Cyclic External Loading Research Articles

Low-Cycle Corrosion Fatigue Deformation Mechanism for an α+β Ti-6Al-4V-0.55Fe Alloy

Titanium alloys with high strength and good corrosion resistance have become one of the critical bearing structural materials in marine engineering. But in service, corrosion fatigue would occur under the synergetic action of cyclic external load and corrosion environment, threatening the safety of components. In this study, compared with low-cycle fatigue in laboratory air, the low-cycle corrosion fatigue deformation mechanism and fracture characteristic of the Ti-6Al-4V-0.55Fe alloy were investigated in 3.5% NaCl corrosion solution under selected stress amplitudes. The results showed that under low stress amplitude, corrosion fatigue was determined by fatigue damage and corrosion damage, causing a reduction in fatigue life. The local stress concentration caused by corrosion pits and dislocations pile-up accelerated the initiation of fatigue cracks, and other corrosion behavior including crevice corrosion promoted fatigue crack propagation; the corrosion solution increased the surface damage. While under high stress amplitude, due to the short contact time between the sample and solution and higher applied stress, the fatigue life is determined by fatigue damage caused by multiple slips.

Multilayered Semi-Elliptical Arch Structures: A Damping Analysis

Arch structures are used in various load-bearing applications in up-to-date engineering. In some cases these structures are made of multilayered materials with elastic-plastic behaviour. In many applications, the arch structures are subjected to cyclic external loading. This circumstance necessitates developing analyses of damping energy. Such analyses can be very useful for evaluating the effects of a variety of factors and parameters on the damping energy. The present paper is concerned with analyzing of damping energy in a semi-elliptical multilayered arch structure under cyclic bending. The left foot of the arch is clamped. Besides, the arch right foot is restrained by a rotational spring. An approach for determining of the changes in curvature and the coordinates of neutral axis in the portions of the arch is developed. Then damping energy in the arch is derived by integrating the damping energy density in the layers and summation for the arch portions. A parametric investigation of effects of the strength of the rotational spring, ratio of the ellipse semi axes and loading on damping energy is carried-out.

Embankment surface crack pixel-wise identification in UAV images based on a lightweight U-Network with transfer learning

Embankments in challenging environments experience cyclic external influences and loads that render their surfaces susceptible to a range of defects, with cracks particularly menacing. Therefore, accurate and efficient crack identification holds paramount significance for embankment safety. This paper addresses the challenges of low crack recognition rates on complex embankment surfaces and the high training costs associated with deep learning-based crack detection methods. We introduce a novel residual structure as the foundational innovation, followed by developing a streamlined U-shaped network (denoted as U²-Net_Aggregation) that amalgamates multi-scale insights by integrating skip connections. The newly devised U²-Net_Aggregation network demonstrates significant improvements in accuracy, intersection over union (IOU), and F1-score metrics through ablation tests on three distinct crack datasets. Comparisons with commonly employed crack detection methods (FCN, SegNet, U-Net, DeepCrack, and U²-Net) were conducted at the pixel level using an embankment crack dataset. Results unequivocally showcase enhancements in both accuracy and processing speed. To overcome the challenge of a high application threshold resulting from a scarcity of training data specific to embankment cracks, we compile an extensive dataset featuring diverse building crack scenarios, serving as a foundation for transfer learning. Following transfer learning, U2-Net_Aggregation attains recall, IOU, and F1-score values of 87.31%, 81.76%, and 90.06%, respectively. The proposed methodology enables swift and accurate crack detection by directly utilizing Unmanned Aerial Vehicle (UAV) images, offering a novel avenue for embankment crack detection in real-world field scenarios.

How the Direction of Screws Affects the Primary Stability of a Posterior Malleolus Osteosynthesis under Torsional Loading: A Biomechanical Study

Insufficient fixation of a posterior malleolus fracture (PM) can lead to posttraumatic complications such as osteoarthritis and chronic pain. The purpose of this biomechanical study was to test the hypothesis of whether the direction of PM screw fixation has an impact on the primary stability of osteosynthesis of a PM under torsional loading. PM fractures of 7 pairs human cadaveric lower leg specimens were stabilized with posterior to anterior (p.a.) or anterior to posterior (a.p.) screw fixation. Stability of the osteosynthesis was biomechanically tested using cyclic external torsional loading levels, in 2 Nm steps from 2 Nm up to 12 Nm, under constant monitoring with 3D ultrasonic marker (Zebris). The primary stability does not differ between both stabilizations (p = 0.378) with a medium effect size (η2p = 0.065). The movement of the PM tends to be marginally greater for the osteosynthesis with a.p. screws than with p.a. screws. Whether a.p. screws or the alternative p.a. screw fixation is performed does not seem to have an influence on the primary stability of the osteosynthesis of the PM fixation under torsional loading. Although osteosynthesis from posterior seems to be more stable, the biomechanical results in the torsional test show quite equivalent stabilities. If there is no significant dislocation of the PM, a.p. screw fixation could be a minimally invasive but stable surgical strategy.

Modeling of Magneto-Mechanical Damping in Ferromagnetic Alloys: a Brief Review

This review is devoted to the analysis of three components of magneto-mechanical damping (MMD), namely, macroeddy, microeddy, and hysteresis components, in ferromagnetic alloys. Theoretical models and equations for all three components of the MMD are discussed. The application of the classical theory of micro- and macroeddy MMD to determine the influence of the frequency of cyclic external mechanical load on the damping ability of the material is shown. A discussion on the role of various factors, such as temperature, strain amplitude, vibration frequency, and external magnetic field, on the magnitude of the MMD is provided.

Investigation of interlocking effect of crushed stone ballast during dynamic loading

The present paper deals with the experimental investigation of interlocking effect of crushed stone ballast material, assessing it as the relationship with the residual and dynamic stresses under the ballast layer during laboratory dynamic tests with the consideration of different boundary conditions. The laboratory experiments were executed with a scaled model of ballast under the sleeper. The measured pressure at the bottom surface of the ballast has two parts: dynamic and residual. The dynamic part depends on the external loading; the residual part remains after unloading. The measured residual stress was observed up to 3 times higher than the stress due to cyclic external loading. The relationship of the residual stress and interlocking effect to ballast particles angularity is analyzed. A simple interpretation of the distribution of residual stress is proposed, that depends on the measured cyclic stress and the elasticity of bounding walls. The study of interlocking effect of ballast could be potentially useful for many practical problems of railway track design as well as for the track maintenance issues.

Ultimate strength assessment of ship hull structures subjected to cyclic bending moments

The concept of “cyclic ultimate strength” of ship hull structures is proposed. In navigating in waves a ship is subjected to alternate bending moments in hogging and sagging conditions. The present study aims to demonstrate that the ship hull girder can experience a weaker ultimate strength under cyclic loading than under monotonically increasing bending moment as assumed in the conventional analysis. Nonlinear finite element method is employed, using the explicit LS-DYNA solver, to analyse the ultimate strength of a container ship hull structure subjected to monotonic and cyclic bending moments. The numerical analyses under cyclic loading consider the ratcheting effect and the Bauschinger effect in steel. The ship hull girder can break due to the plastic strain accumulation and alternating plasticity collapse when the cyclic external load is greater than the elastic limit but lower than the monotonic ultimate limit. The collapse modes of the hull girder under monotonic and cyclic loadings are compared to reveal their different failure principles.

Self-loosening Analysis of Bolted Joints

Bolted joint is one of the most widely used connections because of relatively simple structure, reliability, easy disassembly and maintenance. However, self-loosening of bolted joints under cyclic external loading is still an unsolved issue and need to make a further research. As is known to all, the self-loosening process can be divided into two distinguishable stages under cyclic transverse loading. The first stage is featured by a short and sharp clamping force reduction with no relative rotation between the nut and the bolt. The loss of the clamping force is due to the accumulation of local cyclic plastic deformation at the root of the engaged thread, which is account for no more than 10-15%. Therefore, the second stage is the primary cause of self-loosening. In this study, a further research on the second stage self-loosening of bolted joints is presented by the ways of combining theoretical analysis with the three-dimensional finite element simulation. The nature of the second stage self-loosening of bolted joints lies in: 1) the variation of the contact pressure between the engaging thread surfaces, 2) lateral micro-slip between the engaging thread surfaces. Both of them result in an apparent slip between the engaging thread surfaces of bolt and nut which finally leads to the self-loosening of bolted joints. In addition, the effects of preload, amplitude of cyclic transverse loading and the friction coefficient of engaging surfaces on self-loosening are studied by the finite element simulation. Moreover, a reasonable explanation of the trend about the self-loosening of bolted joints is given. The corresponding results are very important for the bolted design. It can be concluded that reasonable preload may improve the ability of bolted joints resisting to self-loosening.

Hierarchical modeling method and dynamic characteristics of cutter head driving system in tunneling boring machine

Traditional modeling method is difficult to establish the fully coupled model of the cutter head driving system in tunneling boring machine (TBM) owing to the structural complexity. In this paper, a generalized hierarchical modeling method is proposed to establish the entirely dynamic model of the TBM cutter head driving system. The components are equivalent to beam element with the same mass and stiffness characteristics, and the nonlinear coupling factors such as time-varying mesh stiffness, transmission error and gear backlash of both the multiple pinions driving and planetary gearbox are considered. The overall dynamic equation is formed by assembling the unit matrixes of basic finite elements according to the coupling relationships. Based on the proposed model, the vibration modes of driving system are classified into axial modes, rotational modes and lateral modes with distinct characteristics. The effects of coupling parameters on the natural frequency of driving system are investigated as well. The dynamic model is also simulated to analyze the dynamic response of driving system under two load conditions, cyclic external loads and simulated external loads. The main response frequency components and the effect of damping coefficients on vibration amplitudes are investigated. The proposed modeling method can be generalized to other complex driving system, and the obtained results provide data support for the dynamic design of driving system in TBM.

Finite element modeling of residual stress relaxation in steel butt welds under cyclic loading

This study attempts to develop a numerical method to predict the real stress state in steel butt welds subjected to cyclic mechanical loading. 3-D FE thermal simulation of a butt welding is first performed to obtain the residual stresses. Then, 3-D elastic–plastic FE model which incorporates the cyclic plasticity constitutive equation based on the Chaboche nonlinear kinematic hardening and the isotropic hardening rule is developed to evaluate redistribution of the residual stresses under cyclic external loading. The analytical results have shown that the FE analysis method devised here can effectively predict the cyclic relaxation of the residual stresses.

Effect of mechanical boundary conditions on orientation of angiogenic microvessels

Mechanical forces are important regulators of cell and tissue phenotype. We hypothesized that mechanical loading and boundary conditions would influence neovessel activity during angiogenesis. Using an in vitro model of angiogenesis sprouting and a mechanical loading system, we evaluated the effects of boundary conditions and applied loading. The model consisted of rat microvessel fragments cultured in a 3D collagen gel, previously shown to recapitulate angiogenic sprouting observed in vivo. We examined changes in neovascular growth in response to four different mechanical conditions. Neovessel density, diameter, length and orientation were measured from volumetric confocal images of cultures exposed to no external load (free-floating shape control), intrinsic loads (fixed ends, no stretch), static external load (static stretch), or cyclic external load (cyclic stretch). Neovessels sprouted and grew by the third day of culture and continued to do so during the next 3 days of loading. The numbers of neovessels and branch points were significantly increased in the static stretch group when compared with the free-floating shape control group. In all mechanically loaded cultures, neovessel diameter and length distributions were heterogeneous, whereas they were homogeneous in shape control cultures. Neovessels were significantly more oriented along the direction of mechanical loading than those in the shape controls. Interestingly, collagen fibrils were organized parallel and adjacent to growing neovessels. Externally applied boundary conditions regulate neovessel sprouting and elongation during angiogenesis, affecting both neovessel growth characteristics and network morphometry. Furthermore, neovessels align parallel to the direction of stress/strain or internally generated traction, and this may be because of collagen fibril alignment induced by the growing neovessels themselves.

Fatigue failure prediction of press fitted parts subjected to a cyclic loading condition by finite element methods

The lower control arm is one of the most important components and constrains the longitudinal and vertical movements of the wheel relative to the vehicle. Recently, aluminium alloy has become a constituent material of the arm instead of steel in order to reduce the weight of the vehicle. As a cyclic external load is applied to the ball joint of the arm, the contact force between the steel bush housing and the aluminium arm joined by the press fitting varies through the deformation of the rubber bush. The oscillatory contact force between these components could determine the life of the aluminium alloy arm. Indeed, the failure of the arm was observed at the bush mount where fretting is expected. In this study, nonlinear finite element simulations were carried out to investigate the cause of the failure. As a result, it turned out that the failure of the mount was associated with the fretting introduced by the press fitting and the cyclic stress due to the oscillatory contact force by external cyclic loads. In addition, the failure location and lifetime of the aluminium alloy arm were predicted based on a damage analysis.

Experimental and numerical analysis of dynamic metal hydride hydrogen storage systems

This paper examines the dynamics of metal hydride storage systems by experimentation and numerical modelling. A specially designed and instrumented metal hydride tank is used to gather data for a cyclic external hydrogen load. Thermocouples provide temperature measurements at various radial and axial locations in the metal hydride bed. This data is used to validate a two-dimensional mathematical model previously developed by the authors. The model is then used to perform a parametric study on some of the key variables describing metal hydride systems. These variables are the equilibrium pressure, where the tails and concentration dependence are investigated, and the effective thermal conductivity of the metal hydride bed, where the pressure and concentration dependence are analyzed. Including tails on the equilibrium pressure curves was found to be important particularly for the accuracy of the initial cycles. Introducing a concentration dependence for the plateau region of the equilibrium pressure curve was found to be important for both pressure and temperature results. Effective thermal conductivity was found to be important, and the inclusion of pressure and concentration dependence produced more precise modelling results.

Evaluation of the fretting fatigue behaviour of commercially pure titanium

Fretting fatigue occurs when the contact surfaces of two components undergo small oscillatory movement while they are subjected to a clamping force. A cyclic external load gives rise to the early initiation of fatigue cracks, thus reducing their service life. In this paper, the fretting fatigue behaviour of commercially pure titanium flat samples (1.5mm thick) is evaluated. A fretting device composed of a frame, load cell, and two screw-mounted cylindrical fretting pads with convex extremities was built and set to a servo-hydraulic testing machine. The fatigue tests were conducted under load control at a frequency of 10 Hz and stress ratio R = 0.1, with various contact load values applied to the fretting pads. Additional tests under inert environment allowed assessing the role of oxidation on the wear debris formation. The fracture surfaces and fretting scars were analysed via scanning electron microscopy in order to evaluate the surface damage evolution and its effect on the fatigue crack features. The effect of the fretting condition on the S-N curve of the material in the range of 104-106 cycles is described. Fatigue crack growth calculations allowed estimating the crack initiation and propagation lives under fretting conditions. The effect of the fretting condition in fatigue life is stronger for the lower values of cyclic stress and does not seem to depend on the contact loading value.

An Experimental Investigation of the Effects of Clamped Length and Loading Direction on Self-Loosening of Bolted Joints

An experimental investigation was conducted to study the effects of clamped length and loading direction on the self-loosening behavior of bolted joints by using specially designed fixtures. The experiments mimicked two plates jointed by an M12×1.75 class 10.9 bolt and a nut. The joints were subjected to cyclic external loading. A constant preload of 25kN was used for all the experiments conducted. During an experiment, the relative displacement between the two clamped plates, δ, was a controlling parameter. The reduction in clamping force, the applied transverse load, and the nut rotation were measured cycle by cycle. The relationship between, Δδ∕2, the amplitude of the relative displacement between the two clamped plates, and, NL, the number of loading cycles to loosening is referred to as self-loosening curve and was obtained for different clamped lengths and applied load directions. Similar to a fatigue curve, an endurance limit can be identified from the self-loosening curve. It was found that increasing the clamped length can enhance the self-loosening endurance limits in terms of the controlled relative displacement of the two clamped plates. However, the load carrying capability was not influenced significantly due to the thickness of the clamped plates. For a given bolted jointed structure, an angle of the external load from the pure shearing direction resulted in an increase in self-loosening resistance.

Numerical evaluation of residual stress relaxation by cyclic load

The aim of this work is to evaluate by numerical simulation the real stress state of a welded component subjected to a sinusoidal external load. In particular, the relaxation process of the residual stress, due to the application of a cyclic external load, has been studied. Longitudinal welded joints, laser welded and 3 mm thick, are analysed in this work following the residual stress evolution during a fatigue test. A numerical procedure, implemented by the ABAQUS code, carries out the residual stress field originated by a welding process and it is taken as a pre-stress condition in the present ABAQUS simulations. In order to evaluate the influence of the amplitude level on the residual stress relaxation, eight different sinusoidal loads at the load ratio R = 0.1 are applied to the model. The results show a significant reduction in the initial residual stress level, even after the first load cycle. They offer a very precise explanation of the fatigue behaviour of this kind of welded joints, confirming the behaviour experimentally observed.

On the Effects of Cyclic Transversal Forces on OsseointegratedDental Implants: Experimental and Finite ElementShakedown Analyses

This paper is concerned with the mechanical strength of fixed osseointegrated dental implants subjectedto cyclic external loads, applied mainly in a direction orthogonal to their axis. Such a loading condition,seen as a basic design action for the implant, has been given little attention so far.Experimental results and numerical simulations, performed on two- and three-dimensional FiniteElement models, are discussed. The shakedown theory is used to show that a common implant design(threaded fixture-abutment-connection screw) is susceptible of low-cycle fatigue failure under loadingconditions well within the working range, even if the same design is able to withstand loading of thesame type, but applied monotonically, much in excess of the working values. The shakedown analysesgive an indication of several possible failure modalities: the low-cycle fatigue either of the implant or ofthe connection screw, or the loosening of the connection screw itself.Experimental and numerical results are in good qualitative agreement, and both suggest that the issueof transversal cyclic loading on fixed dental implants should be carefully reconsidered in the designphase.

The energy dissipation of particle-reinforced metal-matrix composite with ductile interphase

The finite element analysis together with cell modeling method has been employed to simulate the behavior of energy dissipation for the particle-reinforced metal-matrix composites (PMMCs) with a ductile interphase in this paper. The influences of strain amplitude ε 0, ratio of elastic constant r, relative thickness R i / R p of the interphase on the energy dissipation ψ of the whole composite at the room temperature have been investigated under cyclic external loading. The results show that, when the elastic constant ratio r is less than 1, the energy dissipation of the composite is much higher than that when the elastic constant ratio r is more than 1. The results also show that, when the interphase is soft, the energy dissipation ψ of the whole composite decreases with the increasing of the elastic constant ratio r, while, it increases when the interphase is hard. And with the ratio of elastic constant r in a certain range (just around r=1), the energy dissipation ψ of the composite decreases with the increasing of the relative thickness, but increases when the ratio of elastic constant is out of the certain range.

The delayed fracture of an isotropic viscoelastic plate with a microcrack under a varying load

The delayed fracture of an isotropic viscoelastic plate due to subcritical growth of a rectilinear microcrack of normal separation is studied. The paper deals with the development of the crack due to the stretching of the plate by uniformly distributed increasing and cyclic external loads applied perpendicularly to the crack line. The investigation is carried out within the framework of the Boltzmann-Volterra theory for resolvent integral operators of difference type, which describe the deformation of materials with time-dependent rheological properties. Numerical calculations are performed for resolvent integral operators with a kernel in the form of Rabotnov's fractional-exponential function. The kinetics of a crack with a tip zone commensurable with the crack length is studied. The results are compared with those obtained on the basis of the hypothesis on the thin structure of the crack tip.

Determination of Limit Load by Means of Elastic-Plastic Finite Element Analysis as Exemplified by a Lateral Nozzle Subjected to Cyclic External Load

Determination of Limit Load by Means of Elastic-Plastic Finite Element Analysis as Exemplified by a Lateral Nozzle Subjected to Cyclic External Load