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There is very little data addressing cartilage response to tensile forces, and no literature attempts to correlate compressive with tensile modalities. Our hypothesis was that the cyclic compression and tension modulate chondrocyte matrix proteoglycan synthetic response differently. Porcine chondrocytes cultured to confluence on a flexible membrane were subjected to cyclic compression (Group A: 13 KPa at 1 Hz) or tension (Group C: 10% strain at 1 Hz) for 16 or 32 h; while controls not subjected to any force were kept (Group B). The chondrocytes were then stained with alcian blue and stained areas quantified with confocal microscopy and image processing software. Two-factor ANOVA with post-hoc tests (Scheffe and Bonferroni) statistical analysis were used. Proteoglycan staining covered 46% (range 28%–61%) and 39% (range 26%–49%) of the surface area following 32 and 16 h of compression respectively, 23% (range 15%–49%) for control, and 19% (range 10%–29%) and 16% (range 9%–25%) following 16 and 32 h tension respectively. Proteoglycan content following all compressions was significantly greater than with cyclic tension or control (p < 0.0001). Our data demonstrate that chondrocytes cultured in vitro respond to compression distinctly different from tension and that it is highly sensitive to mechanical loading, with rapid adaptation to its mechanical environment. These results imply that cartilage grown in culture, with the intention of transplantation, may structurally benefit from an environment of cyclic loading at higher frequencies.

Experimental results obtained from cyclic buckling and postbuckling tests of pre-damaged stiffened CFRP panels are presented in this paper. This work was conducted within the COCOMAT project funded by the EU with the objective of contributing to the reduction of structural weight at safe design. COCOMAT was targeted at establishing a new design scenario for composite stiffened panels which are understood as part of an aircraft fuselage. This design scenario aimed at exploiting considerable reserves in the load carrying capacity in fiber composite fuselage structures by accurate simulation of collapse. The project results cover an experimental database, improved slow and fast computational tools, as well as design guidelines. A reliable simulation of the collapse load requires taking degradation into account. For the validation of the tools, a sound database of experiments is needed which gives information about the progress of damage during the loading process. In this context, the present paper focuses on the investigation of pre-damaged stringer-stiffened panels under cyclic axial compression. A set of four panels of the same design was split into two variants which differ only in the position of an artificial Teflon disbond beneath a stringer. One panel of each variant was tested statically until collapse in one step as reference, while the other panel was tested cyclically with different amplitudes. Before the test, all test structures were assessed by ultrasonic inspection and the geometric imperfections were measured. During the test, advanced measurement systems such as the ARAMIS system for the measurement of the buckling pattern and thermography for monitoring the skin-stringer separation were utilized in addition to strain gauge measurements and the record of the load shortening data. The test structures, their preparations for testing, the buckling test facility, and the measurement systems used are described. The test results as to the influence of the cyclic loading on the damage progression in the skin-stringer connection are presented and discussed.

One of the most important goals of the COCOMAT EU R&D project was the generation of a formulated set of guidelines which could be applied when buckling or postbuckling of CFRP-stiffened panels was considered. In this paper, issues of concern for composite materials are addressed, such as prediction of damage onset and propagation of defects during buckling and postbuckling, under static and cyclic loading. The design and analysis guidelines are the result of good cooperation between partners from the industry, universities and research centers, and are based on extensive theoretical and experimental research performed within the COCOMAT project. The analysis guidelines cover FE types and meshing, damage mechanisms, degradation models, recommendations regarding interface elements in the damage zone and ply failure models. Comparison between various analysis codes is also provided. Lessons learned concerning testing of structures in the buckling regime are presented. The sensitivity of conventional design to damage of various sizes has been investigated, and conclusions are drawn regarding design criteria. Two structural applications are also presented, where some of the above-mentioned guidelines were implemented at Israel Aerospace Industries (IAI).

Earthquake causes wide and severe damage to building structures, due to not just the great ground motion but also secondary actions, such as impact, blast or fire, occurring after earthquake. The extreme combined loading scenario should be considered for safety of buildings and lives. Taking fire for example, the combined load can be considered as an event in which the structures are first partially damaged under an earthquake and then attacked by fire. In order to investigate the post-earthquake loading scenario, it is important to assess the partial damage caused by earthquake on different components of structures. The behavior of welded steel I-beam to hollow square tubular columns is investigated herein. A detailed experimental study is presented in which two groups of unstiffened welded steel connections, with the same configurations, subjected to static and cyclic loading are considered. The flexibility and strength of the connections are measured, while the damage phenomena and failure modes are explored during the tests. The connection damage is found to be a cumulative fracture developing process which leads to significant gradual degradation of the mechanical properties of the connection. The quantificational evaluations of the cyclic loading induced damage are also carried out to investigate the connection damage level according to different loading intensities. A finite element modeling numerical study is also carried out to validate the experimental results and a good agreement is achieved. The test results and FE modeling provide a benchmark data for the unstiffened welded connections and can be used for further investigations of the connections subjected to combined actions such as post-earthquake fire.

Earthquake causes severe damage to buildings and infrastructure, due to not only the ground motion but also secondary actions, such as impact, blast and fire which would occur after an earthquake. In order to investigate the post-earthquake loading scenarios, it is important to assess the partial damage of structures caused by earthquake. This paper presents the behavior of double-angle bolted steel I-beam to hollow square tubular column connections under static and cyclic loading. A detailed experimental study is presented in which two groups of bolted steel connections with different column wall thickness are considered. The flexibility and strength of the connections are measured, while the damage phenomena and failure modes are explored in the tests. The connection damage under cyclic loading is found to be an accumulative developing process of fracture which leads to significant gradual degradation of the mechanical properties of the connections. Quantitative evaluations of the cyclic loading induced damage are carried out to investigate the damage level of connections according to different loading scenarios. The test results herein provide a detailed understanding of the behavior of the double-angle bolted connections under seismic loading, which would be useful for further investigations under post-earthquake actions.

With an emphasis on the integrated deterioration of railway track geometry and components, a new hybrid numerical-analytical method is proposed for the predictive analysis of track geometrical vertical levelling loss (VLL). In contrast to previous studies showing a dependency on the number of cycles, this research unprecedentedly incorporates the influence of operational, vehicle and track conditions. The numerical models are carried out using an explicit finite element (FE) package under cyclic loadings, and then, their outcomes are iteratively regressed by an analytical logarithmic function that accumulates permanent deformations in order to quantify VLL over a long term. The results are first compared with other previous studies, indicating a very good agreement with them. Then, field measurements have been used to further verify the results. In this study, parametric simulations are performed varying three key parameters: axle load, train velocity and ballast tangent stiffness. The parametric studies exhibit that the rate of VLL raises about 50% if the axle load increases only from 30 ton to 40 ton for a freight train running at 70 km/h on a stiffer ballast track. In contrast, for a 25-tonnes-axle-load train running from 60 km/h to 100 km/h on a similar track, the vertical levelling degradation reduces by approximately 20%. The main findings suggest that higher axle loads contribute significantly to the VLL due higher contact forces and, on the other hand, a lower train speed does not necessarily imply a low rate of VLL since the influence of train velocities on track geometry (VLL) is associated with the natural frequencies (or wavelengths) of the ballasted railway track. The insight demonstrates that the load frequencies play a key role on the deterioration of VLL.

The principle of this study is experimental measurement and description of behavior of transpedicular fixation during cyclic loading due to convergence of screw insertion. Investigations were made of three configurations of assemblies of posterior stabliization with converging screws at 0°, 20° and 40°. The experiment was inspired ASTM Standard F1717 and modified to minimize the effect of other parameters. The MTS 858.2 Mini Bionix testing system was used during the experiment, in conjunction with the Interface 1010ACK load cell. Data processing and analysis were carried out by Matlab R 20102b, MathWorks. The probed assemblies were cyclically loaded until structural failure occurred, always at the screwbone (or PUR block) interface, i.e., the "windshield wiper" effect. The measurement results show that while the rigidity of the assembly increases with increased convergence of transpedicular screws, they also indicate an increased initial rate of assembly damage accumulation, together with assembly failure during a reduced number of cyclic loading cycles. The mechanical behavioral study of transpedicular fixation is limited by the conditions of simplification of interpretation of complex movements and spinal pathophysiology in the attempt to minimize the effect of other parameters and exaggerated measurements.

This paper assesses the effectiveness of vibration in accelerating bone remodeling and orthodontic tooth movement. Databases of PubMed, Web of Science, and ScienceDirect were searched from January 2017 to March 2019 for randomized or quasi-randomized controlled trials that evaluated the effectiveness of vibration in accelerating bone remodeling and orthodontic tooth movement. The inclusion criteria were as follows: (i) studies that assessed the efficacy of vibration (cyclic loading) in bone remodeling and orthodontic tooth movement and (ii) those that employed groupings (experimental vs. control/placebo groups) on the basis of the use of vibration (cyclic loading). Eight clinical trials were included in this short review. Five studies met the eligibility criteria for bone remodeling and orthodontic tooth movement. Four studies found that low-magnitude high-frequency vibration could accelerate bone remodeling. However, contradictory results were obtained with regard to the acceleration of orthodontic tooth movement by vibration in human participants. Low-magnitude high-frequency vibration can accelerate bone remodeling and orthodontic tooth movement. However, this acceleration is dependent on the magnitude and frequency. Further research is necessary to determine the most feasible protocols for investigating the effects of magnitude and frequency of vibration on the acceleration of orthodontic tooth movement in human participants.

This paper proposes a combined discrete–finite element model to investigate the dynamic behavior of ballasted railway tracks. The discrete element method (DEM) is adopted to model the discrete ballast materials. The shapes of ballast particles resembled clumps of overlapping spheres which are obtained by the growth of spheres inside convex polyhedrons. The finite element method (FEM) is used to analyze the continuous embankment and foundation. The transmission between DEM and FEM at the ballast-embankment interface is processed according to the interaction force based on the principle of virtual work. The dynamic behavior of ballasted railway track under cyclic loading is simulated with the developed DEM–FEM model. The settlement of the sleeper and the deformation of the embankment and foundation, the force chains in the ballast and stress distributions in the embankment and foundation are obtained. The developed model is helpful in better understanding the mechanical characteristics of ballasted railway tracks.

A computationally efficient and robust simulation method is presented in this work, for the cyclic modeling of reinforced concrete (RC) structures. The proposed hybrid modeling (HYMOD) approach alleviates numerical limitations regarding the excessive computational cost during the cyclic analysis and provides a tool for the detailed simulation of the 3D cyclic nonlinear behavior of full-scale RC structures. The simplified HYMOD approach is integrated in this work with a computationally efficient cyclic concrete material model so as to investigate its numerical performance under extreme cyclic loading conditions. The proposed approach adopts a hybrid modeling concept that combines hexahedral and beam-column finite elements (FEs), in which the coupling between them is achieved through the implementation of kinematic constraints. A parametric investigation is performed through the use of the Del Toro Rivera frame joint and two RC frames with a shear wall. The proposed modeling method managed to decrease the computational cost in all numerical tests performed in this work, while it induced additional numerical stability during the cyclic analysis, in which the required number of internal iterations per displacement increment was found to be always smaller compared with the unreduced (hexahedral) model. The HYMOD provides for the first time with the required 3D detailed FE solution tools in order to simulate the nonlinear cyclic response of full-scale RC structures without hindering the numerical accuracy of the derived model nor the need of developing computationally expensive models that practically cannot be solved through the use of standard computer systems.

This paper presents the results of an experimental and numerical investigation on the behaviour of U-shaped walls subjected to lateral uniaxial and biaxial cyclic loading. The test specimens have been sized and detailed according to EC8 code. The tests aimed at studying the behaviour of a full scale U-shaped wall in uniaxial and biaxial bending and shear, and compared the alternative design requirements of two versions of EC8. A 3D multilayer thin shell finite element formulation, incorporating a concrete model with a smeared cracking capability, was used to simulate the experimental behaviour of this specimen. Comparison between numerical and experimental results shows that the model is capable of predicting with acceptable accuracy the most important characteristics of the measured global and local response. Based on the findings of both experimental and numerical results, some additional recommendations for design and analysis have resulted.

In this paper, the damage prediction of shear-dominated reinforced concrete (RC) elements subjected to reversed cyclic shear is presented using an existing damage model. The damage model is primarily based on the monotonic energy dissipating capacity of structural elements before and after the application of reversed cyclic loading. Therefore, it could be universally applicable to different types of structural members, including shear-dominated RC members. The applicability of the damage model to shear-dominated RC members is assessed using the results from reversed cyclic shear load tests conducted earlier on eleven RC panels. First, the monotonic energy dissipating capacities of the panels before and after the application of reversed cyclic loading are estimated and employed in the damage model. Next, a detailed comparison between the analytically predicted damage and the observed damage from the experimental tests of the panels is performed throughout the loading history. Subsequently, the effects of two important parameters, the orientation and the percentage of reinforcement, on the damage of such shear-dominated panels are studied. The research results demonstrated that the analytically predicted damage is in reasonably good agreement with the observed damage throughout the entire loading history. Furthermore, the orientation and percentage of reinforcement is found to have considerable effect on the extent of damage.

A simple damage model for concrete is presented. The basic version of the model is a combination between the elastic-damage part of the elastoplastic-damage model of Faria & Oliver and the damage theory of Mazars. This basic version of the model takes into account most of the basic traits of concrete under monotonic static and dynamic loading, like the different response under compression and tension, the stiffness reduction with the increase of external loading and the appearance of softening behavior. This model is further enhanced with a sensitivity to the rate of loading and the ability to simulate cycling behavior, characteristics which are necessary for general cases of dynamic loading. The main advantages of this model are the employment of only one damage parameter and the investigation of the damage state in concrete under static or dynamic loading. The model is implemented into a general three-dimensional finite element program capable of treating static and general dynamic problems. The validation and performance of the proposed model is demonstrated by characteristic numerical examples.

AA7075-T6 is widely used in the aviation and aerospace industries thanks to its interesting strength-to-weight ratio. The fatigue life of the structural components of aircrafts and rockets can be altered as a consequence of the impact of alien objects. This work investigates the fatigue behavior of an AA7075-T6 hourglass specimen induced by the impact of a steel ball at its minimum cross-section. The specimen is then tested with a rotating bending moment. The stress distributions after impact and during cyclic loading are estimated using the Finite Element (FE) method. The failure mechanism was clarified by the observation of the failed specimen surface.

Monopile foundations contain welding residual stresses and are widely used in industry to support offshore wind turbines (OWTs). The monopiles are subjected to hammering loads during installation and cyclic loads during operation, therefore the influence of residual stress redistribution as a result of fatigue cycles must be evaluated in these structures. The existing empirical models to predict the residual stress redistribution in the presence of cyclic loading conditions are strongly dependent on the material, welding process and loading conditions. Hence, there is a need to predict the residual stress redistribution using finite element simulations. In this study numerical analyses have been conducted to predict the initial state of residual stress in a simplified weld geometry and examine the influence of subsequent cyclic loads on the relaxation behavior in residual stress profiles. The results have shown that fatigue cycles have a severe effect on residual stress relaxation with the greatest reduction in residual stress values observed in the first cycle. Moreover, the numerical prediction results have shown that the stress amplitude plays a key role in the extent of residual stress relaxation in welded structures.

Results from 23 cyclic tests, including 18 cantilever-typed steel bridge piers and five beam-to-column connections, are presented to investigate their ductile fracture behavior as related to the seismic design of steel bridge structures, and based on shell and fiber models, two evaluation methods of ductile crack initiation are proposed. The effect of various parameters, including plate width-thickness and column slenderness ratios, cross-section shape, loading history, repeated earthquakes and initial weld defect is investigated experimentally. Among these parameters, width-thickness ratio, loading history and initial weld defect are shown to have significant influence on ductile fracture behavior. The test data suggest that for unstiffened box specimens, current seismic design provision limits on ultimate strain may not provide sufficient ductility for seismic design. On the other hand, based on the experimental results, two damage index-based evaluation methods respectively using shell model and fiber model are successfully employed to predict ductile fracture of steel bridge structures. Comparisons between experimental and analytical results show that they can predict ductile fracture behavior with good accuracy across the specimen geometries, steel types, loading histories and initial weld defects.

The common characteristic of self-centering (SC) post-tensioned (PT) connections is the gap opening and closing at the beam-column interface, which often face the problem of deformation incompatibility with floor system and cause severe damage of floor slab. To address this drawback of PT frame expansion, an innovative friction-damped SC coupled-beams (CBs) that incorporate PT strands to provide a SC capacity along with friction devices (FDs) to dissipate energy is presented. The global mechanics of this system are briefly explained. An individual FD is tested to investigate the wear resistant properties and friction coefficients of friction materials for the energy dissipation system of SC-CBs. Detailed three-dimensional finite element models (FEMs) of a single-story single-bay frame using SC-CBs are developed with consideration of different FDs installation positions. Analytical results indicate that SC-CBs can sustain large lateral deformations without structural damage under cyclic loading. Then, simplified computational models are validated based on the solid models and applied in nonlinear dynamic analysis of a five-story MRFs using SC-CBs. The frame using SC-CBs minimizes the residual drift and displays good energy dissipation capacity under both the design basis and maximum considerable levels of seismic loading.

Ductility and energy dissipation capacity of the beam column joints are the two prominent characteristics which govern the stability of the entire structure constructed in the seismic prone areas. In this paper, the effect of potassium-activated geopolymer concrete in the exterior beam column joint application is investigated under low frequency cyclic loading. Numerical analysis has been done by using the finite element software Abaqus and compared with the experimental work. From the load deformation relationship, parametric studies are carried out in the aspects of ductility, stiffness degradation, energy dissipation capacity, drift ratio and cracking pattern. The use of potassium-activated geopolymer technology in the exterior beam column joint application resulted in the improved ductility, energy dissipation capacity with superior ultimate load carrying capacity of 1.05% over conventional cement reinforced concrete beam column joints with special confining reinforcement confirmed by IS 13920 due to the enormous polymerization activated by high molecular potassium ions. There is an improved energy dissipation capacity of 2.78% of potassium-based geopolymer specimen resulting in lesser number of non-structural cracks and 11.26% more deformation under 11.96% enlarged drift ratio than the conventional reinforced concrete specimen. From the observed results, it is clearly noted that the implementation of potassium-activated green polymer technology in the beam column joints possessed enhanced ductility characteristics to protect the structure susceptible to seismic environment and resulted in innovative, economical and sustainable mode of seismic-resistant building construction.

It is an established fact that multiscale modeling is an effective way of studying materials over a realistic length scale. In this work, we demonstrate the use of sequential and concurrent multiscale modeling to study the effect of cyclic loading on both the atomic and continuum regions, of graphene, a material which comes with its own set of unique properties. Moreover, to further strengthen this work, we have studied the temperature effects during the cyclic loading, by analyzing the effect of loading and varying temperature gradients.

Research on two-dimensional (2D) materials, such as graphene and molybdenum disulfide (MoS₂), now involves researchers worldwide, implementing cutting edge technology to study them. However, when considering using 2D materials in such promising applications, one of the major concerns is the mechanical failure, which remains heavily underexplored. In this work, we demonstrate the use of sequential and concurrent multiscale modeling to study the effect of various fracture modes on graphene and molybdenum disulfide (MoS₂). Some of the fracture modes explored are pure tensile loading, shear loading, and a combination of tensile and shear loading and cyclic loading to investigate fatigue.