Stresses In Structural Elements Research Articles

Influence of Nonstationary Processes in Drill Rigs on the Durability of Structural Elements

Assessing the effects that nonstationary dynamic processes have on the durability of structural elements belongs to an important trend in modern dynamics and technical diagnostics of machines. Normally, fatigue strength calculations are performed taking into account only periodically variable stresses, as steady operating modes of machines are much longer in comparison with transient modes. However, a significant role in fatigue failure in machines and engineering structures is also played by nonstationary loads. This is explained by emerging intensive oscillations in the mechanical system during accelerating, braking, or changing the operation mode of a machine unit, which often lead to the accumulation of fatigue damages in the materials of parts in heavy loaded assemblies. The combination of stationary and nonstationary dynamic loads manifests itself, particularly in drilling rigs, where technological cycles include steady motion modes, starts, and stops. This paper represents a generalized mathematical model describing nonstationary processes in the lift system of a drill rig, which considers the relationship between electromagnetic processes in asynchronous motors and mechanical oscillatory phenomena, with the purpose of determining dynamic loads and stresses in structural elements of the rigs. Nonlinear physical systems include mechanical members with both concentrated and clearly expressed distributed parameters. The durability of structural elements is evaluated by means of a computer algorithm for analysis of crack growth rates using the NASGRO equation obtained with the presence of plastic deformation zones. An example of the crown block axis illustrates the influence of nonstationary dynamic processes in drill rigs on the durability of structural elements.

Development of a Method for Assessing Thermal Stresses in Structural Elements of Aircraft

Development of a Method for Assessing Thermal Stresses in Structural Elements of Aircraft

Development of a Numerical Model Designed to Calculate the Temperature Field and Thermal Stresses in Structural Elements of Aircrafts

Development of a Numerical Model Designed to Calculate the Temperature Field and Thermal Stresses in Structural Elements of Aircrafts

АНАЛИЗ ОБЩЕГО НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ В ЗОНЕ СОЕДИНЕНИЯ СИЛОВОЙ НЕРВЮРЫ И ПАНЕЛИ КРЫЛА

Ensuring the assigned fatigue life of the structural elements of the wing is achieved by choosing structural materials, the level of design stresses, structural and technological methods for improving the fatigue resistance characteristics, both regular and irregular zones of the structure. The value and character of local stresses in structural elements significantly affect fatigue life. Therefore, knowledge of the character of stresses (especially in irregular zones) makes it possible to increase the fatigue life of the structure. The article analyzed one of the irregular zones on the wing – joint of the rib and the lower skin. The SIEMENS NX CAD system was used to build the calculation model. For further analysis, the model was imported into the ANSYS CAE system. All geometric characteristics of structural elements were specified (thickness and cross-sectional area, which were obtained from the sizing analysis), and appropriate materials were assigned (which were chosen for each elements from design considerations to increase weight efficiency and fatigue life). When high – lift devices is used, the general stress-strain state of the wing changes. In addition to the torque moment due to the mismatch between the center of pressure and the center of rigidity of the wing section, when the high-lift devices are retracted, an additional force acts and changes the values and influence of the torque moment at specific wing section. For design cases, such as cruising, takeoff and landing, aerodynamic characteristics were obtained (depending on the position of slats and flaps, flight speed and altitude, mass characteristics), which were subsequently used to obtain the stress-strain state of the analyzed section of wing. The analyzed section of wing was chosen with the presence of a slat and flap zone. In the course of the analysis, the stresses in the skin were compared in each case near the rib-skin joint. This area has a complex stress state due to the action of tensile forces and shear flow (between the skin and the rib). For further study of fatigue life in this area, depending on the level of loading, maximum stresses and shear stresses were analyzed.

Measurement of Residual Stresses in Structural Elements of Ships Using Sectional Method.(Dept.M)

Measurement of Residual Stresses in Structural Elements of Ships Using Sectional Method.(Dept.M)

Kontrolisanje zaostalih napona - razvoj najnovijih inženjerskih metoda za merenje bez razaranja i povoljnu preraspodelu zaostalih napona

Welding is used widely in the automotive industry for joining a variety of structural components and parts. Of paramount importance in these structures are their engineering properties, such as fatigue life, distortions, dimensional stability and corrosion resistance that can be affected considerably by the presence of residual stresses (RS). The knowledge of RS and the ability to control their distribution in welded structures is critical when evaluating their fatigue life and preventing catastrophic failures. An engineering concept of residual stress management (RSM) has been developed that addresses all aspects of residual stresses in structural elements. RSM includes three major stages in stress management, i.e. RS determination, RS analysis and RS redistribution. Using this approach, stresses in structures and materials can be evaluated in each specific case either theoretically or experimentally and the performance and fatigue behavior of such structures optimized. This paper is built as an overview of the RSM concept and its application in the fields of non-destructive measurement of residual and applied stresses and in treatment of structures with residual stresses to achieve better performance and longer fatigue life. All three stages of the RSM concept will be discussed and for each of the stages practical engineering approach examples will be given. An example of a project in which the residual stress distribution in a filet welded joint was measured, analyzed and changed by post-weld treatment will be presented to demonstrate the effectiveness of the RSM approach. The advancements in the modern tools used in this project for non-destructive measurement of residual stresses using an ultrasonic computerized complex for residual stress measurement and the ultrasonic peening for redistribution of the residual stresses that allowed improving the quality of the welds and increasing their fatigue life will also be presented.

Online Determining Heat Transfer Coefficient for Monitoring Transient Thermal Stresses

This paper proposes an effective method for determining thermal stresses in structural elements with a three-dimensional transient temperature field. This is the situation in the case of pressure elements of complex shapes. When the thermal stresses are determined by the finite element method (FEM), the temperature of the fluid and the heat transfer coefficient on the internal surface must be known. Both values are very difficult to determine under industrial conditions. In this paper, an inverse space marching method was proposed for the determination of the heat transfer coefficient on the active surface of the thick-walled plate. The temperature and heat flux on the exposed surface were obtained by measuring the unsteady temperature in a small region on the insulated external surface of a pressure component that is easily accessible. Three different procedures for the determination of the heat transfer coefficient on the water-spray surface were presented, with the division of the plate into three or four finite volumes in the normal direction to the plate surface. Calculation and experimental tests were carried out in order to validate the method. The results of the measurements and calculations agreed very well. The computer calculation time is short, so the technique can be used for online stress determination. The proposed method can be applied to monitor thermal stresses in the components of the power unit in thermal power plants, both conventional and nuclear.

Development of a numerical model designed to calculate the temperature field and thermal stresses in structural elements of aircrafts

Development of a numerical model designed to calculate the temperature field and thermal stresses in structural elements of aircrafts

Effect of stress concentration in a beam of rectangular cross section in the region of attachment of the longitudinal efforts

Relevance. To ensure the safe operation of buildings and structures, it is necessary to more accurately determine the stress-strain state (SSS) of structural elements, to identify areas of stress concentration. The distribution of stresses in the region of the fastening bars in three-dimensional formulation is relatively little studied. In these areas, there may be significant stress concentrations that contribute to the occurrence and development of cracks and splits, which are a harbinger of destruction. The development of modern methods of calculation, software systems and the growth of computing capabilities allow refining the design scheme: to move from one-dimensional to two-dimensional calculation scheme, from two-dimensional to three-dimensional calculation scheme. All this makes it possible to more accurately assess the SSS of structural elements and structures, to identify areas of stress concentration, as well as to investigate the effect of the Poisson's ratio on the stress concentration. Methods of research. It is noted that cracks and breaks in the edges under the influence of longitudinal loads occur in the rods (racks) of square cross-section. Three-dimensional elements based on the spline version of the finite element method and the LIRA computational complex are used to estimate the stress-strain state. The spline finite element method, thanks to the synthesis of the idea of parametrization and the finite element method (FEM) with cubic approximation of all three required variables within each element, allows obtaining consistent three-dimensional finite elements. On the basis of the mentioned methods and complexes, numerical studies of the stress concentration in the bars of square and rectangular cross-sections fixed at one end and perceiving the tensile forces at the other end are performed. Conclusions. It is found that in the angular points of the cross section in the area of fastening of straight bars, perceiving axial tensile forces, there are stress concentrations. Away from the mounting area of the bar, the voltages are aligned. By increasing the Poisson's ratio, the stress concentration increases faster than at low values. The transition from a onedimensional design model to a two-dimensional one, and even more so to a three-dimensional model allows to determine the stress concentration, both in plan and in thickness. Information about the concentration of stresses in elements of structures will allow designers to more accurately design structures and facilities, and the operators to promptly identify the defective region.

Barlow's method of superconvergence for higher-order finite elements and for transverse stresses in structural elements

In this paper, Barlow's method for determining superconvergent stress/strain points is examined for higher-order finite elements, with particular interest in ones proven in lumped mass explicit dynamic methods, and for transverse stresses in structural elements computed via equilibrium equations. Others have found that for Lagrange polynomial finite elements with equidistant nodes, Barlow and Gauss-Legendre points are coincident for linear and quadratic elements, but are different for cubic and higher order interpolations. The reason for this non-coincidence is identified herein, whereby it is shown to be a result of poor-performing interpolation functions using equal nodal spacing Lagrange polynomials, which results in poor nodal displacement predictions—a violation of a basic premise of Barlow's method. The paradox is shown to be eliminated by using better interpolation functions with “optimal” nodal locations defined by Gauss-Lobatto points. These “optimal” higher-order elements are shown to compute exact superconvergent nodal displacements for one-dimensional bars, and consistent with lower-order elements, the stress/strain superconvergent points predicted by Barlow's method are found to be Gauss-Legendre points of the same precision rule as that of the Gauss-Lobatto rule used for the nodal locations. Barlow's method thus has a sound theoretical foundation for higher-order elements. The coincidence of Barlow with Gauss-Legendre points has been confirmed to be the case for up to 9th order “optimal” nodal location elements in one, two, and three dimensions, which retains the same number of degrees of freedom and also noticeably increases the critical time increment in explicit methods. In addition, calculation of transverse stresses in structural elements, via integration of three-dimensional equilibrium equations, is examined from Barlow's superconvergence viewpoint. Barlow's method provides a distinct means to determine superconvergent points for higher-order derivatives of displacement fields, as necessary for this procedure, and explains the observations of others in computing static transverse stresses from equilibrium equations. A brief catalogue of the newly defined Barlow points for transverse stresses in common element types is provided.

Development of the numerical model for evaluating the temperature field and thermal stresses in structural elements of aircrafts

An approximate method for estimating the thermal stresses of the aircraft key components of the simple geometric shape (the edges of the hull and wings, the nose fairing) has been developed. The mathematical model of such estimates is based on the solution of the quasi-static thermoelasticity problem. The solution is evaluated in the area with curvilinear boundaries, and the shape of these boundaries changes under the influence of thermal and mechanical loads. Thus the computational domain is transformed to an area where the regular Cartesian (structured) grid can be introduced. The initial validation and verification of the developed numerical methodology was carried out. Numerical modeling of temperature fields and thermal stresses in the simplest components of aircraft structures (cylinder blunted over the sphere and the shell) is performed.

Taking into account the residual stresses during transverse calculation of the load frame to extend the service life above the warranty period

We study the possibility of accounting for the influence of residual stress in structural elements of a high-duty item on the strain–stress state and predicting the residual life of cyclical strength throughout the entire part for an extension of the safety period. Checking calculation of the static strength checking by computer simulation reveals that the influence of residual stress on the stress field from the external load is more difficult to determine than by simple algebraic addition. It is shown that the part under the action of external cyclical stresses with the maximum peak intensity of external stresses should be considered as a structure under multicycle fatigue conditions, since the intensity of total (external and residual) stresses is higher than the endurance limit.

Instrumental Inspection of the Repair Quality of Stator Core Suspension in Turbogenerators

Results of instrumental inspection of the condition of the elastic suspension of the stator core in a high-power turbogenerator are considered. The effectiveness of repair is assessed by analyzing the dynamic characteristics of suspension elements during repair. Integrated diagnostic inspections regularly reveal (1 - 3) loosening of the joints between upper keybars and lamina- tions in large turbogenerators (165 MW and higher) with axial suspension of the stator core. The occurrence and de- velopment of such a defect is accompanied by increased vibration and sign-variable mechanical stresses in structural elements of loose sections of the suspension, which shortens their fatigue life. Moreover, impacts of the dovetail of a loose keybar on the core locally intensify the wear of the "dove- tails" of the keybar and the core. In the most critical cases, upper laminations of the stator core break and spall (Fig. 1). Fragments of laminations are direct danger to the insula- tion of the stator winding because they can pass through the ventilating passage and reach the surface of bars, where they, vibrating in variable magnet field, can cut the frame insula- tion to copper and, thus, cause earthing. Theoretical and full-scale studies show that a cause of in- creased local vibration is a decrease of the natural frequen- cies of loose keybars toward the resonance at the frequency of "magnetic" vibration of the stator core (100 Hz) caused by the radial attraction by the rotor poles. Currently, such defects are detected using an instrumen- tal method for testing the joints between the keybars and the core. The method involves measurement and spectral analy- sis of the free vibrations of certain sections of keybars after shock excitation (1, 2). Unlike visual inspections required by the Scope and Standards of Testing Electrical Equipment, the method allows us to quantitatively assess the actual condi- tion of the suspension. This substantially improves the reli- ability of detection and the sensitivity to incipient defects. Preserving and extending the residual life and ensuring the high operational reliability of high-power heavy-duty generators is a challenge which can be resolved with an inte- grated approach involving not only timely and reliable detec- tion of failures, but also development of repair measures and control of repair quality. Here we will analyze the experience of using the shock- pulse method to assess the effectiveness of repair measures intended to restore the vibration-isolation properties of the elastic suspension of the stator core of a high-power turbo-

USING OF ELECTRICAL METHODS OF MEASURMENTS FOR TESTING OF BRIDGES AND OTHER STRUCTURES

Purpose. With the introduction of new designs of bridges and other structures the using of electrical measurement techniques in which the measured deflections, stresses and vibrations, using transmitters, are converted into electrical signals in their testing is of current interest. It is very important to ensure a phase-sensitivity and proportionality between the measured values of mechanical and electrical signals. Methods. For electrical measurements transducers are connected to electrical bridge-type circuits. Calculations of the Wheatstone’s bridge connections forvarious transducers were made on the basis of Kirchhoff's laws for branched electrical networks and using circuitry transformation for which in the original scheme of the Wheatstone’s bridge the group of resistances is replaced by another, equivalent resistances group. Calculations were performed for the cases of presence and absence of internal resistance of the bridge’s power supply with connecting of transducers into one, two and four arms of a Wheatstone’s bridge. Findings. From the calculations the tables and graphs of the functions fi () and f0i () , that determine the change of current in the measuring diagonal of a Wheatstone’s bridge are derived, where  is relativechange in resistance of transducers. For one active sensor functions f1() and f01() are characterized by a high non-linearity, but for strain gages at   0,01...0,02 , acceptable linearity and accuracy of the measurements are provided. Originality. Based on the result analyses for calculations conducted, relations of the current in the measuring diagonal of the Wheatstone’s bridge are determined for various connecting circuits of transducers for measurement of the stress-strain state in structural elements during tests. Also the value of the internal resistance of the bridge’s power supply is taken into account. Calculations were performed using Kirchhoff's laws and by equivalenttransformation of branched circuit. Practical value. Obtained plots for functions fi () and f0i () and the above table allow to estimate the conditions for ensuring the linearity of derived dependencies for different transducers’ connections. Bridge-type circuits for insertion of strain gages allow to distinguish stresses in structural elements from the action of normal forces as well as vertical and horizontal bending moments.

Predicting the Residual Stresses in Steel T-Section Beams Using Acoustic Emission Technique

The Mechanical machining processes such as machining, cutting and welding induce residual stresses in structural elements. Welding is considered one of the common causes of such residual stresses due to metal shrinkage effects. Mild steel T-sections are widely used in many structures such as (i.e. bridges, offshore platforms and marine vessels). Since, local heating during welding causes severe thermal gradients welding in the welded structure and the uneven cooling that follows produces residual stresses and distortion. The aim of this paper is predicting the presence of residual stresses using acoustic emission (AE) technique. Series of laboratory bending tests are carried out on four welded T-section beams. Four different welding sequences have been applied to generate different values of residual stresses. The raw AE signal was analyzed in time and frequency domains. The results showed promising indications that the AE could be used as a screening technique for monitoring residual stresses presented in the welded structures.

Impact of wind effects on the loading of hydraulic cranes-manipulators with articulated booms

This article analyzes the influence of intensity of the longitudinal and transverse wind effects on its loading, using universal mathematical models for investigating of the dynamics of a hydraulic crane boom when moving units. The authors showed that the wind load not only causes additional stresses in structural elements, but also affects the kinematic and dynamic motion parameters of links articulated boom crane.

The influence of operating loads on the state of stress and strain in selected load-bearing elements of a tower-type headgear structure / Wpływ obciążeń eksploatacyjnych na stan naprężenia oraz przemieszczenia wybranych elementów nośnych konstrukcji basztowej wieży szybowych

Abstract The headgear structure allows the conveyance to be moved over the shaft top to the loading (unloading) point, at the same time it keeps in place the rope pulleys while tower-type headgear structures also accommodate the entire winder installations. The headgear is where the final stage of the hoisting installation is located and where the surface transport systems begin. These aspects strongly impact the actual shape of the tower, its height and in some cases determine the design of the entire winding gear. In order that all the headgear functions should be provided, it is required that the ultimate state conditions should be maintained throughout its entire service life. In order to assess the critical service conditions, the computation procedure should be applied based on design loads and fatigue endurance parameters. The computations of characteristic loads acting on the headgear structure use the developed model of the system based on the dynamic analysis carried out for a specific case: a hoisting installation operated in one of the underground collieries in Poland. The maximal and minimal loads acting on a Koepe pulley and those required for the system operation are determined accordingly. The laws of dynamics provide a background for finding the forces and moments of forces acting in the components of the driving system (including the electric motors and pulley blocks) for the specified loading of the Koepe pulley. Underlying the numerical FEM model of the tower-type headgear structure are the technical specifications of the analysed object and FEM calculations followed by endurance analysis to find the state of stress in structural elements of the headgear under the typical service conditions. The results help in assessing how the design of the hoisting installation should impact on safety features of load-bearing elements in the headgear structure.

Stability of layered coatings with outer layer made of ceramics

Introduction. The factors responsible for the performance of coatings in various elements of modern metallurgical equipment [6, 7] (such as the combustion chamber and air tuyere of a blast furnace) include: external conditions (temperature and stress in structural elements, composition and intensity of corrosive components in the gas flow); internal conditions (composition of the coating, material of the substrate, coating technology). The reactions to external and internal factors include thermal fatigue, physical and chemical development and formation of interlayers and, as a consequence, local bulging of coatings; erosive damage and cracking; change in the behavior of the interaction between the components of the gas flow and the materials of the layered coating (hydrogenation) and the oxidation processes, which give rise to new surface layers. One of the possible causes of bulging may be the surface microbuckling of the coating. The condensed metal coatings currently used for protection purposes are capable of resisting the erosive action of the gas flow. Note that the more functions the coating performs, the more layers it consists of and the higher the probability that it will lose stability. This, in turn, may lead to failure of such elements of metallurgical equipment, with all that ensues. It is of interest to study the surface instability of layered coatings in tuyeres and the effect of the basic mechanical, physical, and chemical processes in the blast furnace on this process. The three-dimensional stability of layered coatings with variable structure was addressed in [4, 8–13] where a general problem-solving approach was developed and some specific results were discussed. In [3, 14], the effect of high temperature is incorporated in buckling problems by correcting the values of the mechanical characteristics of the layers at set temperature of the coating. Following [3, 14], we will study (using a plane strain assumption) the effect of the outer ceramic layer on the stability of a layered coating in the operating conditions of the air tuyere of a blast furnace. We will use a piecewise-homogeneous medium model for linear elastic bodies. We will give a problem statement, outline a problem-solving method, and solve specific surface-buckling problems for layered coatings with outer layer made of ceramics. Small subcritical strains in the composite under biaxial thermomechanical loading will be assumed. 1. Problem Formulation. Governing Equations. The air tuyere of a blast furnace can be modeled by a layered

The hole drilling technique for on site deduction of the stresses states in stone masonry by using eight strain gages

The hole drilling technique for the on site deduction of the stresses in structural elements of cultural heritage structures has been applied on two monuments: the Saint Jakobs Church (Leuven, Belgium) and the Cathedral of Tarazona (Zaragoza, Spain). The adjustment of the technique becomes an experimental methodology very different from the one employed for the deduction of residual stresses. There are two main differences in the experimental methodology: the experimental dimensions are thirty times greater and eight strain gages are placed instead of three. The latter is done to have sufficient redundancy in the system and to be able to make an objective analysis of the quality of the results. The paper basically deals with the derivation of the stress states from the strains caught by the eight strain gages in agreement with the general methodology of the hole drilling technique.

Modeling of weak blast wave propagation in the lung

Blast injuries of the lung are the most life-threatening after an explosion. The choice of physical parameters responsible for trauma is important to understand its mechanism. We developed a one-dimensional linear model of an elastic wave propagation in foam-like pulmonary parenchyma to identify the possible cause of edema due to the impact load. The model demonstrates different injury localizations for free and rigid boundary conditions. The following parameters were considered: strain, velocity, pressure in the medium and stresses in structural elements, energy dissipation, parameter of viscous criterion. Maximum underpressure is the most suitable wave parameter to be the criterion for edema formation in a rabbit lung. We supposed that observed scattering of experimental data on edema severity is induced by the physiological variety of rabbit lungs. The criterion and the model explain this scattering. The model outlines the demands for experimental data to make an unambiguous choice of physical parameters responsible for lung trauma due to impact load.