Types Of Imperfections Research Articles

Bifurcations with imperfect SO(2) symmetry and pinning of rotating waves

Rotating waves are periodic solutions in SO(2) equivariant dynamical systems. Their precession frequency changes with parameters and it may change sign, passing through zero. When this happens, the dynamical system is very sensitive to imperfections that break the SO(2) symmetry and the waves may become trapped by the imperfections, resulting in steady solutions that exist in a finite region in parameter space. This is the so-called pinning phenomenon. In this study, we analyse the breaking of the SO(2) symmetry in a dynamical system close to a Hopf bifurcation whose frequency changes sign along a curve in parameter space. The problem is very complex, as it involves the complete unfolding of high codimension. A detailed analysis of different types of imperfections indicates that a pinning region surrounded by infinite-period bifurcation curves appears in all cases. Complex bifurcational processes, strongly dependent on the specifics of the symmetry breaking, appear very close to the intersection of the Hopf bifurcation and the pinning region. Scaling laws of the pinning region width and partial breaking of SO(2) to Z m are also considered. Previous as well as new experimental and numerical studies of pinned rotating waves are reviewed in the light of the new theoretical results.

Effective properties of periodic fibrous electro-elastic composites with mechanic imperfect contact condition

In this work, two-phase parallel fiber-reinforced periodic piezoelectric composites are considered wherein the constituents exhibit transverse isotropy and the cells have different configurations. Mechanical imperfect contact at the interface of the composites is studied via linear spring model. The statement of the problem for two phase piezoelectric composites with mechanical imperfect contact is given. The local problems are formulated by means of the asymptotic homogenization method (AHM) and their solutions are found using complex variable theory. Analytical formulae are obtained for the effective properties of the composites with spring imperfect type of contact and different parallelogram cells. Some numerical examples and comparisons with other theoretical results illustrate that the model is efficient for the analysis of composites with presence of parallelogram cells and the aforementioned imperfect contact.

Thermal Transport across Solid Interfaces with Nanoscale Imperfections: Effects of Roughness, Disorder, Dislocations, and Bonding on Thermal Boundary Conductance

The efficiency in modern technologies and green energy solutions has boiled down to a thermal engineering problem on the nanoscale. Due to the magnitudes of the thermal mean free paths approaching or overpassing typical length scales in nanomaterials (i.e., materials with length scales less than one micrometer), the thermal transport across interfaces can dictate the overall thermal resistance in nanosystems. However, the fundamental mechanisms driving these electron and phonon interactions at nanoscale interfaces are difficult to predict and control since the thermal boundary conductance across interfaces is intimately related to the characteristics of the interface (structure, bonding, geometry, etc.) in addition to the fundamental atomistic properties of the materials comprising the interface itself. In this paper, I review the recent experimental progress in understanding the interplay between interfacial properties on the atomic scale and thermal transport across solid interfaces. I focus this discussion specifically on the role of interfacial nanoscale “imperfections,” such as surface roughness, compositional disorder, atomic dislocations, or interfacial bonding. Each type of interfacial imperfection leads to different scattering mechanisms that can be used to control the thermal boundary conductance. This offers a unique avenue for controlling scattering and thermal transport in nanotechnology.

Surface deposits in laser brazing

Laser brazing is a joining technology which is typically employed in areas with particularly high quality requirements, e.g. to create zero gap roof joints in automotive production. These seams have to be leak proof and are situated in an area of the car body which is in direct view of the customer. Hence, seam defects and imperfections are inacceptable and require extensive rework whenever they occur. Resulting high rework costs can be avoided by investigating the causes of these imperfections in order to prevent their formation. One type of imperfection which may occur during the laser brazing of galvanised steel are dark deposits on the seam surface. This paper examines the mechanisms leading to the formation of this type of imperfection. Results on the properties of surface deposits are presented along with conclusions regarding the systematic reasons behind their formation. Finally, strategies for preventing the occurrence of surface deposits are recommended.

Finite element analysis on thermal upheaval buckling of submarine burial pipelines with initial imperfection

In-service hydrocarbons must be transported at high temperature and high pressure to ease the flow and prevent the solidification of the wax fraction. The pipeline containing hot oil will expand longitudinally due to the rise in temperature. If such expansion is resisted, for example by frictional effects over a kilometer or so of pipeline, compressive axial stress will be built up in the pipe-wall. The compressive forces are often so large that they induce vertical buckling of buried pipelines, which can jeopardize the structural integrity of the pipeline. A typical initial imperfection named continuous support mode of submarine pipeline was studied. Based on this type of initial imperfection, the analytical solution of vertical thermal buckling was introduced and an elastic-plasticity finite element analysis (FEA) was developed. Both the analytical and the finite element methodology were applied to analyze a practice in Bohai Gulf, China. The analyzing results show that upheaval buckling is most likely to build up from the initial imperfection of the pipeline and the buckling temperature depends on the amplitude of initial imperfection. With the same amplitude of initial imperfection, the triggering temperature difference of upheaval buckling increases with covered depth of the pipeline, the soil strength and the friction between the pipeline and subsoil.

Carrier Frequency Offset and I/Q Imbalance Compensation for MB-OFDM Based UWB System

Two types of wireless communication system imperfections, namely carrier frequency offset (CFO) and in-phase/quadrature-phase (I/Q) imbalance, are addressed in this paper. We propose an efficient time domain CFO and I/Q imbalance estimation and compensation scheme for multi-band orthogonal frequency division multiplexing based ultra-wideband system. In this scheme, a data-aided CFO estimation algorithm, which is robust to a large I/Q imbalance, is presented. Also, a time domain I/Q imbalance estimation algorithm based on partially CFO compensated preambles is introduced. Finally a two-step joint CFO and I/Q imbalance compensation scheme is developed. Taking full advantage of the ECMA-368 preamble symbols, the proposed scheme is competent for large I/Q imbalance (2-dB gain error and 20-deg Phase error) and carrier frequency offset (50 ppm), and the results are confirmed by simulations.

Influence of imperfections on the out-of-plane flexural strength of post-tensioned masonry walls

The influence of imperfections on the response of simply-supported post-tensioned masonry (PTM) walls subject to out-of-plane loading is investigated. Two types of imperfections of the masonry and/or tendons are studied: a linear misalignment and an initial deflection defined using a sinusoid. Walls with a range of slenderness ratios are considered, in addition to walls with restrained and unrestrained tendons. By means of finite element studies, imperfections were found to affect the nominal flexural strength of the walls by as much as 10% for a linear imperfection and 20% for an initial deflection. However, the tolerance limits on the placement of masonry and post-tensioning tendons, coupled with the conservatism implicit in the nominal flexural strength formulas developed for design use in the US, appear to be sufficient to offset those differences, thus precluding consideration of accidental eccentricity for PTM walls.

Robust topology optimization accounting for misplacement of material

The use of topology optimization for structural design often leads to slender structures. Slender structures are sensitive to geometric imperfections such as the misplacement or misalignment of material. The present paper therefore proposes a robust approach to topology optimization taking into account this type of geometric imperfections. A density filter based approach is followed, and translations of material are obtained by adding a small perturbation to the center of the filter kernel. The spatial variation of the geometric imperfections is modeled by means of a vector valued random field. The random field is conditioned in order to incorporate supports in the design where no misplacement of material occurs. In the robust optimization problem, the objective function is defined as a weighted sum of the mean value and the standard deviation of the performance of the structure under uncertainty. A sampling method is used to estimate these statistics during the optimization process. The proposed method is successfully applied to three example problems: the minimum compliance design of a slender column-like structure and a cantilever beam and a compliant mechanism design. An extensive Monte Carlo simulation is used to show that the obtained topologies are more robust with respect to geometric imperfections.

Wood Knot Classification using Bagging

The quality of the wood is determined by the number of defects and its distribution. In a piece of timber, the most common type of imperfection is called knot that decreases the strength of the wood. Manual selection and classification process of knots is tedious and time consuming job. An automatic sensing machine is able to inspect wood automatically and correctly identify the defects it possess, and its effect on the quality of the final product. In this paper, it is proposed to detect and classify the knots in timber boards. The image of knots is pre processed using Hilbert transform and Gabor filters. The features obtained from pre processing, is classified using data mining techniques and compared with bagging technique. Index Terms Wood Knots, Hilbert Transforms, Gabor Filter, Naive Bayes, Radial Basis Function.

Numerical estimation of stresses in railway axles using a train–track interaction model

The fatigue design of railway axles requires that the stresses arising in the axle in real service are accurately quantified. This paper describes a method to compute the dynamic stresses arising in railway axles as the effect of train–track interaction, based on the numerical simulation of the dynamic interaction between a flexible wheelset and a flexible track. The wheelset is modelled as a flexible rotating body using an Eulerian approach, whereas track is regarded as an infinite periodic system with the rail modelled as a Timoshenko beam resting on discrete elastic supports, considering the inertia associated with the sleepers.The paper presents an application of the proposed procedure to the calculation of the dynamic stresses caused in the axle by different types of geometric imperfection occurring on the wheel and rail surfaces, considering the cases of a single harmonic rail corrugation, random rail roughness and a wheelflat.

A numerical evaluation of the influence of defects on the elastic modulus of single and multi-walled carbon nanotubes

Finite element models of single-walled and multi-walled carbon nanotubes in their perfect and fundamental forms (zigzag and armchair) were constructed. Then, after obtaining the mechanical properties of perfect carbon nanotubes, three types of imperfections, i.e., doping with Si atoms, carbon vacancy and perturbation of the ideal location of the carbon atom were introduced in different amounts to the perfect models to make them imperfect. Finally, the mechanical properties of the imperfect carbon nanotubes were numerically simulated and compared with those of perfect ones. Simple relations which predict the change of Young’s modulus as a function of the imperfection percentage were derived. The results show that the existence of any kind of imperfection in the perfect models leads to lower stiffness values. This study allows to realistically judge any simulation based on perfect structures and gives for the first time a good estimate to which extend the values based on perfect structures must be lowered in order to account for common imperfections to predict the mechanical properties of carbon nanotubes which are nowadays used in the production of advanced nanocomposites and reinforced materials.

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150°C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~50J/m−2 for as-sprayed TBCs to ~120J/m−2 for the TBCs exposed at 1150°C for 200h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.

Statistical characterisation and modelling of random geometric imperfections in cylindrical shells

Thin cylindrical shells are the most prevalent and important structural component of vessels across the process industries. Such structures are prone to accidental buckling due to inadvertently induced vacuum. Minor deviations in the nominal geometry of the shell can affect the apparent initial buckling load. One common deviation is that the radius of the vessel is not constant but rather varies randomly with location on the shell. This paper presents extensive experimental data permitting a full statistical characterisation of defects of this nature. The data was obtained from detailed measurements of 39 replicate test vessels at the laboratory scale. Both amplitude and frequency content of this type of imperfection is quantified. Furthermore a methodology whereby the variation in radius is characterised as a two dimensional random field is outlined. An algorithm to generate realisations of this field is developed and the output is shown to be consistent with the measured results.

Experimental Behaviour of Concrete Filled Steel Tubes (CFST) with Initial Concrete Imperfection Subjected to Eccentric Compression

Gap between steel tube and concrete core could be recognized as a type of initial concrete imperfection in concrete-filled steel tubular (CFST) members. This paper is an attempt to study the effect of gap on the behaviour of concrete-filled steel tubular (CFST) columns subjected to eccentric compression. A total 14 specimens were tested and the main parameters were the gap type (circumferential gap and spherical-cap) and gap ratio. The influence of gap on the failure mode and ultimate strength of CFST columns were experimentally examined.

Supervisory Control of Multiple Robots

A military multitasking environment was simulated to examine the effects of an intelligent agent, RoboLeader, on the performance of robotics operators. The participants' task was to manage a team of ground robots with the assistance of RoboLeader, an intelligent agent capable of coordinating the robots and changing their routes on the basis of battlefield developments. In the first experiment, RoboLeader was perfectly reliable; in the second experiment, RoboLeader's recommendations were manipulated to be either false-alarm prone or miss prone, with a reliability level of either 60% or 90%. The visual density of the targeting environment was manipulated by the presence or absence of friendly soldiers. RoboLeader, when perfectly reliable, was helpful in reducing the overall mission times.The type of RoboLeader imperfection (false-alarm vs. miss prone) affected operators' performance of tasks involving visual scanning (target detection, route editing, and situation awareness). There was a consistent effect of visual density (clutter of the visual scene) for multiple performance measures. Participants' attentional control and video gaming experience affected their overall multitasking performance. In both experiments, participants with greater spatial ability consistently outperformed their low-spatial-ability counterparts in tasks that required effective visual scanning. Intelligent agents, such as RoboLeader, can benefit the overall human-robot teaming performance. However, the effects of type of agent unreliability, tasking requirements, and individual differences have complex effects on human-agent interaction. The current results will facilitate the implementation of robots in military settings and will provide useful data to designs of systems for multirobot control.

Dynamics of simply supported fluid-conveying pipes with geometric imperfections

In this paper, the dynamics of simply supported fluid-conveying pipes with geometric imperfections is examined, by considering the integral–partial–differential equation of motion. The effect of sinusoidal wave or parabolic variations of imperfections is investigated for the four-degree-of-freedom (N=4) model of the system. Linear analysis shows that each type of imperfections affects the natural frequency of only one single mode. For half-sinusoidal wave or parabolic variation of imperfections, the critical flow velocity at which buckling instability occurs is higher than that for a pipe without imperfections. In all cases, the pipe remains in its undeformed static equilibrium state at low flow velocity. At high flow velocity; however, nonlinear analysis predicts that the pipe would be attracted to one of two other nontrivial equilibria, which, more importantly, may be asymmetric due to the presence of imperfections. For pipes with imperfection in the form of half-sinusoidal wave or parabolic variation, interestingly, the nonlinear theory predicts that a small buckling displacement would occur at flow velocities slightly lower than the critical flow velocity predicted by the linear theory.

System analysis of a tilted field-widened Michelson interferometer for high spectral resolution lidar

High spectral resolution lidars (HSRLs) have shown great value in aircraft aerosol remote sensing application and are planned for future satellite missions. A compact, robust, quasi-monolithic tilted field-widened Michelson interferometer is being developed as the spectral discrimination filter for an second-generation HSRL(HSRL-2) at NASA Langley Research Center. The Michelson interferometer consists of a cubic beam splitter, a solid arm and an air arm. Piezo stacks connect the air arm mirror to the body of the interferometer and can tune the interferometer within a small range. The whole interferometer is tilted so that the standard Michelson output and the reflected complementary output can both be obtained. In this paper, the transmission ratio is proposed to evaluate the performance of the spectral filter for HSRL. The transmission ratios over different types of system imperfections, such as cumulative wavefront error, locking error, reflectance of the beam splitter and anti-reflection coatings, system tilt, and depolarization angle are analyzed. The requirements of each imperfection for good interferometer performance are obtained.

VIBRATION OF CIRCULAR BLADED DISK WITH IMPERFECTIONS

The steady state response of a model of circular bladed disk with imperfection is investigated. Disk imperfection results from additional two groups of damping heads fixed on opposite ends of one diameter. These damping heads are introduced into the computing model as additional point mass, damping and stiffness. Such type of imperfection causes the bifurcation of double eigenfrequencies into pairs of close eigenfrequencies. The effect of imperfection is examined both numerically on three-dimensional nonrotating FE-model and analytically on a simplified split 2DOF model of rotating disk excited by single point harmonic force. Nonlinear friction connection is analyzed and equivalent linear damping coefficient is derived and used in the calculation procedure. It is shown that nonproportional distribution of damping strongly influences the high of resonance peaks. Some examples of response curves illustrate the dynamic properties of stationary and rotating disks with mass-damping-stiffness imperfection.

Preliminary study on ductile fracture of imperfect lattice materials

The ductile fracture behavior of two-dimensional imperfect lattice material under dynamic stretching is studied by finite element method using ABAQUS/Explicit code. The simulations are performed with three isotopic lattice materials: the regular hexagonal honeycomb, the Kagome lattice and the regular triangular lattice. All the three lattices are made of an elastic/visco-plastic metal material. Two typical imperfections: vacancy defect and rigid inclusion are introduced separately. The numerical results reveal novel deformation modes and crack growth patterns in the ductile fracture of lattice material. Various crack growth patterns as defined according to their profiles, “X”-type, “Butterfly”-type, “Petal”-type, are observed in different combinations of imperfection type and lattice topology. Crack propagation could induce severe material softening and deduce the plastic dissipation of the lattices. Subsequently, the effects of the strain rate, relative density, microstructure topology, and defect type on the crack growth pattern, the associated macroscopic material softening and the knock-down of total plastic dissipation are investigated.

La conoscenza e le reti: gli orizzonti competitivi del caso italiano e una riflessione metodologica sull’economia di impresa

This paper underlines the close relationship between knowledge and networks, even going so far as to argue for the importance of organising the production of knowledge in a network, and stressing the fact that networks are the ideal models for the development of knowledge itself. The paper, after pausing to consider the salient points concerning the Ford era, during which decision taking power was considered to be of the greatest importance, goes on to show how companies are no longer based on work of the manipulative type but rather how companies have become cognitive systems, based on work which aims to develop further knowledge and consolidate what is already known, in so doing it provides further value. This new concept leads us to see the importance of increasing knowledge within the economic system, and takes us then to the definition of networks as systems for communication and relationship forming, on two levels, as well as being fed by a morphogenetic process. From a methodological point of view, then, we move on to look at reality where imperfect network types exist and we present the American model (service economy) and the Japanese one (knowledge sharing). After that we present the Italian case too, which must include the ability to act on renovating industrial poilicy required in order for these networks to be created. Some possible alternatives are suggested regarding this matter including both flexibility and risk-sharing.