Stress-controlled Cyclic Tests Research Articles

The failure type-dependent characterization of liquefaction resistance by small-strain shear modulus for saturated sand

In this study, a re-visit of existing laboratory tests was carried out to study the relationship between the liquefaction resistance (CRR) and the small-strain shear modulus (Gmax) of saturated sands with consideration of the failure types of both cyclic mobility and flow liquefaction. The result indicates that the relationship between CRR and Gmax is failure type-dependent, and the liquefaction resistance of cyclic mobility grows faster than that of flow liquefaction. Then discrete element method was used to explore the relationship between the liquefaction resistance and the small-strain shear modulus for a typical saturated granular material, with consideration of different liquefaction types. A series of undrained stress-controlled cyclic tests together with shear wave velocity measurements were simulated, and results similar to the laboratory studies were obtained. Then a new expression is proposed to describe the relationship between CRR and Gmax for granular material. The corresponding micromechanical mechanism is further explored, which shows that there are two kinds of distinct behaviors of the particle contact structure when the granular material changes from loose to dense state under the same confining stress. Compression behavior is dominant for flow liquefaction while collapse behavior is dominant for cyclic mobility type of liquefaction. The different behaviors of the particle contact structure influence the growth of the mechanical contacts, which further leads to the failure type-dependent growth of liquefaction resistance for a saturated granular material.

The Liquefaction Behavior of Leachate-Contaminated of Sand under Cyclic Loads

Uncontrolled solid waste affects the stress-strain behavior of the sand as well as causes an environmental problem. In this research, the effect of leachate on the dynamic behavior of sand has been studied and the results were compared with clean sand with different saturation degrees. The modified split molds at 50 mm diameter and 100 mm height were used to prepare the leachate-contaminated sand. Both contaminated and clean sand samples were prepared by dry pluviation method. After preparation, the sand samples in the modified mold were put in the leachate wastewater to keep for different cure times. Then they were installed on the dynamic triaxial test system at 0.1 Hz and the same cyclic stress ratio. During the saturation process to prevent the structure of contaminated sands from being disturbed, samples were not passed through CO2 and distilled water. Stress-controlled cyclic tests were conducted at on both clean and contaminated sand consolidated at 100 kPa isotropically. After consolidation phase the measured B values varied between 0.45-0.93. Results show that leachate-contaminated sands are more liquefiable depending on cure time at the same saturation value and saturated clean sand samples and partially saturated leachatecontaminated sand samples show similar liquefaction behavior under the same cyclic ratio.

Effects of consolidation conditions on the dynamic shear modulus of saturated coral sand over a wide strain range

The utilization of coral sand as a construction material for ports and other infrastructure systems in coral reefs is prevalent, wherein the soil elements within slopes and embankments typically experience an anisotropic stress state. It is imperative to accurately characterize the degradation behavior of the stratigraphic material's stiffness across a wide range of strains. In this study, the maximum and strain-dependent shear modulus (G0 and G) characteristics of saturated coral sand under isotropic and anisotropic consolidations were investigated by performing multi-stage stress-controlled undrained cyclic torsional shear tests with cyclic hollow cylinder shear apparatus. The experimental results indicated that G0 and G generally rose with increasing initial effective consolidation pressure but decreased with increasing inclination of initial major principal stress from the vertical (αc). In contrast, the effect of consolidation stress ratio (kc) on G0 and G was nonmonotonic, and that of kc and αc on the strain-dependent shear modulus reduction (G/G0) curve was slight. By defining anisotropic consolidation indexes δ0 and δr used for quantifying the effect of consolidation-induced anisotropy on the G0 and the G/G0, respectively. A unified formula of G0 obtained by multiplying Hardin's model by δ0, as well as a unique formula of G/G0 obtained by correcting γr in the Davidenkov skeleton curve expression as a linear function of δr were established under various consolidation conditions, respectively. The proposed G formula over a wide strain range is well validated by the experimental data for siliceous sandy soils from the literature.

Monotonic and fatigue behaviour of notched glass fibre-reinforced polymer laminates immersed in seawater

This paper studies the monotonic and fatigue behaviour of notched glass fibre-reinforced polymers after long-term exposure in seawater. Uniaxial quasi-static tensile tests and uniaxial stress-controlled cyclic tests were carried out for different immersion times, ranging from 0 days to 910 days, using rectangular cross-section specimens with a central hole. The geometric discontinuity was monitored, via digital image correlation, to better understand the damage accumulation process and the fracture mechanisms. A three-dimensional progressive damage model was also implemented, using the finite element method, to simulate the stress–strain response and the failure pattern. Fatigue life of the immersed laminates was predicted from the S-N curve of the dry laminates along with the three-dimensional progressive damage model. Overall, the proposed predictive approach captured well the detrimental effect of seawater on fatigue lifetime.

Role of geofoam inclusions on the liquefaction resilience of transportation geostructures

Though the effectiveness of geofoam buffers for improving the dynamic load response and seismic performance of road embankments and retaining walls is well-established, their role on soil liquefaction is not investigated by many. This study explores the potential of Expanded Polystyrene (EPS) geofoam inclusions for liquefaction mitigation in sands through a series of stress-controlled constant volume cyclic simple shear tests. The study quantifies the increase in liquefaction resistance of sand with geofoam inclusions and reduction in modulus degradation and shear deformations and increase in energy dissipation with loading cycles. Effects of geofoam density and thickness of the layer on the liquefaction response were also investigated through systematic series of experiments with geofoam of three different densities and two different thicknesses. Findings from this experimental study suggest that providing a thick low-density geofoam layer provided the maximum benefit among all the inclusions. Fundamental mechanisms governing this improved response are explored through a series of bender element tests. Results from this study confirm the effectiveness of geofoam buffers in mitigating liquefaction in granular soils and recommend their use in road embankments and retaining walls being constructed in zones of high seismicity to avoid liquefaction and associated lateral spreading failures in these structures.

The effect of cooling rate and degassing on microstructure and mechanical properties of cast AZ80 magnesium alloy

The effect of casting cooling rate and degassing on the mechanical properties of cast AZ80 alloy is investigated. A water-cooled copper mould with a wedge geometry is chosen for the casting, which provides different cooling rates at different sections. The casting is performed with and without the addition of hexachloroethane degassing agent. Macrohardness, hot compression, quasi-static tensile, and stress-controlled cyclic tests are performed to characterize the mechanical properties of the cast samples. The microstructure of the material is studied using optical microscopy and X-ray computed tomography methods. The connection between the microstructure and mechanical test results shows that the morphology of primary α-Mg and porosity defects affect the mechanical properties of cast AZ80. The casting cooling rate controls both of these microstructural features. Higher cooling rates result in a finer dendritic morphology of α-Mg that improves mechanical properties and creates a finer and more discrete morphology of β intermetallic particles. The fine dendritic morphology results in smaller interdendritic regions, which in turn prevents pore coalescence and negates the detrimental effect of porosity on the quasi-static tensile and cyclic properties. In lower cooling rates, where the coarse grain structure of the cast material cannot prevent pore coalescence, degassing benefits the mechanical properties, especially fatigue resistance, by reducing the volume fraction of porosity defects. Moreover, the hot deformation behavior of cast samples shows that the addition of the degassing agent and casting with a higher cooling rate improves the range of deformation conditions where cracking will not occur.

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.

Experimental study on the dynamic properties of rigid polyurethane foam in stress-controlled cyclic uniaxial tests

Frequent major earthquakes in Japan necessitate earthquake-resistant infrastructure. The dynamic deformation characteristics of rigid polyurethane foam, used as a lightweight embankment material, is examined in this study. The effect of the initial static stress on the shear modulus of single-layer samples was small, while the shear modulus of two-layer samples increased with increasing initial static stress. The damping ratio of single-layer samples converged to 4–5%, but decreased with increasing initial static stress up to 30 kPa in two-layer samples before increasing above 40 kPa. Results from cyclic uniaxial and triaxial tests were compared. The dynamic properties depended on restriction conditions and the deformation direction.

The Ʃ3 twin dependence of thermo-mechanical fatigue of a polycrystalline high-purity Cu film

This study conducts stress-controlled cyclic tension-tension tests from room temperature (RT) to 200 °C, at 50 °C intervals, on high-purity Cu films to comprehensively understand the temperature-dependent fatigue life and the related microstructural and fracture characteristics. As evidenced by the electron backscattering diffraction analysis, the Ʃ3 boundary fraction decreases and the low-angle grain boundary fraction increases as the temperature increases. The significant change in the grain structure at high temperatures leads to a reduced fatigue life. The fatigue striations are closely spaced at RT and the fracture surface develops into the typical fatigue striation and the void zones.

Experimental study on uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium alloy with different plastic deformation mechanisms

The uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium (Mg) alloy is investigated by uniaxial stress-controlled cyclic tests at room temperature and with addressing the roles of different plastic deformation mechanisms. Different stress levels are prescribed to reflect the cyclic plasticity of the alloy controlled by diverse deformation mechanisms (i.e., dislocation slipping, deformation twinning and detwinning ones), and then the influences of stress level and stress rate on the ratchetting and fatigue life are discussed. The experimental results demonstrate that different evolution characteristics of whole-life ratchetting and fatigue life presented during cyclic tests with various mean stresses, stress amplitudes and stress rates are determined by the dominated plastic deformation mechanisms. It's worth noting that the ratchetting can occur in the compressive direction even in the cyclic tests with a positive (tensile) mean stress, and the fatigue life increases first and then decreases with the increase of mean stress on account of the interaction between dislocation slipping and twinning/detwinning mechanisms. Comparing the fatigue lives obtained in the asymmetric stress-controlled and symmetrical strain-controlled cycle tests, it is seen that the ratchetting deformation causes an additional damage, and then leads to a shortening of fatigue life.

Study on properties of SLM-NiTi shape memory alloy under the same energy density

Abstract In recent years, selective laser melting (SLM)-NiTi had developed rapidly due to the ability to achieve the complex shape and internal features, as well as high dimensional accuracy. The choice of parameters was particularly critical to the forming and performance of SLM-NiTi. In this work, we had designed and prepared five sets of SLM-NiTi shape memory alloys with the same energy density (range of 40–90 J/mm3). The microstructure, phase transition characteristic, mechanical properties and shape memory effect of SLM-NiTi shape memory alloys were investigated through various characterization methods of X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and stress-controlled cyclic tensile tests, etc. The results showed that the surface forming quality of SLM-NiTi was not only related to the energy density, but also related to the value of P/V, which higher than 0.3 or lower than 0.1 would lead to the formation of surface pores. Interestingly, the SLM-NiTi showed ultrahigh failure strength of 735 MPa and elongation of 10.88% under room temperature tensile conditions. In addition, stress-controlled cyclic tensile tests under 400 MPa indicated that the SLM-NiTi had excellent shape memory effect of 76.1% recovery ratio and 3.95% recoverable strain after ten times loading–unloading cycles. The design of multi-parameter variables can not only optimize the surface quality, but also provide a basis for the prediction of SLM-NiTi phase transition temperature.

Monotonic and Fatigue Response of Heat-Treated Friction Stir Welded Al 6061-T6 Joints: Testing and Characterization

The present study evaluates the influence of post-weld heat treatment on the fatigue strength of friction stir welded aluminum 6061-T6 joints. The solution-treatment artificial aging (STA) was applied to the friction stir welding samples prior to cyclic tests. Hardness distributions along the centerline of the welded specimens as well as the tensile properties and microstructural features were initially examined. The stress-controlled cyclic tests were then conducted at constant amplitude loading with the load ratio of R = 0.1. Experimental results revealed that tensile strength of STA heat-treated samples increased 73 % as compared to the as-welded (AW) joints. Furthermore, the STA heat-treated specimens experienced 28 % higher elongation than those of AW samples. A 3D finite element analysis was developed to (i) simulate residual stress distribution of the welded joints after heat treatment processing and (ii) analyze stress/strain components at weld toe induced as a result of applied loading cycles. The heat-affected zone with critical local stress/strain values was found to be the most vulnerable zone for cracking. Predicted fatigue lives by means of the Smith, Watson, and Topper model were found in close agreement with those experimentally obtained values.

Modelling and analyses of the mechanical response of P92 steel in stress-controlled cyclic tensile-loading tests with holding periods

ABSTRACT The current study investigates the mechanical behaviors of P92 steel in the stress-controlled cyclic tensile-loading tests with holding period. First, the testing conditions and some typical experimental results of P92 steel in those kind of tests are reviewed. To simulate the mechanical behaviors of P92 steel, a three-stage creep model is adopted. The material parameters in the model are identified by using the experimental data of pure creep tests, where the low- and high-stress ranges are considered separately. The constitutive equations of the model can be solved through a conventional numerical approach. With the obtained numerical results, the responses of P92 steel samples in the cyclic tensile-loading tests with holding periods can be predicted at a quantitative level. Some key features of the experimental results are also analyzed. Furthermore, the damage evolution property of P92 steel in the tests can be revealed. The current modelling work will be helpful for the residual lifetime evaluation and the safety design of P92 steel components under practical loading conditions.

Cyclic plasticity of an interstitial high-entropy alloy: experiments, crystal plasticity modeling, and simulations

The development of high-entropy alloys (HEAs) comprising multiple principal components is an innovative design strategy for metallic materials from the perspective of thermodynamic entropy. However, despite their potential candidacy for engineering applications, the lack of research on the cyclic loading responses as well as constitutive modeling of the HEAs is a major constraint. Therefore, the present work focuses on the cyclic plasticity of a typical carbon-doped interstitial HEA (iHEA) with nominal composition Fe49.5Mn30Co10Cr10C0.5 (at.%). The results of stress-controlled cyclic tests with nonzero mean stress showed that the iHEA exhibits significant cyclic hardening and stress level–dependent ratcheting. Owing to its improved cyclic hardening, the saturated ratcheting strain rate of the iHEA is lower than that of conventional steels such as the 316L stainless steel. Furthermore, microscopic characterizations revealed that the cyclic deformations caused massive martensitic phase transformation and hierarchical structures in the iHEA. The experimental results were used to develop a physical mechanism-based crystal plasticity constitutive model that is capable of describing the cyclic plasticity of the iHEA, which was implemented into a finite element framework. The simulation results showed that the loading stress significantly affected the microstructural evolutions, leading to a stress level–dependent cyclic plasticity. Thus, this investigation provides a fundamental basis for fatigue tests and service life prediction/optimization of the iHEA in the future, which can promote its engineering applications.

Ratcheting behaviour of flash butt welds in heat-treated hypereutectoid steel rails under uniaxial and biaxial cyclic loadings

Experimental study consisting of both uniaxial and biaxial stress-controlled cyclic tests was performed to investigate the ratcheting behaviour of new flash butt welds in a heat-treated hypereutectoid steel rail used in Australian heavy haul railways. Digital image correlation technology was applied to capture the heterogeneous strain field and its evolution at every point on the surface of the weld specimen. The results show that the softened zone with a significant hardness drop is more sensitive to plastic deformation and results in a higher ratcheting strain than the region around the bond line. Moreover, the distribution of ratcheting strain can be correlated to the longitudinal hardness profile within the heat-affected zone of the weld. At the initial stage of cyclic tests, the softened zone tends to exhibit cyclic hardening while the region around the bond line is more likely to exhibit cyclic softening. Compared with the parent rail, the softened zone shows much worse ratcheting resistance, while the region around the bond line demonstrates slightly better ratcheting resistance. Metallographic analysis indicates that the microstructure and resulting ratcheting resistance of the weld vary with the longitudinal position. High ratcheting strain in the softened zone is mainly attributed to the existence of the spheroidised microstructure with a high amount of ferrite. The outcomes of this study can provide valuable information for further establishing the constitutive models to quantify the heterogeneous ratcheting of rail welds under practical wheel-rail contact situations by finite element simulations.

Experimental investigation on electro-mechanically coupled cyclic deformation of laterally constrained dielectric elastomer

A series of electro-mechanically coupled cyclic tests at large deformation are carried out to characterize the cyclic deformation of a laterally constrained dielectric elastomer (DE) in this work. In the strain-controlled cyclic tests of the dielectric elastomer (e.g., VHB 4910 DE) with a constant or cyclic voltage, cyclic stress softening occurs and is influenced by the phase-angle difference between the applied cyclic strain and cyclic voltage. In the stress-controlled cyclic tests of VHB 4910 DE with a constant or cyclic voltage, ratchetting (a cyclic accumulation of inelastic strain) takes place; the ratchetting strain is considerably enhanced by applying higher voltage level, higher stress level and lower stress rate, and is also affected by the phase-angle difference between the applied cyclic stress and cyclic voltage. Moreover, the remarkable recovery of residual strain after the cyclic tests demonstrates that the cyclic stress softening and ratchetting of VHB 4910 DE mainly stem from the viscoelasticity. The comprehensive experimental observations are very useful to develop, calibrate and validate an electro-mechanically coupled constitutive model of dielectric elastomers.

Assessment of Dynamic Response of Cohesionless Soil Using Strain-Controlled and Stress-Controlled Cyclic Triaxial Tests

Soils subjected to earthquake motions undergo random variations in stress, strain and frequency during the entire period of shaking. The spatial and temporal complexity of the strong motions is commonly addressed through simplistic strain-controlled or stress-controlled cyclic loading tests as a part of laboratory investigations. Such a methodology is utilized and reported in the present study for assessing the dynamic response of cohesionless sand obtained from River Brahmaputra, India. In order to assess the dynamic response and liquefaction potential of the cohesionless Brahmaputra sand, both stress-controlled and strain-controlled cyclic triaxial tests were performed on reconstituted cylindrical specimens prepared at different relative densities (Dr) ranging from 30 to 90%. The specimens were subjected to varying effective confining pressures (σ′c: 50–200 kPa), shear strain amplitudes (γ: 0.015–4.5%) and cyclic stress ratios (CSR: 0.05–0.3). For all the tests, the frequency of the applied harmonic loading was maintained at 1 Hz. The magnitude of excess pore-water pressure (PWP) generated during successive loading cycles of the strain-controlled tests was found to be considerably lower than that generated in a stress-controlled test. The strain-controlled test reveals a reduction in the development of excess PWP with the increase in confining pressure and relative density, thereby indicating a decrease in liquefaction potential with increasing confining pressure. The stress-controlled test highlighted that based on a particular CSR, an increase in confining pressure results in the requirement of higher deviatoric stress and smaller numbers of cycles are required to initiate liquefaction. Although apparently misleading, it is imperative that during earthquake, same deviatoric stress is applied on the entire soil deposit. Thus, for specimens at larger depths having higher confining pressure, the CSR value is smaller, and thereby successive deeper layers require more number of stress cycles to liquefy. Hence, the study revealed that both stress-controlled and strain-controlled tests can be successfully used to assess the dynamic properties and liquefaction potential of cohesionless specimens. Based on the findings, the developed cyclic shear stress ratio (CSR) = 0.5 and cyclic shear strain amplitude (γ) = 0.5% are considered as the limiting conditions to achieve the onset of liquefaction through strain-controlled and stress-controlled approaches, respectively.

Cyclic deformation behavior and related micro-mechanisms of a special CVD Ni processed with bimodal grain structures: Ultrafine (UF) grains and large grains with UF/nano twins

Stress-controlled cyclic tests were conducted on bulk sheet Ni-carbonyl Chemical Vapor Deposited material (CVD Ni) with bimodal grain structures: ultrafine (UF) grains and large grains with UF/nano twins. The tests were run with the cyclic stress ratio R = 0.05 and peak stress level of 0.9–1.5 times of the material's yield strength. Results show that within the applied peak stress ratio of 0.9–1.1, the material demonstrated cyclic hardening behavior first, followed by stress–strain saturation till fracture; upon increasing the ratio to 1.4–1.5, an additional softening stage was activated and continued till fracture. By transmission electron microscope (TEM) examination, it was found that such cyclic deformation responses were associated with the stability of the ultrafine- and nano-twin structures. Initial hardening was found mainly due to the increase in dislocation density and the activities of dislocations especially with their strong interactions with the dense twin boundaries (TBs). The saturation was contributed by the simultaneous operations of the softening effect due to massive detwinning and the hardening behavior due to dislocation interactions with existing TBs. Newly formed dislocation walls and cell structures were further found in samples with stress ratio of 1.4 at fracture, corresponding to the softening stage. Furthermore, such microstructural evolution, which were observed also through annealing and monotonic deformation of the same material, is identified as a consistent energy reduction path for the material. Thus, an energy criterion is further established to predict the massive detwinning events that cause the major softening phenomena under cyclic deformation.

Cyclic Deformation Behavior and Related Micro-Mechanisms of Ni-Carbonyl Chemical Vapor Deposited (CVD) Material with Bimodal Grain Structures: Ultrafine (UF) Grains and Large Grains with UF/Nano Twins

Stress-controlled cyclic tests were conducted on bulk sheet Ni-carbonyl Chemical Vapor Deposited material (CVD Ni) with bimodal grain structures: ultrafine (UF) grains and large grains with UF/nano twins. The tests were run with the cyclic stress ratio R=0.05 and peak stress level of 0.9 to 1.5 times of the material's yield strength. Results show that within the applied peak stress ratio of 0.9-1.1, the material demonstrated cyclic hardening behavior first, followed by stress-strain saturation till fracture; upon increasing the ratio to 1.4-1.5, an additional softening stage was activated and continued till fracture. By transmission electron microscope (TEM) examination, it was found that such cyclic deformation responses were associated with the stability of the ultrafine- and nano-twin structures. Initial hardening was found mainly due to the increase in dislocation density and the activities of dislocations especially with their strong interactions with the dense Twin Boundaries (TBs). The saturation was contributed by the simultaneous operations of the softening effect due to massive detwinning and the hardening behavior due to dislocation interactions with existing TBs. Newly formed dislocation walls and cell structures were further found in samples with stress ratio of 1.4 at fracture, corresponding to the softening stage. Furthermore, such microstructural evolution, which were observed also through annealing and monotonic deformation of the same material, is identified as a consistent energy reduction path for the material. Thus, an energy criterion is further established to predict the massive detwinning events that cause the major softening phenomena under cyclic deformation.

The hysteretic curve characteristics on Q235 steel under asymmetrical cyclic loading

Ratcheting and Bauschinger effect usually occur in the structures which are subjected to cyclic loading. It is necessary to research on the evolution rules of ratcheting and Bauschinger effect changed with cycle numbers and further to look into the influence on material mechanics property. In this work, asymmetrical stress-controlled cyclic loading tests on Q235 steel have been conducted to obtain variation trends of strains. Furthermore, the relationships between ratcheting and loading conditions, Bauschinger effect and loading conditions are analyzed, respectively. The results indicate that Q235 steel shows cyclic softening characteristic ultimately during the tests. Ratcheting occurs when stress peak of applied loading is larger than lower yield stresses, and the initial cycle number of ratcheting is decided by the stress level of applied loading. No matter how the mean stress and the stress amplitude changed, the yield value of stress at the first loop is almost identical, and tensile yield stress is larger than compressive yield stress. It suggests that Q235 steel shows obvious Bauschinger effect under asymmetrical stress cyclic loading, and the influence of mean stress on Bauschinger effect is more significant than stress amplitude.