Constant Stress Amplitude Research Articles

Effect of corrosion environments on the mechanical properties of friction stir welded aluminum alloy AA3003

The article presents an experimental study on the corrosion resistance of the 3003 Aluminium alloy Friction Stir Welded (FSW) joint in different working environments. Three types of corrosive media at different temperatures were tested to see how they affected the FSW joint mechanical properties. The media were sodium chloride, hydrogen chloride, and ethylene glycol. In parallel with the study of the effect of temperature, static tensile tests were carried out on specimens taken perpendicular to the welding direction. Also, micro-hardness profiles were used to determine the influence of the parameters studied on the various weld zones. Finally, the effect of the most aggressive corrosive medium, temperature, and the inclusion of ethylene glycol on the crack propagation resistance of an FSW joint were studied. Fatigue-propagation tests at constant stress amplitude were carried out to understand what affects the fatigue crack propagation of welded joints after corrosive attack. The results showed that uniform and pitting corrosion were combined to cause FSW joint surface degradation. Corrosion damage does not appear to alter yield strength and ultimate stress values. However, FSW joints show a significant degradation in mechanical properties with increasing temperature and with the aggressiveness of the corrosive medium. The inclusion of ethylene glycol was found to delay the rate of crack propagation during testing, leading to an improvement in the service life of pre-corroded FSW joints.

An activation mechanism for cyclic degradation of clays in bounding surface plasticity

AbstractDuring undrained cyclic loading, clayey soils experience substantial stiffness and strength degradation when subjected to shear amplitudes exceeding a critical threshold. This paper presents an enhanced bounding surface rate‐independent plasticity model, an evolution of the previous SANICLAY model, tailored to capture this specific behavior during cyclic loading. A distinguishing feature of the proposed model is the introduction of an activation mechanism. This mechanism triggers degradation modeling based on the applied cyclic shear amplitude. To measure this amplitude, the activation mechanism incorporates a novel state variable that serves as a proxy for the applied cyclic stress. The effectiveness of the proposed model is demonstrated by comparing it to experimental data from various materials subjected to cyclic shearing under undrained conditions. The study encompasses a broad range of constant strain or stress amplitudes. Compared to the reference model, the proposed model exhibits improved predictive accuracy for the stress‐strain response of clays at small amplitudes of cyclic loading and large number of cycles. Furthermore, it accounts for strength degradation due to cyclic loading.

Study on dynamic characteristics and non-constant fractional dynamic creep model for frozen silty clay

The long-term dynamic characteristics of frozen soil are important theoretical basis for the dynamic stability evaluation of geoengineering in cold regions. Compared to unfrozen soil, the dynamic creep behaviour is more complicated owing to its rheological property. In this study, triaxial tests under cyclic loads with different constant stress amplitudes and confining pressures for frozen silty clay (FSC) are carried out. The long-term dynamic creep process and deformation mechanism under different dynamic stress amplitudes were investigated. The test results show that with the cyclic numbers increasing, the dynamic elastic modulus and the hysteretic loop area decrease because of the damage accumulation in the samples. Also the dynamic strength decreases with an increase in failure cyclic numbers under different confining pressures. Based on the fractional calculus theory, replacing the Newton's dashpot in the traditional Maxwell model with fractional Abel's dashpot, a fractional dynamic creep model is established. Considering the melting and crushing of the ice inclusion, the slip effect in frozen soil is increasingly significant, the viscosity coefficient of dashpot element is decreasing with an increase in loading time. In the proposed model, a non-constant dashpot element is introduced to clarify the constitutive relation of the FSC in the accelerated creep stage. The comparison results confirm that the proposed constitutive model is valid and suitable for reflecting the long-term dynamic creep behaviours of the FSC.

High-Cycle Fatigue Life Behaviour of Fabricated Glass Fibre-Reinforced Polymer

This study focuses on the fatigue behaviour analysis of glass fibre-reinforced polymer (GFRP) composite specimens under high-cycle fatigue loading conditions. Therefore, property validation is recommended in the material development process upon further investigation of the fabricated GRFP. This study aims to evaluate the behaviour of the fabricated GFRP fatigue specimen when subjected to high-cycle fatigue loads and compare it to existing studies. A GFRP fatigue test sample was fabricated using the hand layup process into a flat rectangular panel, which was then cut into a small dimension of 28×2×0.2 cm fatigue specimen. Fatigue tests were performed on five flat specimens at different constant amplitude loads or stress levels between 40% and 80% of ultimate tensile strength to obtain the stress–life curve for the fabricated GFRP. Results showed that the high-stress levels of 80% contributed to the most reduced fatigue life cycle of GFRP. This result is consistent with previous studies and lies within the published life cycle range, validating the fabricated GRFP. A new parameter called the failure modulus, or Mf, may be used to quantify a particular set of fatigue tests.

Effect of Self-Healing by Dicyclopentadiene Microcapsules on Tensile and Fatigue Properties of Epoxy Composites.

Microcapsules of urea-formaldehyde (UF) containing dicyclopentadiene (DCPD) were synthesized by the in situ polymerization technique for self-healing of epoxy. The dispersion of microcapsules in the epoxy matrix was achieved using ultrasonication. Composites of epoxy, having 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules capable of self-healing, were prepared. The shape and size of the microcapsules were determined by field emission electron microscopy. Spherical capsules of DCPD, with an average diameter of 172 nm, were obtained. Investigation of tensile properties indicated a decrease in the tensile modulus with an increase in wt.% of microcapsules. There was a reduction of 22%, 27%, 39%, and 30% in the elastic modulus of composites for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively. Tensile strength was found to increase with an increase in wt.% of microcapsules. The tensile strength of the composites increased by 33%, 20%, 8%, and 21% for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively, in comparison with that of neat epoxy. The fatigue life of composites was investigated by conducting uniaxial tension-tension fatigue tests at constant stress amplitudes of 20, 25, 30, and 35 MPa, at a constant stress ratio (R = 0.1) and a frequency of 3 Hz. The fatigue life of composites increased with an increase in wt.% of microcapsules in comparison with that of neat epoxy. It was found that the fatigue life of the composites decreased with 1.5 and 2.0 wt.% of microcapsules in comparison with composites with 0.5 and 1.0 wt.% of microcapsules. The fracture surfaces of the tested samples were examined with the help of scanning electron microscopy (SEM) to understand the various mechanisms responsible for the change in modulus, strength, failure strain, and fatigue life of composites.

Fatigue Life Estimation of High Strength 2090-T83 Aluminum Alloy under Pure Torsion Loading Using Various Machine Learning Techniques

The ongoing effort to create methods for detecting and quantifying fatigue damage is motivated by the high levels of uncertainty in present fatigue-life prediction approaches and the frequently catastrophic nature of fatigue failure. The fatigue life of high strength aluminum alloy 2090-T83 is predicted in this study using a variety of artificial intelligence and machine learning techniques for constant amplitude and negative stress ratios (). Artificial neural networks (ANN), adaptive neuro-fuzzy inference systems (ANFIS), support-vector machines (SVM), a random forest model (RF), and an extreme-gradient tree-boosting model (XGB) are trained using numerical and experimental input data obtained from fatigue tests based on a relatively low number of stress measurements. In particular, the coefficients of the traditional force law formula are found using relevant numerical methods. It is shown that, in comparison to traditional approaches, the neural network and neuro-fuzzy models produce better results, with the neural network models trained using the boosting iterations technique providing the best performances. Building strong models from weak models, XGB helps to predict fatigue life by reducing model partiality and variation in supervised learning. Fuzzy neural models can be used to predict the fatigue life of alloys more accurately than neural networks and traditional methods.

STRAIN BEHAVIOR UNDER ALTERNATE REPEATED LOADING WITH CONSTANT STRESS AMPLITUDE AFTER LARGE TENSION OR SHEAR

This research paper describes elasto-plastic behaviors obtained under repeated loading after applying a large pre-deformation. In our previous research, the yield surface shape under finite deformation has been investigated to the test specimens of tough pitch copper using Natural Strain theory that is a reasonable for representing a large deformation. Moreover, the variation of yield stress with increase of the number of repetitions has been investigated when the repeated loadings are applied to the test specimens that the anisotropy for yield surface had already been formed. In those studies, the elasto-plastic behaviors measured under constant strain amplitude have been mainly examined, however, behaviors for constant stress amplitude have not yet been researched in detail. Therefore, in the present study, the strain behaviors obtained under repeated loadings with constant stress amplitude are investigated using specimens that have already undergone the large pre-deformations for tension or simple shear. Then, the relation between these strains’ behaviors and the anisotropy for the yield surface shape formed by large pre-deformation is revealed in this study.

Full-scale experimental research on the bending fatigue performance of post-tensioned prestressed concrete pipe piles

Post-tensioned prestressed concrete pipe piles (large cylinder piles) have been widely used in port and bridge engineering because they are economical and deliver excellent performance, and thus, have potential for use in offshore engineering. However, limited research has been devoted to the bending fatigue of large cylinder piles, which is essential for offshore engineering design. This study evaluated the bending fatigue of large cylinder piles by performing a series of full-scale bending fatigue tests on three specimens (L = 12 m, D = 1.2 m) and numerically analyzing the static cracking patterns in them. The static performance of large cylinder piles, including the bending moment of static cracking (Mcr) and its patterns, was analyzed by using a validated finite element model. Three-point bending fatigue tests with cyclic loading under a constant amplitude (stress ratio R = 0.4, peak fatigue load = 37.72%–80.00% Mcr) were then conducted to examine the development of mid-span displacement and stiffness degradation in the specimens. A critical damage factor (Dcr), which uses the cracking of concrete as the indicator of termination of the test, and a modified approach to prediction were then proposed and applied, based on findings of the fatigue test and past literature, to predict the number of cycles that it took for the large cylinder piles to crack. Finally, bi-linear S–N curves were plotted by considering the effective prestress according to the experimental and estimated data. The results here provide a theoretical basis for the fatigue-based design and application of large cylinder piles.

Uniaxial ratcheting behavior and molecular dynamics simulation evaluation of 316LN stainless steel

In this study, a series of uniaxial ratcheting experiments under different mean stresses with a constant stress amplitude were performed on a 316LN stainless steel at room temperature. Meanwhile, molecular dynamics (MD) simulation was carried out to reveal the microstructure evolution mechanism at the initial stage of ratcheting deformation at atomic scale. The results showed that the initial stage of ratcheting deformation could be divided into three stages: the stage of beginning (ratcheting strain growth rate (RSGR, dεr/dN) >0.01%), the stage of decreasing ratcheting rate (RSGR at 0.00001–0.01) and the stage of elastic/plastic shakedown (RSGR <0.00001) based on the changes of ratcheting strain rate. The shakedown ratcheting strain was linearly increased with increase of the mean stress in both MD simulation and experiment. And the ratchet deformation at different stages strongly depended on dislocation evolution. At the stage of beginning, the dislocation originated from coincident site lattice (CSL) boundaries and formed pile-up structures at random angle grain boundaries (RAGBs); while at the stage of decreasing ratcheting rate, the dislocation tangled at CSL grain boundaries or inside the grain, the dislocation density gradually increased and tended to be stable. When the given stress was insufficient to produce more plastic strain, the dislocation density balanced dynamically and the ratchet deformation entered the stage of elastic/plastic shakedown.

Application of Modified Fatigue Curve for Evaluation of Fatigue Damage of Steels at Variable Stress Amplitudes. Part 1. Calculation Model and Initial Data at Constant Stress Amplitudes

Application of Modified Fatigue Curve for Evaluation of Fatigue Damage of Steels at Variable Stress Amplitudes. Part 1. Calculation Model and Initial Data at Constant Stress Amplitudes

Fatigue of titanium alloy Ti6Al4V with diamond structure obtained by Laser Power Bed Fusion method

This paper presents the results of fatigue tests conducted on Ti6Al4V titanium alloy with diamond structure obtained by the Laser Power Bed Fusion method. Samples used in tests were printed with porosities: 81%, 73%, 50%, 34% and near-zero porosity. Samples were subjected to cyclic tests with a constant stress amplitude. The number of cycles until sample failure was registered. Obtained fatigue test results made it possible to determine simple, semi-empirical dependencies making it possible to forecast the fatigue life of Ti6Al4V titanium alloy with diamond structure obtained by the Laser Power Bed Fusion method under conditions of uniaxial, cyclically variable loads. The experimental results revealed that the initiation of the macro-crack occurred already with a small number of cycles. This was caused by the presence of two types of notches: technological micro-notches between particles of melted powder and notches related to the shape of the diamond structure itself. Microscopic observations of the fatigue fractures of samples were carried out, both on those subjected to low-cycle tests and those subjected to high-cycle tests. This made it possible to identify crack initiation and damage accumulation mechanisms as well as to propose numerical dependencies for samples of the tested structure. For this purpose, it is necessary to determine only the tensile strength of the given metamaterial and the fatigue characteristic for the given porosity.

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types.

Effects of strontium addition and T6 heat treatment on uniaxial fatigue of A356 aluminum alloy: Fatigue life and cyclic behavior

AbstractThe effects of strontium addition and T6 heat treatment on fatigue behavior of A356 aluminum alloy were studied in this paper. Uniaxial fatigue tests were carried out by using constant stress amplitude. In addition, microstructural analysis of porosity and fracture surface were performed on the material. Results showed that the alloy at T6 conditions presented the best fatigue life, mainly in low cycle fatigue. In high cycle fatigue, the fatigue life of as‐cast and heat treatment alloys almost converged to the same alternating stress. The strontium modified alloys presented higher porosity levels than unmodified ones, which reduced the fatigue life. The stress–strain hysteresis loops showed that the A356 + T6 alloy reached the highest total strain ranges and that depending on the stress level applied, the alloy may or not present cyclic hardening. The fracture analysis revealed porosity near the surface was the main fatigue failure nucleation site for strontium modified alloys.

Observation of the Formation Process of Fatigue Dislocation Structure in Micro-sized Ni Single Crystal

The purpose of this study is to diretly observe the formation process of characteristic fatigue dislocation structure in micro-sized nickel (Ni) single crystal specimens with a single slip orientation. Dog-bone shaped specimens with a cross-sectional area of 2 μm×2 μm were prepared by means of a focused ion beam processing system. Although tension-compression cyclic tests under a constant stress amplitude were performed on them, they were interrupted after different cycles (N = 1, 100, 1000 and 3500) and the inside was observed by a UHV-TEM after thinning. In the early stage of the test (N = 1 and 100), dislocations were rapidly multiplied. After N = 1000 where intrusions/extrusions appeared on the specimen surface, the dislocations gathered at the local areas. Then dislocation walls like those observed in a ladder-like structure in PSB was formed after N = 3500 cycle. This implies that vein, which is the important dislocation structure in fatigued bulk material, cannot be formed in micro-sized metals due to the tiny dimension.

Investigation of fatigue behavior of Kevlar composites with nano-Graphene filled epoxy resin

The fatigue behaviour of Kevlar based fibre-reinforced composites is often less studied and requires further investigation. The objective of this study is to investigate the fatigue behaviour of the Kevlar composites reinforced epoxy infused with Graphene nanoparticles. Most of the engineering components used in defense, aerospace, biomedical, automobile industries and military appliance use aramid fiber as primary structural component. The Kevlar has low compressive strength and other cons there by limiting its mechanical and electrical behavior in various applications. In this study, kevlar-49 and graphene nanoparticle infused epoxy are sandwiched together using vacuum resin infusion technique. The difference in visual appearance of Graphene, Graphite and Carbon Nanotubes often leads to misconception; hence to visualize the graphene nanoparticle in a non-destructive format, the Raman Spectroscopy was performed. The fatigue test was then conducted at constant stress amplitude having an R ratio of 0.2 with frequency of 10 Hz, in accordance to ASTM D3479. The fatigue limit was observed at 1,005,498 million cycles. The visual view of the fatigue experiment illustrates the damage propagation of the developed composites which is started by matrix cracking followed by fiber breakage and delamination between Kevlar fiber layers once the developed composite reaches the failure point. Thus, this development of Nanoparticle reinforced composite shows good agreement findings and has high potential to replace or reduce the usage of general synthetic fibers.

Advanced fatigue assessment - the future of wind turbine towers

One of the sustainable development goals of the Green Deal established by the European Union is the demand of affordable and clean energy. Among the renewable energies, the wind energy is already crucial in reaching the ambitious goals and will become even more important in the future. Since economic aspects will gain more importance, an efficient use of materials and resources is essential, especially those related to the fatigue design and service life prediction of wind turbine towers made of steel.This paper introduces an advanced fatigue assessment method, using the strain-life approach and the crack propagation method, named Two Stage Model. This model combines the two phenomenological aspects of crack initiation and propagation for a more reliable fatigue service life prognosis compared to commonly applied and simplified methods. By investigating both parts of the Two Stage Model separately, the paper shows the effect of selected input parameters on the service life for constant stress amplitudes. A Monte-Carlo Simulation is applied to validate the Two Stage Model as a vulnerable and reliable fatigue assessment concept.

Formation process of fatigue slip bands with unique configurations of ultrafine-grained high-purity Cu fabricated by severe plastic deformation

Fatigue-induced grain growth was observed in ultrafine-grained (UFG) metals processed by the severe plastic deformation technique, and the slip bands (SLBs) formed on coarse grains served as potential crack initiation sites. The SLBs in conventional grain-sized materials are characterized as parallel linear-like configuration along with primary slip orientation. By contrast, four typical configurations of SLBs were commonly observed in UFG materials, including granular, square lattice-like, triangular lattice-like, and parallel linear-like configurations. In the present study, stress-controlled fatigue tests were conducted on oxygen-free copper processed by equal-channel angular pressing under constant stress amplitudes. In addition, two-step block loading fatigue tests were carried out to observe the formation behavior of SLBs in a large dynamically recrystallized grain subjected to a higher cyclic stress. The objective of this study was to investigate the formation process of SLBs with a variety of configurations in UFG high-purity copper based on the microstructural evolution and the change in surface morphology because of cyclic stressing.

EFFECTS OF OVERLOAD AND SUPPLYING OIL ON CRACK GROWTH BEHAVIOR IN MG ALLOYS

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

Numerical investigation on the effect of thickness and stress level on fatigue crack growth in notched specimens

The effect of specimen thickness on the behavior of fatigue crack growth rate (FCGR) requires rigorous investigation because the thickness effect is not independent and could be a function of the specimen geometry, material properties, loading conditions, and environmental conditions. The purpose of this study is to numerically investigate the effect of specimen thickness and stress level on the behavior of FCGR at the specimen-free surface and mid-thickness in notched mild steel specimens. Elastic-plastic numerical calculations were performed based on FEM using the domain integral method to calculate the cyclic J-integral parameter, ΔJ, which was used for calculating FCGR. Different specimen thicknesses and various constant amplitude stress levels were used in the numerical calculations. Crack growth calculations were carried out using the node release technique in which the history of the former crack was considered. The calculated fatigue lives were firstly verified against the experiments taken from the literature. The results of numerical calculations showed that no significant thickness effect was noticed at the mid-thickness for calculations carried out below general yield while when the stress level is close to general yield the thinner specimens showed faster crack growth rates. Further, thickness has a remarkable effect on the crack growth rate at the free surface when the applied stress level is below and close to general yield where the thinner specimens gave faster crack growth rates. The crack-tip driving force ΔJ and the local strains and stresses induced ahead of the crack front could explain the mechanics and behavior of FCGR for the applied thicknesses and stress levels. The distributions of the local strains and stresses could also reveal the effect of the local material ahead of the crack front on the behavior of crack growth.

Monotonic and cyclic behaviour of sand in direct simple shear test conditions considering low stresses

Many geotechnical problems, such as buried pipeline–soil interaction and wave loading on sand seabeds, require the soil behaviour at relatively low effective stresses as compared to that in typical geotechnical practice. Monotonic and cyclic direct simple shear (DSS) tests were conducted on dry sand under constant normal stress, ranging from 12.5 kPa to 400 kPa, to investigate the effects of confining pressure on stress–strain behaviour. The tests were conducted using a combined advanced dynamic cyclic simple shear apparatus, which has a high-precision feedback system for controlling and measuring the forces and displacements while maintaining a high level of accuracy, which is essential, especially for tests at low normal stresses. The monotonic test results with this advanced system show an increased shear to normal stress ratio, thereby the mobilized friction angle, at the low-stress level compared to that of high normal stresses, as observed in some previous studies. The cyclic test results show that the load–displacement behaviour and cyclic compaction are governed by the normal stress. The sand becomes densified when the applied shear strain amplitude is greater than its threshold value. In constant strain amplitude cyclic loading, the lower normal stresses result in higher compaction; however, an opposite trend exists in constant stress amplitude cyclic tests.