Number Of Loading Cycles Research Articles

Experimental investigation of the load-carrying capacity of machine-hammered surfaces with variation of the process parameters

If the load exceeds the load carrying capacity of the tooth on the gear, tooth flanks or tooth root damage occurs. The resilience of gears can be increased with various manufacturing and finishing processes. Machine Hammer Peening (MHP) is currently being researched as an alternative to shot peening to increase the load carrying capacity of gears. Due to adjacent teeth, the machining of the tooth root and tooth flank is only possible with an impact angle β > 0 ° between the hammer head and the normal of the machining surface, depending on the gear geometry. The aim of this work is to gain knowledge about the rolling sliding resistance of hammered contact surfaces under the influence of different impact angles and machining directions. For this purpose, three impact angles and two machining directions are varied during the machining of the analog specimens by the MHP. In fatigue strength tests, the hammered test specimens are compared with a ground and a shot-peened variant. The fatigue strength tests show that the hammered test specimens with an impact angle β ≤ 30 ° achieve a higher number of load cycles on average than the ground reference variant (increase of up to 100 %). For the transfer of the results to the process for hammered tooth flanks, it can be hypothesized that for impact angles β ≤ 30 °, the machining direction locally on the flank in the opposite direction to the sliding speed leads to a higher rolling sliding strength in the range of short time fatigue strength.

Experimental Investigation on the Critical Dynamic Stress of Frozen Silty Clay Under Different Temperature and Moisture Conditions

In this paper, a comprehensive series of dynamic triaxial tests were conducted to delve into the influence of temperature and moisture content on the behavior of frozen silty clay. Upon scrutinizing the experimental outcomes under prolonged reciprocal cyclic loading, insights were gained into how varying temperatures and moisture contents impact the cumulative permanent strain (CPS) and critical dynamic stress (CDS) of frozen clay. The results show that the variation curves of CPS with the number of cyclic loadings show significant changes at different temperatures and moisture contents. Additionally, based on the assessment of vertical CPS recorded at the 100th and 1000th loading iterations, criteria for assessing the plastic stability and plastic creep threshold of frozen silty clay were devised. Consequently, an analysis was conducted to delineate the correlation between the variation in vertical cumulative strains and the dynamic stresses applied within the frozen clay, resulting in the formulation of a series of correlation curves. The relationship between the changes in CDS affected by different temperatures and water contents were analyzed. The CDS under the plastic stability and plastic creep limits showed a slowly increasing trend with decreasing temperatures and a slowly decreasing trend with increasing water contents.

Experimental and Numerical Simulation on the Generation and Development of Cumulative Plastic Strain of Cement Sheath in Salt Cavern Gas Storage Well

ABSTRACTSalt cavern gas storage is an important technical means to balance the demand for staggered energy supply. Due to the repeated injection and extraction of natural gas in gas storage facilities, sealing integrity failure in the wellbore of gas storage facilities frequently occurs. In response to this, considering the cyclic loading and unloading of the pressure load inside the casing, mechanical tests of set cement were carried out under alternating loads, quantifying the cumulative plastic strain change law of set cement and revealing the deterioration characteristics of its mechanical properties. A numerical model of cumulative plastic strain of casing cement sheath formation under alternating load was established based on the obtained experimental parameters. Comparative verification was conducted using experimental data, and the variation law of cumulative plastic strain of cement sheath was analyzed. The distribution of cumulative plastic strain on the cement sheath bonding surface of the entire wellbore was quantified. The research results indicate that the higher the internal pressure value of the casing, the earlier the plastic strain appears, and with the increase in the number of alternating loads, the cumulative plastic strain increases approximately linearly. After the internal pressure increased by 30 MPa, the cumulative plastic strain increased by a maximum of 46.75%. When the number of loading and unloading cycles under alternating loads is small, reducing the elastic modulus (6 GPa) of the cement sheath can effectively reduce its cumulative plastic strain. However, as the number of loading and unloading cycles under alternating loads exceeds a specific value, the cumulative plastic strain produced by high elastic modulus (15 GPa) cement sheaths decreases. Finally, the distribution pattern of cumulative plastic strain along the wellbore under different gas injection times and complex formation conditions was analyzed. Suggestions for establishing well barriers in salt cavern gas storage during cementing were proposed. The research results can provide theoretical and engineering references for evaluating the sealing integrity of gas storage wells.

Characterising geomaterial shakedown and related deformation accumulation from cyclic triaxial tests

Permanent deformation accumulation in soils and unbound granular materials under repetitive loading leads to surface rutting in roads and deterioration of track geometry in railways, eventually requiring costly and disruptive maintenance. Cyclic or repeated load triaxial tests are a convenient test method to characterise the permanent deformation behaviour of different soils under high numbers of load repetitions and a large volume of data is available. Existing empirical functions either take no account of shakedown, leading to an over-prediction of deformations at a high number of load cycles, or those that do take account of shakedown contain a high number of regression parameters that make them difficult to use. A simple empirical function with only one input parameter is proposed to characterise the accumulation of permanent axial strain in cyclic triaxial tests on a wide range of geomaterials undergoing shakedown deformations. It is shown to give predictions of a similar accuracy to existing functions at low numbers of load cycles and increased accuracy at high numbers of load cycles. It is shown to be applicable to soils ranging from a high plasticity clay to a rail ballast, across a range of stress states, stress history and strain levels provided that ratcheting deformations do not occur. The single input parameter has a physical meaning and is related to the permanent deformation required to reach the shakedown condition. Since the proposed function predicts the slowing rate of deformation accumulation towards shakedown not covered by the commonly-used existing functions, it could lead to more economical designs of applications involving high numbers of load repetitions. Its broad application to both coarse granular materials and fine-grained soils should streamline the design of applications involving layers of both these material types without the need to change empirical function.

Damage accumulation in carbon steel under low-cycle loading at the crack initiation stage and strain ageing temperature

Based on the strain criterion for fatigue fracture and experimental studies, the damage accumulation kinetics from recoverable plastic strain (plastic strain in the tensile half-cycle), from the unilateral accumulation and elastic strain with the increasing number of loading cycles was studied. The influence of stress ageing on the redistribution of the levels of these components of damage, depending on the number of cycles before destruction, was studied. Depending on the load level, the limit states (formation of a surface macro-crack or buckling failure of strain stability due to the formation of an internal crack in the quasi-static fracture range) were established. Strain ageing reduces the range of quasi-static fracture. It is shown that the critical values of strains and stresses correspond to the limit states. The parameters determining the capacity of structural materials under static and cyclic loading under conditions of stress ageing are confirmed.

Modifying Bitumen with Recycled PET Plastics to Enhance Its Water Resistance and Strength Characteristics.

This study investigates the modification of bituminous mixtures by varying percentages of PET particles (1%, 3%, 5%, 8%, 10%, and 12% PET). The following methods were employed to analyze the samples: the ring-and-ball softening point determination method (ASTM D36/D36M-14), the Fraass breaking point determination method (EN 12593: 2015), the elongation determination method (EN 13589: 2014), and the needle penetration depth determination method (EN 1426: 2015). Optimal bitumen/PET ratios were identified to obtain modified bituminous mixtures (MBMs) with enhanced operational characteristics (5% and 8% PET). The physical and mechanical properties of the investigated bitumen samples and PET were determined. A comparative analysis of the modified bituminous mixture samples based on their physical and mechanical properties was conducted. Microstructures of the surface of modified bituminous mixture samples with varying modifier contents were obtained. An X-ray structural analysis was performed on the samples of modified bituminous mixtures with varying PET contents. The dependencies of the moisture absorption rate on time were determined for the samples of modified bituminous mixtures with different modifier contents. The values of the stress intensity factor were determined based on the number of loading cycles in fatigue tests using three-point bending for the samples of modified bituminous mixtures with varying modifier contents.

Damage indicators evolution during cyclic loading of composite plate with hole

Novel experimental method, which provides quantitative description of damage indicators evolution caused by fatigue loading of composite specimens with stress concentrators, was developed. Damage parameters are derived as deformation response to artificial notch inserting. This notch is extended from the edge of central through hole in plane rectangular specimen under constant external load. Current values of damage indicators are obtained at different stages of fatigue loading. These data reveal dependencies of required parameters on loading cycle number. The damage accumulation function for involved cycle range is quantitatively constructed on this base. It is found that this function is related to the first stage of the process investigated. The results obtained represent the essential link in the design of further experiments.

Steel stresses and shear forces in reinforcing bars due to dowel action

AbstractReinforcing bars in structural concrete are typically designed to carry axial forces. Nevertheless, due to their bending stiffness, the bars can also carry transverse forces, that are associated with the localized bending mechanism (dowel action) resulting from the relative displacements (or slip) wherever a crack interface or a discontinuity interface (between two concrete parts cast at different times) intercept the bar. Such localized bending induces stress concentrations in both the bars and the concrete. The relative displacement can occur at interfaces either perpendicular to the bar or inclined with respect to its axis. Thanks to steel ductility, the bending stresses in the bars due to dowel action do not impair the sectional capacity at the ultimate limit state. Fatigue verifications, however, require an accurate evaluation of these stresses under imposed transverse displacements or shear forces. As well known, dowel action can be described by means of the traditional unidimensional Winkler's model (beam on an elastic foundation), where the bearing stiffness of the concrete embedment is typically introduced through a couple of parameters, namely the bar diameter and the concrete strength in compression. The actual behavior of a dowel, however, is definitely more complex and for such a reason, improvements are needed for the Winkler's model to introduce other parameters typical of actual structures. Hence, a new formulation is introduced in this study for the bearing stiffness, that is calibrated based on mechanical considerations and measurements with optical fibers. The proposed formulation also accounts for the following parameters: angle between the crack and the bar, concrete‐cover thickness, number of load cycles and the softening effect caused by the local secondary cracks radiating from bar ribs during the pull‐out process. The predictions of the model—implemented with the proposed bearing stiffness—fit fairly well the test results under both monotonic and cyclic loads, in terms of shear force–transverse displacement response and peak stress in the reinforcing bars.

Stiffness degradation and mechanical behavior of microfiber-modified high-toughness recycled aggregate concrete under constant load cycling

The mechanical properties of High-Toughness Recycled Aggregate Concrete (HTRAC) were investigated in this study as an innovative and environmentally friendly construction material, along with its potential applications in structural stability. Small-scale specimens with six levels of micro-steel fiber content were made, and a series of cyclic tests with constant loads were carried out. Using In-Situ 4D CT technology, the damage characteristics of the microstructure of HTRAC and the reinforcing effects of fibers on key mechanical parameters (peak stress, peak strain, ultimate strain, post-peak modulus, and toughness indicators) were analyzed. A comprehensive fiber reinforcing factor calculation model was proposed to assess its contribution to strength, deformability, and toughness, and the correlation between the number of cyclic loadings and stiffness degradation was also quantified. it is confirmed that HTRAC exhibits a significant advantage in toughness compared to traditional recycled aggregate concrete (RAC). The findings of this study provide crucial technical support for the further development and application of HTRAC, indicating its promising prospects in the field of sustainable construction materials.

Temporal Homogenization Modeling of Viscoelastic Asphalt Concretes and Pavement Structures under Large Numbers of Load Cycles

Temporal Homogenization Modeling of Viscoelastic Asphalt Concretes and Pavement Structures under Large Numbers of Load Cycles

Investigation of Effect of Part-Build Directions and Build Orientations on Tension-Tension Mode Fatigue Behavior of Acrylonitrile Butadiene Styrene Material Printed Using Fused Filament Fabrication Technology.

This article explores the fatigue characteristics of acrylonitrile butadiene styrene (ABS) components fabricated using fused filament fabrication (FFF) additive manufacturing technology. ABS is frequently used as a polymeric thermoplastic material in open-source FFF machines for a variety of engineering applications. However, a comprehensive understanding of the mechanical properties and execution of FFF-processed ABS components is necessary. Currently, there is limited knowledge regarding the fatigue behavior of ABS components manufactured using FFF AM technology. The primary target of this study is to evaluate the results of part-build directions and build orientation angles on the tensile fatigue behavior exhibited by ABS material. To obtain this target, an empirical investigation was carried out to assess the influence of building angles and orientation on the fatigue characteristics of ABS components produced using FFF. The test samples were printed in three distinct directions, including Upright, On Edge, and Flat, and with varying orientation angles ([0°, 90°], [15°, 75°], [30°, 60°], [45°]), using a 50% filling density. The empirical data suggest that, at each printing angle, the On-Edge building orientation sample exhibited the most prolonged vibrational duration before fracturing. In this investigation, we found that the On-Edge printing direction significantly outperformed the other orientations in fatigue life under cyclic loading with 1592 loading cycles when printed with an orientation angle of 15°-75°. The number of loading cycles was 290 and 39 when printed with the same orientation angle for the Flat and Upright printing directions, respectively. This result underscores the importance of orientation in the mechanical performance of FFF-manufactured ABS materials. These findings enhance our comprehension of the influence exerted by building orientation and building angles on the fatigue properties of FFF-produced test samples. Moreover, the research outcomes supply informative perspectives on the selection of building direction and building orientation angles for the design of 3D-printed thermoplastic components intended for fatigue cyclic-loading applications.

Factors influencing static and cyclic behaviour of polymer-grouted sands

ABSTRACT In this study, thorough laboratory research was conducted in order to increase knowledge of the beneficial effect of water-soluble epoxy resin on the monotonic or cyclic triaxial strength of different sands. Seven sand fractions (with different particle sizes, uniformity coefficients and mineralogical compositions) were injected with solutions of epoxy resin and water, at ratios of 3.0, 2.0 and 1.5. To evaluate the influence of chemical treatment on the static strength behaviour of grouted sands, a series of monotonic triaxial tests on dry specimens and isotropically consolidated undrained (CIU) triaxial tests, were conducted on samples cured for 90 days. CIU cyclic triaxial compression tests were also performed, to assess improvements in the cyclic resistance of treated sands. The experiments showed that this resinous grouting resulted in an appreciable increase in cohesion values, particularly for fine sands and well-graded sands, ranging from 0.15 to 0.66 MPa. Despite the fact that friction angle values were reduced, the overall shear strength increased over a wide range of stresses. Moreover, all grouted sands exhibited a substantial improvement in cyclic properties, with failure occurring at significantly elevated stress levels and after enduring a larger number of loading cycles, compared to their untreated counterparts.

Exploring Fatigue Crack Initiation in Helical Gears: Impact of Carburization and Frictional Influences

A computational model is presented to analyze the fatigue crack initiation period in helical gears, incorporating considerations for heat treatment via carburization and friction effects. The aim is to determine the number of stress cycles necessary for the initiation and growth of initial cracks, while investigating the impact of dynamic behavior. This study employs a dynamic gear model with two degrees of freedom in torsion, developed using MATLAB, and incorporates the Papadopoulos criterion for fatigue analysis. The computational results are benchmarked against the strain-life method. The findings underscore the significant influence of the friction coefficient between surfaces, heat treatment, and dynamic loads on the growth of initial fatigue cracks. For instance, with a friction coefficient (μ) of 0.5, the initial crack forms on the tooth surface (y=0) in both methods, with the (ε-N) method suggesting 28-65 cycles and the Papadopoulos criterion indicating 101-156 cycles. The latter accounts for the greater influence of residual stresses. Additionally, untreated gears exhibit a minimum number of loading cycles for initial crack appearance at 1.07 × 104 cycles. In contrast, carburized gears demonstrate an extended lifespan, requiring 1.45 × 105 loading cycles for crack initiation in our example. This emphasizes the efficacy of carburization in enhancing gear durability.

A Contact Mechanics Model for Surface Wear Prediction of Parallel-Axis Polymer Gears.

As surface wear is one of the major failure mechanisms in many applications that include polymer gears, lifetime prediction of polymer gears often requires time-consuming and expensive experimental testing. This study introduces a contact mechanics model for the surface wear prediction of polymer gears. The developed model, which is based on an iterative numerical procedure, employs a boundary element method (BEM) in conjunction with Archard's wear equation to predict wear depth on contacting tooth surfaces. The wear coefficients, necessary for the model development, have been determined experimentally for Polyoxymethylene (POM) and Polyvinylidene fluoride (PVDF) polymer gear samples by employing an abrasive wear model by the VDI 2736 guidelines for polymer gear design. To fully describe the complex changes in contact topography as the gears wear, the prediction model employs Winkler's surface formulation used for the computation of the contact pressure distribution and Weber's model for the computation of wear-induced changes in stiffness components as well as the alterations in the load-sharing factors with corresponding effects on the normal load distribution. The developed contact mechanics model has been validated through experimental testing of steel/polymer engagements after an arbitrary number of load cycles. Based on the comparison of the simulated and experimental results, it can be concluded that the developed model can be used to predict the surface wear of polymer gears, therefore reducing the need to perform experimental testing. One of the major benefits of the developed model is the possibility of assessing and visualizing the numerous contact parameters that simultaneously affect the wear behavior, which can be used to determine the wear patterns of contacting tooth surfaces after a certain number of load cycles, i.e., different lifetime stages of polymer gears.

Analysis of short-term self-healing performance of asphalt mixture under a three-point bending fatigue test

To evaluate the self-healing performance of asphalt mixture under repeated loads, indoor three-point bending fatigue tests and self-healing tests were conducted on asphalt mixture AC-13. The stress control mode was selected to test the fatigue resistance of the mixture, and the dissipated energy recovery value and visco-elastic ratio were proposed to evaluate the self-healing performance of the mixture. The experimental results show that there is a good linear relationship between fatigue life and fatigue stress, and the damage degree to the mixture is exponentially related to the load times. The numerical dispersion of flexural tensile modulus is large, which is not suitable for evaluating the self-healing performance of the mixture. As the number of loading cycles increases, the hysteresis dissipated energy of the mixture gradually decreases and tends to stabilize. After the loading interval, due to the self-healing effect, the visco-elastic performance of the asphalt material is partially restored. The initial dissipated energy before and after interval can be used as an evaluation index for self-healing behavior, and there is a good linear relationship between the initial dissipated energy and the damage degree. Due to the deformation migration of the mixture during the initial loading stage before and after interval, the visco-elastic ratio has changed, and this change becomes more pronounced with the damage degree of the mixture, indicating that the self-healing ability of the mixture has also been improved.

Development of improved finite element formulations for pile group behavior analysis under cyclic loading

AbstractThe effect of cyclic loading is an essential factor leading to progressive soil strength degradation. Therefore, a comprehensive analysis of the pile‐soil system behavior under cyclic loading is required to ensure the stability of pile group. There is room for improvement in the inherent constraint of the conventional numerical model in terms of approximating the soil resistance distribution along the pile by point loads at element nodes, necessitating a specific element that integrates considerations of pile group effect and cyclic loading within a unified framework. This study aims to develop a newly specific type of element for efficiently predicting nonlinear behavior within the pile‐soil system, addressing simulations involving nonlinear pile‐soil interaction, pile group effect, and cyclic loading. Modified element formulations based on soil stiffness matrices and soil resistance vectors specifically address pile group effect and consider parameters that influence pile behavior under cyclic lateral loading. The numerical solution procedure with Newton‐Raphson iteration allows the calculation of pile responses in geometric and material nonlinear analyses. The validation of the proposed method includes several examples, comparing it with existing numerical solutions and experimental tests of single piles and pile groups under cyclic loading. These comparisons further support the consistency of the proposed method with measured data and validate its accuracy in considering group effect and cyclic loading. The parametric study illustrates the ability of the proposed method to capture cyclic loading parameters while considering the influence of the number and magnitude of load cycles, the cyclic load direction, and the installation methods.

Fatigue-induced fracture assessment for FRP-steel bonding joints after seawater exposure

This paper investigates fatigue durability of FRP-steel double-lap shear bonded structure in seawater environment. Three simulated environments (original-salinity seawater dry-wet cycle, triple-salinity seawater dry-wet cycle, and triple-salinity seawater immersion) with 30-, 60-, and 90-day maintenance duration are tested. Experimental results indicate that both the quasit-static and fatigue strength are degraded by the seawater maintenance, and longer maintenance will lead to continued deterioration of the bonded joints. The failure/damage of bonded joints primarily originates from the bonding failure in the docking zone and extend towards the two ends. For a specific bonded joint, the load amplitude (∆F) and cycle number (N) can be approximated fitted by a linear logarithmic curve. Based on this observation, a prediction model of the cycle number (N) for the bonded joints is proposed, and comparison demonstrated that the prediction model can effectively assess the fatigue strength of GFRP-steel double-lap shear bonded joints after seawater maintenance. The findings of this study can offer both theoretical and experimental foundations for assessing the durability of FRP-steel bonded structures in seawater corrosion environments.

Model test on dynamic response of coral sand in island dredging airport under wave, tidal and aircraft landing load

The construction of an island airport on a coral reef will inevitably encounter wave, tidal and aircraft takeoff and landing loads. To study the dynamic response of the coral sand island airport under these loads, a 1:50 laboratory model test was conducted with Meiji Reef Airport as the engineering background. The model test studied the pore water pressure characteristics of the coral sand strata beneath the airport runway under wave loads with different wave heights, tidal loads with different tidal heights, and aircraft impact loads with different cyclic loading times. Additionally, the displacement characteristics of the coral sand strata beneath the airport runway under the aircraft impact loads with different cyclic loading times were also studied. The results show that the pore water pressure of coral sand strata under the airport runway decreases with the increasing offshore distance under the wave and tidal loads. The change of wave height has little influence on the pore water pressure of coral sand strata, while the change of tidal water level has a significant effect on it. As the number of aircraft impact load cycles increases, the cumulative excess pore pressure in the coral sand strata under the airport runway no longer increases and plastic deformation occurs. The research can provide a reference for the runway foundation treatment of the island airport under construction and planning.

Experimental Research on Breakage Characteristics of Feed Pellets under Different Loading Methods

Particle breakage is a common phenomenon during the processes of production, storage, and transportation. Because of the requirements for pellet integrity in poultry farming, research on the breakage characteristics of feed pellets is necessary. In this paper, repeated compression tests under different loading forces and repeated impact tests under different air pressures were carried out with feed pellets as the research object. The breakage behaviors were described, and the particle size distribution of feed pellets was analyzed quantitatively. The results revealed a positive correlation between crack density in feed particle beds and loading force. The compression process was divided into three stages based on force–displacement curves. The size of the feed pellets during repeated impacts decreased continuously and was negatively correlated with air pressure. The Weibull function accurately described the particle size distribution, with R2 values exceeding 0.97 and 0.96. The Weibull parameters showed a steady breakage degree in compression tests and a growing breakage degree in impact tests. The variation in energy and pulverization rate under different loading conditions was examined as the number of loading cycles increased. The relationship between energy and pulverization rates was fitted, showing that both parameters increased with loading cycles in different loading methods. The model of Vogel and Peukert could describe the relationship between energy and pulverization rate well, with R2 values exceeding 0.94. The minimum energy required for pellet breakage was higher in compression than in impact due to the compaction of the feed particle bed during repeated compression. The results can provide basic theory and data support for breakage characteristics and quality evaluation of feed pellets.

Dynamic characteristics of compacted landfill waste material from cyclic triaxial tests

This study conducted a series of cyclic and monotonic triaxial tests on reconstituted landfill waste material from a closed landfill site in Sydney, Australia, to assess its dynamic behaviour under various testing conditions. Specifically, the effects of cyclic deviatoric stress, loading frequency, and effective confining stress on the cumulative plastic axial strain, resilient modulus, and damping ratio under undrained cyclic loading conditions were investigated. Results indicated that the plastic deformation, resilient modulus, and material damping are significantly influenced by dynamic stress and confining stress, with a lesser impact from loading frequency. Notably, as the number of loading cycles increased, the cumulative plastic axial strain and resilient modulus exhibited an increase, whereas the damping ratio decreased. Furthermore, increasing cyclic deviatoric stress led to an increase in both cumulative plastic axial strain and damping ratio, while an increase in confining stress resulted in a decrease in these parameters. Conversely, the resilient modulus showed an increase with rising cyclic deviatoric stress and confining stress. The influence of loading frequency on cumulative plastic axial strain and resilient modulus was minor, and its effect on the damping ratio was rather negligible. The study observed that initial loading cycles caused rearrangement and reorientation of waste components and the mobilisation of fibres with tensile forces as loading progressed, suggesting that these landfill waste samples behaved comparably to fibrous soil with randomly distributed fibres. Through nonlinear regression analysis, an empirical relationship for cumulative plastic axial strain incorporating cyclic deviatoric stress, confining stress, number of cycles, and frequency was derived. This research contributes valuable insights into the behaviour of compacted landfills as railway subgrades, providing a foundation for informed decision-making in the design of transport infrastructure over closed landfill sites.