Cyclic Plastic Strain Research Articles

Analysis of high temperature and strain amplitude effects on low cycle fatigue behavior of pitting corroded killed E350 BR structural steel

High-tension structural steels are prone to accelerated fatigue damage from pitting corrosion and high-temperature. Despite adverse effects, research on their low cycle fatigue (LCF) behavior is limited. Specifically, studies analyzing temperature-dependent pit sensitivity effects, considering pit-related material’s susceptibility to surface topographic features variation and stress concentration are lacking. This study conducts LCF tests on pitting corroded killed E350 BR structural steel at multiple strain amplitudes and high temperatures. It develops temperature-dependent parameters, such as the cyclic softening pit sensitivity factor, suitable for integration into existing approaches like total cyclic plastic strain energy density (CPSED), power-law, average strain energy density (SED), Coffin-Manson, and pit stress intensity factor (pit-SIF). Further, it proposes multiple linear regression-based prediction models relating total CPSED and average SED with strain amplitude and temperature. Corroded specimens show higher plastic deformation and reduced peak stress, fatigue life, and total CPSED compared to uncorroded ones. The developed parameters, integrated with average SED approach, predicts LCF life within an error band of ±1.5, while power-law relationship reduces it to ±1.2. Moreover, pit-SIF approach estimates fatigue life within an error band of ±1.5. The findings provide critical knowledge for enhanced component design, leading to structural safety, performance, and fire resilience.

Effect of cluster chemistry on the strengthening of Al alloys

When a supersaturated Al solid solution is held at room temperature, natural ageing occurs through the formation of clusters/zones providing a strengthening of the material. Recently, it was shown that this process can be greatly amplified by applying a cyclic plastic strain to a supersatured Al alloy. The cyclic strengthening also leads to a microstructure dominated by clusters/zones but the strengths are much higher than those obtained by natural aging, and can even exceed the peak aged strength.This work uses a combination of atom probe tomography and small angle x-ray scattering to provide a quantitative comparison of the cluster states in commercial Al alloys AA2024 and AA7075, after both natural aging and cyclic strengthening. We show that the chemistry of the clusters formed by these processes is different, and the clusters formed by cyclic strengthening tend to have chemistries closer to the composition of the relevant local equilibrium precipitate.The average force required to shear an individual cluster/zone is back-calculated from the strengthening observed experimentally and a strong correlation is found between the shearing force and the solute content of the cluster (i.e. 1-XAl, where XAl is the Al content of the cluster). This correlation applies not only for AA2024 and AA7075, but also AA6111 and AA7050 data taken from the literature, as well as some precipitates formed at elevated temperatures. This work shows that the first order effect controlling cluster strengthening is simply the cluster chemistry and its proportional effect on the force required to shear a cluster.

Heat transfer analysis and thermo-mechanical property of M-shape metal-rubber at high-temperature

Particular attention is given to the M-shape metal-rubber (MMR), which is a porous material with cushioning characteristics and excellent heat resistance coated in pipelines. The heat transfer characteristics and thermo-mechanical property of MMR has not been clear due to its complicated micro-porous structures and the coupling effect between structure and performance. In this work, the influence of different parameters (emissivity, density and gap distance) on the equivalent thermal conductivity of MMR were considered at the mesoscopic model level. In addition, thermal stress at different temperatures was used as predefined fields for thermal-mechanical coupling to analyze the transformations of performance. The results indicate that surface emissivity and cavity emissivity during heat transfer exhibit an inverse relationship with MMR thermal conductivity. Furthermore, it is observed that natural convection has the most significant impact on thermal conductivity. And, the thermal conductivity is negative correlation with density and gap distances. Under cyclic loading, stress concentration and plastic strain occur in MMR, and residual thermal stress is generated. The secant stiffness exhibits a positive correlation with temperature, while the loss factor demonstrates a negative correlation, primarily due to thermal expansion.

Damage assessment for steel structures subjected to cyclic pre-strain Part2: SEM in-situ observations clarifying the effect of stress state on damage evolution during plastic deformation

The authors conducted fracture toughness test results after the cyclic plastic strain and compared them with virgin steel in the previous report (Part 1 [1]). In this previous report, it was revealed that prestraining under high triaxiality stress state made more degradation of toughness. In this study, in order to deeply investigate the effect of triaxiality on material damage, tensile tests in a scanning electron microscope (SEM) were performed on two types of specimens with different triaxiality histories. First, material damage was evaluated by in-situ observation of the evolution of material damage under tensile loading using the EBSD (Electron Back Scattered Diffraction Pattern) method. As a result, it was confirmed that material damage tended to be more significant in the specimen with a higher triaxial field at the same equivalent plastic strain level and that material damage propagated more rapidly. The results of the slip evaluation based on the slip model for crystals in steel materials suggest that the ease of damage development associated with plastic deformation in a high triaxial field is due to the magnitude of movement of a particular slip plane.

Influence of Strain Amplitude on Low-Cycle Fatigue Behaviors of a Fourth-Generation Ni-Based Single-Crystal Superalloy at 980 °C

Total strain-control, low-cycle fatigue experiments of a fourth-generation Ni-based single-crystal superalloy were performed at 980 °C. Scanning electron microscopy and transmission electron microscopy are employed to determine fracture morphologies and dislocation characteristics of the samples. As the strain amplitude increased from 0.6 to 1.0%, the cyclic stress and plastic strain per cycle increased, the cyclic lifetime decreased, more interfacial dislocation networks were formed, and the formation rate accelerated. Cyclic hardening is associated with the reaction of accumulated dislocations and dislocation networks, which hinder the movement of dislocations. The presence of interfacial dislocations reduces the lattice mismatch between the γ and γ′ phases, and the presence of dislocation networks that absorb mobile dislocations results in cyclic softening. At a strain amplitude of 1.0%, the reaction of a high density of dislocations results in initial cyclic hardening, and the dislocation cutting into the γ′ phase is one of the reasons for cyclic softening. The crack initiation site changed from a near-surface defect to a surface defect when the strain amplitude increased from 0.6 to 0.8 to 1.0%. The number of secondary cracks initiated from the micropores decreased during the growth stage as the strain amplitude increased.

Constitutive model of duplex stainless steel S22053 and its weld under cyclic large plastic strain loading

This study investigated the mechanical properties and constitutive models of duplex stainless steel S22053 under cyclic large plastic strain loading. To this end, smooth round bar specimens of the base metal, deposited metal, and heat-affected mixing zone material were tested under monotonic tensile loading and cyclic loading. Furthermore, the failure modes and stress–strain curves of the specimens were analysed. Based on the test results of the smooth round bar specimens under cyclic symmetric constant amplitude and monotonic tensile loading, the N3L1 and S10 parameters of the Chaboche model for duplex stainless steel S22053 were calibrated, respectively. Then, the accuracy of the calibrated Chaboche model was verified against the test results of the smooth round bar and circular notched bar specimens. The results showed that the base metal and heat-affected mixing zone material have higher ductility, and the strength of the deposited metal is the highest. Furthermore, the N3L1 parameters can accurately simulate the behavior of smooth round bar specimens under cyclic medium plastic strain loading, while the S10 parameters are more suitable for simulating the mechanical response of circular notched bar specimens under cyclic large plastic strain loading. The accuracy of the Chaboche model depends on the matching of the loading strain range of the test data for calibrating the model with that of the simulation cases. The results of this study provide fundamental data for analysing the seismic and ductile fracture performance of duplex stainless steel and its welded structures.

Fatigue crack growth modeling considering a hybrid propagation strategy

AbstractFatigue results from the occurrence of several damage mechanisms and their interactions. The cyclic plastic strain and damage accumulation at the crack tip are widely pointed as the main agents behind fatigue crack growth (FCG). In this work, the authors propose the prediction of FCG through a node release numerical model that offers several possibilities regarding the modeling of the mechanisms behind fatigue. A hybrid propagation method is presented where both cumulative plastic strain and porous damage represent parallel propagation criteria. Accordingly, the node is released once either a critical plastic strain or a critical porosity, at the crack tip, is reached. The Gurson–Tvergaard–Needleman (GTN) damage model is employed to predict porous damage evolution through the processes of nucleation and growth of micro‐voids. The model is validated through comparison with experimental data for the AA2024‐T351 aluminum alloy. Finally, the interactions between plastic strain, porous damage, crack closure, and stress triaxiality are accessed.

Role of Persistent Slip Bands and Persistent Slip Markings in Fatigue Crack Initiation in Polycrystals

The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno.

Cyclic failure mechanisms of saturated marine coral sand under various consolidations

The undrained cyclic response of coral sand in earthquake is an important issue that must be considered in ocean engineering. A series of undrained cyclic triaxial tests was conducted on saturated coral sand using GDS dynamic triaxial apparatus. This study discusses the influence of consolidation ratio (kc), cyclic stress ratio (CSR), and relative density (Dr) on the undrained cyclic response of saturated coral sand. The characteristics of excess pore water pressure (EPWP), axial strain (εa), effective stress path, and undrained cyclic resistance of saturated coral sand are analyzed. The test results show that two axial strain generation modes exist in saturated coral sand: cyclic mobility and plastic strain accumulation. The EPWP (ue) generation modes are quite different subjected to various CSRs under isotropic consolidation, while that is consistent under anisotropic consolidation. Suitable models are selected to describe various EPWP generation patterns. Consolidation condition can significantly affect the development of effective stress path. The residual effective stress becomes large with the increase of kc. A new index called stress parameter (λ) is introduced to consider the effect of kc and CSR on undrained cyclic resistance comprehensively. The critical line to distinguish the failure point from non-failure point of coral sand specimens can be clearly found in the correlation between λ and the relative density after consolidation (Drc). The existence of the critical line in the relationship between stress parameter and relative density is also verified by using the literature data.

Electropulsing suppresses extrusion growth in polycrystalline copper subjected to fatigue loading

The effect of electropulsing on the extrusion due to persistent slip bands (PSBs) and the defects in PSBs was investigated. To extend the service life and improve structural safety, technology is needed to repair fatigue damage such as cracks and dislocations directly. It has been shown that electropulsing of damaged metallic materials delays fatigue crack initiation and extends fatigue life, but the detailed mechanism of this has not been explained. When cyclic strain is localized into a PSB, the PSB forms lamella, thin walls that contain rich dislocation, and thick channels that contain poor dislocation. The cyclic plastic strain within a PSB causes extrusions and intrusions, called persistent slip markings, to appear on the surface. Cracking is initiated at the tip of the intrusion. Fatigue tests were performed using an electromagnetic force fatigue testing system while applying electropulsing every 2000 cycles to polycrystalline copper. The extrusion height before and after electropulsing was observed through atomic force microscopy. The resulting extrusion growth was slowed by the electropulsing. Polák’s model was applied to examine the effect of electropulsing on the extrusion growth. When the vacancy concentration reaches a steady state in the channel and electropulsing is performed, the vacancies migrate and are assumed to be annihilated at the matrix or walls. Therefore, the vacancy concentration in the PSB decreases with electropulsing. The decrease in vacancy concentration in the PSB is thought to suppress the extrusion growth until the vacancy concentration reaches a steady state again.

Ultra-low cycle fatigue performance of grid structure with bolted spherical joints

This study aimed to investigate the effect of the specification of each grid structure component on the ultra-low cycle fatigue performance of the rod element of a grid structure with bolted spherical joints. Thus, an ultra-low cycle fatigue test of three identical composite steel tube and bolted sphere specimens was performed. The test was performed under three kinds of constant amplitude loading regimes for large axial cyclic plastic strain loads. Based on steel ductility damage and initial geometric defects, a finite element simulation of the test was then conducted using Abaqus/Explicit, which was calibrated according to the test results. In addition, parametric numerical simulations were performed to analyze the effects of different steel tube slenderness ratios, bolt diameters, and steel tube wall thicknesses on the seismic performances of the composite steel tube and bolted sphere specimens. The results indicated that the specimen dissipated energy mainly through material damage in the plastic hinge region in the middle of the steel tube. The steel tube slenderness ratio mainly affected the fatigue life and energy dissipation capacity of the specimen. Further, the ultimate tensile and compressive bearing capacity as well as deformation capacity of the specimen both increased with increasing steel tube wall thickness. Finally, the energy dissipation pattern of the specimen was most sensitive to the bolt diameter; thus, the energy dissipation capacity of the steel tube can be fully utilized only by selecting the bolt specification that matches the rod element.

Enhanced fatigue endurance limit of Cu through low-angle dislocation boundary

How to remarkably elevate the high-cycle fatigue resistance of metallic materials under cyclic loading is a crucial, yet technically challenging issue for major structural applications. Either traditional strengthening approaches or newly-developed hierarchical nanostructural modifications has been demonstrated to usually display a limited ability of enhancing the 107-cycle fatigue endurance limit. Here, we introduce massive nano-scaled low-angle dislocation boundaries in coarse grained pure Cu by means of simple drawing process, not only enhancing the tensile strength obviously, but also imparting a fatigue limit as high as 130 MPa and a fatigue ratio of 0.35, which is a record for pure Cu. Upon cyclic loading, the extensive activation of strongly confined dislocation slip within homogeneous dislocation cells (with cell size of about 300 nm) effectively mediates cyclic plastic strain with slightly cyclic softening. The elevated stress resistance of DC Cu to cyclic loading is primarily attributed to the high density of built-in low-angle dislocation cells, which are strong and also suppress local surface roughening and crack initiation, thereby contributing to an enhanced fatigue life.

Influence of sloping ground conditions on cyclic liquefaction behavior of sand under simple shear loading

An experimental program of undrained cyclic simple shear tests was undertaken on Ticino sand, covering a broad range of initial relative densities and static stress levels, to investigate the effect of an initial static shear stress on failure mechanism, cyclic resistance, and pore water pressure generation. The results indicated that two types of failure mechanisms occurred, namely cyclic mobility and plastic strain accumulation, depending on the magnitude of the initial static shear stress with respect to the applied cyclic shear stress. Test results showed that the cyclic resistance of Ticino sand may either increase or decrease with an increasing initial static shear stress depending on the density state of the sand. The Kα correction factors measured under cyclic simple shear loading were compared with the values reported in the literature based on both cyclic triaxial and simple shear tests, suggesting that Kα could be dependent on the particle gradations and grain shape for a given depositional mechanism. Finally, it was verified that the initial static shear stress has a remarkable impact on the development of cyclic pore-water pressures (PWP) of the sands so that a modified PWP generation model has been effectively proposed in the present study.

Effect of Different Holding Times on The Facture Behavior for Low Cycle Fatigue of Aluminum Alloy aa6063 at 350 ° c Temperature

low Cycle Fatigue behavior of aluminum alloy AA6063 heat treated at 350 ° c with different holding times was studied . Constant S – N Curve is designed with the main fatigue parameters . The results are verified with that obtained numerically . Using different holding times with low cycle fatigue properties AA6063 have been observed . Holding times of 2 , 4 , 6 and 8 hours at 350 ° C temperature was used . The yield stress and the ultimate tensile strength were decreased with the increasing the holding time . The strength Coefficient K and the strain hardening index n results were increased as the holding time temperature increased . Also the exponent b of fatigue strength and the exponent C of fatigue ductility were increased as the holding time increased . Cyclic elastic Strain do not change much with increasing the time at Same temperature 350 ° C and the situation is also the Same for fatigue strain coefficient , for the cyclic plastic strain - Optic Micrograph images were taken using variable magnification before and after fatigue test . Moreover the failure mechanisms with microstructural features are studied using scanning electron microcopy (SEM)

Achieving 1 GPa fatigue strength in nanocrystalline 316L steel through recovery annealing

Nanostructured metals offer an improved fatigue limit and high cycle fatigue performance compared to their microcrystalline counterparts. However, even the small cyclic plastic strain amplitudes present at these cyclic loading regimes can induce grain growth, a prerequisite for fatigue crack initiation. Retarding cyclically induced grain growth, for instance by alloying, should thus further improve the fatigue performance. Here, we followed a different strategy and investigate the role of recovery heat treatments, known to shift the onset of plasticity to higher stress levels. Indeed, we observe for the recovered specimens an unprecedented increase of the fatigue limit, reaching for 316L steel values of almost 1 GPa at a stress ratio of R=-1. First experiments further indicate that this strategy could also be used to improve the fatigue limit of conventionally cold worked austenitic structures.

Digital-image-correlation observation of cyclic plastic strain field during the damage-accumulation mode of fatigue crack propagation under pure cyclic mode II loading for cold-rolled SUS430 steel

Fatigue cracks propagate intermittently by the cyclic initiation and coalescence of voids in front of the fatigue crack tip during fatigue crack propagation in the damage accumulation mode. In this study, digital image correlation is used to measure the evolution of the measured plastic strain distribution near the crack tip with loading and cyclic loading during one cycle. The evolution is estimated based on strain distribution measurements results and mechanical considerations. The strain is set at the unloading point because results indicate that the crack propagated in the region where the experimental shear plastic strain is the maximum in the first quarter cycle (at the first maximum load); the measured plastic strain is almost constant in the region where the crack propagates, and the crack is initiated by a subsequent cyclic strain localization and coalescence with the main crack. This study clarified the damage accumulation mode fatigue crack propagation mechanism with the measured strain behavior, and presents a method to use continuum plastic strain at the crack tip as a mechanical driving force for this mode.

Investigation of metal fatigue using a coupled entropy-kinetic model

A new method is proposed to predict the fatigue life of metals based on the fracture fatigue entropy (FFE) criterion. This method enables a rapid prediction of the cyclic plastic strain energy and the temperature evolution using the concept of kinetic strength of solid materials. Theoretical predictions are compared with experiments results for Al 7075-T6, LCS 1018, API 5L X52, and MCS 1045. It is shown that this model can reliably predict the temperature evolution, plastic strain energy dissipation, and the fatigue life of metals.

Fatigue life prediction of hypervapotron for CFETR divertor

The engineering design of China Fusion Engineering Testing Reactor (CFETR) will be finished in the end of this year. Compared with Experimental Advanced Superconducting Tokamak (EAST), this is a large development which means closer to commercial fusion reactors. The design objective of CFETR divertor requires a steady-state thermal load of 10–20MW/m2 exhaust. There are several design options for the CFETR divertor. Hypervapotron with its superior performance has been researched for several decades as one important candidate for future fusion device divertor. In this paper, a new design of hypervapotron plasma facing unit (PFU) was developed. Horizontal and longitudinal ribs were adopted to generate more vaporized cores to enhance heat transfer. Some optimizations were performed based on the allowable max heat flux (MAHF) to get the final design. Rensselaer Polytechnic Institute (RPI) boiling model of Eulerian Multiphase fluid and k-epsilon model of Viscous were chosen to deal with sub-cooled boiling. 3Sm rule and Bree-diagram was used to confirm the rationality of the structure after thermal-mechanical analysis. Numerical simulation results suggested that CuCrZr alloy will experience cyclic plastic strain. Based on this, fatigue life prediction of the hypervapotron PFU at a steady-state heat flux of 10MW/m2 were carried out through local stress-strain method, neuber curve and design fatigue life curve. Eventually, a fatigue life of approximately 1.2489 year was verified for CFETR divertor. This paper provides a systematic method for fatigue life prediction and reliability analysis of hypervapotron.

On the kinetic formulation of fracture fatigue entropy of metals

AbstractThe kinetic concept of the strength of solids is used to determine the fatigue life of stress‐controlled cyclic loading and evaluate the fracture fatigue entropy. Unlike the traditional stress or strain‐based models, this theory uses the applied stress amplitude and the specimen's surface temperature to determine the lifetime. The degradation fraction in each cycle is used to calculate the life cycle. Then the material parameters of the final life equation are obtained for two materials: low carbon steel 1018 and aluminum 7075‐T6. The number of cycles to fracture and the cyclic plastic strain energy ( ) are used to calculate the fracture fatigue entropy (FFE). It is shown that the life cycle prediction of the proposed model agrees well with the experimental results from the literature. Results provide further evidence for the constancy of FFE.

Experimental investigation of failure mechanism of grid structure with bolted spherical joints

Under strong earthquakes, the grid structure members may frequently bear plastic strain so that the grid failure, which is characterized by ultra-low cyclic fatigue. In the present study, the ultra-low cycle fatigue test of three full-scale grid structures, which are under vertical cyclic large plastic strain load, is carried out. The hysteretic and skeleton curves, energy consumption capacity, ductility performance, bearing capacity degradation, and stiffness change are obtained. Also, the macroscopic morphology of rod fracture is analyzed. In addition, the corresponding recommendations are made based on the failure mechanism of the rod and the deformation characteristics of the joint. It was found that the ultra-low cycle fatigue damage of the lower chord is the main cause of grid failure. Also, the failure behavior of the rod in the structure is very similar to that in the member test. The coupling effect of ultra-low circumferential fatigue and local buckling causes the strain to concentrate in the middle of the rod. The computational analysis of such structures under strong earthquakes should consider the degradation of joint stiffness.