Preload State Research Articles

A Three-Dimensional Finite Element Analysis of A Two-Axially Pre-Loaded Plate Exposed To A Dynamic Force

In this paper, the forced vibration analysis by a harmonically time-dependent force of an elastic plate covered rigidly by a rigid half-plane is given. The plate layer is subjected to bi-axial normal initial force, into lateral sides separately. Here, the preloading state is exactly static and homogeneous. To eliminate the disadvantage of such a nonlinear model, the problem formulation is modeled in terms of the fundamental consideration of the theory of linearized wave in elastic solids under a pre-loaded state (TLWESPS) in a plane-stress case. For this purpose, considering Hamilton’s principles, the system of the partial equations of motion and the boundary-contact conditions are found. Based on the virtual work and the fundamental theorem of calculus of variation, the three-dimensional finite element method (3D-FEM) is used to understand the dynamic behavior of the plate. A numerical validation process is established. Next, influences of certain problem parameters such as Young’s modulus, aspect ratio, thickness ratio, pre-loaded parameter, etc. on the frequency mode of the pre-stressed system are given.

Research on the Influence of Disc–Drum Connection Bolt Preloading Rotor Assembly Modal Characteristics and Diagnosis Technology

The drum rotor of an aero-engine is connected by one or multiple mounting edges through bolts, and their dynamics are significantly influenced by the preload state of the bolts. Long working hours in challenging environments can result in the deterioration of bolt pre-tightening during assembly or service, which impacts the rotor’s dynamic stability and overall performance. Currently, there are no available methods for detecting the dynamic characteristics of the drum connection components. This paper analyzes the impact of the natural characteristics of the drum composite structure of a high-pressure aero-engine turbine based on the refined finite element method when the preloading state changes. Two conditions of deviation and uneven stiffness distribution were applied to the connected components of the drum. The analysis focused on the impact of the pre-tightening state on its natural frequency. After analyzing the feasibility of identifying the pre-tightening state, two methods are proposed. These methods focus on changes in natural frequency and mode shape, specifically the sensitive natural frequency change method and the mode step change method. The methods proposed in this paper can serve as a reference for evaluating the quality of assembling complex disc–drum structures with multiple bolt connections.

Dynamic and Residual Static Behavior of Axially Loaded RC Columns Subjected to Low-Elevation Impact Loading

Columns can suffer heavy damage due to dynamic impact effects, which are ignored during their design. The impact effect could be a vehicle crash to columns of streetside buildings, parking garages or bridges. However, the effect of impact loading on the behavior of reinforced concrete columns has not been sufficiently studied. In this study, an experimental and numerical investigation is carried out on the impact behavior of axially loaded reinforced concrete columns. Dynamic experiments were carried out by dropping a mass from different heights to apply low-elevation impact on axially loaded, full-scale (30 × 30 × 320 cm) columns. After evaluating the performance of the columns under varied impact loadings, the residual load carrying capacities of the columns were also obtained by static loading. Additionally, a three-dimensional finite element model was developed and validated by using drop weight experimental results. The effect of increasing the impact energy on the behavior of RC columns was also examined numerically. As a result of the research, it has been observed that, as the applied impact energy increases, the dynamic damage/failure mode changes from flexure to shear. When a column was impacted by 75.8% of its total impact energy capacity, a decrease of 38.1% in its stiffness and a decrease of 49.7% in its load carrying capacity were determined compared to its previous unimpacted state. Additionally, the static energy dissipation capacity loss of the column was reached, up to 81.7% of its preloading state. The developed finite element model can also be utilized to determine the dynamic performance and the damage modes of columns under vehicle collision-type low-elevation impacts, which can be a guide for structural engineers in the design of such vulnerable columns and will contribute to safer structural designs.

Local subsidence behavior of linear guideway assembly resting on a rough elastic foundation

The accurate stiffness estimation of linear guideway assembly is determined by the structural deformation of carriage and supporting rail-pedestal-bolt joint. This paper proposes an analytical model to investigate the stiffness and local subsidence behaviors of linear guideway assembly. The different preload states between the upper and lower raceways are revealed, and the retraction behavior of carriage wing is found. Considering the local subsidence of the rail below a carriage, the proposed model reports the non-uniform load distribution along the finite-length rail, which is placed on the rough foundation and assembled by bolts. Loading tests are performed to verify the local sinking effect and the model accuracy. The developed model is promising to serve to the deformation error compensation.

EFFECT OF CHORD PRELOADING AND BRACE INCLINATION ON AXIAL RESISTANCE OF FULL-WIDTH RHS X-JOINT

According to current representative design standards, the influence of brace angle on the axial resistance of rectangular hollow section (RHS) X-joints is assumed to be independent of the chord preloading state. To investigate the appropriateness of this assumption, extensive test-validated finite element (FE) analyses are performed on full-width RHS X-joints with varying brace angles and chord preloads. The analyses showed that an appreciable coupling effect between the brace angle and chord preload exists on the joint resistances. The most adverse coupling effect occurs under the condition of high brace inclination and large compressive chord preload, leading to the concern that the current design resistance formula may overestimate the ultimate joint strength. Consequently, a revised chord stress function is sought for inclined full-width RHS X-joints that accounts for the coupling effect between chord preload and brace angle. In the FE analyses, high strength steel joints are included as well as conventional mild steel joints. The chord stress effect in high strength steel joints is found to be more detrimental compared to geometrically identical mild steel joints, implying that the chord stress effect may be influenced by the class (slenderness) of the chord cross-section. Therefore, a proper limitation on the chord section slenderness, such as the Class 2 requirement in design codes, appears essential.

Free Vibration Characteristics Analysis of Metal-Rubber Cylindrical Shells Based on Viscoelastic Theory

The cylindrical shell made of metal rubber has a strong ability to reduce and absorb vibration, which widens its application in the industrial field. Therefore, it is of great significance to study the vibration characteristics of metal-rubber cylindrical shells (MRCSs). However, there is relatively little research on this aspect. Based on this, the dynamic properties of MRCS are investigated in this paper based on viscoelastic theory, the Rayleigh–Ritz method, and the Gram–Schmidt orthogonal polynomials. The correctness of the proposed model was verified by comparison with the literature and experimental verification. The results show that the preloading state and boundary conditions have significant effects on the natural frequency and modal loss factor of MRCS. The effect of the Pasternak elastic foundation on the natural frequency and modal loss factor of MRCS varies with the change of the axial half wave number m. The effect of the Pasternak elastic foundation on higher-order vibrations is similar to that of the artificial spring technique.

Analysis of the contact angle of ball screw with Gothic profile considering raceway profile manufacturing error and radial clearance

The ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. The elastic contact angle is considered as the key parameter for the ball screw mechanism with Gothic profile, and it is hard to measure this parameter directly. This paper investigates a calculation method for elastic contact angle of the ball screw considering the raceway profile manufacturing error and radial clearance. The method is based on two deformation model of balls under two preload states in ball screw. The deduced formula from the model on free contact angle is found to be identical with results of split inner-ring ball bearing. The effect of the radial clearance and raceway profile manufacturing error are investigated in this paper. The results show that the radial clearance is a key factor affecting the elastic contact angle. The research provides a theoretical basis for error analysis and optimization design of the new type double-nut ball screw mechanism.

Design of a compliant adjustable constant-force gripper based on circular beams

Compliant grippers have been widely used in engineering systems to achieve precision operations. In this paper, a compliant adjustable constant-force gripper based on circular beams is developed for realizing the compatible and stable constant-force operation. In the developed gripper, a stiffness-combination constant-force mechanism (SCCFM) is applied as its basis. A pseudo-rigid-body PPRR model and particle swarm optimization algorithm are utilized for circular-beam modeling and identification of the SCCFM, respectively. A devised preload mechanism realizes the adjustment of the initial preloading state of the SCCFM. A series of experiments are conducted to demonstrate the effectiveness of the developed SCCFM and the gripper.

Research on the identification method of the pre-tightening state of the matching surface of the aero-engine disk-drum rotor

The aero-engine rotor system is connected by one or more stages of mounting edges through bolts, and the pre-tightening state of the mating surface will directly affect the dynamic performance of the rotor and even the whole machine. However, there is little research on the identification method of the pre-tightening state of the mating surface of the disk-drum rotor with a multi-bolt flange connection. Therefore, based on the vibration theory, this paper analyzes the feasibility of the pre-tightening state identification method of the mating surface. Then, from the perspective of the assembly process and structural characteristics of the rotor system, this paper takes the high-pressure compressor disk-drum rotor, high-pressure turbine disk-drum rotor, and high-pressure rotor system as the research objects, respectively, and uses the finite element method and experimental method to analyze the influence of the pre-tightening state change of the mating surface on the rotor dynamic characteristics. Finally, according to the obtained regularity cognition, three pre-tightening state identification methods of multi-bolt connection mating surface are proposed: low-order vibration phase difference method, amplitude variation method of higher-order frequency response function (FRF), and axial tension-sensitive natural frequency offset method. The proposed methods can effectively identify the preload state of the mating surface of the disk-drum rotor in engineering due to improper assembly or harsh environment and provide a new research idea for the dynamic characteristic detection and assembly quality evaluation of aero-engine rotor.

МОНИТОРИНГ СОСТОЯНИЯ ПАР ТРЕНИЯ ЭНЕРГОСИЛОВЫХ УСТАНОВОК НА ОСНОВЕ КЛАСТЕРОВ ОДНОВИТКОВЫХ ВИХРЕТОКОВЫХ ДАТЧИКОВ

The tasks of on-line assessment of the state of friction pairs of high-powered power plants are considered. The approaches to the solution of the tasks are offered, too. It is noted that the development of the appropriate methods and means for monitoring the parameters of the condition of bearing units of this class of machines will allow not only to detect timely the bearing damage and appearance of the pre-load state of the power plant, but also to estimate the residual life of the friction pair at each current point in time. This, in turn, facilitates the transition from the traditional routine maintenance and operation of power plants to their condition-monitored maintenance and operation. The proposed solutions are based on the use of original single-coil eddy-current sensors with sensitive elements in the forms of a wire segment or a single current circuit, as well as their cluster compositions (sensor ‘s groups or sensors with a group of sensitive elements). The simplicity of the sensor’s design and the technological solutions embedded in it ensure high technical and operational characteristics of the primary transducer and give the opportunity to use the sensors in real operating modes of power plants. It is proposed to use the information about the presence of metal wear particles of the friction pairs, washed with oil, in the lubrication system of the power plant and the axial displacement of the shaft in a radial thrust bearing as the main monitored parameters. The estimation of the bearing residual life resource can be given on the base of the information about the state of friction pair. A brief description of the methods for obtaining the information about the desired parameters is given. The principles of the construction of monitoring systems implement the methods is provided on the base of generalized structural scheme.

Fully elastic multilayered triboelectric energy harvester made of polymer thin films and elastic tubes

This paper presents a triboelectric energy harvester that can be stacked in multiple layers and fabricated using an elastic hollow tube, polymer film, and conductive tape. Using the compression deformation characteristic of the elastic tube, the proposed device can stably generate high average powers at large displacements and generate energy even at small vibrations, where the two materials are not entirely separated. The proposed device was fabricated by attaching a conductive tape to the film and a self-assembled monolayer coating. Peak-to-peak values of 33.5 V and 11.5 μA were measured when a force of 50 N at 4 Hz was applied to a single-layer device. When the device was pressed by 1 mm in advance, the outputs decreased to 19.2 V and 6.6 μA. A three-layer device exhibited the same open-circuit voltage, whereas the maximum short-circuit current was measured to be 33 μA, approximately three times higher. When the device was operated in the full contact–separation mode, the maximum power was measured to be 837 µW, whereas when a 1 mm preload state was maintained, the measured power was 419 µW.

60188 The coupled dynamic analysis of hydroelectric unit and powerhouse considering the bolt connection characteristics

As the increasing of the capacity and water head of the hydropower stations, the hydroelectric unit’s vibration and its induced powerhouse vibrations become a critical problem. The system of hydraulic-mechanical-electric-structural is coupled and with strong non-linear characteristics. The frame and head cover are all connected with the powerhouse pier via high-strength bolt. The paper is concentrated on the dynamic coupling mechanism and simulation model considering the bolt’s connection and load transmission characteristics. With the theory analysis and numerical simulation, the connection and transmission behaviour of the high-strength bolt would be studied and a more accurate and simple mechanical expression would be derived. The connection sliding law and dynamics modelling method of bolts are discussed. Based on the displacement-load curve of single lap and single bolt analysis it is concluded that the increasing preloading will improve the tangential bearing capacity of the bolted joint connection. Then the FEM coupling model of the lower bracket and powerhouse is established with the virtual medium method. Based on the free-vibration and dynamic response analysis the effects of the loosening of the bolt on the static stiffness of the coupling structure are discussed. The von-Mises stress’s increasing due to some of the bolt’s loosening would result to the material fatigue damage. Finally five preloading schemes of the bolts are discussed and the simulation results show that the preloading force has a greater lateral impact on the structure than on the longitudinal direction. The preloading force improves the coupling structure’s stiffness and reduces the vibration amplitude of the frame. As for the preloading force, when it is small the connection effect is not strong enough and the vibration level may be higher, so the actual preloading state should be exactly simulated. When the bolt preloading reaches 60-70% of the material yield strength, the rigid connection can be directly used in the modelling for the convenience of calculation.

Experimental Studies of the Process of Tightening an Asymmetric Multi-Bolted Connection

The paper presents experimental studies of an asymmetric multi-bolted connection at the preloading state. Design assumptions and structure of a stand for testing forces in bolts in a multi-bolted connection were introduced. The tightening process was conducted with a wrench, monitoring the values of the forces in the bolts using a calibrated measuring system based on resistance strain gauges. The measurement data was processed using the Matlab R2018b Simulink program. Two methods of bolt tightening were tested: in one pass (in which the bolts were tightened in a specific sequence and each bolt was tightened immediately to the full preload value) and in several passes (in which the bolts were also tightened sequentially but the full preload value was applied to the bolts in three tightening cycles). The influence of the method and sequence of bolt tightening on the distribution of force values in bolts during and at the end of the preloading state was investigated. The results were statistically processed and summarized in the form of diagrams showing the distributions of normalized force values in the bolts for all the considered tightening cases. The tests were carried out for the selected connection and an assembly method was proposed which would make it possible to achieve the most even distribution of the force values in the bolts at its end, and thus before the connection exploitation state.

Dyspnea in Chronic Low Ventricular Preload States.

Dyspnea in low-preload states is an underrecognized but growing diagnosis in patients with unexplained dyspnea. Patients can often experience debilitating symptoms at rest and with exertion, as low measured preload often leads to decreased cardiac output and ultimately dyspnea. In the present article, we performed a review of the literature and a multidisciplinary evaluation to understand the pathophysiology, diagnosis, and treatment of dyspnea in low-preload states. We explored selected etiologies and suggested an algorithm to approach unexplained dyspnea. The mainstay of diagnosis remains as invasive cardiopulmonary exercise testing. We concluded with a variety of nonpharmacological and pharmacological therapies, highlighting that a multifactorial approach may lead to the best results.

Cardiovascular Findings in Severe Malaria: A Review.

Background:Severe malaria remains a leading cause of death worldwide. A greater understanding of its impact on multiple organ systems is essential in reducing the burden of disease. In this review we will summarize previously reported cardiovascular parameters of both adults and children with severe malaria.Method:For this systematic review we searched MEDLINE and PUBMED for all papers published on cardiac function in severe malaria from January 1, 1990 until September 1, 2019. Severe malaria was defined as per World Health Organization. Publications were included if there was data from echocardiography, Pulse Contour Cardiac Output (PiCCO), or Pulmonary Arterial catheters (PAC) reported. Studies were excluded if related to medication induced cardiac dysfunction, malaria in pregnancy, or included subjects with known pre-existing heart disease.Results:Twenty-four studies met inclusion criteria, the majority of which were studies of adult patients or a mixed cohort. Six solely involved pediatric patients. Significant heterogeneity existed in the cardiac parameters measured and results reported. One pediatric and one adult study suggested a reduced preload state during severe malaria. Cardiac systolic function was reported primarily within, or above, normative numeric ranges established in uninfected pediatric patients without anemia. Extensive variability existed in adult studies with reports of an elevated cardiac index in two studies, normal cardiac function in two studies, and descriptions of decreased function in two studies. Two reports suggest afterload in pediatric severe malaria is reduced. Reports of changes in the systemic vascular resistance of adults with severe malaria are inconsistent, with two trials demonstrating an increase and two suggesting a decrease. Studies demonstrated a mild rise in pulmonary pressure in both pediatric and adult patients that normalized by discharge.Conclusion:Based on limited data, the cardiovascular effects of severe malaria appear to be heterogeneous and vary depending on age. Further detailed studies are required to explore and understand the overall hemodynamic effects of this high burden disease.

Preload state detection for precision spindle bearings based on multi-level classification

In this study, a multi-level state classification method based on support vector data description (SVDD) is proposed to detect bearing preload state. Firstly, a three-category classification support vector data description algorithm is proposed to establish the three-state non-aliasing hypersphere model, which can combine the kernel principal component analysis (KPCA) and membership degree. Then, by classifying the first-level training sample model of uniform and non-uniform preload states, the second-level training sample model of non-uniform preload states is established based on SVDD algorithm. Moreover, the balanced multi-label propagation classification criterion is defined that can be used to identify the preload state level based on the training sample model. Finally, a preload state detection system is developed, which can accurately simulate uniform/non-uniform preload states for spindle bearings. The experimental results demonstrate that the proposed algorithm can effectively classify the preload states of spindle bearings with average accuracy higher than 94%.

Nonlinear Static Analysis and Structural Optimization of Rigid Clamp Band Connection Device between Launch Vehicle and Satellite

The Rigid Clamp Band Connection Device (RCBCD) is a novel satellite-rocket connection method adapted to the heavy lift trend of the launch vehicle. The static analysis model for the Rigid Clamp Band (RCB) and the Docking Ring (DR) under preloading state is established, and the structural strength and connection stiffness characteristics under the axial loading are analyzed, and the axisymmetric equivalent and parametric modeling techniques are combined to optimize the section shape parameter, which improves the overall connection performance. The results show that the structural stress concentrated on the "line-to-face" contact position between RCB and DR. Increasing the axial dimension of RCB, reducing the V-type angle and deepening the occlusion depth of RCB and DR can effectively improve the connection rigidity of RCB. By optimizing the section shape, the connection performance of RCBCD can be improved by above 70% under the structural strength and mass constraints.

Mechanical Responses of a Combined Support Structure for a Nb3Sn Sextupole Magnet During Assembly and Thermal Cycle

The Institute of Modern Physics (IMP) in Lanzhou, China, is currently developing a complicated superconducting magnet system composed of four solenoids and six sextupole coils based on a sextupole-in-solenoid configuration. The magnet is made of a single Nb3Sn wire and designed for a fourth-generation ECR ion source operating at frequency of 45 GHz. To effectively prevent the wires motion from performance degradations and a quench, a pre-stress is essential for the superconducting magnet winding and strengthening. Focusing on the sextupole magnet's assembly and the pre-load procedure, a combined support structure with temporary bladders, load pads, and load keys, as well an aluminum shell was designed and fabricated in this work. A whole assembly process was fulfilled with a dummy structure at room temperature and a cooling-down and warming-up cycle to show its validity. Some low-temperature resistance strain gauges, together with a half-bridge compensation, were used to monitor strain of the aluminum shell during the pre-load control performance and thermal cycle process. A parametric study on the equivalent friction of the combined structure was carried out for the mechanical characteristics based on FEM simulation to show a reasonable agreement with the experiment measurements as a proper friction coefficient chosen. Finally, a real sextupole magnet was assembled with the combined support structure and tested for excitation. It was illustrated that a proper pre-load state realized by the support structure and a high excitation current about 800 A was reached so that the excited magnetic field gained 7 T as the designed value.

Finite Element Simulation of Tensile Preload Effects on High Velocity Impact Behavior of Fiber Metal Laminates

As primary structures and protective structures, fiber metal laminates (FMLs) in practical application are always in preload states before possible impact damage threats. In this work, a nonlinear finite element (FE) model is developed to study the impact damage scenario where tensile preloaded FMLs are impacted by a hemispherical nose projectile at high velocities. In the modeling, the intra-layer damage in aluminum sheets and composite laminates and the delamination between adjacent plies are entirely included. The FE model is verified with available experimental results in non-preload condition and then applied to predict the high velocity impact behavior in uniaxial and biaxial tensile preload cases. The ballistic performance and damage response of representative FMLs with different preloads and impact velocities are discussed and analyzed in detail. The numerical results and modeling strategy here provides applicable reference for preloaded impact issues in other FMLs.

Coupled Static‐Dynamic Tensile Mechanical Properties and Energy Dissipation Characteristic of Limestone Specimen in SHPB Tests

To investigate the dynamic splitting tensile mechanical property of limestone under coupled static and dynamic state, the dynamic split tensile tests of limestone under one‐dimensional coupled static and dynamic load with different strain rates were performed with the help of modified split Hopkinson pressure bar (SHPB) equipment. The dynamic splitting tensile mechanical property and energy dissipation characteristic under two stress states were also compared in this research. Test results indicated that the dynamic tensile strength of the limestone specimen increased with the increase of average strain rate, exhibiting an obvious strain rate effect. In addition, dynamic tensile strength under uniaxial state was higher than that under one‐dimensional coupled static and dynamic load state under the same test condition. Moreover, the deformation modulus increased with increasing average strain rate under uniaxial state, while it decreased with increasing average strain rate under coupled static and dynamic state. Both the reflected energy and absorbed energy linearly increased with increasing incident energy. The preload in the radial direction could increase the reflected energy and decrease the absorbed energy. Moreover, the transmitted energy with preload state was slightly lower than that under uniaxial state. Finally, the dynamic tensile strength of limestone specimen increased as a power function with increasing absorbed energy.