Cyclic Stress Levels Research Articles

Responses of frozen, fine-grained soils due to multiple packets of cyclic stress with variable amplitude and an empirical prediction model

A suite of stress-controlled cyclic triaxial tests were performed on frozen, fine-grained soils to assess the characteristics of the accumulated and cyclic behaviours under packets of cyclic stress with variable amplitude. The influence of cyclic stress sequences is highlighted, and the results of the variable amplitude cyclic tests indicate that the applied cyclic stress amplitude sequence is of importance regarding the final accumulated deformations. A step change is observed in the accumulated strain when the applied cyclic stress amplitude is beyond that of the previous stress package, while a decreasing level of cyclic stress below the previous loading package results in slight additional strain accumulation. The results of these tests also indicate that the cyclic stiffness is dependent on both the past accumulated strain and the current cyclic stress amplitude. For frozen soil, a higher past accumulated strain and lower cyclic stress levels yield a higher cyclic stiffness. An empirical approach for representing the response of frozen, fine-grained soils under multiple packets of cyclic loading is proposed and then verified by the test data. The results show that the proposed empirical model is able to extract and predict the accumulated and cyclic behaviours under multiple packets of cyclic loading.

Undrained cyclic loading response of lignosulfonate treated high plastic clay

In this study, a non-traditional and environment friendly chemical stabilizer, lignosulfonate has been used to stabilize high plastic clay and its effectiveness has been investigated through a series of static and cyclic loading tests. Unconfined compression tests were conducted for various percentages of lignosulphonate contents (0.5, 0.75, 1.5, 2.0, 2.5 and 3.0%) by weight of dry soil at curing periods of 5, 10 and 21 days. Shear strength and deformation characteristics of stabilized specimens were examined, corresponding to optimum content of lignosulfonate at which unconfined compressive strength was maximum, for different confining pressures and cyclic stress levels under unconsolidated undrained conditions. Test results indicated a significant increase in unconfined compressive strength and shear strength under static loading; whereas, reduction in axial strains and rate of permanent strain development, and increased resilient modulus under cyclic loading. In addition, SEM analysis has also been conducted to understand the microstructural changes occurring in the soil due to addition of lignosulfonate.

Investigation of mechanical and energy evolution characteristics for sandstone after high temperature damage under cyclic loading

During the production of underground engineering, investigating the mechanical behaviors of rock materials after high temperature damage is of great importance for controlling the stability of the underground structure. In this paper, for revealing the deformation and energy evolution laws of rock materials after high temperature damage under cyclic loading, a series of uniaxial compressive and cyclic loading experiments were conducted on sandstone after various high temperature damages, i.e., 200, 300, 400, 500, 600, 700, 800, and 900°C, to study the effect of high temperature on primary wave velocity, microstructure, deformation, strength, energy, etc. It can be observed that the primary wave velocity and microstructure weakened seriously with the development of high temperature damage and the uniaxial compressive strength of samples increases toward the maximum value with a high temperature damage of 300°C and then decreases gradually. The energy proportion index was established to characterize the influence of high temperature damage and cyclic stress levels on energy evolution laws of samples. With an increase in temperature, compared with input energy, elastic strain energy proportion decreases and dissipated energy proportion rises up, which indicate that the property of samples transforms from elasticity to plasticity. Finally, the variation laws between the burst tendency of the sample and high temperature damage were well described by presenting the average elastic energy index, which provides references for studying failure characteristics of rock materials.

Undrained Cyclic Response and Resistance of Saturated Calcareous Sand considering Initial Static Shear Effect

Sand elements in the natural or manmade field have often undergone initial static shear stresses before suffering cyclic loading. To explore the effect of static shear stress, a series of undrained cyclic triaxial tests were performed on dense and loose calcareous sand under different initial and cyclic shear stresses. The triaxial test results are used to describe the effect of static shear stress on the cyclic response of the calcareous sand with different relative density. Cyclic mobility, flow deformation, and residual deformation accumulation are the three main failure modes under varying static and cyclic shear stress levels. The cyclic resistance of dense sand is greater than that of loose sand, but the initial static stress has different effects on the cyclic resistance of the two kinds of sand. The dense sand owns a higher cyclic resistance with SSR increasing, while for the loose sand, 0.12 is the critical SSR corresponding to the lowest value of the cyclic resistance. The dense sand has more fast accumulation of dissipated energy, compared with loose sand. Additionally, an exponential relationship is established between static shear stress, relative density, and normalized energy density.

Numerical and experimental investigation into the fatigue life of FRP bonded to concrete and anchored with bidirectional fabric patches

Although extensive research has been conducted into the use of fibre reinforced polymers (FRPs) in strengthening of concrete structures under static loads, their fatigue behaviour when subjected to cyclic loading still requires investigation. The issue of fatigue performance of FRP strengthening systems is mostly relevant to the strengthening of concrete bridges which are subjected to significant traffic loading cycles throughout their lifetime. In order to offset some of the shortcomings of externally bonded FRPs, anchorage systems are commonly used in bridge strengthening applications in conjunction with FRPs externally bonded to concrete in order to mitigate premature debonding failure. This paper presents a summary of numerical and experimental investigations conducted to evaluate the fatigue performance of externally-bonded FRP laminates anchored with bidirectional fibre patch anchors. The anchored laminates were bonded to reinforced concrete (RC) blocks and subjected to various cyclic loading scenarios. Parameters such as the stress range, peak cyclic stress level, and the corresponding number of cycles prior to failure were noted. The results were used to generate an S-N curve relationships. A finite element model was developed and calibrated based on the experimental results. Good correlation was achieved between the experimental and FE results in relation to the maximum number of cycles, strain distribution, and mode of failure. The outcomes of this study showed that no fatigue degradation was observed when the peak cycling stress level was less than 60% of the ultimate static capacity. Further details of the results and recommendations for future research work are also provided.

Remodeling of an in vitro microvessel exposed to cyclic mechanical stretch.

In the lungs, vascular endothelial cells experience cyclic mechanical strain resulting from rhythmic breathing motions and intraluminal blood pressure. Mechanical stress creates evident physiological, morphological, biochemical, and gene expression changes in vascular endothelial cells. However, the exact mechanisms of the mechanical signal transduction into biological responses remain to be clarified. Besides, the level of mechanical stress is difficult to determine due to the complexity of the local distension patterns in the lungs and thus assumed to be the same as the one acting on the alveolar epithelium. Existing in vitro models used to investigate the effect of mechanical stretch on endothelial cells are usually limited to two-dimensional (2D) cell culture platforms, which poorly mimic the typical three-dimensional structure of the vessels. Therefore, the development of an advanced in vitro vasculature model that closely mimics the dynamic of the human lung vasculatures is highly needed. Here, we present the first study that investigates the interplay of the three-dimensional (3D) mechanical cyclic stretch and its magnitude with vascular endothelial growth factor (VEGF) stimulation on a 3D perfusable vasculature in vitro. We studied the effects of the cyclic strain on a perfusable 3D vasculature, made of either human lung microvascular endothelial cells or human umbilical vein endothelial cells embedded in a gel layer. The in vitro 3D vessels underwent both in vivo-like longitudinal and circumferential deformations, simultaneously. Our results showed that the responses of the human lung microvascular endothelial cells and human umbilical vein endothelial cells to cyclic stretch were in good agreement. Although our 3D model was in agreement with the 2D model in predicting a cytoskeletal remodeling in response to different magnitudes of cyclic stretch, however, we observed several phenomena in the 3D model that the 2D model was unable to predict. Angiogenic sprouting induced by VEGF decreased significantly in the presence of cyclic stretch. Similarly, while treatment with VEGF increased vascular permeability, the cyclic stretch restored vascular barrier tightness and significantly decreased vascular permeability. One of the major findings of this study was that a 3D microvasculature can be exposed to a much higher mechanical cyclic stress level than reported in the literature without any dysfunction of its barrier. For higher magnitudes of the cyclic stretch, the applied longitudinal strain level was 14% and the associated circumferential strain reached the equivalent of 63%. In sharp contrast to our findings, such strain typically leads to the disruption of the endothelial barrier in a 2D stretching assay and is considered pathological. This highlights the importance of 3D modeling to investigate mechanobiology effects rather than using a simple endothelial monolayer, which truly recapitulates the in vivo situation.

Bending fatigue behavior of thin Zr61Ti2Cu25Al12 bulk metallic glass beams for compliant mechanisms application

In this work, bending fatigue behavior of thinner Zr61Ti2Cu25Al12 (ZT1) BMG beams with thicknesses of 500 μm and 100 μm were investigated with three-point bending (3PB), to evaluate the reliability and safety of this potentially material used in compliant mechanisms under cyclic bending load. Fatigue endurance limits (FELs) of ZT1 beams with thicknesses of 500 μm and 100 μm were determined to be 470 MPa, and ∼0.3 of its tensile strength. Fatigue life and FEL of ZT1 beams in bending are substantially independent of the beam thickness, in the scale from 500 μm down to 100 μm. Fatigue cracks of all ZT1 beams initiated at the extrinsic microvoids, either indirectly derived from surface scratches during mechanical polishing or originated from as-cast process. However, fatigue crack propagation behavior is highly related to the magnitude of cyclic stress level. By interrupted fatigue tests, larger proportion about 60–70% of fatigue life was spent on crack propagation under high stress level, while crack initiation occupied larger proportion about 80–85% under low stress level. The medium stress level of about 590 MPa almost shared the crack-initiation life and crack-propagation life. In addition, there are two crack deflection mechanisms during cracks steady propagation stage. One mechanism is crack deflection along the interacted shear bands ahead of crack-tip, the other one is crack “jump” from one shear band to another shear band at a certain angle. These two mechanisms alternate dominate the crack propagation process, generating typical staircase-like crack propagation trajectory. Fatigue endurance limits of ZT1 beams in the form of either stress amplitude or strain amplitude are superior to traditional candidate materials in the field of compliant mechanisms, showing that the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG even under cyclic loading.

Mechanical Responses of a Deeply Buried Granite Exposed to Multilevel Uniaxial and Triaxial Cyclic Stresses: Insights into Deformation Behavior, Energy Dissipation, and Hysteresis

This article presents the results for cyclic uni/triaxial tests on the deeply seated granite samples drilled from a −915 m deep tunnel in Sanshandao (SSD) gold mine. The monotonic and cyclic tests were carried out to observe the mechanical responses of the granite samples under different loading regimes. The disparities concerning the strain evolution and compressive strength of granite samples considering monotonic and cyclic uniaxial and triaxial loading are presented. Deformation behaviour, dissipated energy, and hysteresis are documented and evaluated. Quantitative correlations between strain evolution and cyclic stress levels are revealed. The amount of energy transformation during uniaxial and triaxial cyclic loading is determined. The impacts of confining pressure level on ultimate strain, energy dissipation, and stress‐strain phase shift are presented. The mechanical responses of the granite samples subjected to different stress paths and loading strategies are summarised, and corresponding interpretations are given to clarify the differences of mechanical behaviour encountered in distinct loading methods.

Strengthening effects of cyclic load on rock and concrete based on experimental study

Cyclic load affects the mechanic behavior and stability of rock mass. Although cyclic load effect has been extensively studied for yielding stage, it is somewhat unclear in early stage of rock stress-strain curves. This paper describes an experimental study, carried out cyclic load test in elastic deformation phase, on sandstone, limestone, and concrete specimens, to research the mechanical behavior. The result showed strengthening effect of cyclic loads in elastic deformation stage within the first 100 cycles of the cyclic loads. The irreversible deformation under cyclic loads displayed similarity to creep curves, and it increased along with cyclic unloading stress level. Further analysis on cyclic times and unloading stress level demonstrated that there's threshold of strengthening effects under cyclic loads in elastic deformation stage, which could be developed in two aspects: (1) Strength increased with cyclic times until it reached an optimal cycling time then it decreased, and (2) the strength initially increased with unloading stress level then it decreased when the optimal unloading stress level was attained. The optimal cyclic times and optimal unloading stress were calculated according to quadratic function. The result showed that the optimal cyclic times increased with the decrease of unloading stress level. The optimal cyclic unloading stress level is about 30% of elastic limitation stress under same cycling times. Different specimens showed different strengthening potential under uniaxial cycling load since the initial porosity of specimens are different. The conclusion offered by this work would be beneficial to decisions of geoengineering and constructions.

Fatigue behaviour and constitutive model of yellow sandstone containing pre-existing surface crack under uniaxial cyclic loading

In engineering practice, rock masses are often subjected to cyclic loading. Under cyclic loading, the mechanical properties of rocks are significantly different from those under conventional loading conditions. The mechanical behaviour and fatigue of intact rock has been extensively studied. However, the mechanical fatigue properties of rock masses with surface cracks and related damage evolution models are not well understood. In this study, a series of fatigue damage tests were carried out on yellow sandstone specimens; five different pre-existing surface crack angles (0°, 15°, 30°, 45° and 60°) and three different maximum cyclic stress levels (19.1 MPa, 25.6 MPa and 30.3 MPa) were considered. The results showed that with the increase in the number of cycles, Young's modulus gradually stabilized, while the secant modulus decreased non-linearly. The maximum strain and residual strain continuously accumulated. The influence of macro cracks on the damage of rocks was studied through the increase in strain energy. Based on the macro–micro coupling damage, a constitutive model was proposed for the fatigue test. The theoretical results were fitted with the experimental data, showing that the proposed model can effectively describe the fatigue damage characteristics of fractured rock masses. The research results have certain reference value for the long-term stability evaluation in fractured rock mass engineering.

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.

Strain response and energy dissipation of floating saline ice under cyclic compressive stress

Abstract. Understanding the mechanical behavior of sea ice is the basis of applications of ice mechanics. Laboratory-scale work on saline ice has often involved dry, isothermal ice specimens due to the relative ease of testing. This approach does not address the fact that the natural sea ice is practically always floating in seawater and typically has a significant temperature gradient. To address this important issue, we have developed equipment and methods for conducting compressive loading experiments on floating laboratory-prepared saline ice specimens. The present effort describes these developments and presents the results of stress-controlled sinusoidal cyclic compression experiments. We conducted the experiments on dry, isothermal (−10 ∘C) ice specimens and on floating-ice specimens with a naturally occurring temperature gradient. The experiments involved ice salinities of 5 and 7 ppt, cyclic stress levels ranging from 0.04–0.12 to 0.08–0.25 MPa and cyclic loading frequencies of 0.001 to 1 Hz. The constitutive response and energy dissipation under cyclic loading were successfully analyzed using an existing physically based constitutive model for sea ice. The results highlight the importance of testing warm and floating-ice specimens and demonstrate that the experimental method proposed in this study provides a convenient and practical approach to perform laboratory experiments on floating ice.

The effect of stress state on rafting mechanism and cyclic creep behavior of Ni-base superalloy

A three-stepped specimen of Ni-base superalloy was designed in the paper to conduct cyclic creep tests under different stress levels. The experiment aimed at studying the effect of loading cyclic numbers and the stress state on the rafting mechanism and the cyclic creep behavior. The design of three-stepped specimen can help to simultaneously analyze the extent of microstructure evolution under different stress states, which included uniaxial states in gauge sections and multiaxial stress states in beveled sections. Metallographic observations revealed that both the rafting behavior and topological inversion behavior of γ/γ′ microstructure occurred during the cyclic creep tests. The specific connectivity number of the γ′ phase NA (γ′) was introduced to assess the extent of these microstructure evolutions, which was found to be related to the loading cyclic numbers and stress levels. Then a parameter of normalized cyclic number Tr was proposed to consider the contribution of these two factors. The extent of microstructure evolution was observed increasing with the normalized cyclic number. And the relationship between evaluation parameters NA (γ′) and Tr was proved to follow an asymptotic equation. The finite element model that established in the paper assisted to qualitatively analyze the rafting or topological inversion process. It successfully predicted the cyclic lives and acquired the normalized cyclic number Tr. The magnitude and distribution gradient of principal stress field in finite element model effectively account for rafting behavior under different stress states.

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.

Dependence of fatigue limit on stress ratio and influence of cyclic stress on shear strength for an adhesive lap joint

ABSTRACT The improvement in adhesion technology for joining different types of materials has led to expectations of weight reduction in cars and commercial vehicles. Various levels of stress and stress ratios are loaded onto the joints of such transport vehicles. Therefore, the fatigue shear strength of an adhesive lap joint is important. This study is related to the fatigue properties of structural adhesive joints. Lap-joint specimens using epoxy adhesive were prepared, and fatigue tests with four different stress ratios were conducted. Subsequently, the fatigue limit diagram was plotted, and the effect of stress ratio on the fatigue limit was investigated. Moreover, the strength of the specimens subjected to various cyclic stress levels was investigated systemically. With cyclic shear stress, the influence of the shear stress amplitude on the resistance to fatigue failure was more significant than that of the number of cycles. The specimens that endured the fatigue tests maintained approximately the same shear strength as that of the as-received specimens. The findings of this study can provide important guidance for the design of a car body.

Nonlinear damping properties of recycled aggregate concrete short columns under cyclic uniaxial compression

Although recycled aggregate concrete (RAC) has been studied intensively over past two decades, its damping property is still not analyzed sufficiently. In this paper, a nonlinear damping model for RAC prisms subjected to cyclic uniaxial compression are presented. Then with the combination of the compressive strength and damping property of RAC, an optimum recycled concrete aggregates (RCA) content including particle size and replacement percentage are recommended. The cyclic uniaxial compression test was conducted to investigate the dynamic modulus and nonlinear damping property of RAC, meanwhile tests for the RAC’s static compressive strength and modulus of elasticity were also carried out. Then based on the nonlinear damping model and compressive strength, relationships among the compressive strength, damping property, load indicators and RCA content are analyzed to obtain the optimum RCA content. Test results indicate that RAC obtains a higher loss factor than NAC. The loss factor of RAC is an increasing power function of RCA particle size and a decreasing power function of RCA replacement percentage. However, the compressive strength and dynamic modulus of RAC decreases compare with that of natural aggregate concrete. The different choice of cyclic uniaxial compression load indicators (cyclic stress range, stress level or strain range) has significance impact on RAC damping property and optimum RCA content.

Fatigue of carbon cord-rubber composites: Effect of frequency, [formula omitted] ratio and life prediction using constant life models

The influence of frequency and R ratio on tensile fatigue of carbon cord reinforced HNBR composites has been studied. Frequency was found to have negligible effect on fatigue life, due to minimal heat build-up even at the highest frequency. Longer fatigue life at higher R ratios at a given maximum cyclic stress level was associated with the dynamic process of forming and melting of crystallites during loading and unloading, leading to a crack-arresting effect. The applicability of various analytical constant life diagram (CLD) models showed that the modified Harris’s CLD and piecewise linear CLD gave a reasonably good agreement with experimental fatigue data.

Viscoplastic Ratcheting Response of Materials under Step-Loading Conditions at Various Cyclic Stress Levels

The present study intends to assess ratcheting response of rate-dependent SS316L, SS316L(N), and 63Sn37Pb alloys under various step-loading conditions at room temperature through use of the Ahmadzadeh-Varvani (A-V) model developed based on a viscoplastic flow rule. The choice of viscoplastic description enabled the model to control plastic strain increments over loading progress at various stress rates. Predicted ratcheting over asymmetric loading cycles and multi-step cycles was influenced by cyclic stress levels, stress rates, and sequence of loading steps. Predicted ratcheting in steel samples SS316L and SS316L(N) over the second loading step in Low-High and High-Low loading sequences was primarily influenced with the stress amplitude in the second step. Multi-step-loading sequences High-Low-High and Low-High-Low in solder samples revealed how noticeable the impact of mean stress on ratcheting is. Predicted ratcheting values by means of the A-V model closely agreed with experimental data of alloys examined in this study.

The pore pressure and deformation behavior of natural soft clay caused by long-term cyclic loads subjected to traffic loads

The undrained deformation behavior of soft clay, may be subjected to traffic loads, and has drawn extensive concern. In order to study the pore water and stress–strain behavior of soft clay subjected to traffic loads, one-way cyclic laboratory experiments were designed and investigated by a hollow cylinder apparatus (HCA). Different cyclic stress levels and confining pressures are considered in this article. It can be concluded that pore pressure and axial strain of the samples are dependent on cyclic stress ratios (CSR) and effective confining pressures significantly. With higher CSR and effective confining pressures applied, the values of pore pressure and axial permanent strain increase. The empirical formulas are established to predict the value of axial permanent strain and pore water pressure. Moreover, the axial and torsional degradation index of soft clay, as well as the axial and torsional stress–strain relationships, are obviously dependent on CSR.

Study on characteristics of fatigue strength of ARALL composite in constructive elements

Due to Fatigue Testing fatigue life of specimens made of various materials and with various flange types was estimated. Specimens with dish-shaped and ring-shape flanges were tested. Specimens were made of aluminum alloy D16chATV sheets (thickness 1.2 mm) and of ARALL with a thickness of 1.2 mm based on aluminum layers of 0.5 mm thickness. Specimens were tested under 3 levels of cyclic stress: σmax=120, 100, 80 MPa with minimum stress of σmin=20 MPa. Some of ARALL specimens were tested only under σmax=100 MPa stress level. Under each stress level were tested 5 – 6 specimens. According to the results of all specimen tests the authors plotted plotted fatigue curves. Using the Finite Element Method the stress state of specimens during cyclic stress tests was analyzed. Calculations of fatigue life of specimens were made on the basis of the offered method. Full-element and numerical experiments revealed that fatigue resistance of monolithic alloy specimens with dish-shaped flange under all stress levels is higher than of specimens with a ring-shaped flange. ARALL specimens with dish-shaped and ring-shaped flanges both according to test results have a higher fatigue resistance than monolithic alloy specimens.