Stress-controlled Cyclic Loading Research Articles

Fractional cyclic cohesive zone model for time-dependent fatigue behavior of soft adhesives under mode-II loading

Soft adhesives exhibit nonlinear viscoelastic behavior during cyclic loading, suggesting that their cycle-dependent fatigue behavior can be significantly affected by the time-dependent deformation behavior. Soft adhesives are frequently subject to various cyclic conditions in practical applications, presenting potential safety risks. This paper has proposed a fractional cyclic cohesive zone model (CZM) to characterize the time-dependent fatigue behavior of soft adhesives. To capture the nonlinear evolution of fatigue damage under asymmetric stress-controlled cyclic loading, a fatigue damage model that considers the contribution of in-situ stress/strain was incorporated into fractional-order viscoelastic model. The proposed CZM was validated through single-lap shear creep tests at various stress levels and fatigue tests at various stress amplitudes and loading periods. Furthermore, the impacts of stress ratio and peak stress level on fatigue failure under different loading periods were investigated using the proposed CZM. The results indicate that under a short loading period, the failure of the soft adhesives is primarily influenced by cycle-dependent fatigue damage, while under a long loading period, it is primarily affected by time-dependent creep deformation. Overall, this work offers a numerical strategy to describe the fatigue failure of nonlinear viscoelastic soft adhesives and to understand their time-dependent fatigue failure mechanism, with implications for product design and optimization.

Numerical simulation and experimental validation of ratchetting deformation of short fiber-reinforced polymer composites

This paper proposes a micromechanical model based on the finite element method (FEM) to analyze the ratchetting deformation of short glass fiber-reinforced polymer composites (SFRPCs). To reasonably consider the complex mechanical characteristics of the polymeric matrix under asymmetrical stress-controlled cyclic loading, a damage-coupled nonlinear viscoelastic-viscoplastic (VE-VP) cyclic constitutive model was constructed. The cyclic constitutive model of the matrix was introduced into the FEM model utilizing a user-defined material, and the corresponding experimental validation of the matrix was conducted. Subsequently, the cyclic constitutive model was introduced into the analysis of the micro-FEM model of the composites. The random orientation was considered to generate fibers in the FEM. Based on the analysis of the microscale representative volume element (RVE) model, the effects of different fiber content and cyclic peak stress on the ratchetting of SFRPCs were studied in detail. After validation, the effects of the fiber length and fiber stiffness on the ratchetting deformation of SFRPCs are discussed based on the developed micro-FEM model. The results show that the increase in the fiber length and fiber stiffness reduced the ratchetting deformation of SFRPCs within a specific content range. The model can assess the ratchetting deformation and damage evolution within the composites to provide more accurate predictions of the ratchetting deformation of SFRPCs and guide the expanded application of SFRPCs in structures.

A numerical approach for simulation of stress-controlled cyclic shear loading in a soil column considering partial drainage

Liquefaction strength curves (LSCs) are commonly generated based on undrained stress-controlled cyclic testing to define the liquefaction potential of sands. However, these LSCs do not consider the effect of partial drainage on liquefaction potential. This is mainly because there are very limited experimental techniques to run stress-controlled tests for partially drained sands. This manuscript proposes a procedure to numerically simulate a saturated sand column with a free drainage boundary at the top (or top and bottom), accelerated horizontally at the bottom with constant acceleration cycles of duration designed to generate constant cyclic shear stress histories in the column. This is done for columns subjected to low and high overburden pressures. The paper starts with numerical simulations of a typical undrained cyclic direct simple shear, CDSS tests by applying a stepped velocity wave to a single soil element, an already established approach. This approach only works for a single element in undrained condition which is fixed at the base. Then, the proposed procedure is implemented by applying stepped acceleration time histories at the base of soil columns having one or two drainage boundaries. In these stepped acceleration runs, the durations of the acceleration cycles are controlled to achieve a partially drained stress-controlled CDSS kind-of-loading at specific elevations within the soil columns. The technique is used to show the effect of partial drainage on sand liquefaction behavior at both low and high overburden pressures. The results show that the effect of drainage is much more significant at higher overburden. The existence of both top and bottom drainage boundaries resulted in less liquefaction vulnerability, as compared to having only one drainage boundary at the top of the sand column. The results show that liquefiable sand layers in the field may be less prone to liquefaction under high overburden pressure than predicted by the current state-of-practice, which relies mainly on undrained small-scale cyclic tests.

High temperature strain characteristics and life prediction of Sanicro25 alloy under stress-controlled cyclic loading

High temperature strain characteristics and life prediction of Sanicro25 alloy under stress-controlled cyclic loading

Fatigue crack propagation in fine-grained magnesium under low temperature tension-tension cyclic loading

Fine-grained magnesium was tested under stress-controlled tension-tension cyclic loading at -30 °C and the tested sample was observed using scanning electron microscope and electron backscatter diffraction to explore the fatigue behavior and crack propagation. The fatigue data showed that the material experienced cyclic softening followed by cyclic hardening before the final fracture failure. The microscopic observations demonstrated that the cracks were almost perpendicular to the loading direction with some zigzags and the cracks progressed along both small angle grain boundaries and large angle grain boundaries. Although the cracks were mainly propagated along large angle grain boundaries, the value of grain boundary angle was not the primary factor to determine the crack propagation direction. The local residual strain from the rolling process was released due to the crack propagation and there was more strain relaxation at regions closer to the cracks.

Piezomagnetic Behavior Corresponding to the Variations of the Toughness of X70 Steel Under Cyclic Loadings

The goal of this study is to investigate magnetic field characteristics corresponding to variations of the toughness of X70 pipeline steel under cyclic loading. After a series of stress-controlled cyclic loadings, the specimens were subjected to static tensile tests. Changes in the magnetic field were recorded simultaneously during the test. The results suggest that cyclic loadings will reduce the toughness of the material. The initial value and slope of the magnetic field-strain curve obtained in the static tensile test are related to the number of cyclic loadings. The evolutions of magnetic field and the stress-strain curve can reflect variations in the toughness of materials. Compared with the stress-strain curve, the magnetic field demonstrates the fatigue degradation process more markedly. The high sensitivity of the magnetic field to toughness may be used to estimate the degree of fatigue damage of steel.

The effect of stress amplitude on ratcheting strain development in an extruded AZ31B magnesium alloy under different mean stresses

The ratcheting behavior in an extruded AZ31B magnesium alloy was investigated under uniaxial stress-controlled cyclic loading at room temperature. The results showed that basal <a> slip, prismatic <a> slip and {10–12} extension twinning/detwinning are the deformation mechanisms accommodating cyclic strain. The weight of the slips or the twinning/detwinning that relates to stress amplitude affects the valley strain and ratcheting strain. The increase of the weight of the dislocation slips results in an increment of strain hardening rate. When the stress amplitude is 105 MPa, the hardening rate is higher than that at the amplitudes of 45 MPa and 75 MPa, which leads to a ratcheting strain development deviating from the regular pattern.

Re-liquefaction resistance of lightly cemented sands

The re-liquefaction resistance of cemented sands under multiple liquefaction events such as pre-shock, main-shock, and after-shock earthquakes is a complex phenomenon because the response may alter due to bond breakage. A series of multistage liquefaction–re-consolidation soil element tests under undrained stress-controlled cyclic loading condition using cyclic triaxial were carried out to assess the liquefaction and re-liquefaction resistance of cemented sands with varying degrees of cementation. Lightly cemented specimens were reconstituted using Sydney sand and high early strength Portland cement with cement content ranging from 0.25% to 1% and unconfined compression strength from 15 to 80 kPa. The results showed that the re-liquefaction resistance of cemented sands with different amounts of cement decreased after the first liquefaction event and then increased for succeeding liquefaction events. While the trend of residual excess pore water pressure ratio and cyclic stiffness degradation index of untreated sand under successive liquefaction events remained consistent, the corresponding responses for cemented sands altered for the second to the fifth liquefaction events. In fact, the residual excess pore water pressure ratio and cyclic stiffness of cemented sand increased and degraded faster during the early cycles of loading for the second to fifth liquefaction events.

Inspection of ratcheting models for pathological error sensitivity and overparametrization

Accurate analysis of plastic strain accumulation under stress-controlled cyclic loading is vital for numerous engineering applications. Typically, models of plastic ratcheting are calibrated against available experimental data. Since actual experiments are not exactly accurate, one should check the identification protocols for pathological dependencies on experimental errors. In this paper, a step-by-step algorithm is presented to estimate the sensitivities of identified material parameters. As a part of the sensitivity analysis method, a new mechanics-based metric in the space of material parameters is proposed especially for ratcheting-related applications. The sensitivity of material parameters to experimental errors is estimated, based on this metric. Moreover, a relation between pathological error sensitivity and overparametrization is established. This relation gives rise to a new criterion of overparametrization. The advantages of the new overparametrization criterion are exposed and its plausibility is checked by alternative criteria, like the consideration of correlation matrices and validation of identified parameters on “unseen” data. For demonstration purposes, the accumulation of irreversible strain in the titanium alloy VT6 (Russian analog of Ti-6Al-4V) is analysed. Three types of phenomenological models of plastic ratcheting are considered. They are the Armstrong-Frederick model as well as the first and the second Ohno-Wang models. Based on real data, a new rule of isotropic hardening is proposed for greater accuracy of simulation. The ability of the sensitivity analysis to determine reliable and unreliable parameters is demonstrated.

Effect of Nano-Clay Particles and Heat Treating on Pure and Fretting Fatigue Properties of Piston Aluminum Alloy under Stress-Controlled Cyclic Bending Loading

Effect of Nano-Clay Particles and Heat Treating on Pure and Fretting Fatigue Properties of Piston Aluminum Alloy under Stress-Controlled Cyclic Bending Loading

Fatigue behavior of modified ZK60 magnesium alloy after pre-corrosion under stress-controlled loading

Three kinds of modified layers were prepared on the surface of ZK60 magnesium alloy (BM) by micro-arc oxidation (MAO), laser shock penning (LSP), and composite of LSP and MAO method. Four kinds of specimens (BM, MAO, LSP, LSP / MAO) were pre-corroded in a simulated body fluid (SBF) with pH7.4 for 12 h. Fatigue tests were performed for non-corroded and pre-corroded specimens under stress-controlled cyclic loading. The cyclic deformation behavior of the specimens at different stress ratios and different stress amplitudes involves four deformation mechanisms: dislocation slip, twinnig-detwinning transfer to dislocation slip, partial twinning-partial detwinning, complete twinning-complete detwinning. The correlation between the cyclic deformation mechanism and fatigue life is discussed using a proposed evaluation parameter, β. The fatigue life of the material decreases with the β values increasing when cyclic deformation is dominated by twinning-detwinning. The proposed total strain energy model can better predict the fatigue life of modified magnesium alloys under different loading conditions.

Modeling cyclic deformation of austenitic stainless steels at elevated temperatures using a physically-based mesoscale crystal plasticity framework

Austenitic stainless steels demonstrate significant changes in key macroscale deformation responses at elevated temperatures; these changes are relatively well-understood at micro- and macroscales in the framework of dynamic strain aging (DSA). At the microscale, temperature-dependent behavior is attributed to enhanced solute mobility and subsequent pinning of dislocations by solute atmospheres. At the macroscale, phenomenological equations are used to approximate microscale material behavior. However, the connection between unit dislocation processes at the microscale and macroscale deformation behaviors is unclear. In the current study, a dislocation density-based crystal plasticity model is used to relate micro- and macroscale responses for cyclic loading of austenitic stainless steels at elevated temperatures. Specifically, the variation in deformation behavior at elevated temperatures is attributed to modifications in the heterogenous distribution of dislocations on the mesoscale. The proposed framework demonstrates good agreement with experiments for strain- and stress-controlled cyclic loading scenarios. The model is also shown to be able to address key phenomena associated with DSA, i.e., negative strain rate sensitivity and stress-strain curve serrations.

Experimental study and life prediction on fatigue failure of NiTi shape memory alloy under multi-axial one-way shape memory cyclic loadings

In this paper, the thermo-mechanical coupling fatigue experiments of NiTi shape memory alloy (SMA) micro-tubes involving one-way shape memory effect (OWSME) are first carried out under the uniaxial and multiaxial stress-controlled cyclic loadings with some specific loading paths and stress levels. Experimental results show that the OWSME cyclic degradation and fatigue lives of NiTi alloy micro-tubes are dependent on the prescribed loading paths and stress levels; and the multi-axial loadings accelerate the reorientation-induced ratchetting, resulting in more obvious OWSME cyclic degradation in NiTi SMA micro-tubes. Based on the experimental results, the dissipation energy is defined as the damage variable, and the damage evolution equations of mechanical loading and thermal loading parts are established from the correspondent experimental results by considering the influence of loading path. Then, combined with the established failure criterion, the fatigue lives of the NiTi alloy presented during the multi-axial OWSME cyclic loadings are predicted. It is demonstrated that all the predicted lives are located within the twice scatter band, and most of them are within the scatter band of 1.5 times; the predicted lives of the proposed multi-axial fatigue life-prediction model agree with the experimental ones well.

Fatigue Performance of Concrete–Polyurethane Composite Materials under Compression

Polyurethane grouting technology has been applied in the maintenance and reinforcement of transportation infrastructure such as airports, expressways, and high-speed railways. For rigid pavements with voids underneath concrete slabs, it can make a quick and efficient fix. However, polyurethane grouting materials applied underneath slabs for base enhancement are subjected to cyclic compressive loading from traffic, so their fatigue performance is critical in the concrete–polyurethane composite structure. There have not been many studies of this aspect. This paper investigated the influence of several key design factors, including grout thickness, grout density, and void shape, on the stress–strain and fatigue performance of the concrete–polyurethane composite structure. Laboratory compressive tests were conducted at various stress levels following a four-factor, three-level orthogonal experiment design. The results showed that the strain response of composite specimens under stress-controlled cyclic loading may be divided into three stages: an initial stage in which the cumulative strain changes little with the number of load repetitions, a second stage in which the cumulative strain increases steadily with the number of load repetitions, and a third stage in which the cumulative strain increases rapidly with the number of load repetitions. The third stage represents the occurrence of fatigue failure. The rank of the influences of the four factors on fatigue performance is, from high to low, grout thickness, stress ratio, void shape, and grout density.

Micromechanical crystal plasticity back stress evolution within FCC dislocation substructure

Experiments and theories have suggested the evolution of intragranular back stress is primarily dependent on the characteristics and development of dislocation substructure in face-centered cubic (FCC) metals and alloys. Despite this, continuum modeling of complex cyclic loading phenomena such as inelastic ratchet strain accumulation typically appeals to one of several phenomenological back stress forms with weak connections to physical mechanisms. In the current work, a micromechanically-based back stress evolution law is derived and implemented in a crystal plasticity framework for FCC metals that directly considers the evolution of dislocation substructure. The model is used in a case study of stainless steel 316L (SS316L), and good agreement is found with experiments on single- and poly-crystalline specimens subjected to monotonic, fully-reversed cyclic loading, and stress-controlled cyclic loading with mean stress. Ratcheting is attributed to dislocation substructure formation, dissolution, and stabilization within this physically-based framework.

Influences of roughness and heat treatment on high-cycle bending fatigue properties of A380 aluminum alloy under stress-controlled cyclic loading

In the present study, the effect of the T6 heat treatment on high-cycle bending fatigue properties of the A380 (AlSiCu) aluminum alloy has been investigated. The fracture behavior of materials was also examined under stress-controlled cyclic loadings. For such an objective, standard specimens were prepared and tested by a rotating bending fatigue machine. In addition, the microstructural characterization and the fracture surface analysis were performed by the scanning electron microscopy. Using the T6 heat treatment, the solution treatment was performed at 510°C for 5 hrs and after quenching in the air, the ageing treatment was carried out at 200°C for 3 hrs. The main aim of this work is to investigate the heat treatment effect on fatigue properties of AlSiCu alloys. Moreover, the roughness influence was also studied on the fatigue lifetime. Based on experimental results, the heat treatment changed the distribution and the size of silicon phases and enhanced the microhardness. Under the highest and lowest stress levels, the heat treatment increased the high-cycle bending fatigue lifetime by 26% and 85%, respectively. Samples with low roughness had a greater fatigue lifetime than those with high roughness. The fracture surface analysis demonstrated that in both as-cast and heat-treated specimens, several microcracks occurred through intermetallic compounds. The heat-treated and as-cast materials showed a brittle fracture behavior due to cleavage planes on the fracture surface of specimens. However, some dimples and quasi-cleavages were also observed on the fracture surface of as-cast samples.

Microscopic substructures of stainless steel 304 undergoing a uniaxial ratcheting deformation

Under asymmetrical stress-controlled cyclic loading accompanied by ratcheting, the evolution of microscopic substructures of stainless steel 304 (SS304), a face-centered cubic metal, was detected using transmission electron microscopy (TEM). This observation demonstrates that dislocation slip is the main mechanism of uniaxial ratcheting in the non-steady stage (stage I). Dislocation proliferate rapidly duo to the high stress level of the cyclic tests, and dislocation density increases continuously with the the number of cycles. The ratcheting rate decreases continuously due to the interaction of moveable dislocations, and the planar dislocation substructures (dislocation pileups and tangles) are the dominant patterns in this stage. In the later stage I, distinct lath $$\alpha $$ -martensite was observed in the material due to the nucleation of martensite and the phase transformation zones increase gradually with the growth of axial ratcheting strain in the stage II of uniaxial ratcheting. The multiply and cross-slip were activated gradually in increasing numbers of grains, and the prevailing dislocation configurations evolve into more complicated and stable ones (dislocation walls and cells). In addition the dislocation configurations after various prescribed tensile strain of the monotonic tension and creep deformation with three different holding times were also observed by TEM. Comparing the evolution of microscopic substructures in various loading modes, the physical mechanism of uniaxial ratcheting of SS304 can be revealed as combination of the evolution of dislocation configurations and martensite transformation in the stage II of uniaxial ratcheting.

Evolution of transformation characteristics of shape memory alloys during cyclic loading: transformation temperature hysteresis and residual martensite

This paper investigates the evolutions of transformation temperatures and residual martensite of shape memory alloys during cyclic loading. Pseudoelastic NiTi specimens were firstly subjected to stress-controlled cyclic loading; then differential scanning calorimetry and x-ray diffraction characterizations were conducted on samples cut from specimens after different cycles as well as virgin one. Results show that (i) transformation temperatures rise while the transformation temperature hysteresis increases at initial cycles and then remarkably decreases when the SMA is highly distorted; (ii) due to the accumulation of residual martensite, the more fatigue cycles applied to the SMAs, the less latent heat absorbed/released during the transformation. Furthermore, residual martensite volume fraction was calculated with respect to plasticity and a linear character was observed.

A crystal plasticity-based constitutive model for ratchetting of cyclic hardening polycrystalline metals

A crystal plasticity-based constitutive model is developed by modifying the second item of Armstrong–Frederick (A–F) nonlinear kinematic hardening rule and introducing self-hardening and latent hardening representing the dislocation interaction between slip systems. With the help of β scale-transition rule, the cyclic stress–strain responses of polycrystalline metals can be obtained from the single crystal constitutive model. Then the developed model is applied to describe the cyclic deformation behavior of a face-centered cubic polycrystalline metal, i.e., 316L stainless steel. The predicted results of the model compared with the experiments of 316L stainless steel show that the model can not only simulate the uniaxial and multiaxial ratchetting of the face-centered cubic crystal material during the asymmetrical stress-controlled cyclic loading, but also describe the cyclic hardening characteristics of materials under symmetrical strain-controlled cyclic loading. Meanwhile, the model is capable of predicting uniaxial ratchetting of different orientations at single crystal scale.

The application of Chaboche model in uniaxial ratcheting simulations

Abstract This article presents the application of Chaboche nonlinear kinematic hardening model in simulations of uniaxial ratcheting. First, the symmetrical strain-controlled cyclic tension/compression tests for PA6 aluminum samples were done. Using the experimental stress–strain curve, initial material hardening parameters were determined by the ABAQUS software. The experimental curve was compared with the numerical one. For better fitting of both curves, the optimization procedure based on the least-square method was applied. Using the determined hardening parameters, numerical simulations of the ratcheting were done by the finite element analysis software. Numerical results were then compared with the experimental data obtained in the stress-controlled cyclic loading test.