microstructural evolution, electron microscopy has had an important effect on materials investigation. Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Electron Diffraction, Operando Electron Microscopy, and Aberration-Corrected Electron Microscopy offer the investigation on understanding of nanoscale material properties and structure. The present research covers the basics, advantages and disadvantages, and material-related applications of various electron microscopy techniques. TEM is useful for investigating atomic arrangements and imperfections in materials, while SEM offers micro- to nanoscale topographical, morphological, & compositional information. EDS, frequently employed with SEM or TEM, analyzes elements and compounds to determine material compositions. Opando Electron Microscopy allows researchers to observe and assess materials during catalytic reactions and battery charge/discharge cycles. This approach is vital for knowing how dynamic processes influence nanoscale material characteristics and behaviour. Another advanced technique, Aberration-Corrected Electron Microscopy, corrects lens aberrations that interfered with electron microscope resolution. This adjustment enables imaging at sample-limited resolutions, allowing further studies of atomic structures & flaws. These electron microscopy methods significantly improve nanomaterial microstructural evolution understanding. These have enabled extensive study of flaws, transitions between phases, and formation mechanisms, which are essential to producing optimal materials.

Local relaxation of residual stress in HFMI-treated high-strength steel welded joints subjected to high-peak loads

This study investigates the effect of high-peak loads on local relaxation of residual stress and fatigue damage in HFMI-treated high-strength steel welded joints. The finite element simulations are applied to consider the combined effect of geometry, material, and residual stress. This simulation model specifically contains the actual treated surface profile with material imperfections. As a result, the material imperfections highly influence the local elastic-plastic behavior under load conditions, including a high tensile and compressive peak load, thus the local relaxation of residual stress and fatigue damage. The results provide further understanding towards a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

Finite Elements modelling and assessment of ceramic rollers with edge cracks

To ensure reliability of hybrid cylindrical roller bearings each ceramic roller is inspected, and scrapped if a surface imperfection above the critical size is detected. This inspection aims to reduce the potential root cause of Rolling Contact Fatigue (RCF) which can occur during operation, shortening the bearing life. The rejection criterion is based on experimental and theoretical knowledge, which for the last decade was developed in SKF for the material imperfections mainly located on the rolling element raceway. These imperfections are subjected to high contact pressure and therefore are considered as the primary root cause of RCF failure. Regarding rollers, however, imperfections can be present beyond the raceway, i.e. at the roller chamfer where the lower risk of RCF is expected, because the edge imperfections are typically out of the rolling contact zone. Nevertheless, the risk associated with these features should be assessed too, chiefly because the size of edge imperfections can be rather large. In our previous study, the imperfection termed as a Missing Material was studied, combining the semi-analytical tool for the contact mechanics and the Finite Elements (FE) method for the stress analysis. In the current work, another imperfection type is considered, and this is a surface crack located at the chamfer of ceramic roller. The RCF analysis is based on the semi-analytical evaluation of the rolling contact pressure (between a ceramic roller and a steel inner ring), and computational fracture mechanics for the estimation of fatigue crack propagation.

Quasi-3D free and forced vibrations of poroelastic microplates in the framework of modified couple stress theory

In this study, a comprehensive study on the vibration behaviour of functionally graded thick microplates with material imperfections is presented for free and forced vibrations within a quasi-3D model and the modified couple stress (MCS) theory. Axial, transverse, rotation, and stretching motions are considered for modelling the thick microplate in the framework of the modified power-law scheme and quasi-3D and MCS theories. The microplate deformation is assumed to be infinitesimal and is modelled using the linear strain–displacement relationships. Using a virtual work method, the governing motion equations are derived. For fourfold coupled (axial-transverse-rotation-stretching) characteristics, the partial differential equations components are discretised via trigonometric expressions and the corresponding natural frequencies and time histories (time-dependent deflections) are determined numerically. The methodology and model are initially validated through a comparative analysis between the natural frequencies of the macrolevel simplified structure and those obtained using finite element software. Additionally, the simulation is compared, for validation purposes, with other simplified versions from the literature. Once model's validity is confirmed, an investigation into the quasi-3D free and forced vibrations of the structure is conducted for the new model. The results demonstrate the considerable effect of thickness stretching and material imperfection on the vibration characteristics of the poroelastic thick microplates.

O.I.C.-based design of extruded and welded aluminum I-sections

Current standards rely on simplified approaches to predict the resistance of aluminum structural elements that can be shown to inadequately account for the effects of strain hardening, instabilities, and heat reduced properties. This paper summarizes investigations towards the development of an alternative design method for aluminum open cross-sections, based on the Overall Interaction Concept (O.I.C.). This innovative design approach relies on the interaction between resistance and stability, and also allows to consider geometrical and material imperfections. Moreover, it allows to obtain direct, precise and consistent resistance predictions using continuous buckling curves.A numerical finite element model was developed to accurately predict the resistance of aluminum cross-sections. Its efficiency was validated by comparing its numerical predictions of resistance to available experimental test data. Extensive parametric studies were then conducted, allowing to study the impact of varying geometries, aluminium alloys and load cases on the resistance. Using the results from more than 2300 numerical simulations, O.I.C.-type design proposals were formulated for the local resistance of extruded and welded aluminum sections.The performance of the design equations was evaluated by comparing their resistance estimates to the reference numerical results and to resistance predictions from the Canadian, European, and American aluminum design standards. The comparisons showed that the O.I.C. design proposal leads to much more accurate and consistent results than these standards, while remaining simpler and more efficient.

Multi-physics model development and application to real-case alkaline electrolyzer

The realization of mathematical, multi-physics models for alkaline electrolyzers is crucial for advancing this technology. Lumped parameter models offer shorter simulation times, compared with other approaches, and practical industrial applications. If electrochemical models provide the polarization curve, the hydrogen production rate, and the device efficiency, thermal models solve the equations involving the electrolyte temperature. With a coupled approach, the two models can be linked together by considering the device voltage as temperature dependent. Despite the relevance of such models, few instances of their direct application on existing electrolyzers can be found in the literature, and combined electrochemical-thermal simulations are rare.This study presents a multi-physics model applied on an alkaline electrolyzer and validated against measurements acquired on a dedicated experimental test bench.The physics-based model accurately predicts the polarization curve, exhibiting a high precision match with experimental data. Additionally, it identifies material or geometrical imperfections in the electrolyzer, allowing for optimization in the design phase. The thermal model successfully converges to the desired electrolyte temperature of 72 °C under stationary conditions. Additional transient simulations demonstrate an average deviation of only 0.25% compared to the measured temperature trend. Finally, a sensitivity analysis explores the coupling of the electrolyzer with a wind turbine under different wind conditions.The study showcases the effectiveness of the coupled electrochemical-thermal model in predicting electrolyzer performance, with a direct application to a wind turbine power output.

Formation of non-uniform fibre distribution and its effect on the flexural performance of pultruded GFRP box beams

This study investigates the effect of non-uniform fibre distribution (NUFD) as a defect in the pull-winding manufacturing process on the mechanical properties of pultruded glass fibre-reinforced polymer (GFRP) box section profiles. These profiles exhibit balanced mechanical properties but are susceptible to NUFD during production, negatively affecting local buckling capacity. Experimental and numerical analyses were conducted on pultruded GFRP profiles manufactured under three winding tension configurations, resulting in a 5% variance in load capacity during bending. Results show that corner NUFD influences local buckling capacity more than flange NUFD. Specifically, corner NUFD decreases load capacity by up to 20%, while flange NUFD increases it by up to 3%. Conversely, the effect of NUFD location is insignificant to the failure determined by the material strength without buckling instability. Moreover, a linear relationship between the rotational restraint coefficient and corner fibre volume fraction provides insight into the impact of material imperfections on load capacity.

Coupled five-parameter dynamics of Mindlin and third-order shear deformable FG graphene-platelets reinforced viscoelastic plates with geometric and material imperfections

The present work investigates the coupled five-parameter dynamics, in particular, the coupled five-parameter structural resonance frequency, of functionally graded (FG) graphene-platelets reinforced viscoelastic plates in the presence of geometric and material (porosity) imperfections, based on the Mindlin theory and the third-order shear deformation plate theory (TSDPT). Various imperfections are often found in structures as a result of the challenges in manufacturing and operations over time. Geometric imperfection is considered in the FG reinforced plate structure by modelling an initial curvature in the out-of-plane direction. Material imperfections, in the form of closed-cell voids, are formulated as uniform and non-uniform FG porosity distributions. Uniform and thickness-wise FG gradation effects of graphene-platelets are accounted for in the determination of the effective material properties using a two-dimensional Halpin-Tsai micromechanics model, in conjunction with the rule of mixtures. Viscoelasticity of the FG imperfect plate structure is modelled by employing the Kelvin-Voigt model to consider energy dissipation. Using the Mindlin plate theory and the TSDPT, five equations of motion, which are coupled through the five parameters u, v, w, ϕ1, and ϕ2, are derived using Hamilton's variational principle. An assumed mode method is then employed to obtain the real and the imaginary components of resonance frequencies. The combined influences of the different effects on the structural resonance are examined. The results obtained offer a comprehensive understanding of how the FG graphene-platelets, FG porosity, plate thickness, geometric imperfection, and the utilisation of two different shear deformation plate theories collectively influence the coupled five-parameter dynamics of the FG graphene-platelets reinforced viscoelastic plate structure with imperfections. It was found that the difference in frequency resulted from the use of the two plate theories is most dominant with the FG GX and/or FG PX graphene-platelets reinforced porous plates, and that geometric imperfection was found to alter a graphene-platelets reinforced plate's sensitivity towards material imperfection.

(Invited) A Closer Look at Impact Ionization Coefficients in Wide Band Gap Semiconductors: Theoretical Models and Measured Data

The direct measurement of ionization coefficients in wide band gap semiconductor materials is challenging due to the need to operate at high field strengths and the requirement of specific test structures that enable single carrier injection. More often than not, ionization coefficients are inferred from current multiplication data measured in p-n junctions or transistors structures. Unfortunately, sub-par material quality, processing issues and inappropriate measuring techniques have led to ionization coefficients values that are contradictory and with large variation among different datasets.As a result, theoretical models that attempt to predict the values of the carriers’ ionization coefficients are important since they provide an estimate of the expected values and a first order evaluation of the material performance. Developing theoretical models presents its own set of challenges. The established approach is based on a full electronics structure description of the semiconductor material and a suitable model to quantify the interaction of carriers with phonons, impurities and material imperfections. While this methodology has been successful with conventional elemental and compound semiconductors, and it has been applied to some wide band gap materials such as GaN, 4H-SiC [1,2] and diamond, a number of open issues still exists. The main difficulty is the determination of the carrier-phonon scattering strength at high electric field strengths that cannot be inferred from low-field mobility measurements.Among all wide band gap semiconductors, 4H-SiC [3] and GaN [4] are the material for which ionization coefficients have been measured by several groups. In the case of GaN, different measured ionization coefficients datasets are available and seems to provide a consistent indication of the expected values. Corresponding theoretical models based on full-band Monte Carlo simulations have been able to predict the correct trends, namely the fact that holes dominate the ionization process, but a quantitative agreement between predicted and measured values has only been achieved recently [5].This talk will provide an overview of the currently available experimentally measured ionization coefficients for wide band gap semiconductors and compare them to the ones obtained with theoretical models of different complexity. Specifically the development of the theoretical models for high-field transport for 4H-SiC, GaN will be discussed. Three different approaches, one based on empirical carrier-phonon scattering rates, one using the rigid pseudo ion model, and the direct evaluation of the interaction parameters based on a ab-initio DFT approach will be compared, and the outcome for each one of them benchmarked against the measured values. Additionally, the models will be applied to determine the ionization coefficients in AlGaN alloys, cubic BN and diamond.[1] E. Bellotti et al., J. Appl. Phys., 87, (8), p.3864, 15 April 2000.[2] F. Bertazzi et al., J. Appl. Phys., Vol.106, N6, p.063719, 15 September 2009.[3] A.O. Kostantinov, Appl. Phys. Lett. 71, 90, 1997, T. Hatakeyama, Appl. Phys. Lett. 85, 8, 2004[4] Cao et al., APL, N.112, 2018, Maeda et al., JAP, N.129, 2021, McClintock et al., APL, N.90, 2007, Ji et al., APL, N115, 2019[5] E. Bellotti and M. Matsubara, unpublished.

Facile Modification on Buried Interface for Highly Efficient and Stable FASn0.5Pb0.5I3 Perovskite Solar Cells with NiOx Hole‐Transport layers†

Comprehensive SummaryFormamidinium (FA)‐based Sn‐Pb perovskite solar cells (FAPb0.5Sn0.5I3 PSCs) with ideal bandgap and impressive thermal stability have caught enormous attention recently. However, it still suffers from the challenge of realizing high efficiency due to the surface imperfections of the transport materials and the energy‐level mismatch between functional contacts. Herein, it is demonstrated that the modification on buried interface with alkali metal salts is a viable strategy to alleviate these issues. We systematically investigate the role of three alkali metal bromide salts (NaBr, KBr, CsBr) by burying them between the NiOx hole transport layer (HTL) and the perovskite light‐absorbing layer, which can effectively passivate interface defects, improve energy‐level matching and release the internal residual strain in perovskite layers. The device with CsBr buffer layer exhibits the best power conversion efficiency (PCE) approaching 20%, which is one of the highest efficiencies for FA‐based Sn‐Pb PSCs employing NiOx HTLs. Impressively, the long‐term storage stability of the unencapsulated device is also greatly boosted. Our work provides an efficient strategy to prepare desired FA‐based ideal‐bandgap Sn‐Pb PSCs which could be applied in tandem solar cells.

Strain, crack, stress and shape diagnostics of new and existing post-tensioned structures through distributed fibre optic sensors

Prestressed concrete is commonly used in the construction of safety–critical structures such as large-span girders or slabs. The article focuses on post-tensioned elements, in which curvilinear multiwire strands are tensioned after concrete’s hardening to transfer the compressive force to the concrete. The overall idea is to confront external forces and achieve extended benefits, e.g. longer spans or reduced cracking. However, the design, construction and maintenance of such elements are particularly challenging because of their complexity, including spatial stress state in the anchorage zones and also the standard features of concrete structures like heterogeneity, material imperfections and, most importantly, local discontinuities (cracks). Reliable diagnostic solutions are therefore essential for both new and existing components. Distributed fibre optic sensing (DFOS) allows the measurements to be taken continuously over the entire structural length and its advantages have already been proven in civil engineering applications, especially those related to concrete structures. However, the novelty of the presented research lies in 1) simultaneous analysis of strains, stresses, cracks and displacements (shape changes); 2) direct comparison of results from monolithic sensors embedded in the concrete (for new structures) and those installed in pre-made near-to-surface grooves (for existing structures); 3)the application of the embedded composite strain sensor to the bi-directional analysis of the stress field within the anchorage zones; 4) possibility of strain transfer analysis between the cracked concrete, cable duct, injection and prestressing tendon during mechanical bending tests. The DFOS-based strain, crack and displacement results were also compared with reference measurements made with conventional spot techniques, achieving very good compliance. The research proved that a well-designed DFOS-based system, including appropriate measurement techniques, optical sensors, installation methods and post-processing algorithms, is the relevant tool for the diagnostics of post-tensioned concrete structures.

Efficient quantification of composite spatial variability: A multiscale framework that captures intercorrelation

Composite structures suffer from material imperfections. Non-deterministic models at the micro- and mesoscale propagate this spatial variability. However, they become impractical when the structure size increases. This paper proposes a numerically efficient multiscale methodology that links structural behaviour with the spatial variability of material imperfections on smaller scales. Fibre strength variability is accounted for through a fibre break model. A mesoscale model considers fibre volume fraction and fibre misalignment variability using random fields. Measurements provide probabilistic data for these imperfections. Subsequent homogenisation results in intercorrelated material properties on the structural macroscale that are modelled effectively with vine copulas. The methodology is verified by predicting the failure load of a coupon model. Predictions are very similar to those obtained by directly modelling spatial variability on the structural scale.

Theory of Imperfections of Three-Dimensional Tuning Plate Gyroscope. Part I: Material Imperfections

Theory of Imperfections of Three-Dimensional Tuning Plate Gyroscope. Part I: Material Imperfections

A new microwave nondestructive testing method of relative permittivity and conductivity for stratified media based on dyadic Green's function I: Theory

Stratified composite materials are employed in many industrial scenes, including bridge construction, vehicle, ship, aviation, etc. However, minor imperfections in composite materials can have significant effects and even result in disasters when used. In this work, a multi-parameter inversion method that can simultaneously reconstruct the conductivity and permittivity parameters of layered media is devised and presented for microwave nondestructive testing (NDT) of the minor imperfections in stratified composite materials. Firstly, the calculations for the electric field in the layered medium model with the emission source as the point source are provided. Secondly, the spatial electric field variations (the Fréchet derivative) caused by changes in conductivity and permittivity of arbitrary layers are calculated. Then the permittivity and conductivity are reconstructed using the Distorted Born iterative method (DBIM) technique. The numerical examples at microwave frequencies show that the inversion technique has a strong capacity to reconstruct for a single parameter (relative permittivity or conductivity) and multi-parameter (relative permittivity and conductivity). Our suggested multi-parameter inversion technique in this research has excellent potential for use in the identification of multilayer composites and subsurface inclusions.

Influence of Opening Type in Axially Compressed Castellated Columns

Recently, castellated columns have been increasingly used in buildings because of their flexibility, easy compatibility and potential savings due to less steel consumption. However, there is a lack of research related to the subject. In view of that, this work evaluates the influence of the opening type and size on the resistant capacity of the column, considering a nonlinear plastic analysis by the finite element method (792 simulations). For this, columns without openings were modeled to validate the methodology proposed. After validation, a nonlinear buckling analysis of castellated columns was performed. In this study, different cut patterns of the beam web are evaluated, as well as residual stresses, material and geometric imperfections, based on a statistical design. The loads obtained by castellated columns and columns without web are compared. Also, the resistance reduction factors associated with global buckling (χ) for castellated profiles are compared to the values proposed by ABNR NBR 8800 (2008). Furthermore, statistically, there are no differences between the opening types evaluated. Finally, in order to contribute to the design of castellated columns, equations based on regression models were found to predict the load capacity of these elements.