Imperfect Geometry Research Articles

Self‐Assembled Rolled‐Up Microcoils for nL Microfluidics NMR Spectroscopy

AbstractSeveral attempts are made to downscale nuclear magnetic resonance (NMR) spectroscopy systems and to enable high resolution chemical analysis of small sample quantities. However, miniaturization is nontrivial due to stringent demands on precise analyte sampling within the detector while performing local excitation of the sample and signal detection with a microsized coil. Imperfect coil geometry and inhomogeneities in the coil's surrounding environment have a detrimental impact on the signal quality, hampering further development of miniature NMR systems. To solve this challenge, a new type of monolithic wafer‐scale self‐assembled microcoils with a detection volume of 1.5 nL are integrated into a microfluidics circuit. The microcoils are fully encapsulated in polydimethylsiloxane (PDMS) allowing for simplified and precise supply of an analyte through the interior of the detector. Due to their construction, with the inner winding touching the analyte, the microcoils have an almost 100% filling factor. Magnetic field inhomogeneities are reduced through well‐defined microtubular architectures and susceptibility matched conductors. This approach results in a spectral linewidth of only 8 ppb with shimming and 22 ppb without shimming. The demonstrated methodology promotes the realization of next generation miniaturized analytical NMR systems for product monitoring, safety verification, medical testing, and material evaluation.

Porosity, mass and geometric imperfection sensitivity in coupled vibration characteristics of CNT-strengthened beams with different boundary conditions

Structures face different types of imperfections and defects during the fabrication process, installation and working environment. In this paper, the imperfection effects in the coupled vibration behaviour of axially functionally graded carbon nanotube (CNT)-strengthened beam structures with different boundary conditions are analysed considering porosity as well as geometric and mass imperfections in the structure. Porosity is modelled using different types of formulations for simple-cell, open-cell and closed-cell porous structures. The porosity is assumed to be either uniform or by varying through the thickness of the hollow beam using different functions. Mass imperfection effect is added to the system by considering a concentrated mass in the system affecting the mass homogeneity of the structure. Geometry imperfection is also considered by having an initial deformation in the structure which could be caused by an improper fabrication process. Coupled axial and transverse equations of motion are obtained using Hamilton’s principle and the von Karman geometrical nonlinearity. Governing equations are solved for different types of boundary conditions using a semi-analytical modal decomposition technique. It is shown that strengthening the base matrix with CNT fibres can improve the vibration behaviour of imperfect structures and the influence of CNT volume fraction and distribution through the length of the beam is discussed. The results provided in this paper may be used as a benchmark to validate future experimental results to prevent imperfection, delamination and stress singularities in the structures.

Impact Behavior of Additively Manufactured Stainless Steel Auxetic Structures at Elevated and Reduced Temperatures

Metamaterials produced using additive manufacturing represent advanced structures with tunable properties and deformation characteristics. However, the manufacturing process, imperfections in geometry, properties of the base material as well as the ambient and operating conditions often result in complex multiparametric dependence of the mechanical response. As the lattice structures are metamaterials that can be tailored for energy absorption applications and impact protection, the investigation of the coupled thermomechanical response and ambient temperature‐dependent properties is particularly important. Herein, the 2D re‐entrant honeycomb auxetic lattice structures additively manufactured from powdered stainless steel are subjected to high strain rate uniaxial compression using split Hopkinson pressure bar (SHPB) at two different strain rates and three different temperatures. An in‐house developed cooling and heating stages are used to control the temperature of the specimen subjected to high strain rate impact loading. Thermal imaging and high‐speed cameras are used to inspect the specimens during the impact. It is shown that the stress–strain response as well as the crushing behavior of the investigated lattice structures are strongly dependent on both initial temperature and strain rate.

Image-based material characterization of complex microarchitectured additively manufactured structures

Significant developments in the field of additive manufacturing (AM) allowed the fabrication of complex microarchitectured components with varying porosity across different scales. However, due to the high complexity of this process, the final parts can exhibit significant variations in the nominal geometry. Computed tomographic images of 3D printed components provide extensive information about these microstructural variations, such as process-induced porosity, surface roughness, and other undesired morphological discrepancies. Yet, techniques to incorporate these imperfect AM geometries into the numerical material characterization analysis are computationally demanding. In this contribution, an efficient image-to-material-characterization framework using the high-order parallel Finite Cell Method is proposed. In this way, a flexible non-geometry-conforming discretization facilitates mesh generation for very complex microstructures at hand and allows a direct analysis of the images stemming from CT-scans. Numerical examples including a comparison to the experiments illustrate the potential of the proposed framework in the field of additive manufacturing product simulation.

Influence of local deformation on critical current of high temperature superconductor tape

In many applications, high temperature superconductor (HTS) tapes are placed on top of uneven surfaces. Because the electrical properties of HTS tapes are sensitive to mechanical loading, mechanical stress generated at such local imperfections can considerably decrease the critical current of a HTS tape. Then a method of predicting degradation of critical current from a numerical simulation would be helpful.We present the results of 3D mechanical finite element model analysis, using the software ANSYS for optimising the geometry of a round CORT (Conductor-On-Round-Tube) cable. In these simulations, the HTS tape is bent around a circular former of certain diameter with a surface imperfection. We used three parameters to characterize the imperfection geometry: diameter of the former, the height and the width of imperfection. In order to validate the mechanical simulation results, we performed in-situ measurement of critical current of HTS tape.

Stress-based topology optimization under uncertainty via simulation-based Gaussian process

Gaussian processes (GP) form a well-established predictive tool which provides a natural platform for tackling high-dimensional random input data in challenging simulations. This paper introduces a generic framework for integrating Gaussian Processes with risk-based structural optimization. We solve robust and reliability-based design problems in the context of stress-based topology optimization under imperfections in geometry and material properties, and loading variability. We construct independent GPs for primal and adjoint quantities, namely the global maximum von Mises stress and its sensitivity where we enhance the computational efficiency by leveraging the information from multiresolution finite element simulations. The GP framework naturally lends itself to modeling noise in data. We investigate the effect of numerical modeling error in high-fidelity simulations via a noisy GP emulator and provide a pareto curve that shows the robustness of optimal design with respect to the noise level. We provide a posteriori error estimates that quantify the discrepancy between the noisy emulator and true simulations, and verify them with a numerical study. We demonstrate our approach on a benchmark L-shape structure which exhibits stress concentration, a compliant mechanism design and a heat sink design. We also provide practical guidelines for estimation of failure probability and its sensitivity to facilitate reliability-based topology optimization.

Design optimization and experimental performance test of dynamic flow balance valve

This paper considers a solution to the serious problem of hydraulic maladjustment in a dynamic flow balance valve for small diameter and low-pressure drop working conditions. The pressure drop compensation coefficient revising method was proposed to optimize the profile line of the valve core variable opening area. The flow rates of the valve before and after optimization under different displacements were simulated using Computational Fluid Dynamics (CFD) methods. The flow control accuracy of the balance valve before and after the optimization is compared and analyzed to the test values. At the same time, the influence of three key parameters: spring stiffness, the shell structure of the valve core, and geometry imperfections change due to machining burrs on the surface of the valve core for flow control accuracy was discussed. The results show that flow control accuracy after optimization satisfies the requirements of 5% flow rate control. Spring stiffness, the shell structure of the valve core and machining burrs on the surface of the valve core are shown to have an influence on the flow rate of the balance valve. This is especially true for machining burrs which has a larger effect with an average error of 10.1.

On the reinforcement of thin-walled cold-drawn C-channel beams

The paper considers the problem of stability of a thin-walled channel. Options for increasing the stability of the channel by changing its cross-section are proposed. Stiffness increased by adding triangular folds, as well as by adding stiffeners. The influence of initial imperfections of geometry in the channel model on the critical load of stability loss and postcritical behavior of the model is investigated. Imperfections were defined as deviations of the geometry of the pseudo-random amplitude.

Designing efficient grid structures considering structural imperfection sensitivity

At the initial design stage of a grid structure, shape optimisation is an effective way to find the optimal structural form. However, most of the shape optimisation methods do not take into consideration the imperfections, thus the actual buckling load capacity of the optimised structure is usually low. In this paper, an improved shape optimisation method is proposed, one that is considering the effect of structural imperfection sensitivity. In this method, the bending strain energy ratio is taken as a constraint, and when the total strain energy decreases, yet there is a certain proportion of bending strain energy in the structure. Consequently, the resulted shape is not sensitive to the initial geometry imperfection, and therefore, an efficient structure with higher buckling load capacity and low imperfection sensitivity is obtained. In order to evaluate the redundancy performance of the optimised structure, an index called structural overall redundancy, based on damage model is proposed herein. The damage model is simulated by removing a key rod of the structure. The results demonstrate that the overall redundancy of the structure obtained by the proposed method is higher than that obtained by the traditional method, thus an optimal design of a grid structure is obtained.

Reviving the inter-laboratory comparison measurement results

Key comparison measurements serve as an ultimate tool of quality assurance of results. Whenever the inter-comparison results indicate inconsistency, the participating laboratory needs to take the corrective actions. Practically, the systematic errors involved in the measuring system confines the achievable accuracy. Therefore, the corrective action involves either empirically determine the influences afresh or intuitively reassigns these error values. Alternatively, an analytical method based on inter laboratory comparison results is proposed. The novelty of the proposal is considering task-specific errors in the model that is used for the analysis of interlaboratory comparison results. Without accounting the uncertainties of task-specific errors, the analysis grows complicated and even sometimes it is not feasible. To supplement the proposed method, task-specific errors due to the imperfect geometry of ring gauge, practical inability in implementing the measurement, and unattended environmental influences are explored. The proposed method is demonstrated using some internal diameter key comparison data. The systematic errors responsible for the outlier in the measurement comparison are clearly distinguished.

Influence of AlSi10Mg particles microstructure on heat conduction during additive manufacturing

Powder-based metallic additive manufacturing (AM) technology is opening new avenues to fabricate highly complex components from metallic powders. However, most of the powder-scale modeling methods are limited to single track process and ideal particle microstructure. Nevertheless, the presence of hollow particles significantly influences the heat conduction during AM processing and experimental quantification of the heat conduction between hollow particles is extremely challenging. Herein, we have used X-ray micro-computed tomography (μCT) to reconstruct 3D structures of AlSi10Mg particles. The morphology, location and distribution of intact and hollow particles are studied to analyze their role in AM processing. Based on X-ray tomography images, two 3D image-based finite element models of statistically representative particles with imperfect geometry are reconstructed and compared to simulate the thermal conduction in the powder bed. Simulation results shows that thermal conduction is governed not only by cell topology but also by cavities in particles induced by powder production. The calculation results are consistent with the Serial-Parallel Model, which is based on the reconstruction geometry model and statistical results. The results reveal that the presence of cavities in particles significantly influences the thermal conduction and, consequently, reduces the sintered density during selective laser sintering (SLS).

Investigation on single-bolted cold-formed steel angles with geometric imperfections under compression

The behaviour of cold-formed steel angle members with bolted connections under compression is usually complicated, whereas not significant number of experimental tests is found in the literature. This study offers a better insight on the issue, by comparing experimental tests with a thorough numerical study, both carried out at the Institute of Steel Structures of NTUA. Significant attention is paid for the elaborate measurement of initial geometric imperfections along the member's length. An in-house built deformation plotter was designed and upgraded for this reason. The compression failure loads of the experimental tests are illustrated both in graphical and tabular form and compared with similar tests of the literature. The results denote a prediction of the EN 1993-3-1 towards the safe side, for the case of the examined bolted cold-formed angles. A simple formula is proposed for the accurate prediction of angle columns' buckling deformation, wherein the rigidity of the connection is also included. Finally, interesting conclusions are derived for the numerical simulation of cold-forming effects, as well as the shape and amplitude of the initial imperfect geometry, on the basis of the critical buckling load. Further research is in progress, aiming to capture the stochastic buckling response for the case of bolted cold-formed steel angles.

Symmetry-guided nonrigid registration: The case for distortion correction in multidimensional photoemission spectroscopy

Image symmetrization is an effective strategy to correct symmetry distortion in experimental data for which symmetry is essential in the subsequent analysis. In the process, a coordinate transform, the symmetrization transform, is required to undo the distortion. The transform may be determined by image registration (i.e. alignment) with symmetry constraints imposed in the registration target and in the iterative parameter tuning, which we call symmetry-guided registration. An example use case of image symmetrization is found in electronic band structure mapping by multidimensional photoemission spectroscopy, which employs a 3D time-of-flight detector to measure electrons sorted into the momentum (kx, ky) and energy (E) coordinates. In reality, imperfect instrument design, sample geometry and experimental settings cause distortion of the photoelectron trajectories and, therefore, the symmetry in the measured band structure, which hinders the full understanding and use of the volumetric band mapping datasets. We demonstrate that symmetry-guided registration can correct the symmetry distortion in the momentum-resolved photoemission patterns. Using proposed symmetry metrics, we show quantitatively that the iterative approach to symmetrization outperforms its non-iterative counterpart in the restored symmetry of the outcome while preserving the average shape of the photoemission pattern. Our approach is generalizable to distortion corrections in different types of symmetries and should also find applications in other experimental methods that produce images with similar features.

BEHAVIOUR OF VERTICAL CYLINDRICAL TANK WITH LOCAL WALL IMPERFECTIONS

Design of modern thin-walled metal structures is widely used around the world. In recent decades, more comprehensive research is carried out to investigate the behaviour of various thin-walled structures. Generally, the structure with regular geometry is investigated. In various countries such as USA, Russia, and the European Union issued the standards on regulation of the construction, design and maintenance of thin-walled structures. The actually used period of tanks usually is longer than recommendatory period. Recommendatory maintenance period of metal tanks is 15–20 years. Therefore, for such structures one of the most considerable questions is the residual load bearing capacity beyond the end of the maintenance period. This phase of using of structures is associated with complex investigation and numerical analysis of thin-walled structures. In this paper the load bearing capacity of the steel wall of the existing over-ground vertical cylindrical tank in volume of 5,000 m3 with a single defect and with a few contiguous local defects of the shape is analyzed. Calculations carried out are taking into account all the imperfections of the wall geometry. A major goal of the research – developing a realistic numerical model of the object analyzed, taking into account all the imperfections, determining the wall stress and strain state, exploring the places of extreme points, calculating the residual load bearing capacity of the tank and scrutinizing possible strengthening schemes for defective areas.

Parallel experiments of numerical and full-scale models of flange joints of the beam-column type

The paper discusses a multi-factor experiment method for determining the strength and stiffness of flanged beam-to-column connections. The method suggests two identical models – full-scale and numerical development. Models are matched by geometry and initial imperfections, physical properties of materials, boundary conditions and full loading history. They are matched by displacement and deformation at each loading stage of the models. When the results match, the conducted experiments can be stated and obtained mathematical models are correct and valid, hence using expensive full-scale models in further research can be avoided.

Effects of Sample Geometry Imperfections on the Results of Split Hopkinson Pressure Bar Experiments

The problem of specimen geometry imperfections for ductile materials in the split Hopkinson pressure bar (SHPB) experiments is presented in this paper. Impact of five types of imperfections most frequently encountered in experimental practice and the resulting errors in the position of the specimen in relation to the axis of the bars on the reflected and the transmitted wave profile and on the shape of the stress-strain curve was analysed. The problem was considered based on numerical analyses using a finite element method. It was found that imperfections disturb mainly the beginning and end portions of the reflected and transmitted pulses, which is reflected in the stress-strain curve profile. However, for ductile materials, influence of specimen geometrical imperfections is small, and therefore the SHPB experiments results can be considered reliable from a practical point of view. In the case of all the analysed imperfections, it can be assumed that for imperfection angles α ≤ 0.3°, errors in determination of the stress-strain curves can be omitted.

The effect of longitudinal imperfections on thin-walled conical shells

The load carrying capacity as well as the buckling and post-buckling behavior of conical thin-walled shells exposed to pressure loads are very sensitive to imperfections in the initial geometry. In this study, comprehensive work on the overall longitudinal imperfections created by welding and their effects on external pressure load carrying capacity has been performed by using finite-element models. The models were modified to include either one or two-line imperfection with amplitudes of six different magnitudes. The results presented here have confirmed some of the existing theories and provided new information concerning buckling of thin-walled conical shells. The load carrying capacity from buckling analysis was double the result from the Jawad theory for perfect models (without imperfection). In this research, increasing the cone height increased the imperfections’ weakening effect. In nonlinear analyses, the presence of an imperfection increased the buckling load capacity. However, increasing the number of imperfections resulted in less buckling load capacity than the model with one imperfection.

Deep appearance models for face rendering

We introduce a deep appearance model for rendering the human face. Inspired by Active Appearance Models, we develop a data-driven rendering pipeline that learns a joint representation of facial geometry and appearance from a multiview capture setup. Vertex positions and view-specific textures are modeled using a deep variational autoencoder that captures complex nonlinear effects while producing a smooth and compact latent representation. View-specific texture enables the modeling of view-dependent effects such as specularity. In addition, it can also correct for imperfect geometry stemming from biased or low resolution estimates. This is a significant departure from the traditional graphics pipeline, which requires highly accurate geometry as well as all elements of the shading model to achieve realism through physically-inspired light transport. Acquiring such a high level of accuracy is difficult in practice, especially for complex and intricate parts of the face, such as eyelashes and the oral cavity. These are handled naturally by our approach, which does not rely on precise estimates of geometry. Instead, the shading model accommodates deficiencies in geometry though the flexibility afforded by the neural network employed. At inference time, we condition the decoding network on the viewpoint of the camera in order to generate the appropriate texture for rendering. The resulting system can be implemented simply using existing rendering engines through dynamic textures with flat lighting. This representation, together with a novel unsupervised technique for mapping images to facial states, results in a system that is naturally suited to real-time interactive settings such as Virtual Reality (VR).

Design and analysis on a model sphere made of maraging steel to verify the applicability of the current design code

ABSTRACTA bolt-connected deep-sea manned cabin made of maraging steel 18Ni(250) with an inner diameter of 2.1 m is under development, which will be applied to full ocean depth. As a promising guideline, the design formula by CCS is considered to be applied. However, the formula is originally derived based on titanium alloys with a strength level of 800 MPa. Whether it can be used for manned cabin of maraging steel in case of no change in the design coefficients should be checked. A model sphere will be designed and tested to collapse under uniform external pressure inside a high-pressure chamber to check the effectiveness and practicability of the design formula and the manufacture technology. Results of ultimate strength predicted or calculated by the design formula and direct finite element methods are compared with the experimentally obtained value. The influence of simplification on material properties and initial geometry imperfection is considered and discussed.

A Framework for Assumption-Free Assessment of Imperfect Geometry of a Linac C-Arms

In the present paper a general setup for determination of imperfect geometry of radiotherapeutic devices has been proposed that base on geometric algebra framework. To account for this imperfect geometry, two methods of a calibration were presented, consisting of determining for each angular position of a gantry a correction shift which must be applied to the origin of a laboratory frame of reference to place it along a radiation axis for this angular position. Closed form solutions for these corrections are provided.