Local Structural Instabilities Research Articles

Preliminary design assessment of an alternative repair method for corroded shear panels in ship hull structures

Thickness wastage due to corrosion is a typical consequence of steel plated structures that operate in a sea environment. Corrosion below a given threshold calls for the performance of rehabilitation actions in order to avoid local structural instabilities. An innovative repair technique that is based on the application of composite stiffeners in ship plated structures is evaluated numerically and at a preliminry basis. The stiffeners act as means for strengthening the plating against buckling. The adequacy of the new repair design is evaluated with particular focus on a shear panel, through numerical experimentation, with regards to the required number of stiffeners, their cross section, their material and their length. Methods sourcing from Design of Experiments (DoE) have been employed. Initially a Full Factorial analysis (2k) has been performed so as to identify the significant design variables. In the following, a procedure based on Response Surface Methodology (RSM) and Central Composite Design (CCD) has been developed for deducing the feasible design space with repair solutions that satisfy the imposed requirements. In the practical range of the design variables, the performed study has provided initial findings which show the effectiveness of the proposed method. Depending on the design criteria and requirements imposed by the engineer, different design points may be found through the application of optimization techniques.

Anomalous nonlinear damping in metallic glasses: Signature of elasticity breakdown.

Solid materials, whether crystalline or glasses, are characterized by their elasticity. Generally, elasticity is independent of the probing strain if it is not exceeding the yielding point. Here, by contrast, we experimentally capture a pronounced strain-dependent elasticity in metallic glasses, as manifested by nonlinear mechanical damping in the apparent elastic deformation regime (∼1/100 of the yielding strain). Normal damping behaviors recover at higher temperatures but still below the glass transition. Atomistic simulations reproduce these features and reveal that they could be related to avalanche-like local structural instabilities. Our findings demonstrate that the standard elasticity is not held for metallic glasses at low temperatures and plastic events can be triggered at small perturbations. These results are consistent with previous simulations of model glasses and a scenario of hierarchical free-energy landscape of mean-field theory.

Energy-Tuned Photoelectron Spectroscopy of Lithium-Ion Battery Cathodes: Revealing Oxygen Redox Activity and Investigating New Materials

With the quest to find better performing electrode materials for Li-ion batteries becoming hotter by the day, advanced spectroscopic techniques are being employed to better understand the processes at interfaces of the battery components and deeper in the bulk of the active materials. One of those techniques is energy-tuned photoelectron spectroscopy (XPS), typically performed at synchrotron facilities, which offers the possibility to irradiate a sample with soft or hard X-rays and detect electrons with kinetic energies up to 10,000 eV. This builds up a true surface vs. bulk picture including formation of solid electrolyte interphases on anodes, oxidation products on cathodes and redox activity in the bulk of particles, such as that of transition metals or oxygen. A class of cathode materials which are of particular interest are those exhibiting oxygen redox activity, providing capacity beyond that of the traditional transition metal redox. We have investigated a number of such lithium-excess Ni, Co, or Mn oxide materials, which are known to exhibit oxygen redox, by energy-tuned XPS. The analysis reveals important details of the differences between surface and bulk, which DFT calculations have helped rationalise. Lithium-rich oxyfluorides (Li2MO2F where M = V, Fe, Mn etc.) are another class of material that have gained increased attention in recent years. These cathode materials have promising potential with high theoretical capacities (420 mA h g-1 for Li2VO2F) at ~2.5 V with minimal volume expansion, but suffer from local structural instabilities. We are using energy-tuned XPS (Figure 1) to study their degradation mechanisms and how they might be mitigated using transition metal doping or coatings. Figure 1. O 1s and V 2p XPS spectra of a Li2VO2F electrode for the EU H2020 ‘LiRichFCC’ project, at two photon energies: 2005 eV (black, top), 6015 eV (red, bottom). Figure 1

Hardness of silicate glasses: Atomic-scale origin of the mixed modifier effect

The origin of the various manifestations of the mixed modifier effect in silicate glasses remains poorly understood. Here, based on molecular dynamics simulations, we investigate the origin of a negative deviation from linearity in the hardness of a series of mixed alkaline earth aluminosilicate glasses. The minimum of hardness is shown to arise from a maximum propensity for shear flow deformations in mixed compositions. We demonstrate that this anomalous behavior originates from the existence of local structural instabilities in mixed compositions arising from a mismatch between the modifiers and the rest of the silicate network. Overall, we suggest that the mixed modifier effect manifests itself as a competition between the thermodynamic driving force for structural relaxation and the kinetics thereof.

Ab initio piezoelectric properties of Al0.5Sc0.5N : Impact of alloy configuration on the d33,f piezoelectric strain coefficient

Ab initio density-functional theory simulations of scandium and aluminum nitride (${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$) alloys have been carried out to investigate the impact of alloy configuration on their piezoelectric activity. The ${d}_{33,f}$ piezoelectric strain coefficient has been calculated for six ordered III-V configurations for ${\mathrm{Al}}_{0.5}{\mathrm{Sc}}_{0.5}\mathrm{N}$ alloys---including chalcopyritelike and CuPt-like structures. It varies from 9.3 to $41.4\phantom{\rule{0.28em}{0ex}}\mathrm{pm}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}$, while its value for a randomized configuration is 28.3 $\mathrm{pm}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}$. The alloy configurations have an impact on the degree of competition between near-neighbor coordination of the parent compounds (AlN, ScN) which is known to be responsible for local structural instabilities. This in turn affects (i) the strain sensitivity along the $c$ axis of the wurtzite internal parameter $u$ for Al and Sc sites and (ii) the stiffness of the material, both identified as key contributors to ${d}_{33,f}$. These findings suggest that spontaneous ordering of the alloy during film deposition or post-treatments should be considered as a way to optimize the piezoelectric activity of ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$ alloys.

Thermometer Effect: Origin of the Mixed Alkali Effect in Glass Relaxation.

Despite the dramatic increase of viscosity as temperature decreases, some glasses are known to feature room-temperature relaxation. However, the structural origin of this phenomenon-known as the "thermometer effect"-remains unclear. Here, based on accelerated molecular dynamics simulations of alkali silicate glasses, we show that both enthalpy and volume follow stretched exponential decay functions upon relaxation. However, we observe a bifurcation of their stretching exponents, with β=3/5 and 3/7 for enthalpy and volume relaxation, respectively, in agreement with Phillips's topological diffusion-trap model. Based on these results, we demonstrate that the thermometer effect is a manifestation of the mixed alkali effect. We show that relaxation is driven by the existence of stressed local structural instabilities in mixed alkali glasses. This driving force is found to be at a maximum when the concentrations of each alkali atom equal each other, which arises from a balance between the concentration of each alkali atom and the magnitude of the local stress that they experience.

Constraining the thickness of Europa’s water–ice shell: Insights from tidal dissipation and conductive cooling

The time of crystallization of a 100km thick ocean on Europa is estimated using a Stefan-style solidification solution. This solution is then extended to estimate the present thickness of the ice shell. It is assumed that the shell is initially in a steady-state conductive regime, and the ocean is taken to be an infinite liquid half space cooling from above. We find that in the absence of tidal heating and without the presence of low-eutectic impurities to serve as anti-freezes, a 100km thick ocean solidifies in about 64Myr. Conversely, when considering the present thickness of Europa’s ice shell, if tidal heating is included at a global dissipation rate of ∼1TW, the shell is found to be, on average, approximately 28km thick. However, if this dissipative heating is solely restricted to the shell, the local rate of heating may vary significantly due to crustal compositional heterogeneities and it is shown that this process may, in turn, produce thermal maxima in the crust, which could lead to local melting and structural instabilities, perhaps associated with the formation of chaos regions. Our approach is also extended to Ganymede and Callisto in order to estimate the time of solidification of their putative subsurface oceans and the current thicknesses of their ice-I shells.

Fragment molecular orbital calculations reveal that the E200K mutation markedly alters local structural stability in the human prion protein

The E200K mutation of the human prion protein (PrP) is known to cause familial Creutzfeldt–Jakob disease. In order to elucidate the effects of the mutation on the local structural stability of PrP, we performed ab initio fragment molecular orbital calculations for the wild-type human PrP and the E200K variant modeled under neutral and mild acidic conditions. The calculations revealed that this substitution markedly altered the intramolecular interactions in the PrP, suggesting that the local structural instabilities induced by the E200K mutation might cause initial denaturation of the PrP and its subsequent conversion to a pathogenic form. This work presents a new approach for quantitatively elucidating structural instabilities in proteins that cause misfolding diseases.

Electron beam-induced surface modification and nano-engineering of carbon nanotubes: Single-walled and multiwalled

Influence of low and medium energy electron beam (E-beam) irradiation on the single-walled (SW) and multiwalled (MW) carbon nanotube films grown by microwave chemical vapor deposition are investigated. These films were subjected to electron beam energy of 50 keV from scanning electron microscope for 2.5, 5.5, 8.0, and 15 h and 100, 200, and 300 keV from transmission electron microscope electron gun for a few minutes to approximately 2 h continuously. To assess the surface modifications/structural degradation, the films were analyzed prior to and post-irradiation using x-ray diffraction and micro-Raman spectroscopy in addition to in situ monitoring by scanning and high-resolution transmission electron microscopy. A minimal increase in intertube or interplanar spacing (i.e., d002) for MW nanotubes ranging from 3.25–3.29 Å (∼3%) can be analogized to change in c-axis of graphite lattice due to thermal effects measured using x-ray diffraction. Resonance Raman spectroscopy revealed that irradiation generated defects in the lattice evaluated through variation of: the intensity of radial breathing mode (RBM), intensity ratio of D to G band (ID/IG), position of D and G bands and their harmonics (D* and G*). The increase in the defect-induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while SW transforming semiconducting to quasi-metallic character. Softening of the q = 0 selection rule is suggested as a possible way to explain these results. It is also suggestive that knock-on collision may not be the primary cause of structural degradation, rather a local gradual reorganization, i.e., sp2+δ ⇔ sp2+δ, sp2 C seems quite possible. Experiments showed that with extended exposures, both kinds of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and forming intra-molecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. They also displayed curling and closure forming nano-ring and helix-like structures while mending their dangling bonds. High-resolution transmission electron microscopy electrons corroborated these conclusions. Manufacturing of nanoscale structures “nano-engineering” of carbon-based systems is tentatively ascribed to irradiation-induced solid-state phase transformation, in contrast to conventional nanotube synthesis from the gas phase.

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

AbstractInfluence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (ID/IG), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.

Path-integral approach to resistivity anomalies in anharmonic systems

Different classes of physical systems with sizeable electron-phonon coupling and lattice distortions present anomalous resistivity behaviors versus temperature. We study a molecular lattice Hamiltonian in which polaronic charge carriers interact with nonlinear potentials provided by local atomic fluctuations between two equilibrium sites. A path-integral model is developed to select the class of atomic oscillations which mainly contributes to the partition function, and the electrical resistivity is computed in a number of representative cases. We argue that the common origin of the observed resistivity anomalies lies in the time retarded nature of the polaronic interactions in the local structural instabilities.

Anomalous behavior of the lattice vibration of Bi2Sr2Ca1Cu1.96Fe0.04Oy and its possible correlation with Tc

The vibrational motion of Fe atoms in Bi2Sr2Ca1Cu1.96Fe0.04Oy studied by means of Mossbauer spectroscopy was found to be very anharmonic. This anharmonic behaviour can be explained by local structural excitations between two or more states supposedly existing in the material rather than by the usual phonon-phonon interactions. The increase in Tc for the annealed Bi2Sr2Ca1Cu1.96Fe0.04Oy is attributed to the increase in local structural instabilities which enhances the effective electron-electron pairing.

Causes for the variable superconducting transition temperature in uranium metal

The variable superconducting transition temperature of uranium metal appears to depend upon the variability of a low-temperature martensitic transformation. Local structural fluctuations and phase instabilities are suggested as resulting when the transformation is incomplete. The thermodynamic stability of a two-phase modulated structure involving domain formation is discussed along with possible consequences of a domain configuration. Factors affecting T c are used to explain the anamalous thermal property behavior of α-phase uranium.

Mechanism for the ferroelectric transition in the tetragonal-tungsten-bronze structure ferroelectrics

A microscopic model for the mechanism causing the ferroelectric phase transition in the tetragonal-tungsten-bronze structure ferroelectrics is introduced. It is based on an interaction between the ferroelectric A1 phonon displacement and dynamic local structural instabilities in these materials. The resulting description of the ferroelectric phase transition accounts for its essential experimental features including the absence of a “soft” phonon mode. The model is also used to discuss the unusual superconducting properties of the alkali tungsten bronzes.

Local lattice instabilities in the A-15's

A model based on dynamic local structural instabilities arising from the coexistence of phases and the presence of defects is proposed for the A-15's. Deductions from this model are consistent with unusual characteristic lattice features of the high T c A-15's especially their pressure and strain dependence and also with the pressure and strain dependence of T c .

Dynamic instabilities in high-Tcsuperconductors

A simple microscopic model of local structural instabilities in high-${T}_{c}$ materials, which involves the existence of more than one local state, is proposed, and the effects of dynamic fluctuations between these states on the electron and phonon properties are studied. These results give new insight into several important general features of these materials, including their large anharmonicity and also their structural transformations which often limit the attainable values of ${T}_{c}$.

Beam Buckling by Finite Element Procedure

A finite element procedure for determining the critical buckling load for three-dimensional structures idealized by planar two-dimensional elements is presented. This procedure is applicable to structures whose instabilities involve small displacements and elastic behavior, i.e., linear elastic buckling. Local and overall structural instabilities may be treated together with complex loading and support conditions. The smallest eigenvalue corresponding to the smallest buckling load is determined by an inverse iteration procedure. The accuracy of this finite element procedure is evaluated by comparison with a number of problems for which classical solutions are available. A simply supported wide-flange beam with a stiffener, lateral restraints, and various rotational restraints is presented to illustrate the versatility of the procedure as well as the effect of the restraints on the buckling load. In addition, a comparison between the subject procedure and recent experimental results on a continuous wide-flange beam is presented.