Subsequent Cyclic Loading Research Articles

Experimental study on uniaxial ratchetting of VHB 4910 dielectric elastomer

In this work, a series of uniaxial cyclic tests of VHB 4910 dielectric elastomer were carried out to investigate the uniaxial ratchetting and its dependence on the stress level, stress rate, peak/valley stress hold and loading history. The results show that in the uniaxial stress-controlled cyclic tests, VHB 4910 produces obvious ratchetting, and the higher the applied stress level and the lower the stress rate, the more remarkable the ratchetting is. Compared with the experimental results of pure-shear cyclic deformation, the apparent modulus of VHB 4910 during uniaxial cyclic deformation is lower and the dissipation is smaller. In addition, the uniaxial ratchetting of VHB 4910 is also affected by the peak/valley stress hold and loading history. The peak stress hold will promote the generation of ratchetting, but the valley stress hold will hinder the increase of ratchetting strain. The cyclic loading history with a higher mean stress will inhibit the development of ratchetting in the subsequent cyclic loading case with lower mean stress.

Compressive performance of circular hollow section brace with eccentrically installed rib-stiffened splice plates

Hollow section braces are often adopted in steel frame structures because of their superior performance. To connect tubular members with framing elements, field-bolted connections of the slotted-in single splice plate and gusset plate are majorly used because of their ease of construction. However, the eccentricity between the splice and gusset plate axes reduces the compressive strength of the brace and induces local connection failures under seismic excitations. To improve the compressive strength, this paper proposes a new connection configuration for circular hollow section braces in which cruciform rib-stiffened splice plates are eccentrically installed such that the gusset plate axis coincides with the brace axis. The concept of the proposed connection was first described with theoretical considerations. To demonstrate the effectiveness of the proposed connections, compressive loading tests and subsequent cyclic loading tests were conducted for four test specimens. Furthermore, finite element analyses with long-brace models were performed to demonstrate the good performance of the proposed connection for a wide range of slenderness ratios.

Autofrettage of high-pressure components made of ultra-high-strength-steel

This work is primarily concerned with the fatigue life of high-pressure bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with orthogonally intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The Autofrettage process induces advantageous, life-time prolonging residual compressive stresses in the highly stressed areas of the components. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components.In previous works, fracture mechanics based approaches were used to describe the crack propagation behaviour for autofrettaged specimens made of the quenched and tempered steel 42CrMo4. Results showed that crack arrest has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. As efforts are made to increase the injection pressures of fuel injection systems, in this work, the benefit of using ultra high strength steel for the application described is investigated. In order to achieve reliable results, material testing with samples made of the ultra-high-strength steel W360 was performed. The resulting test data were used to describe the initial loading and cyclic loading behaviour of the material with a suitable material model. Finite element analysis was then performed to simulate the Autofrettage process and subsequent cyclic loading. Based on the simulation results, possible crack initiation was determined. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required.

The deformation characteristics of a kaolin clay under intermittent cyclic loadings

Metro train loading or machine vibration is characterized by intermittent cyclic loading interspersed with resting periods between loading cycles. The deformation of soft clay subjected to such unique cyclic loading may be different from that under consecutive cyclic loading, as reconsolidation occurs during the resting or drainage period. A series of cyclic triaxial tests were performed on a kaolin clay under both intermittent and consecutive cyclic loading patterns. The effects of including drainage periods between loading cycles on the deformation characteristic of the clay are investigated under different cyclic stress amplitudes, resting periods, consolidation confining pressures and over consolidation ratios. The results indicate that including drainage periods between loading cycles improves the cyclic resistance of the clay during subsequent loading cycles. However, compared with those under continuous cyclic loading, the overall axial strain and excess pore water pressure accumulation are much greater under the intermittent loading patterns, and the shorter the duration of resting period, the greater the overall accumulation. This means that including reconsolidation between loading cycles would enlarge the final deformation of soft clay. For the normally consolidated samples, the strain accumulation after reconsolidation highly depends on that developed in the first cyclic loading stage. Excess pore water pressure accumulated during each cyclic loading stage is negligible at given cyclic stress in over consolidated clay, whilst the axial strain accumulates notably in the first cyclic loading stage, but marginally after that. This may be attributed to the attenuation adjustment of microstructure in soil as cyclic loading continues.

Physical Modelling of a Strip Footing on a Geosynthetic Reinforced Soil Wall Containing Tire Shred Subjected to Monotonic and Cyclic Loading

In this study, the mechanical behavior of geosynthetic reinforced soil (GRS) walls has been investigated through physical modeling subjected to strip footing monotonic and cyclic loads at various stress paths. The influence of footing location, stress level, post-cyclic behavior and sand-tire shred admixture on the lateral deformations of the wall facing, bearing capacity and the settlement beneath the footing were assessed. To this aim, 12 physical model tests were conducted with a scale of 1: 4. Results indicated that the bearing capacity has increased with the increase in the offset distance of the strip footing to the wall facing and adding tire shred to the backfill material, but the increase is more prominent by adding tire shred to the backfill material. The location of the footing from the wall facing was a crucial parameter on the deformation of facing and the failure mode of the footing. Failure in the facing was the predominant mode of failure in the near facing footing. However, a rupture in the geosynthetic caused failure in the footing far from facing footing. Also result of cyclic loading tests showed both permanent displacement and residual settlement accumulated with load cycles and a majority of them occurred over the first fifteen cycles. Depending on where strip footing was located, it may or may not induce a magnifying effect on subsequent cyclic loading responses.

Continuous Control of an Underground Loader Using Deep Reinforcement Learning

The reinforcement learning control of an underground loader was investigated in a simulated environment by using a multi-agent deep neural network approach. At the start of each loading cycle, one agent selects the dig position from a depth camera image of a pile of fragmented rock. A second agent is responsible for continuous control of the vehicle, with the goal of filling the bucket at the selected loading point while avoiding collisions, getting stuck, or losing ground traction. This relies on motion and force sensors, as well as on a camera and lidar. Using a soft actor–critic algorithm, the agents learn policies for efficient bucket filling over many subsequent loading cycles, with a clear ability to adapt to the changing environment. The best results—on average, 75% of the max capacity—were obtained when including a penalty for energy usage in the reward.

Effect of variable amplitude block loading on intralaminar crack initiation and propagation in multidirectional GFRP laminate

Studies have shown that the fatigue life of multidirectional laminated fibre composites is reduced for variable amplitude block loading and random load spectra compared to constant amplitude loading. One of the hypotheses in the literature for this is a cycle mix effect that causes extra damage each time the load increases i.e. load amplitude or mean load. However, no explanation for this effect has been provided in the literature. This work aims at identifying the reasons for the reduced fatigue life of variable amplitude block loading tests compared to constant amplitude loading tests. This is done through white light imaging which is used to detect when intralaminar cracks occur in the off-axis layers and from this to determine crack density evolution, crack density rates, as well as initiation and propagation of intralaminar cracks contributing to the total crack density. Combined, these parameters show that there is a slow transition from a high crack density rate in the high load blocks to a lower crack density rate in the low load blocks. The crack density rate in the low load blocks in a variable amplitude loading test is higher than in a constant amplitude loading test at similar damage states. Finally, the high load blocks initiate new cracks in each block but they also cause all existing cracks to continue their propagation in subsequent low load blocks at increased rates. In conclusion, there is evidence that the block changes from high to low loads cause increased crack propagation in subsequent low load cycles, which is an important contribution to explain the damaging effect of cycle mixing.

ВЛИЯНИЕ НЕСОМКНУТЫХ ТРЕЩИН В СЖАТОЙ ЗОНЕ БЕТОНА НА НЕСУЩУЮ СПОСОБНОСТЬ ИЗГИБАЕМЫХ ЖЕЛЕЗОБЕТОННЫХ ЭЛЕМЕНТОВ

One of the most important issues when calculating buildings and structures for seismic effects is taking into account the presence of damage in the compressed zone of concrete. It is known that the current norms of the Russian Federation [4] on earthquake-resistant construction assume the development of plastic deformations in structural elements. When determining the loads, the possibility of developing plastic deformations in structural elements is taken into account by introducing a decreasing coefficient K1, while neglecting the effect of plastic deformations on the strength of bearing reinforced concrete elements. The presence of plastic deformations in the reinforcement leads to the appearance of residual cracks in the compressed zone of concrete and, as a consequence, to a decrease in the bearing capacity of bending elements in subsequent loading cycles. The influence of unclosed cracks in compressed zone of concrete of bending elements on their bearing capacity for symmetrical and non-symmetrical reinforcement, for different values of reinforcement coefficient and for different values of coefficient of plasticity is considered. Based on the results of calculations the bearing capacity data for bending reinforced concrete elements with residual cracks in compressed zone of concrete have been obtained. The approximate method of determination of residual cracks depth in compressed zone of concrete in terms of coefficient of plasticity in the first semi cycle of loading is proposed.

Autofrettage of component-like ultra high Strength Steel Specimens with intersecting Holes

This work is primarily concerned with the fatigue life of high-pressure-bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components. In previous works, results showed that besides crack initiation, crack arrest behaviour has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. In order to achieve reliable results, material testing with samples made of the ultra high strength steel W360 was performed. The resulting test data were used to simulate the Autofrettage process with finite-element analysis. Calculated residual stress distributions were used to determine at which levels of subsequent cyclic loading crack initiation would occur. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably.

The effects of unloading on undrained deformation of a kaolin clay under cyclic loading

Excavation induced unloading may induce additional shear stress in soils. Such additional shear stress or unloading history may affect the long-term deformation of soft clay under subsequent cyclic loadings. A series of undrained cyclic triaxial tests were performed on a kaolin clay to study unloading effect on cyclic behaviour of the soil. The effect of unloading sequence on deformation of the clay under cyclic loading was simulated by reducing cell pressure in between loading cycles. The dependence of axial strain accumulation on the amplitude of cell pressure reduction and unloading schemes was investigated under different initial confining pressures and cyclic loading frequencies. The results indicate that reducing confining pressure accelerates the strain accumulation during subsequent loading cycles and the greater the reduction in cell pressure, the greater the accumulated strain and excess pore water pressure. Such acceleration of axial strain accumulation after unloading would be attributed to the variation of stress state and the soil's ability in memorizing the unloading history. The direction of strain accumulation is controlled by the shear stress ratio rather than the amplitude of unloading. The final accumulated strain is not much affected by unloading schemes if the samples are the same in the initial confining pressure and the amplitude of unloading. An empirical model was developed to fit the axial strain accumulation both before and after unloading. The effects of cell pressure reduction on the parameters of the empirical model were also analysed.

Effects of intermittent loading time and stress ratio on dwell fatigue behavior of titanium alloy Ti-6Al-4V ELI used in deep-sea submersibles

Different components of deep-sea submersibles, such as the pressure hull, are usually subjected to intermittent loading, dwell loading, and unloading during service. Therefore, for the design and reliability assessment of structural parts under dwell fatigue loading, understanding the effects of intermittent loading time on dwell fatigue behavior of the alloys is essential. In this study, the effects of the intermittent loading time and stress ratio on dwell fatigue behavior of the titanium alloy Ti-6Al-4V ELI were investigated. Results suggest that the dwell fatigue failure modes of Ti-6Al-4V ELI can be classified into three types, i.e., fatigue failure mode, ductile failure mode, and mixed failure mode. The intermittent loading time does not affect the dwell fatigue behavior, whereas the stress ratio significantly affects the dwell fatigue life and dwell fatigue mechanism. The dwell fatigue life increases with an increase in the stress ratio for the same maximum stress, and specimens with a negative stress ratio tend to undergo ductile failure. The mechanism of dwell fatigue of titanium alloys is attribute to an increase in the plastic strain caused by the part of the dwell loading, thereby resulting in an increase in the actual stress of the specimens during the subsequent loading cycles and aiding the growth of the formed crack or damage, along with the local plastic strain or damage induced by the part of the fatigue load promoting the cumulative plastic strain during the dwell fatigue process. The interaction between dwell loading and fatigue loading accelerates specimen failure, in contrast to the case for individual creep or fatigue loading alone. The dwell fatigue life and cumulative maximum strain during the first loading cycle could be correlated by a linear relationship on the log–log scale. This relationship can be used to evaluate the dwell fatigue life of Ti alloys with the maximum stress dwell.

Comparison of cyclic liquefaction behavior of clean and silty sands considering static shear effect

In practical engineering, liquefiable sand deposits in natural or manmade slopes, dams, and embankments may contain a certain amount of silty fines and often sustain a static shear prior to subsequent cyclic loading. To understand the effect of static shear stress on the cyclic liquefaction behavior of silty sand, a small amount of crushed fines was added to clean sand, and a series of triaxial tests considering a wide range of static and cyclic shear stress levels were performed. The results indicate that different stress conditions result in two liquefaction failure patterns for silty sand: cyclic mobility and residual deformation accumulation. A comparison of the liquefaction responses of silty and clean sands with the same relative density confirms that the former typically has a moderately higher cyclic resistance than the latter, which can be explained by the reinforced effect of fines on the soil skeleton. It is also demonstrated that the flow deformation type behavior accompanied by rapid development of pore pressure and severely degraded stiffness is more prominent in clean sand, which is responsible for the continuous decrease in its liquefaction resistance when the extensional static stress increases. Furthermore, a unique correlation of equivalent modulus versus cyclic strain amplitude is observed, which depends significantly on the static shear stress level but relatively insensitive to the addition of fines at least up to the considered fines content.

Particle breakage evolution during cyclic triaxial shearing of a carbonate sand

Particle breakage of carbonate sand under cyclic loading might be of interest in some circumstances, while few research results were reported. Drained and undrained cyclic triaxial tests terminated at different loading cycles were conducted on a carbonate sand to investigate the evolution characteristics of particle breakage during cyclic shearing. Considerable breakage was observed after successive cyclic triaxial shearing at moderate confining pressure levels (≤200 kPa). Abrasion of surface asperities and small sharp edges was found to be the primary pattern of particle breakage and the particle size distribution after cyclic shearing showed an increased mass content of fine particles. In drained cyclic tests with constant confining pressure and cyclic stress amplitude, particle breakage appeared to accumulate at a decreasing rate with increasing loading cycles, and the development of particle breakage with loading cycles can be fitted by a logarithmic equation. In undrained cyclic tests, particle breakage exhibited more complicated accumulation patterns because the effective confining pressure and cyclic strain amplitude varied greatly. A cycle-by-cycle incremental breakage evolution model was established, and the model revealed that the incremental breakage generated in subsequent loading cycles decreases with accumulated particle breakage and increases with increasing effective confining pressure and cyclic shearing intensity. The model was validated by comparing the model predictions with the particle breakage evolution observed during undrained cyclic triaxial tests.

Improvements in plate anchor capacity due to cyclic and maintained loads combined with consolidation

Plate anchor technology is an efficient solution for mooring offshore floating facilities for oil and gas or renewable energy projects. When used with a taut mooring, the anchor is typically subjected to a maintained load component and intermittent episodes of cyclic loading throughout the design life. These loads, and the associated shearing, remoulding and consolidation processes, cause changes in the anchor capacity, particularly in soft, fine-grained soils. The changing anchor capacity affects the mooring performance by changing the safety margin and also the overall system reliability. In this paper the changing anchor capacity in reconstituted, normally consolidated natural carbonate silt was assessed through a series of beam centrifuge tests on horizontally loaded circular plate anchors. The results demonstrate that full consolidation under a typical maintained load leads to a 50% gain in the anchor capacity, and subsequent cyclic loading and reconsolidation can triple this increase. An effective stress framework based on critical state concepts is employed to explain and support the experimental observations. This study shows that, when viewed from a whole-life reliability perspective, maintained and cyclic loading provide a long-term enhancement of anchor capacity in soft, fine-grained soils. This beneficial effect is currently overlooked in design practice, but can be predicted using the framework shown here, which can form the basis for a digital twin that monitors the through-life integrity of a plate anchor.

Deformation characteristics of fine-grained soil under cyclic loading with intermittence

Since there is an interval time between two trains traveling in the same direction on an automatically blocked line, the dynamic loading on the railway subgrade is composed of periodic vibration when the train passes and an intermittent stage after a train has passed. Take into account this characteristic, a series of continuous-stopping repeated load triaxial tests were conducted to study deformation characteristics such as plastic strain and elastic strain of fine-grained soil under intermittent loading. Compared to continuous loading, it is found that the intermittent stage has a significant effect on the deformation characteristics of subgrade. The excess pore water pressure accumulated during the loading stage dissipated during the intermittent stage. The development of plastic strain in subsequent loading stages slowed down significantly, and the axial strain of the sample rebounded in the intermittent stage, so cumulative plastic strain was reduced. The intermittent effect on reducing elastic strain was, however, limited. An increase in intermittent time can also improve the resistance of the sample to subsequent cyclic loading. An alternating continuous-stopping loading mode can better simulate intermittent loading of trains in actual train operation and produce more practical results, providing a theoretical basis and guidance for further understanding of deformation characteristics of railway subgrade.

Torsional vibration technique for the acoustoelastic characterization of concrete

There is no nondestructive evaluation method capable of determining stresses in concrete structural members in situ. Here, we propose and evaluate a torsional-vibration testing technique that offers an approach to estimate the stress state in concrete specimens by means of characterizing the material nonlinearity. We axially load three prismatic specimens comprising different concrete mixtures and measure their torsional vibration frequencies during four loading cycles. The fundamental torsional frequency shows a positive correlation with applied compressive stress for both loading and unloading stages after correcting for the effects of non-uniform torsion, geometric nonlinearity and changing boundary conditions. To quantify this behavior, we define the nonlinear parameter $$\beta_{G}$$ to characterize the material nonlinearity (acoustoelasticity). The values of $$\beta_{G}$$ of the initial loading cycle are lower than those of the subsequent loading cycles. However, the latter values of our concrete mixtures are consistent and similar to the values computed from previously published results. An estimate of the $$\beta_{G}$$ parameter of a concrete structural member provides a pathway for nondestructive assessment of in situ compressive stress in the member.

Finite Element Modeling of a Thin Circular Disc Manufactured by Forming Process used in Cyclic Loading and Unloading Applications

Abstract The presented work focuses on numerical modelling and analysis of the forming process of a thin circular disc of negligible thickness, that is later subjected to vibration-induced loading and unloading cycles (fatigue) during application. During the forming process, the material of the pre-form (Mild-Carbon Steel, AISI-1075) is shaped into the required profile of the disc, using a fast moving punch that impacts on a die underneath. The profile consists of a hole at the center of the disc and predefined surface gradations along the periphery. This punching process is followed by heat treatment to relax the residual stresses developed. This paper discusses the development of a detailed thermo-mechanical coupled model using the finite element method, to simulate the different stages of the forming process; namely, the punching process, the instantaneous punch release and structural creep, and the subsequent heat treatment to induce stress relaxation. The numerical model is intended to predict the amount of residual stress and equivalent plastic strain that are developed in the disc at the end of each stage of the forming process. The stress concentration in the disc has serious implications on the subsequent cyclic loading and unloading application. The results obtained from standard tests indicate initiation and propagation of cracks in the disc during application. The intensity and occurrence of crack initiation have a direct connection to the maximum residual stress developed in the disc during the forming process. In addition, the heat treatment model is also intended to investigate the temperature distribution in the disc, during the heating and the subsequent cooling operations. The results of the finite element model are then validated with experimental measurements for each of the stages of the forming process. The finite element packages, Abaqus/Explicit and Abaqus/Standard, are used in conjunction for the aforesaid modeling

Optimum Autofrettage Pressure of Hydrogen Valve Using Finite Element and Fatigue Analysis

The presented article shows an estimation method of optimum autofrettage pressure taking into consideration subsequent cyclic loading. An autofrettage process is used in pressure vessel applications for strength improvement. The process relies on applying massive pressure that causes internal portions of the part to yield plastically, resulting in internal compressive residual stresses when pressure is released. Later applied working pressure (much lower than autofrettage pressure) creates stress reduced by the residual compressive stress improving the structural performance of the pressure vessels. The optimum autofrettage pressure is a load that maximizes the fatigue life of the structure at the working load. The estimation method of that pressure of a hydrogen valve is the subject of the presented work. Finite element and fatigue analyses were employed to investigate the presented problem. An automated model was developed to analyze the design for various autofrettage pressures. As the results of the procedure, the optimum autofrettage pressure is determined. The research has shown that the developed method can profitably investigate the complex parts giving the autofrettage load that maximizes the fatigue life. The findings suggest that the technique can be applied to a large group of products subjected to the autofrettage process.

Cyclic pullout behavior of helical anchors for offshore floating structures under inclined loading condition

Helical anchors are an effective option to support offshore floating structures with mooring and anchoring systems, where wave and tidal forces are the predominant load components leading to cyclic and inclined loading conditions. In this study, the cyclic and inclined pullout load carrying behavior of helical anchors was investigated. Large-deformation finite element (FE) analysis using the coupled Eulerian–Lagrangian (CEL) method was performed to simulate the cyclic pullout load response of helical anchors. Various configuration conditions of the helical anchor and loading direction, including the number and diameter of helical plate, plate arrangement and load inclination angle (θ), were considered in the analysis. Induced displacements were most significant during the first loading cycle, whereas those for subsequent loading cycles were relatively small. The geometry condition of the helical anchor less affected the pullout load carrying behavior as θ increased. The horizontal displacements (δh) were larger than the vertical displacements (δv) when θ was larger than 30°. When θ was smaller than 30°, δv was more dominant component. It was found that the configuration with top-down increasing diameter was more effective to enhance the pullout load carrying behavior than the conventional bottom-up increasing diameter configuration.

The role of pore water plays in coal under uniaxial cyclic loading

Geofluids are common in underground coal mines and have a significant influence on the deformation of coal rock, where the pore pressure of geofluids varies during swelling or shrinkage processes. In order to characterize the role of pore water in coal, the uniaxial behavior of saturated coal under cyclic compression was investigated considering different loading rates, amplitudes, and numbers of cycles. The results show that Young's modulus (E) increased in the first cycle and then stabilized in the remainder of loading cycles. Also, the increment of E in the first cycle was inversely correlated with the loading rate. For the first time, it is found that the radial residual strain gradually decreased in each subsequent loading cycle. This is different from that of water weakening property of coal by immersion of the coal in water without pressurization. In the unloading phase, the pore pressure decayed more slowly than the stress, leading to a high level of deviatoric stress. As a result, the acoustic emission events appeared even at the minimum of cyclic loading. The transverse relaxation time (T2) distribution shows that the pore water pressure mainly changed the primary pores, while its influence on the secondary pores was negligible. From a poromechanical perspective, high pore pressure developed in pores of poor connectivity during initial cyclic loading, causing stress orientation rotation and tensile damage. With further cyclic loading, the pore water was continuously discharged as the pores were compacted into flat cracks. As a result, the onset stress of dilatancy gradually increased in the subsequent loading cycles due to the enhanced frictional resistance to crack slip.