Types Of Imperfections Research Articles

Effect of imperfections on the hyperuniformity of many-body systems

A hyperuniform many-body system is characterized by a structure factor $S\left(\mathbit{k}\right)$ that vanishes in the small-wave-number limit or equivalently by a local number variance ${\ensuremath{\sigma}}_{N}^{2}\left(R\right)$ associated with a spherical window of radius $R$ that grows more slowly than ${R}^{d}$ in the large-$R$ limit. Thus, the hyperuniformity implies anomalous suppression of long-wavelength density fluctuations relative to those in typical disordered systems, i.e., ${\ensuremath{\sigma}}_{N}^{2}\left(R\right)\ensuremath{\sim}{R}^{d}$ as $R\ensuremath{\rightarrow}\ensuremath{\infty}$. Hyperuniform systems include perfect crystals, quasicrystals, and special disordered systems. Disordered hyperuniform systems are amorphous states of matter that lie between a liquid and crystal [S. Torquato et al., Phys. Rev. X 5, 021020 (2015)], and have been the subject of many recent investigations due to their novel properties. In the same way that there is no perfect crystal in practice due to the inevitable presence of imperfections, such as vacancies and dislocations, there is no ``perfect'' hyperuniform system, whether it is ordered or not. Thus, it is practically and theoretically important to quantitatively understand the extent to which imperfections introduced in a perfectly hyperuniform system can degrade or destroy its hyperuniformity and corresponding physical properties. This paper begins such a program by deriving explicit formulas for $S\left(\mathbit{k}\right)$ in the small-wave-number regime for three types of imperfections: (1) uncorrelated point defects, including vacancies and interstitials, (2) stochastic particle displacements, and (3) thermal excitations in the classical harmonic regime. We demonstrate that our results are in excellent agreement with numerical simulations. We find that ``uncorrelated'' vacancies or interstitials destroy hyperuniformity in proportion to the defect concentration $p$. We show that ``uncorrelated'' stochastic displacements in perfect lattices can never destroy the hyperuniformity but it can be degraded such that the perturbed lattices fall into class III hyperuniform systems, where ${\ensuremath{\sigma}}_{N}^{2}\left(R\right)\ensuremath{\sim}{R}^{d\ensuremath{-}\ensuremath{\alpha}}$ as $R\ensuremath{\rightarrow}\ensuremath{\infty}$ and $0l\ensuremath{\alpha}l1$. By contrast, we demonstrate that certain ``correlated'' displacements can make systems nonhyperuniform. For a perfect (ground-state) crystal at zero temperature, increase in temperature $T$ introduces such correlated displacements resulting from thermal excitations, and thus the thermalized crystal becomes nonhyperuniform, even at an arbitrarily low temperature. It is noteworthy that imperfections in disordered hyperuniform systems can be unambiguously detected. Our work provides the theoretical underpinnings to systematically study the effect of imperfections on the physical properties of hyperuniform materials.

Nonlinear behaviour of short elastic cylindrical shells under global bending

A recent computational study identified four distinct domains of stability behaviour at different lengths in thin elastic cylindrical shells under global bending. Cylinders of sufficient length suffer from fully-developed cross-sectional ovalisation and fail by local buckling at a moment very close to the Brazier prediction. Progressively shorter cylinders experience less ovalisation owing to the increasingly strong restraint provided by the boundary at the edges. Very short thin cylinders, however, restrain the formation of even a local buckle and fail through a limit point instability at moments and curvatures significantly in excess of the classical elastic prediction. This limit point behaviour is not caused by ovalisation but by the growth of a destabilising fold on the compressed meridian.The nonlinear behaviour of very short cylinders under global bending is investigated in detail herein, covering a wide range of lengths, radius to thickness ratios and boundary conditions with both restrained and unrestrained meridional rotations corresponding to ‘clamped’ and ‘simply-supported’ conditions respectively. Two types of imperfections are investigated, the critical buckling eigenmode and a realistic manufacturing-related ‘weld depression’. A complex insensitivity to these imperfections is revealed owing to a pre-buckling stress state dominated by local compatibility bending, and the cylinder length is confirmed as playing a crucial role in governing this behaviour. The study contributes to the characterisation of multi-segment shells with very short individual cylindrical segments, often found in the aerospace and marine industries as well as in specialised civil engineering applications such as LIPP® silos.

Communication with Crystal-Free Radios

We consider a communication channel where there is no common clock between the transmitter and the receiver. This is motivated by the recent interest in building system-on-chip radios for Internet of Things applications, which cannot rely on crystal oscillators for accurate timing. We identify two types of clock uncertainty in such systems: timing jitter, which occurs at a time scale faster than the communication duration (or equivalently blocklength), and clock drift, which occurs at a slower time scale. We study the zero-error capacity under both types of timing imperfections and obtain optimal zero-error codes for some cases. Our results show that, as opposed to common practice, in the presence of clock drift, it is highly suboptimal to try to learn and track the clock frequency at the receiver; rather, one can design codes that come close to the performance of perfectly synchronous communication systems without any clock synchronization at the receiver.

Ballast bed cleaning and recycling – influence on stability of continuously welded rail track

The ballast bed main functions are to bind the sleepers to the substructure, to distribute the dynamic loads from the vehicle to the embankment and to ensure the track stability. Fouling may increase in time due to various reasons, under normal dynamic loads. If the ballast bed functions are no longer fulfilled, the welded track can lose its stability with highly damaging results. The purpose of the paper is to investigate the ballast bed fouling degree on welded tracks, with potential loss of stability. Once the ballast bed elasticity decreases and the geometry of the track cross section is modified, the track is subjected to longitudinal axial compression due to thermal loads that can result in stability lost: buckling of the rail. The ballast bed is required to ensure lateral and longitudinal resistance to the track. The lateral resistance force depends on the ballast depth, material properties, maintenance and tamping. The study uses the Energy Method to determine the critical stability force on fouled ballast bed conditions, with irregular settlements due to insufficient track maintenance. Continuously welded track stability was studied for three different fouling cases, both on curved track and on straight line, with various types of horizontal geometrical imperfections, using the maximal thermal effort as a reference. The results of the critical stability force determined using the Energy Method were compared to a real study case situation from a rail sector were ballast bed cleaning was performed. The paper aims to correlate the fouling degree of the ballast bed to continuously welded track stability problems due to thermal effect in connection with insufficient track maintenance. The critical stability force in three various situations were compared to the maximum temperature effort due to thermal load taking into consideration the geometrical aspect of the line, fouling degrees and existing geometrical imperfections.

Buckling and postbuckling of carbon nanotube-reinforced composite cylindrical panels subjected to axial compression in thermal environments

This paper presents an analytical investigation on the buckling and postbuckling behavior of thin composite cylindrical panels reinforced by single walled carbon nanotubes (SWCNTs), exposed to thermal environments and subjected to uniform axial compression. Material properties of isotropic matrix phase and carbon nanotubes are assumed to be temperature dependent, and effective properties of carbon nanotube-reinforced composite (CNTRC) are functionally graded in the thickness direction and estimated by extended rule of mixture. Governing equations are based on the classical thin shell theory taking von Karman-Donnell nonlinearity and initial geometrical imperfection into consideration. Approximate solutions are assumed to satisfy simply supported boundary conditions and Galerkin procedure is applied to derive explicit expressions of buckling loads and load-deflection relation. Effects of volume fraction and distribution type of carbon nanotubes, geometrical parameters, elevated temperature and initial imperfection on the nonlinear stability of CNTRC cylindrical panels are analyzed and discussed. The novelty of the present study is that closed-form results of buckling load and nonlinear load-deflection relation can be readily used to analyze the buckling and postbuckling behaviors of axially loaded CNTRC cylindrical panels.

An Extended Field-Based Method for Noise Removal From Electron Tomographic Reconstructions

Molecular structure determination is important for understanding functionalities and dynamics of macromolecules, such as proteins and nucleic acids. Cryo-electron tomography (ET) is a technique that can be used to determine the structures of individual macromolecules, thus providing the snapshots of their native conformations. Such 3-D reconstructions encounter several types of imperfections due to missing, corrupted, and low-contrast data. In this paper, we demonstrate that extending the reconstruction space, which increases the dimensionality of the linear system being solved during reconstruction, facilitates the separation of signal and noise. A considerable amount of the noise associated with collected projection data arises independently from the geometric constraint of image formation, whereas the solution to the reconstruction problem must satisfy such geometric constraints. Increasing the dimensionality thereby allows for a redistribution of such noise within the extended reconstruction space, while the geometrically constrained approximate solution stays in an effectively lower dimensional subspace. Employing various tomographic reconstruction methods with a regularization capability we performed extensive simulation and testing and observed that enhanced dimensionality significantly improves the accuracy of the reconstruction. Our results were validated with reconstructions of colloidal silica nanoparticles as well as P. falciparum erythrocyte membrane protein 1. Although the proposed method is used in the context of Cryo-ET, the method is general and can be extended to a variety of other tomographic modalities.

On the differentiation of the functional in distributed optimization problems with imperfect contact

We investigate issues of numerical solving of optimal control problems for second order elliptic equations with non-self-adjoint operators - convection-diffusion problems. Control processes are described by semi-linear convection-diffusion equation with discontinuous data and solutions (states) subject to the boundary interface conditions of imperfect type (i.e., problems with a jump of the coefficients and the solution on the interface; the jump of the solution is proportional to the normal component of the flux). Controls are involved in the coefficients of diffusion and convective transfer. We prove differentiability and Lipshitz continuity of the cost functional, depending on a state of the system and a control. The calculation of the gradients uses the numerical solutions of direct problems for the state and adjoint problems.

Nonlinear Dynamics of Imperfect FGM Conical Panel

Structures composed of functionally graded materials (FGM) can satisfy many rigorous requisitions in engineering application. In this paper, the nonlinear dynamics of a simply supported FGM conical panel with different forms of initial imperfections are investigated. The conical panel is subjected to the simple harmonic excitation along the radial direction and the parametric excitation in the meridian direction. The small initial geometric imperfection of the conical panel is expressed by the form of the Cosine functions. According to a power‐law distribution, the effective material properties are assumed to be graded along the thickness direction. Based on the first‐order shear deformation theory and von Karman type nonlinear geometric relationship, the nonlinear equations of motion are established by using the Hamilton principle. The nonlinear partial differential governing equations are truncated by Galerkin method to obtain the ordinary differential equations along the radial displacement. The effects of imperfection types, half‐wave numbers of the imperfection, amplitudes of the imperfection, and damping on the dynamic behaviors are studied by numerical simulation. Maximum Lyapunov exponents, bifurcation diagrams, time histories, phase portraits, and Poincare maps are obtained to show the dynamic responses of the system.

불안정애착유형 모의 관계놀이상담이 모-자녀 간 상호작용에 미치는 영향

불안정애착유형 모의 관계놀이상담이 모-자녀 간 상호작용에 미치는 영향

Sensitivity analysis approach to model epistemic and aleatory imperfection: Application to Land Cover Change prediction model

Most of the research on sensitivity analysis focus on models having only uncorrelated input parameters. In addition, the very few research that focus on models having correlated input parameters presume several assumptions about the parameters.This paper presents a sensitivity analysis approach that addresses the imperfections in a model for Land Cover Change (LCC) prediction. The main objective of this paper is to improve the decisions about the forthcoming LCC. The proposed approach addresses two main problems: (1) different types of imperfection related to the LCC prediction model (aleatory imperfection and epistemic imperfection) and (2) correlation between the input parameters.Results on real world data show that taking into account both types of imperfection in the process of LCC prediction and the correlation between parameters improve the resulting predictions about the land changes. Comparisons with existing methods are also discussed in terms of accuracy and computation time and they show that the proposed approach outperforms existing methods.

Structural and Electronic Properties of Defect‐Free and Defect‐Containing Polypropylene: A Computational Study by van der Waals Density‐Functional Method

This paper presents a study on structural and electronic properties of bulk polypropylene – one of the fastest growing engineering plastics – based on density‐functional calculations. The use of recently developed non‐local van der Waals functionals to approximate the exchange–correlation energy is crucial for a good description of structural properties. Various types of chemical imperfections, their impacts on electronic properties of polypropylene are also studied.

Crashworthiness design for trapezoid origami crash boxes

A thin-walled tube referred to as trapezoid origami crash box is recognized as an energy absorption device. The surface of this tube is prefolded in the light of a developable origami pattern that is delicately designed to introduce a special structure on the surface of a square tube. This special structure, known as trapezoid folded lobe, is employed as a type of geometric imperfection to lower the peak force and as a mode inducer to trigger the complete diamond mode. The quasi-static numerical simulations reveal that the complete diamond mode is successfully triggered. Moreover, geometric and compliance analysis suggest that three key parameters, the number of module M, dihedral angle θ and area ratio ω, could greatly affect the collapse behavior. Based on those analysis, an optimal trapezoid origami crash box is designed to compare with the conventional square and octagonal tubes of identical weight. Furthermore, a series of diamond origami crash boxes are analyzed to compare with the trapezoid origami crash boxes. The comparative results show that the trapezoid origami crash box is the most desirable in terms of energy absorption.

Investigation of optical sensor approaches for real-time monitoring during fibre laser welding

Laser beam welding can be a high productivity joining process, offering significant benefits over conventional welding methods. However, the need for real-time monitoring of laser welding processes is important to provide information on the possible presence of imperfections formed during welding and guarantee quality assurance in production. Laser process monitoring methods are based on the recording of physical phenomena occurring during the laser-material interaction. This paper has investigated an established optical-based monitoring method, using photodiodes responsive to either visible or near-infrared emissions, to correlate the photodiode signals with different types of weld features, imperfections and/or process anomalies. These have included occurrence of internal porosity, changes in penetration depth and beam to joint alignment, deliberate changes in laser power and intentionally poor joint preparation, cleanliness or fit-up. Also, a new type of commercially available process monitoring approach was investigated, based on laser interferometry, to measure keyhole depth. The results of both approaches have confirmed which types of weld features, imperfections and/or process anomalies are most sensitive to these monitoring techniques, with potential to then implement these findings in to a future multisensor solution for real-time process control.

Heterogeneity in polymer networks formed by a single copolymerization reaction: II. Post-gelation structure and pendants

Coarse-grained molecular dynamics simulations are used to study polymer formation, network growth and topology of 3- and 6-functional systems derived from reactive precursors. Part I of this sequence explored the process prior to gelation and showed that crosslinking produces heterogeneous clusters that grow and coalesce at the gel point. Not only are there small, localized defects in the network, but their dendritic structure results in poorly crosslinked regions where clusters coalesce, leading to pendant structures which persist even at high conversion. 6-functional networks exhibit a greater variety of pendants than the 3-functional networks. For 6-functional crosslinkers, some pendants have two- and three-fold connections to the elastically active network and comprise a substantial fraction of local material. Visualizations of these pendants show possible types of imperfection in crosslinked polymer networks. Crosslinked polymers used in many applications are non-ideal networks containing structural and crosslinking defects which inevitably diminish function and durability.

Probability-based methodology for buckling investigation of sandwich composite shells with and without cut-outs

ABSTRACTThe objective of the running EU project DESICOS (New Robust DESign Guideline for Imperfection Sensitive COmposite Launcher Structures) is to formulate an improved shell design methodology in order to meet the demand of aerospace industry for lighter structures. Within the project, this article discusses the development of a probability-based methodology developed at Politecnico di Milano. It is based on the combination of the Stress–Strength Interference Method and the Latin Hypercube Method with the aim to predict the bucking response of three sandwich composite cylindrical shells, assuming a loading condition of pure compression. The three shells are made of the same material, but have different stacking sequence and geometric dimensions. One of them presents three circular cut-outs. Different types of input imperfections, treated as random variables, are taken into account independently and in combination: variability in longitudinal Young's modulus, ply misalignment, geometric imperfections, and boundary imperfections. The methodology enables a first assessment of the structural reliability of the shells through the calculation of a probabilistic buckling factor for a specified level of probability. The factor depends highly on the reliability level, on the number of adopted samples, and on the assumptions made in modeling the input imperfections. The main advantage of the developed procedure is the versatility, as it can be applied to the buckling analysis of laminated composite shells and sandwich composite shells including different types of imperfections.

Labor Market Imperfections, Markups and Productivity in Multinationals and Exporters

This paper examines the links between the internationalization mode of firms and market imperfections in product and labor markets. We develop a framework for modelling heterogeneity across firms in terms of (i) product market power (price-cost markups), (ii) labor market imperfections (workers' bargaining power during worker-firm negotiations or firm's degree of wage-setting power) and (iii) revenue productivity. We apply this framework to analyze whether the pricing behavior of firms in product and labor markets differs across firms that engage in different forms of internationalization. Engagement in international activities is found to matter for determining not only the type of imperfections in product and labor markets but also the degree of imperfections. Clear differences in behavior between firms that serve the foreign market either through exporting or through FDI are observed. Being an exporter introduces allocative inefficiencies in product as well as labor markets as we find export status to be positively correlated with both product market power (markups) and market power consolidated on the labor supply side (workers' bargaining power). But exporting firms where search frictions are inducing wages to vary with revenue are less able to exploit wage-setting power. Firms with foreign subsidiaries, on the other hand, seem to reduce price distortions in product and labor markets. In addition, we observe heterogeneous returns to being an exporter/MNE within an industry and also discern cross-industry differences.

Vibration buckling and fracture analysis of a cracked cylindrical shell

Shell structures are susceptible to various types of imperfections and damage such as cracking, corrosion, chemical attack and time-dependent material degradation, which may impair their structural integrity and affect their service life. The effects of cracks are important considerations in the design of cylindrical shell structures as they influence on load carrying capacity and safety. This present work is a finite element investigation on the vibration, buckling and fracture analysis of a cracked cylindrical shell subjected to a time varying rotating speed. The effects of crack length, orientation of crack and length-diameter ratio of the cylindrical shell are investigated under free-vibration and buckling. The fracture parameters like stress intensity factor and J-integral of the cracked shell are also studied considering the influence of crack length and crack orientation. Results reveal that the crack length and crack orientation can affect the vibration, buckling behaviour of the cylindrical shell significantly.

Vibration buckling and fracture analysis of a cracked cylindrical shell

Shell structures are susceptible to various types of imperfections and damage such as cracking, corrosion, chemical attack and time-dependent material degradation, which may impair their structural integrity and affect their service life. The effects of cracks are important considerations in the design of cylindrical shell structures as they influence on load carrying capacity and safety. This present work is a finite element investigation on the vibration, buckling and fracture analysis of a cracked cylindrical shell subjected to a time varying rotating speed. The effects of crack length, orientation of crack and length-diameter ratio of the cylindrical shell are investigated under free-vibration and buckling. The fracture parameters like stress intensity factor and J-integral of the cracked shell are also studied considering the influence of crack length and crack orientation. Results reveal that the crack length and crack orientation can affect the vibration, buckling behaviour of the cylindrical shell significantly.

Labor Market Imperfections, Markups and Productivity in Multinationals and Exporters

This paper examines the links between the internationalization mode of firms and market imperfections in product and labor markets. We develop a framework for modelling heterogeneity across firms in terms of (i) product market power (price-cost markups), (ii) labor market imperfections (workers' bargaining power during worker-firm negotiations or firm's degree of wage-setting power) and (iii) revenue productivity. We apply this framework to analyze whether the pricing behavior of firms in product and labor markets differs across firms that engage in different forms of internationalization. Engagement in international activities is found to matter for determining not only the type of imperfections in product and labor markets but also the degree of imperfections. Clear differences in behavior between firms that serve the foreign market either through exporting or through FDI are observed. Being an exporter introduces allocative inefficiencies in product as well as labor markets as we find export status to be positively correlated with both product market power (markups) and market power consolidated on the labor supply side (workers' bargaining power). But exporting firms where search frictions are inducing wages to vary with revenue are less able to exploit wage-setting power. Firms with foreign subsidiaries, on the other hand, seem to reduce price distortions in product and labor markets. In addition, we observe heterogeneous returns to being an exporter/MNE within an industry and also discern cross-industry differences.

CNFET With Process Imperfection: Impact on Circuit-Level Yield and Device Optimization

Due to their superior material properties over bulk silicon, low-dimensional materials have been actively explored as potential candidates to replace or complement silicon technology in post-Moore era. As the first low-dimensional based transistor technology widely studied, carbon nanotube field effect transistor (CNFET) technology is among the most promising candidates to outperform silicon in the advanced technology nodes, because of its potential to achieve higher speed, better gate control, and better energy efficiency. However, existing fabrication processes for most emerging low-dimensional material based technologies including CNFETs, suffer from imperfections, which lowers the device and circuit yield. This paper presents a quantitative methodology for estimating circuit-level yield of CNFET CMOS circuits, which takes into account imperfections of CNT substrates based on a recently developed CMOS-compatible self-assembly process. The impact of different types of process imperfection on circuit-level yield is analyzed. The paper also provides a methodology using device sizing optimization to effectively improve circuit level yields with minimal impact on area and energy delay product, consequently relaxing the process requirements to satisfy certain circuit-level yield target. It is shown that an 80% pass rate can be achieved even with today’s process for various 4-stage cascade CMOS circuits through the proposed sizing optimization approach.