Application Of Cyclic Loading Research Articles

Axial Monotonic and Cyclic Testing of Micropiles in Loose Sand

Abstract Micropiles, which are small-diameter deep foundation solutions with diameters that can measure up to 300 mm, are often used to reinforce new and existing foundations. Their use in the foundations of structures with high eccentricity, such as wind towers when subjected to wind loads, may lead to more efficient and economical solutions. As the new generation of wind towers will reach more than 150 m tall, very large and uneconomical gravity foundations are required. In regions of high seismicity this problem is aggravated. To evaluate the behavior of micropiles under variable loading and predict the improvement of the reinforced solution, load tests were performed on steel micropiles under controlled laboratory conditions. A total of 36 tests were conducted on 3-m-long pipe micropiles, both while isolated and in 2 by 2 groups, with three different spacings. The micropiles were installed in a cylindrical container filled with calibrated sand and tested under monotonic and cyclic loading, first without grout, then when low-pressure grouted and retested, with the aim to evaluate the improvement caused by the grout injection, the micropile spacing, and application of cyclic loading both in terms of resistance and stiffness. An improvement both in stiffness and resistance due to the grouting was obtained and, for the applied cyclic loading, there was no clear reduction in micropile cyclic stiffness. The presented results provide a tool for the calibration of numerical models to estimate the behavior of real-scale micropiles installed in higher density sand.

Effect of Mechanical Loading on Osteogenesis of Human Embryonic Stem Cell-Derived Mesenchymal Progenitors within Collagen Microspheres

Mechanical forces and 3D topological environment can be used to control differentiation of mesenchymal stem cells. However, mesenchymal stem cell fate determined by the effect of physical and mechanical cues is not yet fully understood. Understanding how mechanical cues in the microenvironment orchestrate stem cell differentiation provides valuable insight that can be used to improve current techniques in cell therapy. This study investigates the osteogenic effect of mechanical stimulations on soft cellular microspheres loaded with human embryonic stem cell-derived mesenchymal progenitors (hES-MPs) when subjected to dynamic loading and in the absence of chemical stimulation. Microspheres were produced by gelation of bovine collagen type I with 1000 to 2000 hES-MP cells seeded per droplet. Four loading conditions were studied: (1) 10% constant strain was applied by a Bose biodynamic bioreactor for 15 min/day or 40 min/day for 5 or 10 days respectively; (2) 10% adjusted strain was applied (subtraction of polydimethylsiloxane (PDMS) plastic elongation from global strain) using Bose biodynamic bioreactor for the same 4 duration/conditions as in the constant strain protocol. The results indicate that applying mechanical stimulation to hES-MPs/collagen microspheres induced osteogenic differentiation of cells when the loading protocol was adjusted. Alkaline phosphatase activity of samples in the adjusted loading protocol increased significantly on day 14 whilst, the deposited minerals, matrix reorganisation and alignment of collagen fibres enhanced from day 21 post encapsulation onward. Application of cyclic loading to 3D culture of hES-MP cells can be used as a model to regulate mechano-stimulation and linage differentiation in vitro.

Effect of Mechanical Loading on Osteogenesis of Human Embryonic Stem Cell-Derived Mesenchymal Progenitors within Collagen Microspheres

Mechanical forces and 3D topological environment can be used to control differentiation of mesenchymal stem cells. However, mesenchymal stem cell fate determined by the effect of physical and mechanical cues is not yet fully understood. Understanding how mechanical cues in the microenvironment orchestrate stem cell differentiation provides valuable insight that can be used to improve current techniques in cell therapy. This study investigates the osteogenic effect of mechanical stimulations on soft cellular microspheres loaded with human embryonic stem cell-derived mesenchymal progenitors (hES-MPs) when subjected to dynamic loading and in the absence of chemical stimulation. Microspheres were produced by gelation of bovine collagen type I with 1000 to 2000 hES-MP cells seeded per droplet. Four loading conditions were studied: (1) 10% constant strain was applied by a Bose biodynamic bioreactor for 15 min/day or 40 min/day for 5 or 10 days respectively; (2) 10% adjusted strain was applied (subtraction of polydimethylsiloxane (PDMS) plastic elongation from global strain) using Bose biodynamic bioreactor for the same 4 duration/conditions as in the constant strain protocol. The results indicate that applying mechanical stimulation to hES-MPs/collagen microspheres induced osteogenic differentiation of cells when the loading protocol was adjusted. Alkaline phosphatase activity of samples in the adjusted loading protocol increased significantly on day 14 whilst, the deposited minerals, matrix reorganisation and alignment of collagen fibres enhanced from day 21 post encapsulation onward. Application of cyclic loading to 3D culture of hES-MP cells can be used as a model to regulate mechano-stimulation and linage differentiation in vitro.

Effects of Cyclic Thermal Loads on Bone-Implant Interface in Dental Prostheses

Background: Consumption of hot substances may harm the surrounding bone around a dental implant. High temperatures at the bone-implant interface (BII) interferes with local cellular activities involved in the osteointegration. Objectives: The present study was aimed at calculating the temperature distribution through the BII and the jaw bone under application of a transient cyclic thermal load. Methods: In this numerical simulation, finite element method was employed in a commercialized dental implant model drawn by computer-aided design tools based on CT data to find the temperature in superficial and deep bone regions near the BII. The heat load was applied cyclically during the intake time. Results: Results showed that the highest temperature was occurred at the top regions of the interface by magnitude of 48 C. Removal of the thermal loads also was followed by rapid decrease in the bone temperature. Conclusions: Routine beverages of a hot liquid can increase the temperature of the bone beyond the biological thresholds of the bone cells vitality or remodeling functionality, specifically in the delayed loading types of implantation.

Effect of drained static shear on cyclic deformation behavior of K0-consolidated sand

The K0-consolidated soil elements beneath structures like quay walls, bridge abutments, cut and cover tunnels and similar structures may be subjected to static shear involving the rotation of the principal stress prior to cyclic loading, which results in the presence of initial shear stress. A hollow cylindrical apparatus was employed and a series of drained cyclic tests was conducted to examine the effects of the initial shear stress induced by drained static shear prior to the application of cyclic loading. Both the initial vertical stress and the initial torsional shear stress were considered in the drained cyclic deformation behavior. The experimental observations indicate that the initial vertical stress accelerates the accumulation of vertical permanent strain during the first 100 cycles only when the initial vertical stress exceeds the value corresponding to the K0-consolidated stress state. The average growth rate of strain increases due to the presence of the initial torsional shear stress. A Critical Static Shear Line (CSSL), initiating from the K0-consolidated stress state in the (σz−σθ)−2σzθ plane, divides the volume change responses into contraction and dilation in subsequent cyclic loading. To account for the combined effects of the initial vertical stress and the initial torsional shear stress on the deformation behavior, a modified model is proposed to predict the vertical permanent strain.

The Effect of Antibody Size and Mechanical Loading on Solute Diffusion Through the Articular Surface of Cartilage.

Because of the heterogeneous nature of articular cartilage tissue, penetration of potential therapeutic molecules for osteoarthritis (OA) through the articular surface (AS) is complex, with many factors that affect transport of these solutes within the tissue. Therefore, the goal of this study is to investigate how the size of antibody (Ab) variants, as well as application of cyclic mechanical loading, affects solute transport within healthy cartilage tissue. Penetration of fluorescently tagged solutes was quantified using confocal microscopy. For all the solutes tested, fluorescence curves were obtained through the articular surface. On average, diffusivities for the solutes of sizes 200 kDa, 150 kDa, 50 kDa, and 25 kDa were 3.3, 3.4, 5.1, and 6.0 μm2/s from 0 to 100 μm from the articular surface. Diffusivities went up to a maximum of 16.5, 18.5, 20.5, and 23.4 μm2/s for the 200 kDa, 150 kDa, 50 kDa, and 25 kDa molecules, respectively, from 225 to 325 μm from the surface. Overall, the effect of loading was very significant, with maximal transport enhancement for each solute ranging from 2.2 to 3.4-fold near 275 μm. Ultimately, solutes of this size do not diffuse uniformly nor are convected uniformly, through the depth of the cartilage tissue. This research potentially holds great clinical significance to discover ways of further optimizing transport into cartilage and leads to effective antibody-based treatments for OA.

Misfit of Three Different Implant-Abutment Connections Before and After Cyclic Load Application: An In Vitro Study.

This study aimed to evaluate the misfit of three different implant-abutment connections before and after cycling load. One hundred twenty dental implants and correspondent prefabricated titanium abutments were used. Three different implant-abutment connections were evaluated: Morse taper (MT group), external hexagon (EH group), and internal hexagon (IH group). Forty implants and 40 abutments were used per group. The parameters for the mechanical evaluation were set as: 360,000 cycles, load of 150 N, and frequency of 4 Hz. Samples were sectioned in their longitudinal and transversal axes, and the misfit of the implant-abutment connection was evaluated by scanning electron microscopy analysis. One-way analyses of variance, Tukey post hoc analyses (α = .05), and t test (P < .05) were used to determine differences between groups. At the longitudinal direction, all the groups showed the presence of microgaps before cycling load; after cycling load, microgaps were reduced in all groups (P > .05). Transversally, only the MT group showed full fitting after cycling load compared with the other groups (EH and IH) (P < .0001). The application of cycling load produces an accommodation of the implant-abutment connection in internal, external, and Morse taper connections. In the longitudinal direction, the accommodation decreases and/or eliminates the gap observed initially (before load). In the horizontal direction, Morse cone implant-abutment connections experience a complete accommodation with the elimination of the gap.

Numerical investigation of contact stresses for fretting fatigue damage initiation

Fretting fatigue phenomena occurs due to interaction between contacting bodies under application of cyclic and normal loads. In addition to environmental conditions and material properties, the response at the contact interface highly depends on the combination of applied loads. High stress concentration is present at the contact interface, which can start the damage nucleation process. At the culmination of nucleation process several micro cracks are initiated, ultimately leading to the structural failure. In this study, effect of ratio of tangential to normal load on contact stresses, slip amplitude and damage initiation is studied using finite element analysis. The results are evaluated for Ruiz parameter as it involves the slip amplitude which in an important factor in fretting fatigue conditions. It is observed that tangential to normal load ratio influences the stick zone size and damage initiation life. Furthermore, it is observed that tensile stress is the most important factor that drives the damage initiation to failure for the cases where failure occurs predominantly in mode I manner.

Causes of Fracture of Elevating-And-Traversing Stand of Continuous-Casting Machine

The paper shows that the main factors that lead to the fracture of the holder of elevating-and-traversing stand body is the load with alternating signs, the magnitude of which increases by 23% after stand modernization; as well as long service life of elevating-and-traversing stand structure. Alternating load increment apparently provokes the nucleation and growth of fatigue cracks, specific surfaces of which are found in the holder fracture.Among the adverse factors the changes of initial microstructure in the form of rough Widmanstätten structure should be mentioned, which usually has poor crack resistance under the application of cyclic load. The presence of these changes is likely due to the metal recrystallization process in the heat-affected zone of the welded joint of eyelet with the holder ring.In order to prevent similar incidents in future it is necessary to perform check calculation to verify the low-cycle fatigue of its components, taking into account the structural, technological and operational factors.

Dynamic properties of high-density low-alloy PM steels

ABSTRACTPM steels with density higher than 7.5 g cm−3 were prepared from binder-treated powder, and the effects of alloying elements and microstructure on dynamic performance were studied. Although the addition of 1.5 wt-% Cu improves the tensile strength, it impairs toughness of the alloy by the formation of low-toughness pearlite. The 0.5 wt-% Cr addition promotes the formation of Ni-rich martensite and bainite at the expense of pearlite, which is beneficial to strengthen and homogenise the microstructure. The 0.5Cr alloy exhibits improved tensile strength and fatigue strength without sacrificing impact toughness, which exhibits high potential to be used for dynamic and cyclic loading applications. The tensile strength and fatigue endurance strength under 107 cycles of 0.5Cr alloy achieve 1350 and 562 MPa respectively, and impact energy is as high as 21.1 J. The enhanced fatigue life is attributed to the high density, smaller pore size, rounder pores and composite-like microstructure.

Energy-based evaluation of liquefaction potential under non-uniform cyclic loading

Uniform cyclic loading is commonly used in laboratory tests to evaluate soil resistance to earthquake-induced liquefaction, even if the cyclic stresses induced by earthquakes in the field are highly irregular. This paper discusses the use of stress and energy-based approaches to evaluate the liquefaction resistance of sand under irregular loading. Results of undrained cyclic triaxial tests including a large-amplitude singular peak loading cycle are presented and compared to those obtained using uniform loading. Although samples are subjected to loading patterns which would have been deemed equivalent by conventional stress-based methods, the number of cycles required to trigger liquefaction strongly depends on the amplitude and location of the peak within the loading history. Conversely, a unique relationship exists between the accumulation of dissipated energy per unit volume, computed using stress and strain measurements, and the observed residual pore water pressure build-up for all tests, throughout the entire cyclic loading application. This demonstrates that conventional laboratory tests using uniform loading conditions can be employed to determine liquefaction resistance if their interpretation is carried out based on energy principles.

Cyclic magnetic actuation technique for thin film interfacial fatigue crack propagation

Interfacial fatigue crack characterization is challenging for as-deposited thin films due to issues with fixturing and precision application of cyclic loading. A fixtureless and microfabricated test technique is presented to characterize delamination propagation rate as a function of constant amplitude fatigue loading using a novel peel test configuration and magnetic actuation. The test was demonstrated for 1.6μm thick Cu films on a Si substrate, where interfacial fatigue crack propagation was observed to follow Paris Law behavior.

Performance of Reactive Powder Concrete Partial Prestressed Beam-Column Sub-Assemblage Structure System with Partial Prestressed Ratio Exceeds 30%

Research on concrete material in many countries resulted a concrete type of Ultra High Performance Concrete (UHPC) which has a high performance in terms of compressive strength, ductility, durability, and modulus of elasticity using Reactive Powder Concrete (RPC). Research on structural engineering using RPC material shows better performance than normal concrete (NC) to resist gravity and cyclic loads. In this study, the experiments were conducted under the combination of constant axial and cyclic loads on the structure of the partial prestressed interior and exterior beam-column subassemblages with partial prestressed ratio value of 31.72% on the beam. The application of cyclic loading was conducted by displacement control based on the ACI 374.1-05. The purpose of this study was to determine the performance of structures based on three moment frame acceptance criteria presented in the ACI 374.1-05. From the test results, the interior and exterior beam-column subassemblage structure systems showed performance that adequated all of these criteria at the drift ratio of 3.50% and 2.20%, respectively.

Mechanical behavior of zirconia and titanium abutments before and after cyclic load application

Statement of problemEsthetic factors influence the decision to use titanium or zirconia abutments in anterior regions. Clinicians may have concerns about the durability and behavior of these zirconia abutments. PurposeThe purpose of this in vitro study was to evaluate the longitudinal and transverse long axes of the implant-abutment interface before and after the cyclic loading of titanium and zirconia abutments with an external hexagon. Material and methodsForty dental implants with an external hexagon and 40 corresponding abutments made of titanium (Ti) and zirconia (Zr) were subjected to cyclic load (c1) versus no load (c2). The longitudinal and transverse axes of 4 experimental groups (Tic1, Tic2, Zrc1, and Zrc2) were analyzed (vertical/horizontal adjustment) using a scanning electron microscope at ×1000 magnification. The differences among the groups were determined by 1-way analysis of variance (ANOVA) and post hoc Tukey tests (α=.05). T tests were used to identify the statistically significant differences between each group and each condition (α=.05). ResultsSignificant differences were found among the groups with respect to the misfits analyzed in the 2 sections (longitudinal and transverse) before and after load application (P<.05). The behaviors of the groups differed particularly with regard to the accommodation of sets (abutment/implant) after the application of cyclic loads (P<.05). ConclusionThe use of zirconia abutments in titanium implants can cause changes to and/or permanent deformation of the implant hexagon.

The Dynamic Response of Unsaturated Clean Sand at a Very Low Frequency

A series of cyclic triaxial tests at very low frequency was carried out on unsaturated clean sand in order to quantitatively investigate the influence of the degree of saturation on dynamic response. The conventional triaxial testing apparatus, which is usually used on saturated soil, was employed to test the unsaturated soil with the additional pore air pressure controller. During the series of tests, four different degrees of saturation level (Sr = 55%, 70%, 85%, 98%) were applied to the soil specimen based on a single value of effective confining pressure (σ’3). The results revealed that the application of cyclic loading at a very low frequency occurring continuously triggered the decrease of soil resistance. For degree saturation, Sr = 55% revealed that the resistance of soil was stronger in comparison to another level. Furthermore, the experimental results confirmed that applied cyclic loading induced a change in saturation level before and after testing. In addition, at a certain level of saturation, a phenomenon of settlements was likely to occur and the soil specimen then underwent liquefaction.

Performance of a Diaphragm-Type Boundary Pressure Transducer Under Cyclic Loading

Abstract Investigation of soil–structure interaction involves understanding of the complicated rheology and functioning of the pressure sensing devices. As earth pressure cell (EPC) output is used to interpret the stress in the soil at the point of installation of cell, experiments involving careful calibration of the pressure cells are very crucial. In view of the importance of in-soil calibration, the present study is focused on developing a relationship between the applied stress and the measured output, when the soil is subjected to cyclic loading. The effect of soil type and its thickness on pressure cell output under cyclic loading is studied using two types of soils: sand and kaolin, and soil layer thickness in the range of 0.25 DEPC to 2.5 DEPC, where DEPC is the diameter of the EPC. In-soil calibration under the application of cyclic loading demonstrated substantial hysteresis and reduction in the EPC sensitivity. Increase in residual stresses is observed to increase with soil thickness after the first cycle. It is also observed that the critical thickness for a given EPC is not a unique value, but varies with soil type.

Mechanical Behavior Investigation of UHMWPE Composites for Pile Cushion Applications

Presently, there is no specific guiding principle on the utilization of polymeric composites as a pile cushion pad. There is a prerequisite consequently to understand their mechanical behavior during a pile driving process. Hence, this article is concerned with the observations of mechanical behavior under cyclic loading application, compressive strength, coefficient of restitution, and permanent deformation of ultrahigh molecular weight polyethylene (UHMWPE) and activated carbon (AC) composites. UHMWPE/AC composites containing 1, 3, 5, 10 and 20 wt% of AC concentrations were prepared for accomplishing the aim. Prior to those characterizations, the composites were compression molded under 160oC for various operating times (5 to 40 minutes). As a comparison to UHMWPE, applying AC into UHMWPE matrix significantly reduced the cycle time of the compression molding. Compressive strength at 80% deformation, tangent modulus and coefficient of restitution of UHMWPE were inferior to those properties of UHMWPE/AC composites. In terms of the coefficient of restitution, the coefficient values of composites were obviously higher than UHMWPE’s values. Moreover, the coefficient of restitution at 8,000 N/min of all samples was greater than a represented coefficient from the test at 2,000 N/min. UHMWPE/AC composites with 10 wt% of AC loading provided the best mechanically reversible performance. Experimental results also indicated that the mechanical property enhancement associated with an increase in UHMWPE’s molecular weight, while the creep strain of UHMWPE was affected by the applied amount of AC. Permanent deformation data illustrated that the increasing cycles of compression urged the deterioration as same as the results measured via cyclic compression testing. In overview, UHMWPE/AC composites showed a promise for future opportunities as a viable option in replacing a traditional material for manufacturing a pile cushion pad.

Relationship Between Hysteresis Dissipated Energy and Temperature Rising in Fiber-Reinforced Ceramic-Matrix Composites Under Cyclic Loading

In this paper, the relationship between hysteresis dissipated energy and temperature rising of the external surface in fiber-reinforced ceramic-matrix composites (CMCs) during the application of cyclic loading has been analyzed. The temperature rise, which is caused by frictional slip of fibers within the composite, is related to the hysteresis dissipated energy. Based on the fatigue hysteresis theories considering fibers failure, the hysteresis dissipated energy and a hysteresis dissipated energy-based damage parameter changing with the increase of cycle number have been investigated. The relationship between the hysteresis dissipated energy, a hysteresis dissipated energy-based damage parameter and a temperature rise-based damage parameter have been established. The experimental temperature rise-based damage parameter of unidirectional, cross-ply and 2D woven CMCs corresponding to different fatigue peak stresses and cycle numbers have been predicted. It was found that the temperature rise-based parameter can be used to monitor the fatigue damage evolution and predict the fatigue life of fiber-reinforced CMCs.

Constitutive modeling of a paper fiber in cyclic loading applications

The tensile response of dense fiber-based materials like paper or paperboard is mainly dependent of the properties of the fibers, which store most of the elastic energy. In this paper, we investigate the influence of geometrical and material parameters on the mechanical response of the pulp fibers used in paper manufacturing. We developed a three-dimensional finite element model of the fiber, which accounts for microfibril orientation of cellulose fibril, and the presence of lignin in the secondary cell wall. The results showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect, and is independent of the material properties of the fiber, as long as the deformations are elastic. Plastic strain accelerates the change in microfibril orientation and thus makes it material-dependent. The results also showed that the elastic modulus of the fiber has a non-linear dependency on microfibril angle, with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. Based on numerical results acquired from a 3D fiber model supported by available experimental evidence, we propose an anisotropic-kinematic hardening plasticity model for a fiber within a beam framework. The proposed fiber model is capable of reproducing the main features of the cyclic tensile response of a pulp fiber, such as stiffening due to changing microfibril angle. The constitutive model of the fiber was implemented in a finite-element model of the fiber network. By using the fiber network model, we estimated the level of strain that fiber segments accumulate before the typical failure strain of the entire network is reached.

Quantification of biasing effects during fatigue tests on asphalt mixes: non-linearity, self-heating and thixotropy

Various phenomena other than fatigue (so-called “biasing effects”) occur during laboratory fatigue tests on asphalt mixes because of cyclic loading applications, thus altering experimental results and leading to misleading conclusions. The purpose of the study is to isolate and quantify biasing effects, therefore isolating real fatigue damage. In particular, non-linearity, self-heating and thixotropy (defined as a recoverable viscosity reduction after shear application) were evaluated. Six different mixes were produced using three distinct asphalt binders. Tests were performed in tension/compression mode on cylindrical samples. A particular test procedure was followed, consisting of two parts. In the first part, complex modulus measurements were performed at temperatures from 8°C to 14°C and strain amplitudes from 50 to 110 µm/m, at 10 Hz. Regression equations were fitted in order to evaluate variations of norm of complex modulus and phase angle caused by temperature and strain-level changes around common fatigue test conditions (10°C, 100 µm/m). In the second part of the test, five partial fatigue tests (each one consisting of 100,000 cycles at a 100 µm/m strain amplitude) were performed at 10°C, 10 Hz. After each fatigue lag, a 24 hour rest period was imposed. During rest periods, short complex modulus measurements were performed (10°C, 10 Hz) in order to monitor the recovery of mechanical properties. Surface and internal temperature of samples were constantly measured throughout the entire test, in order to monitor self-heating due to repeated loading. A significant temperature increase was observed during each fatigue lag, while, during rest periods, temperature rapidly decreased to the initial value. Self-heating was observed to be correlated to viscoelastic energy dissipation. The procedure used in the study allowed quantitatively estimating biasing effects. Therefore, unrecovered mechanical properties, due to damage accumulation, were obtained. Ninety per cent of total complex modulus and phase angle variations observed during each fatigue lag were found to be completely reversible. Non-linearity and thixotropy appear to influence mechanical properties variations more importantly than self-heating.