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This paper presents a macroscopic constitutive model that is able to reproduce the uniaxial transformation ratcheting behaviors of the superelastic shape memory alloy (SMA) undergoing cyclic loading. A cosine-type phase transformation equation with the initial martensite evolution coefficient that helps to predict the residual martensite accumulation and the nonlinear features of the hysteresis loop with a small number of material parameters is established to describe the phase transformation behaviors of the SMA undergoing cyclic loading. The proposed model simultaneously takes into account the evolution of transformation-induced plastic strain during cyclic loading. The applied loading level and asymmetric tensile and compressive behavior of the SMA on transformation ratcheting are also considered in the proposed model. The evolutions of transformation ratcheting, transformation-induced plastic strain, and transformation stresses are constructed as the function of the accumulated residual martensite volume fraction. The simulated results are compared with the experimental results to show the validity of the proposed model in transformation ratcheting. [ABSTRACT FROM AUTHOR]

Abstract Ten tank designs with nominal capacities ranging from 500 bbl (79.5 m3) to 10,000 bbl (1590 m3) are provided by the American Petroleum Institute Specification 12D (API 12D) to accommodate the needed temporary storage of extracted product in the upstream segment of the oil and gas industry. These tanks are designed to withstand the operational conditions of the storage process and the operating pressure ranges, and are field-welded and mass-produced to accommodate the storage needs of the upstream segment of the facility. The fatigue evaluation, based on ASME BPVC Section VIII, Division 2 is performed in this study for these ten tank designs with a new cleanout detail that is an extended-neck rectangular cleanout with a semicircular top and is surrounded by a reinforcement pad. The stresses found in the junctions connecting the cleanout neck to the bottom plate, the shell to the bottom plate, and the roof to the top angle that is attached to the shell are investigated under the influence of internal pressure, vacuum pressure, and hydrostatic pressure. The tanks were idealized as axisymmetric models using a finite element analysis approach to study the stresses in the roof-to-shell and shell-to-bottom junctions, while a three-dimensional, solid-element, sub-model driven by a three-dimensional shell model of the entire tank was used to idealize the cleanout junction. The stress classification in the junctions of interest were obtained to evaluate the fatigue life by simulating the modeled tanks and loading pattern using elastic stress analyses. The number of permissible design cycles for each of the ten API 12D tanks is presented and discussed in this work. Highlights • Elastic analysis was performed in conformance with the ASME BPVC Section VIII, Division 2. • Both axisymmetric models and solid sub-models were investigated using finite element methods. • Smooth bar design fatigue curve was used for the fatigue life estimates. • Number of permissible design cycles are provided for API 12D tanks. [ABSTRACT FROM AUTHOR]

Highlights • Ratcheting strain behavior tends to decrease after the application of overload which explains the retardation effect in FCG. • The compressive zone of crack-tip residual stress field increases with the decrease in R ratio. • The compressive zone of crack-tip residual stress field increases with the increase of overload. • The compressive part of crack-tip residual stress field increases with the increase of overload ratio and SIF range value. Abstract The present study deals with the investigation of cyclic plastic deformation behavior near crack-tip with/without overload. The effect of overload is studied on single edge notch tension SENT specimen in an elasto-plastic finite element FE analysis. Three different stress ratios, two different overload ratios and a selected stress intensity factor SIF range Δ K value are used. Chaboche model is used for characterizing SA333 Gr 6 C-Mn steel material behavior. Crack-tip cyclic stress/strain hysteresis loops, crack-tip residual stress field and crack-tip ratcheting strain accumulation are simulated with/without overload. Moreover, the effect of overload ratio, stress ratio, background Δ K -level and crack size on controlling the compressive residual stress field, generated near crack-tip after overloading, are discussed. The results show that the progressive accumulation of plastic strain in the crack-tip cyclic plastic zone tends to decrease after overloading. It is also noticed that the maximum magnitude and the size of compressive residual stress zone following a single overload increase with the increase in the overload crack size and the decrease in the background R r a t i o. The finding results provide insight into the understanding of cracked metallic parts' fatigue response under variable amplitude loading. [ABSTRACT FROM AUTHOR]

Abstract It is known that the mechanical properties of electrodes in lithium-ion batteries, such as modulus, yield stress, and interfacial strength, can depend strongly on lithium concentration. Here we show that a thin film island electrode with properties dependent on lithium concentration naturally undergoes plastic ratcheting with accumulative deformation and failure during cyclic charging and discharging. Some key predictions from numerical simulations are validated by galvanostatic tests. Strategies to avoid ratcheting include limiting the electrode size and/or selecting a balanced combination of concentration dependent materials properties. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Z2CND18.12N austenitic stainless steel elbow pipes were studied experimentally under reversed in-plane load of 20 kN and constant pressure of 17.5 MPa after thermal aged 1000 h and 2000 h respectively at the same thermal aging temperature of 500 centigrade degrees (oC). The ratcheting behaviors of pressurized elbow pipes were analyzed compared with original elbow pipe. The results indicate that the ratcheting behavior of pressurized elbow pipe is prominently affected by thermal aging. The uniaxial tensile properties of Z2CND18.12N austenitic stainless steel were also studied under the same condition of thermal aging. It is shown that yield stress of material decreases obviously with the increase of thermal aging period. Chen-Jiao-Kim (CJK) kinematic hardening model was used to evaluate the ratcheting behaviors of pressurized elbow pipes under different thermal aging periods. Simulation results are perfectly agreement with experimental data. Ratcheting shakedown boundaries of thermal aged elbow pipes were determined by CJK model. Highlights • Thermal aging will reduce the yield strength of Z2CND18.12N austenitic stainless steel. • The ratcheting strain of the elbow increases with the increase of thermal aging period. • The ratcheting shakedown boundary decreases with the increase of thermal aging period. • Chen-Jiao-Kim (CJK) kinematic hardening model can evaluate the ratcheting behavior of the elbow aged. [ABSTRACT FROM AUTHOR]

Acoustic emission technique (AET) has been used for studying ratcheting behavior of elbows made of AISI type 304LN stainless steel. High pressure piping and elbows in nuclear power plants are subjected to large amplitude reverse cyclic loading during an earthquake and the stresses are likely to exceed the elastic limit. A strong possibility for the accumulation of plastic strain by ratcheting thus exists in such components. Monitoring cyclic strain and ovality are normally done to predict damage in pressurized components during ratcheting. The novelty of the present work is the use of acoustic emission (AE) for getting information about the damage and formation of cracks during the ratcheting tests in elbow specimens. The tests were carried out under steady internal pressure and cyclic bending. During the ratcheting tests, the first significant increase in AE signal occurred at around 214 cycles and 103 cycles for the two elbows, and this has been attributed to crack initiation. Higher AE generated beyond that has been attributed to crack growth. Different parameters of the AE signal have been used to substantiate the results. [ABSTRACT FROM AUTHOR]

The purpose of this research is determining experimentally the characteristics of tension and cyclic plastic behaviours of as‐received and annealed coppers and studying distribution of stress/strain field near the crack tip. Samples made by pure copper were annealed at 420°C for 40 minutes in electric furnace. To determine the properties of the cyclic plastic behaviour, proper tests with symmetric strain‐controlled conditions were performed on standard samples. Chaboche nonlinear hardening model was used to determine the cyclic plastic behaviour of both materials. According to results, annealing process creates isotropic hardening in the copper and also changes its initial kinematic hardening behaviour. Effects of the annealing and hardening on the variations of the stresses and strains around the crack tip were investigated. Also, ratcheting and mean stress relaxations versus number of cycles, inside the plastic region, were studied. [ABSTRACT FROM AUTHOR]

Abstract Ratcheting and low cycle fatigue are failure mechanisms observed in components subjected to cyclic temperature and mechanical loads. Ratcheting is a global failure mechanism which leads to an incremental plastic collapse of the component whereas low cycle fatigue is a localized mechanism which leads to crack initiation. It is exacerbated by grooves, notches and changes in the geometry of the component. To estimate the remaining life of the component and predict its failure mechanism, it is important to understand how it responds to various combinations of cyclic loads. This paper includes investigation of the ratchet limit and the plastic strain range, which is associated with the low cycle fatigue, of a circumferential butt-welded pipe by using the ratchet analysis method which includes Direct Steady Cycle Analysis (DSCA) within the Linear Matching Method Framework (LMMF). The pipe is subjected to a constant internal pressure and a cyclic thermal load. The investigation is carried out by varying 1) material properties of the weld metal (WM); 2) ratio of inner radius to wall thickness; 3) weld geometry. Within the specified ranges, yield stress and the ratio of inner radius to wall thickness affect the ratchet limit curve. The cyclic thermal load plays a crucial role compared to the internal pressure in influencing the ratchet limit curve. It is observed that the pipe experiences thermal ratcheting at lower yield stress values of the WM. The results obtained are combined to create a limit load envelope, which can be used for the design of welded pipes within the specified ranges. [ABSTRACT FROM AUTHOR]

In the present work, a constitutive model for articular cartilage is proposed in finite elasto-viscoplasticity. For simplification, articular cartilage is supposed to be a typical composite composed of a soft basis and a fiber assembly. The stress tensor and free energy function are hence accordingly divided into two components. The high nonlinear stress-strain response is assumed to be mainly related to the fiber assembly and described by an exponential-type hypoelastic relation. Ratcheting is considered according to the viscoplasticity, the evolution rule of which is deduced from the dissipative inequality by the co-directionality hypotheses. Then, the capability of the proposed model is validated by comparing its predictions with related experimental observations. Results show that the ratcheting behavior and stress-strain hysteresis loops are reasonably captured by the proposed model. [ABSTRACT FROM AUTHOR]

Compressive ratcheting response and creep of non-asbestos fibre composite are tested based on a designed compression–compression clamp for plate-shape specimens. The influences of temperature, stress amplitude and stress rate are discussed in detail. A two-stage constitutive model is developed to describe the compression-resilience relationship of non-asbestos fibre composite. Results present that a crescent-shaped stress–strain relationship of non-asbestos fibre composite is observed under repeated compression, and the proposed two-stage constitutive model can predict the compression-resilience curve for the first cycle very well under various temperatures and stress rates. Additionally, the ratcheting strain of non-asbestos fibre composite obviously enhances with increasing the temperature. However, it has a little change when the stress rate and stress amplitude are varied based on the experimental data. The ratcheting strain rate per cycle reduces promptly during about the first 75 cycles, and it is always less than 1 × 10−4 per cycle during the subsequent cycles and tends to shakedown. Moreover, the ratcheting strain with small stress amplitude is very close to the creep deformation at the same testing time and peak stress under different temperatures. This indicates that the ratcheting strain of non-asbestos fibre composite under fatigue loads with small stress amplitude can be evaluated equivalently by the corresponding creep deformation with high precision. [ABSTRACT FROM AUTHOR]

Abstract Stress-controlled uniaxial "push-pull" fatigue testing was conducted on as-received (cast and extruded) and closed-die cast-forged and extruded-forged AZ80 Mg alloy. The as-cast material possessed random texture and somewhat symmetric cyclic responses. The extruded and forged materials possessed sharp basal texture and asymmetric cyclic responses. All materials exhibited tension/compression asymmetry in their cyclic response to varying degrees, depending on the thermomechanical processing conditions. It was discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as the thermomechanical history. Under fatigue testing, the materials all developed some form of mean strain, with the nature and magnitude of this mean strain being dependent on primarily its texture intensity and propensity to twin in either tension or compression reversals. The type of mean strain (tensile or compressive) depends upon both the orientation and intensity of the starting texture of material. The texture induced ratcheting and resulting mean strain evolution was most pronounced in the as-cast material and had a significant impact on the fatigue life. Following forging, the material exhibited an increase in fatigue life of anywhere from 2 to 15 times for the cast then forged material and more modest yet still significant 8 times longer at stress amplitudes around 140 MPa for the extruded then forged material. The extruded forged material exhibited similar fatigue lives to that of the base material at stress amplitudes which approached the yield strength. The nature of the mean stress development and degree of fatigue life improvement depended on the processing conditions and the type of base material (cast or extruded) utilized to create the forging. Two energy based models were utilized to predict the life of the forged material, and gave a reliable life prediction for a variety of material conditions that were investigated. [ABSTRACT FROM AUTHOR]

Ratcheting and creep of reinforced graphite sheet with stainless steel insert (RGSWSSI) are tested under cyclic stress‐controlled compression by a self‐designed clamp from 500°C to 600°C. The effects of insert type, temperature, stress amplitude, stress rate, creep, and loading sequence are considered. Results present that ratcheting deformations for RGSWSSI with 316‐L stainless steel tanged and bonded insert approach to each other and show little rate dependence, while they slightly increase with the increment of stress amplitude and temperature. Moreover, obvious ratcheting effect takes place under cyclic pulsating loads with the peak stress of 32 MPa at 500°C or higher. The accumulated deformations of RGSWSSI under small stress amplitude only increase during about the first 25 cycles and then always turn to shakedown. It can also be estimated by the corresponding static creep strain in practical engineering with good accuracy at high temperature. This work provides important data and understanding of RGSWSSI under harsh fluctuating loads. Highlights: Accumulated deformations of 5 different types of RGSWSSI are tested under cyclic compression from 500°C to 600°C.The ratcheting strain, compressive moduli, and creep of RGSWSSI are stressed.The effects of insert type, temperature, stress amplitude, stress rate, creep, and loading sequence are considered.The specimens with tanged insert and bonded insert have very similar mechanical feature under the same load. [ABSTRACT FROM AUTHOR]

Uniaxial time-dependent creep and cycled stress behavior of a standard and toughened film adhesive were studied experimentally. Both adhesives exhibited progressive accumulation of strain from an applied cycled stress. Creep tests were fit to a viscoelastic power law model at three different applied stresses which showed nonlinear response in both adhesives. A third order nonlinear power law model with a permanent strain component was used to describe the creep behavior of both adhesives and to predict creep recovery and the accumulation of strain due to cycled stress. Permanent strain was observed at high stress but only up to 3% of the maximum strain. Creep recovery was under predicted by the nonlinear model, while cycled stress showed less than 3% difference for the first cycle but then over predicted the response above 1000 cycles by 4-14% at high stress. The results demonstrate the complex response observed with structural adhesives, and the need for further analytical advancements to describe their behavior. [ABSTRACT FROM AUTHOR]

The phenomenon of ratcheting occurs under the influence of non-zero mean stress during cyclic loading; it singificantly reduces the low cycle fatigue life of engineering structures. The present investigation deals with a detailed comparison on the estimation of dislocation densities in the 42CrMo4 steel induced by ratcheting using two different methods, i.e. X-ray diffraction (XRD) profile analysis and hardness. The dislocation densities in the ratcheted specimens were assessed using XRD profile analysis following the modified Williamson-Hall method as well as hardness measurements following the modified Nix and Gao model. The results showed that dislocation density increased in the ratcheted specimens as compared to the unratcheted ones and increase in accumulation of ratcheting strain was correlated with the increase in dislocation densities in the ratcheted specimens. It was established that both hardness and X-ray diffraction profile analysis methods can very effectively be used to assess the dislocation densities in the ratcheted specimens. [ABSTRACT FROM AUTHOR]

Highlights • Ratcheting is a rate-independent elastoplastic phenomenon. • Ratcheting should not be confused with creep, cyclic softening and fatigue damage. • Within the small strain assumption conditions, ratcheting is not significant for 304L SS. • Outside the small strain assumption conditions, ratcheting is observed in 304L SS under large stress amplitudes. Abstract In this work our goal is to better understand the origin of the cyclic accumulation of the inelastic strain (often called ratcheting) observed in 304L SS subjected to uniaxial cyclic stress control at room temperature. Recent works performed in the frame of small strain assumption attribute this phenomenon essentially to creep (Taleb, 2013). However, outside this frame, it seems that creep is not the only contributor in this phenomenon (Facheris, 2014). New experiments are performed here in order to investigate the role played by creep, cyclic softening, fatigue damage, the mode of control (engineering or true stress) and ratcheting in this observation. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Low cycle fatigue and ratcheting deformation of 25Cr2MoVA steel under cyclic tension were tested at 550°C. The effects of stress rates for 0·1, 0·5, 2·5, 5, 10, and 40 MPa s-1 on ratcheting effect and fracture appearance were discussed systematically. Results indicate that the steady static creep rate very approaches to the ratcheting strain rates for 0·1 MPa s-1, but it is obviously greater than those of greater stress rates tested. Moreover, the steady ratcheting strain rate reduces rapidly with increasing the frequency when the frequency is less than 36·8 h-1, and then changes slightly for the greater frequency. This implies the steady ratcheting rate of 25Cr2MoVA is approximately rate-independent when the stress rate is greater than 10 MPa s-1. Additionally, there are some microvoids in the central fibrous regions of specimens under static creep and cyclic tension with 0·1, 0·5, and 5 MPa s-1, but very few microvoids for the specimen under cyclic tension with 10 MPa s-1 can be observed. Furthermore, the low cycle fatigue life is relatively short when the stress rate is less than 10 MPa s-1, but it enhances exponentially subsequently. [ABSTRACT FROM AUTHOR]

Flotation of buried pipelines at cyclic freezing and thawing of the surrounding soil prevents normal operation of pipelines and may lead to their fracture. This problem is especially relevant for buried pipelines in freezing soils with high water content. The traditional explanation for this emergence of the phenomenon is the Archimedean force (even the term “flotation” indicates this). At the same time, a similar phenomenon known as “strain accumulation” or “ratcheting” is well known in high-temperature engineering. This phenomenon consists in the increase of a structure displacements with the number of loading cycles. Under certain conditions, such accumulation can become stable (the same in successive cycles) and lead to large accumulated displacements after a corresponding number of cycles. The study considers an attempt of applying the methods developed for analyzing displacement accumulation in high-temperature structures of power engineering industry, to the problem of pipeline flotation. It shows that such a mechanism (ratcheting) can play an important role in this task, and the choice of ways to counteract the flotation essentially depends on the process mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The uniaxial fatigue and ratcheting behavior of commercial pure titanium (CP‐Ti) was investigated by asymmetric cyclic stress‐controlled experiments at room temperature. The effects of mean stress, stress amplitude, stress ratio, and peak stress on ratcheting behavior and fatigue life were discussed. It was found that increasing mean stress, stress amplitude, and peak stress or decreasing stress ratio reduced fatigue life and promoted ratcheting behavior. The applicability of different fatigue life models was analyzed, and a new stress ratio‐related failure model was proposed based on the exponential increase of fatigue life with stress ratio. Among all the models investigated in this study, the exponential stress ratio‐related model has more advantage in fatigue life predictions for CP‐Ti under ratcheting‐fatigue interaction. [ABSTRACT FROM AUTHOR]

We introduce a learning framework in which a principal seeks to determine the ability of a strategic agent. The principal assigns a test consisting of a finite sequence of tasks. The test is adaptive: each task that is assigned can depend on the agent's past performance. The probability of success on a task is jointly determined by the agent's privately known ability and an unobserved effort level that he chooses to maximize the probability of passing the test. We identify a simple monotonicity condition under which the principal always employs the most (statistically) informative task in the optimal adaptive test. Conversely, whenever the condition is violated, we show that there are cases in which the principal strictly prefers to use less informative tasks. [ABSTRACT FROM AUTHOR]

In this paper, 4043 aluminum alloy was printed using additive manufacturing (AM) with robot cold metal transfer (CMT) technology. The microstructure and mechanical properties of different parts, including the bottom, middle and upper portions of the printed AM 4043 aluminum alloy, were analyzed. Ratcheting tests were performed using an asymmetric load, and the tensile properties of the pre-ratcheting and un-ratcheted specimens were tested. The results indicate that the dendrites in the middle region are larger than those at the top and bottom regions and that the coarse microstructure associated with the interlayer junction resulted in the decline of the mechanical properties. The tensile strength of the AM specimens is 165 MPa, and the yield strength is 50 MPa. The printed 4043 aluminum alloy exhibits mechanical properties similar to those for conventional cast and welded aluminum alloys. The ratcheting behavior indicates that an increase in the mean stress or stress amplitude leads to an increase in the ratcheting strain, and the accumulation of ratcheting strain causes plastic damage to the materials. Pre-ratcheting behavior causes the tensile and yield strengths to increase compared to the behavior of the un-ratcheted specimens. [ABSTRACT FROM AUTHOR]

As parameters in Chaboche model are difficult to be determined from experimental data, a single objective particle swarm optimization procedure was employed to obtain them. Hysteresis loop and uniaxial and biaxial ratcheting simulations were conducted to validate the determined models. Chaboche models determined by particle swarm optimization give more accurate simulation of ratcheting compared with the model determined by trial and error method. Chaboche models containing different backstress components were studied. Models determined considering uniaxial ratcheting can only predict uniaxial ratcheting precisely, while giving very bad simulation of biaxial ratcheting. The linear hardening rule in the N3L1 model clearly decreases the rate of the accumulation of ratcheting strain, and the N3L1 model gives the best simulations. For biaxial ratcheting, the fourth backstress component can decrease the rate of the accumulation apparently, while it has a little influence on prediction of uniaxial ratcheting. [ABSTRACT FROM AUTHOR]

In this paper, a method is proposed for specifying the parameters of the exponential hereditary function kernels of the endochronic theory of plasticity to describe the ratcheting (cyclic creep) effect of metallic materials under stress-controlled complex non-proportional loading. The method involves the dependence of the difference of plastic moduli on the ratcheting rate on the steady-state portion of the cyclic stress-strain curve. The plastic moduli are determined at the points where the maximum stresses act in different half-cycles of asymmetric loading. It is believed that the greater the difference of plastic moduli towards the mean stress in the cycle, the greater the strain increment in each cycle of loading. The statements of our previously proposed approach were used to determine the rate of plastic strain accumulation at the steady-state stage of deformation under biaxial loading. This approach, based on the data of uniaxial experiments under cyclic asymmetric loading in tension-compression and reversed torsion with the known value of the cycle nonproportionality parameter, is developed to analyze the cyclic loading paths with equal mean and amplitude von Mises stress values. With some simplifications, the expression is proposed to determine the parameters of the exponential kernel of the hereditary function depending on the cyclic path geometry and the known ratcheting rate for the basic loading path. Similar values for the exponential kernel parameters of the hereditary function are obtained in terms of the difference of plastic moduli by using a simpler bilinear model of elastoplastic deformation. The obtained values of the hereditary function parameters were used to simulate the ratcheting effect under uniaxial and biaxial cyclic loading. The loading programs and data of experiments were taken from the literature. The results of simulation have shown that the parameters of the constitutive equations for cyclic plasticity obtained with this method allow one to describe satisfactorily the kinetics of the stress-strain state of metallic materials subjected to biaxial non-proportional loading under cyclic creep. [ABSTRACT FROM AUTHOR]

Uniaxial ratcheting behavior of an automotive-grade dual-phase steel sheet of 1.6 mm thickness has been investigated through engineering stress-controlled asymmetric cyclic loading experiments in the elastoplastic regime at ambient temperature (~ 28°C). Experiments have been performed keeping mean stress, stress amplitude or maximum stress constant. The effect of stress combinations on ratcheting deformation has been rationalized in terms of plastic strain amplitudes, both in loading and in unloading directions. It is found that for all stress combinations the steady-state ratcheting rate follows a perfect power relationship. Scanning electron fractography reveals that failure under asymmetric cyclic loading conditions occurs in a ductile manner through the formation of voids as it happens under tensile loading. [ABSTRACT FROM AUTHOR]

The present research revisits rare experiments which determined elastic shakedown (SD) limit pressures of full scale radial and oblique nozzles partially penetrating spherical vessels. The experiments were conducted at Berkeley Nuclear Laboratories in England [1]. The SD limit pressures were determined via conducting consecutive series of internal pressure cycles and observing cyclic strain variation recorded by strain gauges cemented at predetermined various critical locations within the junctions' vicinities. The Nonlinear Superposition Method (NSM), formulated for computing elastic SD limit loads based on Melan's lower bound SD theorem, is successfully validated against recorded experimental outcomes for both nozzle configurations. Furthermore, full elastic-plastic cyclic loading finite element simulations were executed and illustrated very good correlation to the NSM results. [ABSTRACT FROM AUTHOR]

The present paper aims to perform an experimental and theoretical study regarding the inelastic deformation progressive accumulation observed in a polyvinylidene fluoride (PVDF) specimen machined from flexible pipe internal pressure sheaths under cyclic load-unload tensile tests and in monotonic tensile tests. Experiments show that the elastic mechanical behaviour is not rate-dependent because the Young's modulus does not suffer a considerable variation with change in the strain rate. However, the strain hardening induced by the plastic deformation is strongly rate-dependent. Hence, the mechanical behaviour of this polymer is not elasto-plastic or viscoelastic, but elasto-viscoplastic. Also, the kinematic hardening strongly increases with growth of accumulated inelastic strain, making it much more significant than the isotropic hardening. This paper proposes an elasto-viscoplastic mechanical model capable of describing the cyclic and monotonic inelastic behaviour for an arbitrary loading history. The kinenatic hardening observed for this PVDF makes it possible to identify experimentally all material parameters that arise in the model by performing load-unload tests using flat specimens, eliminating buckling problems and the need to control eventual misalignment in the universal test machine grips. Good agreement was noted when comparing the theoretical results with experiments. [ABSTRACT FROM AUTHOR]

Unidirectional plastic strain accumulation (ratcheting) is one of the main causes of surface crack nucleation in rails and wheels in dry condition. It is related to frictional forces which develop at the contact interface due to sliding, especially in curve and in braking. Surface cracks generated by ratcheting can subsequently lead to severe damage when environmental fluid contaminants (such as rain or snow) are added at the contact interface, due to the complex solid-fluid interaction, which can enhance crack growth. In dry condition, wear and ratcheting can reach an equilibrium, such that the strain field and the crack depth are stationary. Understanding such steady-state deformation regime can be a key factor for predicting the expected crack depth and scheduling a correct maintenance programme. In this paper a model for predicting the strain field at steady-state regime is proposed. Such model, based on an integral equation, allows predicting the strain field and crack morphology at high cycle number with no need of iterative numerical simulation. The potentiality of the model was proven both in characterizing the cyclic plasticity behaviour of materials and in predicting the maximum expected crack depth in full scale railway wheels. [ABSTRACT FROM AUTHOR]

The ratcheting behavior refers to the progressive accumulation of strains resulting from the cyclic loading applied to the specimens. Nowadays, much attention has been devoted to ratcheting pipelines. Hence, the ratcheting behavior of the SS316L U-shaped pipe subjected to internal pressure and cyclic bending force is investigated here. The experiments use the Servo-hydraulic device of Zwick/Roell Amsler HB 100. The local strains at the internal, external, and lateral bending points are specified using the pipe strain gauges. Then, the effect of the internal pressure, average force, and force amplitude on the ratcheting strains are examined and compared. The numerical simulation also employs the Abaqus software and the isotropic/kinematic nonlinear hardening model. It was revealed that ratcheting strain in the axial direction is more critical than in the pipe's peripheral direction. The effect of amplitude and average pressure on thin-wall the ratcheting behavior of thin-wall pipe in the axial direction during four thousand cycles are investigated. According to the observed results, increasing amplitude and average bending force while the internal pressure is constant increases the ratcheting strain. Besides, the variation of the force amplitude affects the ratcheting strain more significantly than the variation of the average force. In order to validate the obtained results, a careful comparison was made between the numerical and experimental results, and a good agreement was observed between them. Notably, the novelty of this study lies in the comprehensive analysis of the ratcheting behavior of the SS316L U-shaped pipe conducted here. • Proposing a framework for examining the ratcheting effect empirically and numerically. • The ratcheting strain in the axial direction is more critical than in the pipe's peripheral direction. • Examining the effect of amplitude and average pressure on thin-wall the ratcheting. • Investigating the behavior of thin-wall pipe in the axial direction during four thousand cycles. • Increasing amplitude and average bending force while the internal pressure is constant. [ABSTRACT FROM AUTHOR]

The prevention of excessive deformation by thermal ratcheting is important in the design of high-temperature components of fast breeder reactors (FBR). In an experimental study that simulated a fast breeder reactor vessel near the coolant surface, it was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. In this paper, we propose a simple screening criterion to prevent continuous accumulation of plastic strain derived from long distance travel of temperature distribution. The major cause of this ratcheting is the lack of residual stress that brings shakedown behavior at the center of yielding area. Because residual stresses are derived from constraint by the neighboring elastic region, we focused on the distance from the center of yielding area to the elastic region. Accordingly, the proposed criterion restricts the axial length of the full-section yield area, which is the double of the above distance. We validated the proposed criterion based on finite element analyses using an elastic-perfectly plastic material. As a result of the validation analyses, we confirmed that the accumulation of plastic strain saturates before second cycles in the cases satisfying the proposed criterion, regardless of the shape of temperature distribution.

Penelitian ini bertujuan untuk mengetahui pengaruh ratcheting anggaran terhadap pencapaian mutu pendidikan. Ratcheting anggaran diukur dengan cara menghitung persentase varian anggaran tahun 2012 dan tahun 2013 pada pos anggaran gaji, perlengkapan dan bahan pembelajaran, serta anggaran non-instruksional. Mutu pendidikan diukur melalui pencapaian hasil rata-rata nilai ujian akhir nasional (NUN). Penelitian ini menggunakan data sekunder dari lembaga pendidikan. Dari 151 lembaga pendidikan yang ada di Wilayah Kota Madiun, yang dapat digunakan untuk penelitian sebanyak 42 lembaga dengan metode penelitian untuk mengukur ratcheting anggaran menggunakan regresi berganda (multiple regression) sedangkan untuk menguji pengaruh ratcheting anggaran terhadap pencapaian mutu pendidikan menggunakan regresi sederhana (ordinary regression). Hasil penelitian menemukan bahwa Ratcheting anggaran berpengaruh negatif terhadap pencapaian mutu pendidikan, sehingga dapat disimpulkan bahwa belum tentu peningkatan ratcheting anggaran diikuti oleh peningkatan pencapaian mutu pendidikan

Ratcheting failure of materials and structures subjected to low cycle fatigue in the presence of significant mean stress is of great interest to researchers. In this experimental and numerical study, the response of 316L stainless steel samples was observed in symmetric strain control uniaxial test followed by post-stabilized monotonic test, uniaxial and biaxial ratcheting tests, in order to determine the Chaboche model parameters and to evaluate ratcheting prediction using finite element analysis. The critical elastic limit was initially obtained from incremental uniaxial cyclic tests. The Chaboche parameters were subsequently extracted from experimental hysteresis and post-stabilized monotonic stress plastic-strain curves using two optimization technics, namely, the Particle Swarm Optimization (PSO) and Genetic Algorithm (GA). The two optimization methods were compared for efficiency, in terms of time and accuracy. The PSO method presented higher efficient results and was subsequently used to derive the parameters from hysteresis and post-stabilized monotonic curves. Different values (by definition) of elastic limit were also used. The Finite Element commercial software ANSYS was utilized with the Chaboche model to predict the uniaxial and biaxial ratcheting behavior of 316L stainless steel pipe. The comparison between experimental and the numerical simulation demonstrates that adopting post-stabilized monotonic curve rather than hysteresis curve and with accurate elastic limit obtained from incremental loading test improves ratcheting prediction significantly.

In this study, the strain ratcheting behavior of piping branch under the influence dynamic bending moments are evaluated. The Chaboche nonlinear kinematic hardening model and combined Armstrong- Fredrick model with isotropic rule are used to predict the plastic behavior of the piping branches. The results of FE method by using the hardening models have been compared with the results of the experimental method and Armstrong-Fredrick kinematic hardening results. The constant parameters of the hardening model and stress-strain data have been obtained from several stabilized cycles of specimens that are subjected to simulated seismic bending cycles. Both the FE and experimental results showed that the maximum strain ratcheting occurred on the flanks in the piping branch hoop stress direction just above the junction. The ratcheting strain rate increases with increase of the dynamic moment levels. The FE results show that initial rate of ratcheting is large and then it decreases with the increasing of loading cycles. In BMS1 sample, the FE hoop strain ratcheting data by using chaboche nonlinear kinematic hardening model comparing with the other hardening models to be near that found experimentally values. In BMS2 and BMS3 components, the FE hoop strain ratcheting data by using chaboche nonlinear kinematic hardening model and combined hardening model comparing with the A-F hardening model to be near that found experimentally values. The hoop strain ratcheting rate by Armstrong-Fredrick model gives overestimated values comparing with the experimental data. [ABSTRACT FROM AUTHOR]

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1. [ABSTRACT FROM AUTHOR]

Strain-life methodologies are commonly employed for fatigue estimation in military aircraft structures. These methodologies rely on models describing the elastoplastic response of the material under cycling. Despite the numerous advanced plasticity models proposed and utilised in various engineering problems over the past decades, the Masing model remains a popular choice in fatigue analysis software, mainly due to its simplicity. However, in the case of military aircraft load spectra including scattered overloads the Masing model fails to represent adequately transient cyclic phenomena, such as mean stress relaxation and strain ratcheting. In this study, four well-known constitutive plasticity models have been selected as potential substitutes for the Masing model within a defence organisation in-house developed fatigue analysis software. These models assessed were the well-known Multicomponent Armstrong–Frederick Model (MAF) and three of its derivatives: MAF with threshold (MAFT), Ohno–Wang (OW) and MAF with Multiplier (MAFM). The models were calibrated with the use of existing experimental data, obtained from aircraft aluminium alloy tests. Optimisation of the parameters was performed through a genetic algorithm-based commercial software. The models were incorporated in the fatigue analysis software and their performance was evaluated statistically and compared against each other and with the Masing model for a series of different flight load spectra for a military aircraft. The results show that all four models have achieved a drastic improvement in fatigue analysis, with the MAFT model giving a slightly better performance. [ABSTRACT FROM AUTHOR]

Elastoplastic investigations of aerospace aluminum are important in the development of an understanding of the possible cyclic transient effects and their contribution to the material performance under cyclic loading. Cyclic plasticity can occur in an aerospace aluminum component or structure depending on the loading conditions and the presence of external and internal discontinuities. Therefore, it is vital that the cyclic transient effects of aerospace aluminum are recognized and understood. This study investigates experimentally the cyclic elastoplastic performance of aluminum 7075-T6 loaded in symmetric strain control, and asymmetric stress and strain control. A combination of cyclic hardening and softening was noticed from high strain amplitude symmetric strain-controlled tests and at low stress amplitude asymmetric stress-controlled tests. From asymmetric strain control results, the extent of mean stress relaxation depended on the size of the strain amplitude. Additionally, saturation of the ratcheting strain (plastic shakedown) was also found to occur during asymmetric stress control tests. The experimental results were further analyzed using published microstructure research from the past two decades to provide added explanation of the micro-mechanism contribution to the cyclic transient behavior. [ABSTRACT FROM AUTHOR]

AISI H13 hot-work tool steel is extensively used as tool material for the aluminum extrusion process and its working temperature is about 500 °C. It is essential to model cyclic responses and conduct lifetime prediction of H13 reasonably for optimization of the tools. In this study, uniaxial tests are conducted to investigate mechanical properties of H13. It can be observed that the material shows significant rate-dependence and obvious cyclic softening effects. In addition, the evolution of ratcheting obviously has three stages: gradually decreasing stage, approximately constant stage, and rapidly increasing stage. Considering above-mentioned characteristics, a modified visco-plastic constitutive model is proposed to describe the cyclic plasticity responses in the whole lifetime by a combination of modified A-F kinematic hardening rule and a new proposed isotropic hardening rule. Furthermore, its lifetime is predicted by involving a failure criterion with the critical size of yield surface. Accelerated ratcheting phenomena and peak stress reduction in failure phase are simulated reasonably. In addition, the simulation results of cyclic responses give a good estimation of plastic deformation under cyclic loading conditions. All in all, the comparisons of simulation and experimental results indicate that the proposed constitutive model predicts mechanical properties, as well as lifetime reasonably. [ABSTRACT FROM AUTHOR]

In this work, we investigate the numerical convergence of a set of plasticity models with different kinematic and directional distortional hardening rules under cyclic plastic loading. In particular, we revisit the results presented in Feigenbaum et al. (2012) in order to more robustly check for convergence during the numerical integration procedure, and show that the results presented in the previous work do not converge. We investigate the role of the step-size and numerical scheme on the convergence of these models when predicting ratcheting. By reducing step-sizes and using a forward Euler scheme during numerical integration, converged solutions are obtained. The new converged results lead to new conclusions. Results still suggest that directional distortional hardening can improve ratcheting predictions, however the addition of directional distortional hardening yields less improvements compared to kinematic hardening alone than previously thought. This new conclusion, strongly suggests the need for additional modeling developments in order accurately predict ratcheting strains under a wide variety of cyclic plastic loadings. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulation-based studies of tensile test and structural evolution of Cu-5 at.% Zr alloy under asymmetric cyclic loading (i.e., ratcheting behavior) considering various stress ratios such as − 0.2, − 0.4 and − 0.6 for different temperatures, viz.≈ 100, 300 and 500 K have been performed using embedded atom model Finnis-Sinclair potential. According to obtained stress-strain response from MD calculation, Cu-5 at.% Zr alloy specimen is pristine in nature as sudden drop in stress just after yield stress and subsequent elastic type deformation are observed for this alloy. Predicted ratcheting strain by MD simulation for Cu-5 at.% Zr alloy varies from 4.5 to 5%. Significant increase in ratcheting strain has been observed with the increase in temperature. Slight reduction in crystallinity is identified at the middle of the each loading cycle from the performed radial distribution function analysis and cluster analysis. [ABSTRACT FROM AUTHOR]

The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and ratcheting during combined cycling at various temperatures ranging from ambient to 923 K, in a type 316LN austenitic stainless steel. The experiments were designed with multi-step load sequences where specific number of small amplitude HCF cycles (referred to as blocks ) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with increase in temperature as well as block-size. The decrease in cyclic life with block-size is more significant at 923 K where multiple damage modes like creep and ratcheting are activated. Dynamic strain aging (DSA) was found to operate in the temperature range, 823–873 K where the decrease in cyclic life with block-size gets saturated. Typically, transgranular fatigue fracture, intergranular creep fracture or dimpled rupture was identified when failure was dictated by LCF, creep and ratcheting respectively. However, synergistic interaction between the above damage modes leading to a mixed mode fracture carrying signatures of fatigue striations, intergranular facets and dimples occurred at specific combinations of block size and temperature. HCF damage played an important role for some specific loading conditions by acting as a link between intergranular (creep) cracks, thus facilitating the crack propagation and final failure. The regimes of dominant failure modes and interactions among them were suitably mapped as a function of temperature and block size. [ABSTRACT FROM AUTHOR]

The work is devoted to the experimental study of steels and alloys deformation properties with account to the mutual influence of the cyclic and static deformation components. Carbon steel and three different chrome-nickel austenitic steels were tested at different temperatures and durations. At strain-controlled cyclic loading these steels demonstrate cyclic hardening (or softening – depending on the previous history) at the running-in stage, then stabilization and subsequent softening at the pre-fracture stage. Experiments show that in case of comparable cyclic and static damage the mutual influence of the cyclic and static material hardening (softening) strongly affects the stress-strain diagrams; this effect should be taken into account in structural analysis. Models that use only the information about static or strain-controlled cyclic loading are incapable to describe this effect. [ABSTRACT FROM AUTHOR]

Ratcheting fatigue behavior of different alloys and their weld joints has been a topic of interest in the past few decades. However, little information is available about the ratcheting response in different zones, when a composite weld section is subjected to asymmetric loading cycle. This work aims at understanding the local ratcheting response in various zones of a dissimilar metal weld (DMW) joint. Digital image correlation technique has been used to measure local ratcheting strain components of a DMW joint. Accumulation of ratcheting strain in various zones of a DMW joint was found to be different, harder region accumulating negligible and softer region accumulating larger local ratcheting strain. Fatigue crack initiation or neck formation occurred in soft region owing to high ratcheting strain accumulation. [ABSTRACT FROM AUTHOR]

Metallic tubes experiences a progressive accumulation of ovalisation, or ratchet, under cyclic inelastic bending. The current paper deals with the ratcheting behaviour of defective small-scale API X80 carbon steel tubes subjected to cyclic bending. The subject appears not to have received due previous attentions but is important in the design and mechanical integrity assessment of the piping systems. Right-angle uniform material loss is used to simulate an external corrosion patch in the test tubes. It is noticed that under inelastic cyclic bending the ovalisation ratchets both with the perfect and defective specimens. The rate of ratcheting increases and the number of cycles to failure decreases in the defective specimens as compared to those in the perfect ones. Both perfect and defective specimens show ratcheting in the circumferential strains, but this quickly ends in shakedown. The local deformation modes in the perfect specimens are characterised by diffusive wrinkling, but by inward kinking in the defective specimens. The perfect specimens demonstrate similar ratcheting rates in the sagging and hogging phases, but the defective specimens not. The modes of local deformation and failure for specimens with identical defects remain almost the same under cyclic and monotonic bending. [ABSTRACT FROM AUTHOR]

Ratcheting occurs easily because of the presence of mean stress during the stress-control fatigue of engineering components. For ductility exhaustion dominated fatigue failure, a new fatigue life prediction model is developed by introducing the mean ratcheting strain rate to incorporate the effects of ratcheting and mean stress on fatigue life. The prediction accuracy of the proposed model was compared with that of the generalised damage parameter, Xia-Kujawski-Ellyin, Walker and Goswami models. Specifically, the model predictions and tested lives were compared using nine sets of experimental data from the literature. In the statistical analysis of these five models, the proposed model provides the highest accuracy and robust life predictions with the lowest model prediction errors. [ABSTRACT FROM AUTHOR]