Post-critical State Research Articles

Acoustic emission during approaching the critical point on stress-temperature diagram of martensitic transformation in Ni48Fe20Co5Ga27 (at.%) single crystal

Recently, the existence of martensite-austenite critical point, CP, on the stress-strain curves in the single crystalline Ni–Fe(Co)-Ga ferromagnetic shape memory alloys has been discovered by disappearance of hysteresis. It was also argued that the shear modulus at the CP and in the postcritical phase should be zero. In the present work we continue the investigation of the above new features on the “compression stress-temperature” phase diagram of the martensitic transformation (MT) of Ni48Fe20Co5Ga27 (at.%) single crystals in the vicinity of the CP by acoustic emission (AE). It is found that the AE activity (amount of events and their amplitudes) drastically dropped down by approaching the CP and tended to stabilize in a postcritical region. The above decrease is mainly due to the decrease of the number of large avalanches, while the exponents of the probability distribution of amplitudes and energies remained approximately the same, illustrating no change in the mechanism of acoustic emission events. The coordinates of the CP were estimated to be (340 ± 50) MPa and (327 ± 40) K. The elastic modulus was reduced with increasing temperature from 20 GPa down to 2 GPa in the critical region, whereby a huge total deformation of about 13 % was achieved. The stress-induced MT also shifted as a function of temperature and its slope decreased by about a factor of two upon approaching the CP. All features of the critical state determined have an obvious practical interest, as well as present a solid basis for theoretical understanding of critical and postcritical states in solids.

Stability of tensegrity-inspired structures fabricated through additive manufacturing

This study encompasses experimental and theoretical research on the stability of tensegrity-inspired structures in view of additive manufacturing technology. The aim of the research is to produce a qualitative and quantitative estimation of the post-critical behavior of struts in 3D-printed tensegrity-inspired modules. Presented results of compression tests performed on modules and single struts show that the bars work in the post-critical state before they reach their load bearing capacity. Comparison of theoretical solutions with experimental tests allowed us to conclude that the struts can carry axial loads significantly higher than the limits resulting from the classical Euler theory.

Stochastic stability of an aeroelastic harvester contaminated by wind turbulence and uncertain aeroelastic loads

The study describes the extension of a recently developed model that examines the stochastic stability of a torsional-flutter-based harvester. The new, extended model accounts for both uncertainty in the aeroelastic loads and wind turbulence effects. The first uncertainty source is a byproduct of the modelling simplifications of the aeroelastic loads, which are described by indicial function approach and ideally applicable to two-dimensional flow. The second source is the flow turbulence that operates by modifying the load through parametric, stochastic perturbations that are applied to the term describing the memory-effect of the load; stochastic perturbations simulate imperfections in the load assessment (e.g., measurement errors). Furthermore, since the “blade-airfoil” used for energy harvesting has finite dimensions, three-dimensional flow effect is simulated through a load reduction parameter applied to the static lift slope, dependent on the aspect ratio of the apparatus; this reduction parameter also leads to approximations and stochastic perturbations. Mean square stochastic flutter stability is examined. Post-critical states are discussed.

Experimental and Numerical Research of Post-Tensioned Concrete Beams.

The purpose of this paper is to present a new approach for the modelling of post-tensioned beams with calibration of the FE model to experimental results until the load capacity and post-critical state are reached. Two post-tensioned beams with different nonlinear tendon layouts were analysed. Material testing for concrete, reinforcing steel and prestressing steel was performed prior to the experimental testing of the beams. The Hypermesh program was used to define the geometry of the spatial arrangement of the finite elements of the beams. The Abaqus/Explicit solver was used for numerical analysis. The concrete damage plasticity model was used to describe the behaviour of concrete with different laws of elastic-plastic stress-strain evolution for compression and tension. Elastic-hardening plastic constitutive models were used to describe the behaviour of steel components. An effective approach to modelling the load was developed, supported by the use of Rayleigh mass damping in an explicit procedure. The presented model approach ensures good agreement between numerical and experimental results. The crack patterns obtained in concrete reflect the actual behaviour of structural elements at every loading stage. Random imperfections found during experimental studies on the results of numerical analyses were also discussed.

Non-convulsive epiletic seizure after electroconvulsive therapy session

IntroductionElectroconvulsive therapy (ECT) is a procedure performed under general anaesthesia involving triggering an intentional brief seizure through small electrical currents through the brain. The anaesthetic depth should be adequate prior to shock and measured with BIS, a processed electroencephalogram (EEG) monitor. Adjusting the hypnotic dose allows to decrease the ictal threshold and thus improve the response to treatment and decrease side effects.ObjectivesOur goal is detecting elements such as spontaneous epileptiform activity after ECT without tonic-clonic activity with the spectral density matrix (SDM).MethodsOur patient: an 87-year-old woman, diagnosed with F20.2 catatonic schizophrenia and under antipsychotic treatment since her youth. She has required multiple hospital admissions due to psychopathological decompensations until starting monthly maintenance ECT sessions in 2014. Since then she no new hospital admissions have been required.ResultsImages 1D and 1E shows the SDM, a spectrogram of the EEG. The X axis show time (minutes), the Y axis shows the frequency (Hz) and the Z axis shows the energy or intensity of that electrical activity in the frequency bands replaced by colors: warm colors (red) reflecting high intensity electroencephalographic activity and cool colors (yellow, blue and green), low activity. Images 2 and 3’s EDM shows spontaneous epileptiform activity after electroconvulsive therapy without tonic-clonic activity. We observed an initial EDM of an awake patient, with warm colours in practically all frequency bands, including the beta band (13-30 Hz), characteristic of waking states. Around 9:50 anaesthetic induction occurs, activity increases in slow frequencies (red colours in alpha, theta and delta), plus an increase of cold colours in beta, reflecting the disappearance of brain activity in that frequency. The asterisk reflects the EEG response to the electrical discharge, followed by a postcritical state with brain activity exclusively in slow waves and high amplitude (delta and some theta) and absence of activity in other frequencies (blue colour in the beta and alpha bands) around 9:57. At about 10:00 there is an abrupt appearance of high intensity brain activity (warm colours) in beta and alpha and delta, mainly, reflecting spontaneous epileptiform activity after treatment and clinically reflected as a patient absent and disconnected from the environment, but without tonic-clonic activity. New postcritical state in which blue colour predominates, reflecting little brain activity, and warmer colours reappear in all frequency bands, including beta, reflecting the progressive recovery of wakefulness.Image:Image 2:Image 3:ConclusionsPsychiatric pathology can be reflected in the SDM, which allows to observe changes in the EEG, correcting the electrical stimulus of the shock and the dose of anesthetic appropriate to the patient to trigger an intentional brief seizure under general anesthesia.Disclosure of InterestNone Declared

Postictal psychosis in a psychiatric pacient, about a case

IntroductionPostictal psychosis is the most frequent psychosis in epileptic patients, appearing between 3-8% of them. As temporary criteria, it must appear in less than a week after the epileptic crisis, with a duration of between 15 hours and 3 months.ObjectivesWe present the case of an 82-year-old patient admitted to the ED due to an epileptic seizure. Request evaluation for agitation and disorientation.MethodsThis is an 82-year-old patient diagnosed with epilepsy for 5 years, with unwitnessed seizures, under treatment with levetirazetam. In the last consultation with neurology, the differential diagnosis was raised with anxiety crises due to normal intercritical EEG.The patient went to the emergency room after a partial crisis, presenting a post-critical state and beginning hours later with disorientation in time and place, manifesting delusions of religious content, as well as visual hallucinations of the same type, presenting agitation that required pharmacological and mechanical restraint.ResultsAdmission to the Neurology Service was decided, with a good response to treatment with a typical intramuscular antipsychotic, with complete remission of the condition in 48 hours. Small areas of ischemia compatible with the patient’s age are observed in the cranial CT and the EEG shows slowed global activity.ConclusionsPostictal psychosis is a phenomenon of low prevalence, however, it is important to take it into account. It is important to recognize the postictal “lucid” period in patients with a family or personal history of psychiatric illness and seizures with compromised consciousness.Disclosure of InterestNone Declared

The effect of load eccentricity on the compressed CFRP Z-shaped columns in the weak post-critical state

The study investigated short, thin-walled Z-shaped carbon-epoxy laminate columns. Two multi-layer composite lay-up configurations were tested [0/45/-45/90]s and [90/-45/45/0]s, each consisting of eight symmetric laminate layers. The Z-columns were loaded in compression, and the loading mode included the scenario of force application eccentric to the center of gravity of the column’s cross-section. The modeled boundary conditions corresponded to the ball-jointed support at the end sections of the column. The study set out to determine the effect of loading eccentricity on the buckling mode, the values of the buckling load (the perfect structure), and the critical force (the structure with initial imperfections). The critical force of the real structure was established using the approximation method, based on experimentally determined post-critical equilibrium paths of the structure. For the numerical finite element analysis (FEM), Abaqus software was employed. In the first step, the FEM solved the eigenvalue problem, which lead to establishing the mode of stability loss and the determination of the corresponding bifurcation load for the Z-column subjected to axial and eccentric loading. The second stage of analysis focused on the nonlinear behavior of the structure with initial imperfections. The critical force was derived from the results of approximation based on the numerical post-critical equilibrium paths. The experimental data then served as the base for the validation of the developed numerical models. The findings enabled determining the effect of the compressive load eccentricity on the buckling mode and the critical force values. The project/research was financed in the framework of the project Lublin University of Technology-Regional Excellence Initiative, funded by the Polish Ministry of Science and Higher Education (contract no. 030/RID/2018/19).

Experimental Investigation on the Vertical Ductility of Rectangular CFST Columns Loaded Axially.

A total of 5 steel and 21 rectangular composite concrete-filled steel tube (CFST) columns of moderate slenderness were tested to investigate their ductility under axial compression. The importance of the vertical ductility of columns was discussed, and a novel ductility measure was proposed and utilized to examine the ductility of tested specimens. The analyses showed that the ductility of axially compressed CFST columns highly depends on their failure mode. The key feature influencing the ductility is their ability to dissipate the energy of imposed loads. The larger the volume of a material that may permanently deform and consequently dissipate the energy, the greater this ability. In consequence, the ductility of specimens exhibiting local failure mode was higher in comparison to the columns that underwent global or mixed global—local failure. It was found that both steel and composite columns were able to carry axial loads in the post-critical state; but due to the limitations of local buckling of the steel cross-section in the concrete core and concrete confinement, all tested composite columns showed greater ductility than their steel counterparts.

Experimental and Numerical Analysis of Post-Critical States and Damage of Thin-Walled Channel Section Columns Made of Carbon/Epoxy Composite

Experimental and Numerical Analysis of Post-Critical States and Damage of Thin-Walled Channel Section Columns Made of Carbon/Epoxy Composite

An experimental investigation of drag and noise reduction from a circular cylinder using longitudinal grooves

Aerodynamic and aeroacoustic characteristics of a cylinder with longitudinal grooves were studied in an anechoic wind tunnel, emphasizing on drag and noise attenuation. The parallel mounted load cells and a single microphone measured the aerodynamic resistance and noise of a circular cylinder with longitudinal grooves at Reynolds number ranging from 5.84×104 to 8.48×104, covering the flow regimes from the sub-critical to the post-critical states. The results show that longitudinal grooves can effectively trigger the boundary-layer transition started at a Reynolds number around 5.84×104, causing the so-called drag crisis. During the transition process, a drag reduction over 50% and a maximum noise attenuation over 15 dB can be achieved by longitudinal grooves. The variations of the drag and noise are linked to the near-field flow measurements using hot-wire in different regimes, enhancing our understanding of the problem.

Experimental investigation on post-flutter characteristics of a typical steel-truss suspension bridge deck

A testing device that was specially developed for large torsional amplitude vibrations was adopted to investigate the nonlinear post-flutter behaviors of a typical steel-truss girder bridge deck. Firstly, a series of free decay vibration tests of a narrow rigid rod model and a section model of the bridge were conducted in still-air to obtain mechanical properties and still-air induced aerodynamic properties. Then, post-flutter experiments of the bridge section model under initial attack angles of −3°, −0°, +3°, +5°, +7°and with different mechanical damping ratios were carried out. Finally, nonlinear and coupling behaviors of the bridge section model during post-flutter were described and analyzed in detail from several different aspects such as aeroelastic center, vibration mode, and wind-induced aerodynamic damping. The results showed that the truss girder section exhibited significant vertical-torsional coupled soft flutter behavior at various attack angles and there was a stable limit cycle oscillation (LCO) and an unstable LCO under a given wind speed near the critical speed. Moreover, the added aerodynamic damping, the coupling vertical deformation, the aerodynamic torsional center, the wind-induced aerodynamic damping, and the soft flutter mode in the post-critical state all exhibited the characteristic of amplitude dependence. Meanwhile, it was found that the coupling vertical deformation was strongly related to the torsional amplitude and wind attack angle. Finally, the effects of mechanical damping on the post-flutter performance were discussed and the results showed that some nonlinear dampers whose damping increased with the increasing amplitude can be adopted to improve the critical flutter speed without deteriorating or even enhancing the post-flutter performance.

Experimental and numerical analysis of stability and failure of compressed composite plates

This paper investigates the stability, load-carrying capacity and failure analysis of thin-walled plate elements weakened by cut-out and subjected to axial compression. Tested plates were made in asymmetric configuration from CFRP composite material with used autoclave technique. The scope of the research included experimental tests on real samples and numerical calculations with using the finite element method in the ABAQUS® program. The influence of the compressive load on the buckling and postbuckling behavior and loss of load capacity of thin-walled composite plates were investigated. Both, the experimental tests and the numerical analysis were carried out in the full load range, up to the structure failure. In the experimental tests, acoustic emissions were used to analyze the damage state of the composite material. Numerical calculations were carried out with the use of progressive failure analysis, based on the initiation of the failure from the Hashin's theory and the further evolution of the failure based on the energy criterion. Numerical results of critical and post-critical state were compare with experimental research showing areas prone to failure of the material. A measurable effect of the conducted research was obtaining a high compatibility of the results of numerical and experimental calculations.

Experimental Research on Concrete Beams Reinforced with High Ductility Steel Bars and Strengthened with a Reactive Powder Concrete Layer in the Compression Zone.

The article describes four-point bending tests of three reinforced concrete beams with identical cross-sections, spans, and high-ductility steel reinforcement systems. Two beams were strengthened in the compressed section with a thin layer of reactive powder concrete (RPC) bonded with evenly spaced stirrups. Their remaining sections, and the third reference beam, were made of ordinary concrete. Measurements of their deflections, strains and axis curvature; ultrasonic tests; and a photogrammetric analysis of the beams are the main results of the study. For one of the beams with the RPC, the load was increased in one stage. For the two remaining beams, the load was applied in four stages, increasing the maximum load from stage to stage in order to allow the analysis of the damage evolution before reaching the bending resistance. The most important effect observed was the stable behaviour of the strengthened beams in the post-critical state, as opposed to the reference beam, which had about two to three times less energy-absorbing capacity in this range. Moreover, thanks to the use of the RPC layer, the process of concrete cover delamination in the compression zone was significantly reduced, the high ductility of the rebars was fully utilized during the formation of plastic hinges, and the bending capacity was increased by approximately 12%.

The Effect of the Type and Amount of Synthetic Fibers on the Effectiveness of Dispersed Reinforcement in Soil-Cements.

The paper deals with mechanical properties of soil-cement composites made with non-cohesive soil and reinforced with dispersed fibers. The research was carried out on the basis of three soil-cement matrices whose compositions varied in terms of the volumetric fraction of cement paste and the water-cement ratio. Two types of polypropylene fibers were used as dispersed reinforcement: single fibrillated-tapes polypropylene fibers (SFPF) and bundles of coiled fibrillated-tapes polypropylene fibers (BCFPF). The fibers varied in terms of their length and mass fraction. The objective of the study was to assess the effect of the addition of fibers to soil-cement composites on their flexural tensile strength and on their behavior in the post-critical state. The studies were carried out after 28 days of curing. Bending tests were carried out to determine post-critical stress values σCMODi, stress values at which the matrix is destroyed (limit of proportionality) σLOP, maximum stress values transferred by the fibers σMOR (modulus of rupture), and total fracture energy Gf,tot as well as compressive strength. The test results obtained, and their analysis, indicate the significant impact of the dispersed reinforcement used on the performance of such composites during bending.

Analysis of the Post-Critical Deformation State of Curvilinear Integral Stiffening of a Thin-Walled Structure Subjected to Torsion

Analysis of the Post-Critical Deformation State of Curvilinear Integral Stiffening of a Thin-Walled Structure Subjected to Torsion

A novel two-degree-of-freedom model of nonlinear self-excited force for coupled flutter instability of bridge decks

This paper aims to propose a novel model for predicting the nonlinear heave-torsion coupled flutter instability of flat bridge decks. The nonlinear oscillatory system during coupled post-flutter instabilities was modeled as a weak perturbation of the classical linear flutter theory. A novel nonlinear self-excited force model was then proposed by introducing extra nonlinear terms in classical linear model to consider the effects of nonlinear aerodynamic damping, amplitude-dependent vibration mode and the coupling of aerostatic deformation. An efficient algorithm for parameters identification and solving of the nonlinear coupled oscillator was also developed. The effectiveness of the proposed analytical framework was validated through an elastically-supported sectional model test in estimating the self-sustained vibrations during post-critical states of a typical closed-box bridge deck.

Progressive Failure Analysis of Thin-Walled Composite Structures Verified Experimentally.

The subject of the presented research was a thin-walled composite column made of CFRP (carbon-epoxy laminate). The test sample had a top-hat cross-section with a symmetrical arrangement of laminate layers [90/−45/45/0]s. The composite structure was subjected to the process of axial compression. Experimental and numerical tests for the loss of stability and load-carrying capacity of the composite construction were carried out. The numerical buckling analysis was carried out based on the minimum potential energy criterion (based on the solution of an eigenvalue problem). The study of loss of load-carrying capacity was performed on the basis of a progressive failure analysis, solving the problem of non-linear stability based on Newton-Raphson’s incremental iterative method. Numerical results of critical and post-critical state were confronted with experimental research in order to estimate the vulnerable areas of the structure, showing areas prone to damage of the material.

Affecting of the Long-Term Deformation to the Stability of RC Frame-Bracing Structural Systems under Special Accidental Impacts

The paper is devoted to investigation of the loading duration affecting to stability of structural elements of reinforced concrete building frames at post-critical states caused by the sudden removal of one of the load-bearing constructions. Modelling of strength and deformation characteristics of reinforced concrete under long-term loading was performed using the theory of plasticity of concrete and reinforced concrete proposed by G.A. Geniev. Following to this theory, the rheological model of long-term strength of concrete was presented by two elements connected in series, one of which characterizes the process of short-term loading, and the second viscous element corresponds to the Kelvin-Voigt model and takes into account the processes of long-term deforming. It was established that at long-term loads acting to operated reinforced concrete structural system the value of critical force corresponding to loss of stability of the structural element decreases. This fact should be taken into account when assessing the resistance of such building frames to progressive collapse under special accidental impacts caused by removal one of load-bearing elements of a structural system.

Analysis of the Functionally Step-Variable Graded Plate Under In-Plane Compression.

A study of the pre- and post-buckling state of square plates built from functionally graded materials (FGMs) and pure ceramics is presented. In contrast to the theoretical approach, the structure under consideration contains a finite number of layers with a step-variable change in mechanical properties across the thickness. An influence of ceramics content on a wall and a number of finite layers of the step-variable FGM on the buckling and post-critical state was scrutinized. The problem was solved using the finite element method and the asymptotic nonlinear Koiter’s theory. The investigations were conducted for several boundary conditions and material distributions to assess the behavior of the plate and to compare critical forces and post-critical equilibrium paths.

Experimental and Numerical Studies of Low-Profile, Triangular Grid-Stiffened Plates Subjected to Shear Load in the Post-Critical States of Deformation.

Constant developments in manufacturing technology have made it possible to introduce integrally stiffened elements into load-bearing, thin-walled structures. The application of thin-walled elements with integral stiffeners potentially increases buckling and critical loads to maintain the mass of the structure and lower production costs. This paper presents the results of experimental investigations and numerical Finite Element Modelling (FEM) analyses of low-profile, isosceles grid stiffened, aluminium alloy plates subjected to pure shear load. Conducted research included analysing buckling and post-buckling states of deformation, taking into account both geometrical and physical nonlinear effects. Use of the Digital Image Correlation (DIC) system during the experimental tests created representative equilibrium pathways and recorded displacement field distributions over the plate surface. The model was initially validated against the experimental results. The results for the stiffened plate were compared to the reference structure in the form of a smooth plate with equivalent mass. Comparative analyses included examining the displacement fields and stress efforts over the plates. The stiffening configuration under examination increased the critical buckling load by 300% in comparison to the unstiffened structure with the same mass. Obtained results also indicate potential problems with areas of concentrated stress in the case of an incorrect geometry design near to the boundary conditions.