Local Loss Of Stability Research Articles

Reasons for Collapse of Steel Silos for Wheat with 1600 M3 Capacity

In 2021, five silos for grain storage were ordered, delivered and mounted for expansion of a warehouse in southern Bulgaria. They were produced of thin-walled galvanized sheets. The sheets were corrugated to prevent local loss of stability. All elements were joined together with bolts to facilitate and speed up the mounting processes. After completion of the mounting works, all five silos were filled up to the roof with grains. Three were filled with wheat, and the remaining two with sunflower seeds. On August 16, 2021, at about 8:30 am, a few minutes after the beginning of the first unloading of one of them, its cylindrical shell had torn, and it collapsed. A team from the University of Architecture, Civil Engineering and Geodesy (UACEG) was invited to inspect the accident site and determine the causes of the collapse. For this purpose, the members of the team carefully inspected the structure of the collapsed silos, tested samples of the used material and made control calculations. Through these activities, they established the most likely causes for steel silos destruction.

Statics and stability of diagonalless beams of particular kind – preliminary studies

Attractive large-scale glazing is currently an architectural trend. However, achieving adequate stiffness for larger glazing spans requires the use of complex cross-sections, generally aluminium sections of considerable height. Members with openwork webs are sometimes used in order to achieve increased load-bearing capacity and stiffness with reduced weight. The disadvantage is that this solution takes up a lot of space inside the building. A recently patented diagonalless member attempts to solve the above-mentioned problems. The member is fully demountable and allows glass units to be installed in the space between the chords. It consists of two chords spaced apart by metal sleeves with bolts passed through them. In this study, preliminary qualitative experimental tests were carried out to determine the behaviour of the member under load and to identify zones sensitive to local deformation. On this basis, numerical models (bar and 3D solid models, including contact interactions) were created and tested. Subsequently, the optimum sleeve spacing was determined, the effect of rotational and translational stiffness reduction at the nodes was investigated, and stress concentration zones and forms of stability loss were identified. A new form of local loss of stability of the chord facewall was identified, the so-called sliding push effect of the chord walls on the sleeve (within the larger openings). This is a completely different type of chord facewall failure from that found in known tubular welded joints. The research programme focused on identifying the phenomena occurring in the new member in order to provide a basis for further, more advanced analyses.

On Local Stability Loss of Modified Composites with Whiskerized Fibers

This article examines the critical compressive stresses required for a modified fiber composite to remain straight while the fibers within it bend. It was assumed that the modified composite consists of three phases: fiber, whiskerized interfacial layer, and matrix. An example of a composite material made up of carbon fibers, a whiskerized layer of carbon nanotubes with an epoxy matrix, and an epoxy matrix was considered. Its physical parameters affecting the critical compressive stresses were assessed, and methods for determining them were proposed. The effective properties of the inclusion and binder composite material were identified using the Voigt and Reis methods. Similarly, the effective properties of the interfacial whiskerized layer were analyzed by the three-phase method. The influence of fiber wavelength and phase shift, which define the destruction of the composite material, on the critical compressive stress value was explored. The wavelengths at which the composite material is destroyed were found. The effect of the volume content of the modified inclusion on the minimum critical compressive stress value was shown. The results for the modified composites were compared with those for the classical composites with a similar volume content of inclusions.

Geosynthetic-encased columns (GEC): A proven case study over 30 years

The construction of embankments on very soft soils is a challenge due to the low shear strength, low permeability, high compressibility, and high-water content of the subsoil. The surcharge load imposed by the embankment can result not only in a local or total loss of stability but also in unacceptable settlements and horizontal deformations or thrusts, which could endanger nearby structures. To overcome these problems, different construction methods are applicable. One particularly interesting and still innovative foundation system is the use of geotextile encased granular columns, which can be considered “State-of-the-Art” in Europe. It was developed and introduced in 1994, and since then many major projects have been realised using geotextile encased columns (GEC). The overall loads and stress concentrations above the column heads induce outwardly directed radial horizontal stresses in the columns. The particularity of the GEC system is that these stresses are counteracted not only by the inwardly acting pressure of the soft soil but also –most importantly – by the radial resistance of the tensile stiff geotextile casing. This paper briefly describes the GEC system and includes the description of a few notable projects using GEC as a foundation system: land reclamation at the Airbus site in Hamburg, Germany, and the Second Niger Bridge in Africa. The geotechnical challenges, design basis, and what can be considered the most important findings of each of these projects are also discussed in this paper.

Badania eksperymentalne zestawu cienkościennych konstrukcji lotniczych

Contemporary aircraft structures, and especially their load-bearing structures, are made almost exclusively as thin-walled structures, which perfectly meet the postulate of minimizing the weight of the structure. While local loss of roofing stability is acceptable under operational load conditions, exceeding the critical load limits of structural elements (frames, stringers) is practically tantamount to the destruction of the structure. The effectiveness of these ideas is influenced by the development of science about materials, processing, and machining processes, as well as the continuous improvement of technological processes. These disciplines allow for the construction of complex, geometrically integral structures that create opportunities not only for a more rational use of material characteristics, but also by their appropriate shaping, significantly increasing the mechanical properties of the supporting structure. The most important advantage in favor of the use of integral systems is economic efficiency, gained by eliminating or limiting assembly operations. Densely ribbed roofing elements belong to the category of load-bearing structure elements which, by reducing the weight which they must support, increase the strength parameters of the load-bearing structure. By reducing the thickness of the coating and, at the same time, introducing sufficiently dense stiffening longitudinal elements, it is possible to obtain a structure with significantly higher critical load values, and consequently a more favorable distribution of gradients and stress levels, which translates directly to an increase in fatigue life. The use of new technologies requires research for evidence purposes, showing that structures manufactured in this way are as safe as those manufactured using conventional methods. For this purpose, the authors conducted tests of the selected structure and performed FEM and experimental verification on the test stand. The results of the tests showed positive results, which confirmed that the method of manufacturing integral structures meets even the stringent requirements set by aviation.

On Wrinkling in Sandwich Panels with an Orthotropic Core.

This paper deals with the local loss of stability (wrinkling) problem of a thin facing of a sandwich panel. Classical solutions to the problem of a facing instability resting on a homogeneous and isotropic substructure (a core) are compared. The relations between strain energy components associated with different forms of core deformations are discussed. Next, a new solution for the orthotropic core is presented in detail, which is consistent with the classic solution for the isotropic core. Selected numerical examples confirm the correctness of the analytical formulas. In the last part, parametric analyses are carried out to illustrate the sensitivity of wrinkling stress to a change in the material parameters of the core. These analyses illustrate the possibility of using the equations derived in the article for the variability of Poisson’s ratio from −1 to 1 and for material parameters strongly deviating from isotropy.

Local Elasto–plastic Buckling of Isotropic Plates With Cutouts Under Tension Loading Conditions

The motivation of the presented study was the observation of the existence of local loss of stability “tension buckling” in the experimental tests of composite and metallic plates with cut-outs subjected to tension. Because of this, the numerical analyses of the aluminum plate with elliptical or circular cutouts at the center and subjected to tensile load are studied in the paper. Although the whole structure is uniformly stretched, the circumferential compressive stresses in the vicinity of the cutout edge are observed. First of all, the linear buckling analysis is carried out for different sizes of the holes. Based on these results, the size of the hole is chosen, where the circumferential stress magnitude in the vicinity of the cutout is the lowest or even comparable to the yield stress of the material. The computations are made for three different values of thickness. Finally, the nonlinear buckling analysis is carried out without and with the plasticity effects included. Generally, in the case of the circular and vertically oriented elliptical cut-out, the loss of stability in the tensed plate is always observed. However, in elastic-plastic analyses, the values of the critical parameters significantly differ from the results obtained for elastic buckling. Finally, the critical geometries for further experimental tests were defined.

Validation of Stainless-Steel CHS Columns Finite Element Models.

Stainless-steel elements are increasingly used in a wide range of load-bearing structures due to their strength, minimal maintenance requirements, and aesthetic appearance. Their response differs from standard steels; therefore, it is necessary to choose a different procedure when creating a correct computational model. Seven groups of numerical models differing in the used formulation of elements integration, mesh density localization, nonlinear material model, and initial geometric imperfection were calibrated. The results of these advanced simulations were validated with published results obtained by an extensive experimental approach on circular hollow sections columns. With regard to the different slenderness of the cross-sections, the influence of the initial imperfection in the form of global and local loss of stability on the response was studied. Responses of all models were validated by comparing the averaged normalized ultimate loads and the averaged normalized deflections with experimentally obtained results.

Investigation of the Fracture of a Semibounded Body Compressed Along a Near-Surface Interface Crack

We consider the problem of compression of a piecewise homogeneous half plane by forces directed along a near-surface crack located on the interface of two materials. This problem belongs to the class of nonclassical problems of fracture mechanics because, for the analyzed loading scheme, the stressstrain state realized in the body is homogeneous and the corresponding expressions for stresses and displacements in the vicinity of the crack do not contain singular components. Due to the fact that the stress intensity factors are equal to zero, for the analyzed problem, the classical Griffith–Irwin fracture criteria are inapplicable. In the indicated situation, the onset of crack propagation is associated with the local loss of stability of the equilibrium state of a part of the material located in the region adjacent to the crack. By using the approaches of the three-dimensional linearized theory of stability of deformable bodies, we propose a mathematical statement of the problem and an approach to its investigation. By using the representations of stresses and displacements in terms of complex potentials, we analyze the case where the roots of the corresponding characteristic equation are identical for each material.

Исследования устойчивости монолитных и слоистых пластин при сжатии

The results of calculations of the stability of monolithic and layered plates, obtained by analytical and numerical methods, are presented, which are compared with experimental data. The obtained results of stability calculations are within the limits of the permissible error. The results of numerical calculations of stability by the finite element method in the ANSYS program turned out to be higher than the values ​​determined by the Euler formula and lower than the results obtained by the formula for calculating the stability of plates. To study the bearing capacity of layered samples under compression, an assessment of their stability was carried out with different numbers and arrangement of layers of alloy and composite material. The optimal layouts of layers in the material for designing a composite panel have been determined. The results were used to design a composite wing panel based on sheets and profiles made of high-strength aluminum-lithium alloy and laminated aluminum-fiberglass. Stability of the composite panel under compression were 7% higher than the experimental values due to the local loss of stability of its elements, which precedes the general loss of stability and reduces the value of the critical load.

GEOTECHNICAL ACCIDENTS OF ROAD STRUTURES AND THEIR LICVIDATION

Geotechnical accidents often occur during road operation. To eliminate their consequences, various measures are required, taking into account the geological features of the site. Three cases of geotechnical accidents of road structures are considered: landslide damage to the bridge cone and the slope of the road embankment; the formation of a local landslide, plastic deformation and loss of stability of the roadbed, settlement of the culvert links; suffusion from the side of the railway cut under the cone of the bridge structure due to damage to the anti-filtration clay screen. In the first case, the reasons for the intensification of the landslide were periodic rises in the level of groundwater, the presence of areas not protected by vegetation, and areas of loose soil. The accident was eliminated by installing an anti-landslide structure made of bored piles, embedded with their lower ends into rocky ground. In the second case, the reasons were waterlogging due to heavy summer rains and melting snow next spring. To eliminate the accident, drainage was arranged in the adjacent territory; the soil was partially replaced, reinforcement with geotextiles was carried out, slopes with a berm were laid, the base of the subgrade was strengthened. In the third case, the removal of soil particles (suffusion) from the side of the railway excavation under the cone of the bridge structure occurred due to damage to the anti-filtration clay screen during the creation of the construction site. After the restoration of the screen and the drainage device, the suffusion was eliminated. The above examples show the importance of engineering and geological substantiation in the design of transport construction facilities.

Calculation of metal pipes with external constraints

The known solutions of the problem on the local stability loss of a pipe are analyzed. A new solution to this problem is proposed on the basis of a nonlinear finite element analysis of the volumetric stress-strain state of a pipe in a compliant medium using the LS-DYNA software package. Keywords pipeline, medium, local stability, calculation, finite element method, LS-DYNA software package. antegor177@mail.ru

Local Buckling and Resistance of Continuous Steel Beams with Thin-Walled I-Shaped Cross-Sections

In modern steel construction, thin-walled elements with Class 4 cross-sections are commonly used. For the sake of the computation of such elements according to European Eurocode 3 (EC3), simplified computational models are applied. These models do not account for important parameters that affect the behavior of a structure susceptible to local stability loss. This study discussed the effect of local buckling on the design ultimate resistance of a continuous beam with a thin-walled Class 4 I-shaped cross-section. In the investigations, a more accurate computational model was employed. A new calculation model was proposed, based on the analysis of local buckling separately for the span segment and the support segment of the first span, which are characterized by different distributions of bending moments. Critical stress was determined using the critical plate method (CPM), taking into account the effect of the mutual elastic restraint of the cross-section walls. The stability analysis also accounted for the effect of longitudinal stress variation resulting from the varied distribution of bending moments along the continuous beam length. The results of the calculations were compared with the numerical simulations using the finite element method. The obtained results showed very good congruence. The phenomena mentioned above are not taken into consideration in the computational model provided in EC3. Based on the critical stress calculated as above, “local” critical moments were determined. These constitute a limit on the validity of the Vlasov theory of thin-walled bars. Design ultimate resistance of the I-shaped cross-section was determined from the plastic yield condition of the most compressed edge under the assumptions specified in the study. Detailed calculations were performed for I-sections welded from thin metal sheets, and for sections made from two cold-formed channels (2C). The impact of the following factors on the critical resistance and design ultimate resistance of the midspan and support cross-sections was analyzed: (1) longitudinal stress variation, (2) relative plate slenderness of the flange, and (3) span length of the continuous beam. The results were compared with the outcomes obtained for box sections with the same contour dimensions, and also with those produced acc. EC3. It was shown that compared with calculations acc. EC3, those performed in accordance with the CPM described much more accurately the behavior of the uniformly loaded continuous beam with a thin-walled section. This could lead to a more effective design of structures of this class.

Методика расчета на прочность волноводов для обеспечения улучшенных массогабаритных параметров

This paper presents a calculation procedure for designing waveguides with iproved mass-dimensional parameters, which ensures the required strength and stiffness characteristics under static, dynamic and deformational loads. The procedure is based on the analysis of methods for determining the stress-strain state of the waveguide-and-distribution systems of spacecraft. The first stage of the procedure involves modelling the waveguide-and-distribution systems in a general formulation by a rod structure with equivalent loading and fixing conditions and determining the stress-strain state of such a system. At the second stage, local areas with the maximum stress-strain state values are selected for a further refined analysis in the rod system. An evaluation of the influence of the waveguide wall thickness on the general stress-strain state is performed. It is shown that when the waveguide wall thickness varies in the range of 0.25 – 2.50 mm, the strength and stiffness of its sections under static loads basically follow the linear law. It is established that under dynamic loading the wall thickness has almost no effect on the resultant stress-strain state of the waveguide arising under the influence of forced fluctuations and quasi-static loading owing to its dependence on the ratios of mass, moments of inertia and resistance, which for some standard sizes of the waveguides will be almost constant. The equations obtained for the rod system cannot be used for evaluating the influence of the wall thickness on local stress-strain state in the form of wall deflection, local loss of stability, etc. In view of this, at the second stage of modelling local areas of interest are selected and transformed into 3D thin-walled structures, with translation into finite element method programs for a more exact analysis. In the future, the proposed technique can be used to solve connected problems of interrelation and the influence of cross-sectional deformations in local zones of the waveguide on the changes in electromagnetic fields with the formation of parasite waves. This will significantly improve the quality of radio engineering characteristics of the waveguide and distribution systems while ensuring their strength, stiffness and minimal mass-dimensional parameters.

RESEARCH OF THE PROBLEM OF LOSS OF STABILITY OF CYLINDRICAL THINWALLED STRUCTURES UNDER INTENSE LOCAL TEMPERATURE EXPOSURE

Presently, three-dimensional printing technology is developing rapidly. This happens due to the need to create products of complex shape, the production of which by existing standard methods is very difficult and disadvantageous, and sometimes technically impossible. The study aims to investigate the problem of temperature stability of a thin-walled cylindrical structure under unsteady local thermal exposure, simulating the motion of a laser beam spot along one of the ends. To solve the problem, the finite element method was used. Since the geometric areas of calculation were parameterized, then for each set of parameters, its own finite element mesh was used for each type of analysis. A parametric spatial finite element model of cylindrical shell pinched at the bottom end was constructed. The local moving heat flux acts on the opposite side, simulating the movement of a laser beam during the additive formation of a thin-walled element. Numerical solutions of the nonstationary dynamic heat conduction problem were obtained, spatiotemporal temperature dependences were obtained, and solutions of the quasistatic problem of the loss of both local and complete loss of stability of the equilibrium state of the cylinder at various times due to the occurrence of local compressive stresses during intense heating were obtained. The dependences of critical heat flux power corresponding to the loss of stability on the cylinder wall thickness and its height were obtained. The above results can be used as calibration parameters of the process of creating thin-walled structures using additive technologies of Laser Melting class for products that require increased requirements for manufacturing accuracy.

Анализ деформирования внешне ограниченных металлических труб

Pipes with an external confinement include structures of the pipe — soil type, in which the modulus of elasticity of the soil is less than that of a metal pipe. The possibility of warping of the pipe wall associated with a local loss of pipe stability when compressed from the medium is investigated. It is assumed that the soil exerts dynamic pressure and local force pressure on the pipe, which is characteristic of seismic loads. A software-digital approach is proposed for calculating the separable extended structure of pipe — soil, which focuses on the construction of 3D finite element models of the structure and calculations in the LS-DYNA software package in a dynamic formulation. The structure is divided into three-dimensional TSHELL finite elements, the pipe material is taken as elastic-plastic, and the medium is linear-elastic. On the side contact surface between the pipe and the medium a normal unilateral constraint is taken, there are no tangential interactions. Calculations are carried out for two types of loading: swaging and swaging with additional local impact. A comparative analysis of the obtained stress states is performed. The appearance of plastic deformations in the pipe is noted. The calculations according to the known two-dimensional calculation schemes of the pipe — soil structure are given.

Thin-walled Integral Constructions in Aircraft Industry

Aviation structures are subjected to a wide spectrum of loads during operation. Each task carried out in flight consists of a number of maneuvers that generate different types of aircraft loading. Strict requirement for modern aviation constructions is high durability and reliability. This requirement means that many, sometimes contradictory restrictions, must be taken into account during the airplane design process. The most important element here is the mass of the structure having a decisive impact on both flight and technical properties as well as economic efficiency. This makes aircraft one of the most complex products of contemporary technology. Modern aviation constructions, or more precisely, their supporting structures are made almost exclusively as thin-walled structures that perfectly fulfill the postulate to minimize the weight of the construction. The structures are commonly used in which the cover is reinforced with longitudinal and transverse elements, providing the structure as a whole with the required rigidity and strength. While in the conditions of service load, local loss of coverage stability is permissible, exceeding the critical load levels of structural body components (frames, stringers) practically results in a structural damage.This specificity forces continuous improvement of both the design methods and the improvement of structural solutions for aircraft structures, as well as manufacturing processes, which is the subject of the project. The development of the science of materials, processing and machining processes as well as continuous improvement of technological processes has an impact on the effectiveness of these ideas. These disciplines allow the construction of geometrically complex integral structures that create opportunities not only for a more rational use of material characteristics, but also through appropriate shaping, significantly increase the mechanical properties of the support structure. An important advantage of the integral systems use is economic savings, gained as a result of elimination or reduction of assembly operations.This paper attempts to analyze the above-mentioned problem more closely, using the example of a densely ribbed rectangular plate.

Совершенствование инженерной методики расчета на прочность стержневых элементов легких стальных тонкостенных конструкций

There has been carried out an improvement of the methodology for determination of reduced sections with the simultaneous action of the entire complex of efforts (including bimoment ones).The solution presented in dimensionless parameters uses the «Section» algorithm and the EUROCODE provisions regarding section reduction. At the beginning, a general dimensionless parameter of efforts is determined, which corresponds to the limiting conditions for ensuring the local stability and section shape. Then, as the specified parameter grows further, the development of the section reduction to the limiting state is considered. As this takes place, the reduced sections are replaced with unreduced ones with loading with fictitious forces that compensate for the reduction. The authors propose a high-speed method for determining the local stability loss coefficients and the cross-sectional shape in dimensionless parameters, which allows obtaining more reliable results of strength control in comparison with those included in the current regulatory literature. The method is illustrated using calculation examples with comparison of the EUROCODE results, which have shown quite significant discrepancies.

Remote inspection by the magnetic tomography method (MTM) to prevent the risks imposed by exploitation of Arctic offshore pipelines

Cold climate areas that provide opportunities for the remote inspection of pipelines include the Barents Sea, the Russian Arctic, the Alaskan Chukchi Sea, the Beaufort Sea and the Canadian Arctic offshore. First, an analysis of several actual projects of contactless diagnostics using the magnetic tomography method of pipelines in Arctic conditions is done. Second, the Risk-Based Inspection methodology for Arctic offshore pipelines is discussed. It involves ensuring pipeline reliability on the basis of data on the technical condition of the metal in actual operating conditions. The magnetic tomography method allows not only to remotely identify areas of anomalies with metal defects, but also to register mechanical stress levels taking into account actual loads. This reduces the risk for the structure to come to the critical state in terms of exceeding local loads. Finally, magnetic tomography technology allows managing risks in cases of local corrosion, stress cracking or loss of stability of underwater pipelines in areas with free spanning. The qualitative indicators of the inspection include the probabilities of identifying, interpreting the degree of danger, missing a dangerous defect. The pipeline diagnostics report provides the parameters of reliability forecasting: the period of incident-free operation, safe working pressure, and pressure coefficient.

Accounting for shear deformations with bending a round plate

This paper presents a solution of the stability problem of a round plate taking into account the influence of shear deformations. The critical load of a round plate is determined taking into account shear deformations and without taking into account the shear deformations. The critical load taking into account the transverse shear with a local stability loss of a round plate is less than the critical load without taking into account the transverse shear. The inclusion of shear deformations in the study of the supercritical behavior of a round plate for isotropic materials does not have practical value, but it makes sense with extremely strong anisotropy of the elastic properties of the material.