Finite Defects Research Articles

On similarity to contractions of class $$C_{\cdot 0}$$ with finite defects

On similarity to contractions of class $$C_{\cdot 0}$$ with finite defects

Laser ultrasound wave pattern analysis for efficient defect detection in samples with curved surfaces

Many production processes involve curved sample surfaces, such as welding or additive manufacturing. These pose new challenges to characterization methods for quality inspection, which are usually optimized for flat extended sample geometries. In this paper, we present a laser ultrasound (LUS) method that can be used to efficiently detect defects (e.g., voids), without extensive scanning effort and without a prior knowledge of the defect location, in finite samples with curved surfaces. The developed method starts with generalized simulations of the LUS wave patterns in samples with varying radii of curvature and width as well as varying excitation size and mechanism (thermoelastic or ablative). Based on the wave pattern analysis, it is possible to predict how every point in the weld can be reached with only few excitation spots. In a second step, we assume a grid of finite size defects at locations at which such voids are most likely formed and perform a thorough simulation analysis that is based on B-Scans to find a few pairs of excitation–detection points most favorable for finding defects anywhere in the weld seam. These results are then compared to the wave pattern analysis, discussing similarities and deviations from the predictions. In a final step, the simulations are compared to experimental results, verifying the almost threefold increase in the detectability of defects by choosing the predicted optimal excitation–detection positions. It is expected that this method will significantly improve the reliability and time efficiency of detecting internal defects in samples with curved surfaces in potential industrial applications.

Revealing Mode Formation in Quasi-Bound States in the Continuum Metasurfaces via Near-Field Optical Microscopy.

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high-quality factors and strong near-field enhancement. Here, the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique, is quantitatively investigated. It is found that the quasi-BIC mode is formed at a minimum size of 10×10-unit cells much smaller than expected from far-field measurements. Furthermore, it is shown that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy.

Diffraction by a set of collinear cracks on a square lattice: An iterative Wiener–Hopf method

The diffraction of a time-harmonic plane wave on collinear finite defects in a square lattice is studied. This problem is reduced to a matrix Wiener–Hopf equation. This work adapts the recently developed iterative Wiener–Hopf method to this situation. The method was motivated by wave scattering in continuous media but it is shown here that it can also be employed in a discrete lattice setting. The numerical results are validated against a different method using discrete Green’s functions. Unlike the latter approach, the complexity of the present algorithm is shown to be virtually independent of the length of the cracks.

A novel method for defects marking and classifying in MFL inspection of pipeline

Magnetic Flux Leakage (MFL) testing is the most widely used non-destructive technique for the inner inspection of oil and gas pipelines. Accurate quantification of defects is a long-standing difficulty in the field of pipeline leak detection. Scientific marking and classification of defects is an important prerequisite for accurate quantification. A novel method of marking and classifying defects with MFL signals is proposed, which is aiming at the problem that it is difficult to mark and classify defects accurately due to the tremendous amount of oil and gas pipeline leakage magnetic detection data. An improved CLIQUE algorithm is used to mark the defect areas of segmented pipelines to estimate the number and location of defects. Then the 3D MFL characteristic signals of the marked areas are studied and extracted. The SSA_BP neural network is trained to classify the defects. The effectiveness and accuracy of the proposed method are tested and verified by using finite element simulation defects and actual defects. The results show that the method is more efficient in marking defects and more detailed in marking areas.

Exploring high-frequency eddy-current testing for sub-aperture defect characterisation using parametric-manifold mapping

Accurate characterisation of small defects remains a challenge in non-destructive testing (NDT). In this paper, a principle-component parametric-manifold mapping approach is applied to single-frequency eddy-current defect characterisation problems for surface breaking defects in a planar half-space. A broad 1–8 MHz frequency-range FE-circuit model & calibration approach is developed & validated to simulate eddy-current scans of surface-breaking notch defects. This model is used to generate parametric defect databases for surface breaking defects in an aluminium planar half-space and defect characterisation of experimental measurements performed. Parametric-manifold mapping was conducted in N-dimensional principle component space, reducing the dimensionality of the characterisation problem. In a study characterising slot depth, the model & characterisation approach is shown to accurately invert the depth with comparable accuracy to an equivalent peak amplitude-phase inversion method. The approach is used to characterise the depth of a sloped slot demonstrating good accuracy up to ≈ 2.0 mm in depth over a broad range of frequencies, indicating applications in geometric feature inversion. Finally the technique is applied to finite rectangular notch defects of surface extents smaller than the diameter of the inspection coil (sub-aperture) over a range of sub-resonant frequencies. The results highlight the limitations in characterising these defects and indicate how the inherent modelling uncertainty around resonance can severely limit characterisation at these frequencies.

Моделювання скінченних неоднорідностей дискретними особливостями

This work focuses on development of a mathematical apparatus that allows to perform an approximate description of inhomogeneities of finite sizes in a continuous bodies by arranging the sources given on sets of smaller dimensions. The structure and properties of source densities determine the adequacy of the model. The theory of differential forms and generalized functions underlies this study. The boundary value problems with nonsmooth coefficients are formulated. The solutions of such problems is sought in the form of weakly convergent series and as an alternative - an equivalent recurrent set of boundary value problems with jumps. A feature of this approach is the ability to consistently improve the adequacy of the description of inhomogeneity. This is important because it allows to qualitatively assess the impact of real characteristic properties on the accuracy of the model description. Reducing the dimensions of inhomogeneities allows the use of efficient methods such as the Green's function and boundary integral equations to obtain a semi-analytic solution for direct and inverse problems. The work is based on a number of partial problems that demonstrate the proposed approach in modeling of inhomogeneities. The problems of modeling of the set of finite defects in an oscillating elastic beam, the set of inhomogeneities of an arbitrary shape in an oscillating plate, fragile cracks in a two-dimensional elastic body under static loading are considered.

Simulating the Shastry-Sutherland Ising Model Using Quantum Annealing

Frustration represents an essential feature in the behavior of magnetic materials when constraints on the microscopic Hamiltonian cannot be satisfied simultaneously. This gives rise to exotic phases of matter including spin liquids, spin ices, and stripe phases. Here we demonstrate an approach to understanding the microscopic effects of frustration by computing the phases of a 468-spin Shastry-Sutherland Ising Hamiltonian using a quantum annealer. Our approach uses mean-field boundary conditions to mitigate effects of finite size and defects alongside an iterative quantum annealing protocol to simulate statistical physics. We recover all phases of the Shastry-Sutherland Ising model -- including the well-known fractional magnetization plateau -- and the static structure factor characterizing the critical behavior at these transitions. These results establish quantum annealing as an emerging method in understanding the effects of frustration on the emergence of novel phases of matter and pave the way for future comparisons with real experiments.

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

Revealing the nature of nonequilibrium phase transitions with quantum trajectories

A damped and driven collective spin system is analyzed by using quantum state diffusion. This approach allows for a mostly analytical treatment of the investigated non-equilibrium quantum many body dynamics, which features a phase transition in the thermodynamical limit. The exact results obtained in this work, which are free of any finite size defects, provide a complete understanding of the model. Moreover, the trajectory framework gives an intuitive picture of the two phases occurring, revealing a spontaneously broken symmetry and allowing for a qualitative and quantitative characterization of the phases. We determine exact critical exponents, investigate finite size scaling, and explain a remarkable non-algebraic behaviour at the transition in terms of torus hopping.

Gravitational interactions of finite thickness global topological defects with black holes

It is well known that global topological defects induce a repulsive gravitational potential for test particles. 'What is the gravitational potential induced by black holes with a cosmological constant (Schwarzschild-de Sitter (S-dS) metric) on finite thickness global topological defects?'. This is the main question addressed in the present analysis. We also discuss the validity of Derrick's theorem when scalar fields are embedded in non-trivial gravitational backgrounds. In the context of the above question, we consider three global defect configurations: a finite thickness spherical domain wall with a central S-dS black hole, a global string loop with a S-dS black hole in the center and a global monopole near a S-dS black hole. Using an analytical model and numerical simulations of the evolving spherical wall we show that the spherical wall experiences a repelling gravitational potential due to the mass of the central black hole. This potential is further amplified by the presence of a cosmological constant. For initial domain wall radius larger than a critical value, the repulsive potential dominates over the wall tension and the wall expands towards the cosmological horizon of the S-dS metric where it develops ghost instabilities. For smaller initial radius, tension dominates and the wall contracts towards the black hole horizon where it also develops ghost instabilities. We also show, using the same analytical model and energetic arguments that a global monopole is gravitationally attracted by a black hole while a cosmological constant induces a repulsive gravitational potential as in the case of test particles. Finally we show that a global string loop with finite thickness experiences gravitational repulsion due to the cosmological constant which dominates over its tension for a radius larger than a critical radius leading to an expanding rather than contracting loop.

Creep turns linear in narrow ferromagnetic nanostrips.

The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.

Effects of tritium content on lattice parameter, 3He retention, and structural evolution during aging of titanium tritide

Effects of tritium content on crystal lattice, 3He retention and structure evolution during aging of Ti tritides were investigated using X-ray diffraction, together with analysis of 3He release. Phase transformation and lattice parameters of the Ti tritides varied apparently depending on tritium stoichiometry. Initial tritium content in the Ti tritides showed significant effects on 3He retention due to the existence of α + δ phase boundaries and lattice symmetry in the Ti tritide. The critical contents of 3He release in the α + δ phase region and ε phase region were found to be smaller than that in the δ phase region. Variation of crystal lattice structures of α + δ, δ and ε phases in the Ti tritides has been investigated, and evolution of 3He during aging is mainly governed by the finite defects of self-interstitial atoms, dislocation loops, 3He bubbles, and dislocations created by formation and growth of 3He bubbles. In the α + δ two phase region, the phase boundaries played an important role to accumulate 3He bubbles and cause inter-bubble fracture. In the ε phase region, a preferred condensation of finite defects in basal plane of Ti tritide lattice and formation of a dislocation network were identified.

Formability of adhesive-bonded steel sheets with artificial finite adhesive defects

In this investigation, the influence of artificial finite adhesive defects with different shapes and sizes, characterized by (1) aspect ratios, a/ b = 0.5, 1 and 1.5 (where a is transverse axis length of defect; b is longitudinal axis length of defect), (2) number of defects and (3) different locations, in the epoxy adhesive layer on the formability of adhesive-bonded blanks, is analysed. The deep drawing quality cold-rolled steel and stainless steel (SS 316L) sheets were used as base materials. The increase in aspect ratio of adhesive defect reduces the ductility of adhesive layer and thereby decreases the formability of adhesive-bonded blanks. This is due to the early failure of adhesive layer at the location of defect where strain is locally concentrated. The strain hardening exponent ( n) of adhesive-bonded blanks has decreased with increase in aspect ratio of adhesive defect in all the regions of deformation. The limit strain of deep drawing quality and SS 316L sheets constituting adhesive-bonded blanks shows decrement with increase in aspect ratio of adhesive defect. It is postulated that the aspect ratio of the finite adhesive defect influences significantly the formability of adhesive-bonded blanks, but not shape of the finite adhesive defects. There is no considerable effect of number of adhesive defects and different locations of adhesive defects on the formability of adhesive-bonded blanks.

Crack in Corrosion Defect Assessment in Transmission Pipelines

Cracks may occur coincident with corrosion representing a new hybrid defect in gas and oil pipelines known as crack in corrosion (CIC) that is not directly addressed in the current codes or assessment methods. Hence, there is a need to provide an assessment of CIC and evaluate the line integrity, as well as identify the requirements for defect repair or line hydrotest. An experimental investigation was undertaken to evaluate the collapse pressures of lines containing corrosion, cracks, or (CIC) defects in a typical line pipe (API 5L Grade X52, 508 mm diameter, 5.7 mm wall thickness). The mechanical properties of the pipe were measured using tensile, Charpy, and J-testing for use in applying evaluation criteria. Rupture tests were undertaken on end-capped sections containing uniform depth, finite length corrosion, cracks, or CIC defects. Failure occurred by plastic collapse and ductile tearing for the corrosion defects, cracks, and CIC geometries tested. For the corrosion defects, the corroded pipe strength (CPS) method provided the most accurate results (13% conservative on average). The API 579 (level 3 failure assessment diagram (FAD), method D) provided the least conservative collapse pressure predictions for the cracks with an average error of 20%. The CIC collapse pressures were bounded by those of a long corrosion groove (upper bound) and a long crack (lower bound), with collapse dominated by the crack when the crack depth was significant. Application of API 579 to the CIC provided collapse pressure predictions that were 18% conservative. Sixteen rupture tests were successfully completed investigating the failure behavior of longitudinally oriented corrosion, crack, and CIC. The pipe material was characterized and these properties were used to predict the collapse pressure of the defects using current methods. Existing methods for corrosion (CPS) and cracks (API 579, level 3, method D) gave conservative collapse pressure predictions. The collapse pressures for the CIC were bounded by those of a long corrosion groove and a long crack, with collapse dominated by the crack when the crack depth was significant. CIC failure behavior was determined by the crack to corrosion depth ratio, total defect depth and its profile. The results showed that the failure pressures for CIC were reduced when their equivalent depths were similar to those of corrosion and using crack evaluation techniques provided an approximate collapse pressure.

Thermal transport of graphene nanoribbons embedding linear defects

Using nonequilibrium Green's function method, the thermal transport properties of zigzag graphene nanoribbons (ZGNR) embedding a finite (semi-infinite or infinite) long linear defect are investigated in this paper. The results show that defect type and defect length have significant influence on the thermal conductance of ZGNR. When the embedded linear defects have the same lengths, thermal conductance of ZGNR embedding t5t7 defect is lower than that of ZGNR embedding Stone-Wales defect. As for the ZGNR embedding finite and the same type defects, their thermal conductance reduce with the increase of the defect length. However, as the linear defect is long enough, the thermal conductance is insensitive to the change of length. By comparing the ZGNRs embedding finite, semi-infinite and infinite long defects, we find that the thermal conductance of ZGNR embedding an infinite long defect is higher than that of ZGNR embedding a semi-infinite defect, while the thermal conductance of the latter is higher than that of ZGNR embedding a finite long defect. This is due to the fact that different structures possess different numbers of scattering interfaces in the phonon transmission direction. The more the scattering interfaces, the lower the thermal conductance is. These thermal transport phenomena are explained by analyzing transmission coefficient and local density of states. These results indicate that linear defects can tune thermal transport property of ZGNR efficiently.

Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3 /SrTiO 3 (001)

A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3 (001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system.

Resonant finite-size impurities in graphene, unitary limit, and Friedel oscillations

Unitary limit for model point scatterers in graphene is known to reveal low-energy resonances. The same limit could be achieved from hybridization of band electrons with the localized impurity level positioned in the vicinity of the Fermi level. The finite size defects represent an easier realization of the effective unitary limit, occurring when the Fermi wavelength induced by the potential becomes of the order of the size of the defect. We calculate the induced electron density and find two signatures of a strong impurity, independent of its specific realization. The dependence of the impurity-induced electron density on the distance changes near resonances from ~r^{-3} to ~r^{-2}. The total number of induced particles at the resonance is equal to one per degree of spin and valley degeneracy. The effects of doping on the induced density are found.

Graphing and grafting graphene: Classifying finite topological defects

The structure of finite-area topological defects in graphene is described in terms of both the direct honeycomb lattice and its dual triangular lattice. Such defects are equivalent to cutting out a patch of graphene and replacing it with a different patch with the same number of dangling bonds. An important subset of these defects, bound by a closed loop of alternating 5- and 7-membered carbon rings, explains most finite-area topological defects that have been experimentally observed. Previously unidentified defects seen in scanning tunneling microscope (STM) images of graphene grown on SiC are identified as isolated divacancies or divacancy clusters.

Investigation on Thermal Release Behavior of Helium-Charged Copper Films by DC Magnetron Sputtering

Helium atoms are introduced into Cu films at room temperature by direct current (DC) magnetron sputtering in a He/Ar mixed atmosphere. The doped helium atoms are distributed evenly in the film and the content can be easily controlled by changing the process parameters. The structure of Cu films with trapped helium was investigated by X-ray diffraction (XRD) technology. With increasing helium irradiation flux, the lattice spacing and width of diffraction peaks increased due to helium effects, corresponding to the increase of finite and infinite size defects in the film. The shape of thermal desorption spectrum (TDS) and the number of peaks strongly depended on the amount of helium introduced into Cu. With increase of helium content, helium release temperature decreases. At the same amount of helium, the peak temperature became higher with increase of heating rate and from this we can obtain a picture which could calculate the activation energy of helium desorption.