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Deformation of rectilinear elements of Du 200 pipeline made of 20 steel under cyclic loading with internal pressure has been studied experimentally. Tests were carried out on a pneumohydraulic bench under conditions close to plane stress state, room temperature, and cycle asymmetry under soft loading mode (stress control). Circumferential and axial strains were recorded at three points of a diametrical section located in the middle of the element under study. Two tubular elements were tested at different values of maximum and amplitude stresses based on 100 cycles of loading. Deformation curves (cyclic creep) of tubes in a circular direction have been obtained, the character of which is significantly non-uniform in comparison with deformation of laboratory specimens. It is shown that even a slight increase in maximum (by 4%) and amplitude (by 8%) stresses significantly (up to 50%) increases the cyclic creep strains in the circular direction. Such growth of deformations can be explained by a peculiarity of 20 steel, the deformation diagram of which in the elastoplastic transition area contains a yield point, which determines intensive deformation of the material. Fixed plastic deformations in the axial direction are more than an order smaller than circular ones, and with increasing loading cycles, they change insignificantly and coincide for the two studied tubular elements. [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]

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]

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]

Uniaxial stress-controlled ratcheting behaviors of expanded PTFE (ePTFE) under cyclic compressive loads were tested. The effects of temperature, stress rate and mean stress on the ratcheting behaviors of ePTFE considering multiple load paths were discussed in detail. Results present that the steady ratcheting strain is rate-independent when the stress rate is less than about 0.1 MPa/s, while it approximately linearly decreases with increasing the stress rate for greater stress rate. Additionally, the steady ratcheting is temperature-independent when the temperature is greater than about 150 °C, but it nearly linearly increases with enhancing the temperature for lower temperature. Especially, the stress rate almost has little effect on the ratcheting strain of ePTFE at 200 °C. Moreover, the accumulated ratcheting strain enhances rapidly in about the first 80 cycles, and subsequently tends to shakedown in the subsequent cycles for each load path. Furthermore, if a higher stress is used in the prior cycling, the greater ratcheting strain may be produced, and a negative ratcheting strain rate can be obtained in the subsequent cycling with lower mean stress due to the greater strain hardening and deformation resistance produced by the previous higher stress. [ABSTRACT FROM AUTHOR]

This study aims to characterize numerically the ratcheting behavior of a girth-welded straight stainless steel pipe in combined action of internal pressure and cyclic bending loading. Finite element (FE) thermal simulation of the girth butt welding process is first performed to identify weld-induced residual stresses. Three-dimensional (3-D) elastic-plastic FE analyses incorporated with the cyclic plasticity constitutive model capable of describing the cyclic plastic performance are next conducted to scrutinize the local (circumferential strain) and global (cross-section diameter change) ratcheting responses of the girth-welded stainless steel pipe under internal pressure and cyclic bending, which take the residual stresses and plastic strains obtained from the preceding thermal simulation as the initial condition. The analytical results demonstrate that welding residual stresses in combination with the internal pressure have significant effects on the hoop strain rate and the in-plane and out-of-plane diameter changes, and the degree and shape of the ovalization which occurs during the multiaxial ratcheting are dependent on the applied loads. [ABSTRACT FROM AUTHOR]

The evolution of the cyclic yield stress for a railway wheel steel (UIC ER7T) during cyclic plastic straining has been characterized at different temperatures from −60 to 600 °C. Different constant strain amplitude levels were examined and for temperatures above 200 °C, hold periods were included to study stress relaxation during constant compressive strain. The results are of use in predicting material deformation and damage. This is demonstrated by the application to improve a criterion for surface initiated rolling contact fatigue damage. [ABSTRACT FROM AUTHOR]

The integrity assessment of structures subjected to cyclic loading must be verified with regard to cyclic type damage including time-independent fatigue and progressive deformation or ratcheting. Cyclic damage is verified simulating the material elastic-plastic loop and looking at the accumulated net plastic strain during each cycle at all points of the structure subjected to the complete time history of loadings. This work deals with the development of a numerical model producing the Chaboche hardening parameters starting from stress-strain data produced by testing of materials. Then, the total plastic strain can be simulated using the Chaboche inelastic constitutive model requested for finite element analyses. This is particularly demanding for pressure vessels, pressurised piping, boilers, and mechanical components of nuclear installations made of stainless steels. A design optimisation by iterative analyses is developed to approach the stress-strain test data with the Chaboche model. The parameters treated as design variables are the Chaboche parameters and the objective function to be minimised is a combination of the deviations from test data. The optimiser calls a macroinstruction simulating cyclic loading of a sample for different material temperatures. The numerical model can be used to produce hardening parameters of materials for inelastic finite element verifications of structures with complex joints like elbows subjected to a combination of steady sustained and cyclic loads. [ABSTRACT FROM AUTHOR]

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633 K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling. [ABSTRACT FROM AUTHOR]

The uniaxial fatigue behavior of the Cu–Nb micro-composite conductor has been studied considering the effect of peak stress and stress ratio (ratio of the minimum stress to peak stress). With the uniaxial tensile test and the three-point bending test, it is found that the conductor is cyclic stable without hardening or softening and its time dependence is slight below the loading frequency of 67 Hz. The ratcheting deformation in the uniaxial tension–tension fatigue test is found to increase with the peak stress while being restrained at higher stress ratio. SEM observations show that there are mainly three fracture modes in the Cu–Nb conductor at different loading conditions: the fracture by necking, the shear fracture and the nominal fracture. The fracture by necking at higher peak stress is caused by microvoid coalescence while the nominal fracture at lower peak stress is due to the progressive interfacial cracks. The shear fracture at intermediate peak stress can be seen as a mixture of the other two modes with visible tortuous dimples and fatigue striations. Given the same peak stress, a higher stress ratio would result in a more brittle fracture as a mixture of the neighboring two of the above three modes. In a macroscopic point of view, the above differences in fracture modes are related with the ratcheting deformation and accumulative cyclic deformation. The ratcheting deformation is responsible for the fracture by necking at higher peak stress with lower stress ratio while the accumulative cyclic deformation leads to the nominal fracture at lower peak stress with higher stress ratio. The shear fracture can still be seen as a combined effect of deformation types. At last, the experimental S–N relations of the conductor are nonlinear in log–log scale in a broad range of peak stress due to different fracture modes. They fit well with a modified Basquin’s Equation proposed in this paper. [ABSTRACT FROM AUTHOR]

To understand the nature of cyclic plastic deformation and damage on a fatigue crack tip, in-depth plane strain elasto-plastic finite element analysis was conducted on a compact tension specimen. In the cyclic plastic zone, low cycle fatigue is the dominant cyclic deformation mode during symmetric load cycling, while ratcheting is dominant during asymmetric load cycling. The size of cyclic plastic zone depends upon the amplitude of stress intensity factor while, the size of monotonic plastic zone depends upon the maximum stress intensity factor. The size of monotonic plastic zone is equal to cyclic plastic zone during symmetric load cycling. [ABSTRACT FROM AUTHOR]

Knowledge of cyclic plastic deformation response at the fatigue crack tip is crucial to understand the nature of cyclic damage acting on the crack tip. Interaction between crack and defect is equally important to understand the cyclic damage progression at crack tip; however isotropic hardening model was adopted in all existing literatures. Isotropic hardening model is suitable to model only the monotonic plastic zone and unable to model the cyclic/reverse plastic zone. Kinematic hardening model is suitable to model the key cyclic plastic deformation responses like Bauschinger effect, ratcheting, and mean stress relaxation. A non-linear kinematic hardening (Chaboche) model is used in this present investigation to represent the material’s cyclic stress–strain response accurately. Effect of micro-cavity positions (angle and distance from crack tip) and sizes on plastic zones near a crack tip is investigated in this study by two dimensional plane strain finite element model of a compact tension specimen. It is observed that the size and shape of the monotonic and cyclic plastic zones are affected by position and size of the micro-cavity. During asymmetric loading condition, the ratcheting strain accumulation direction is also affected by the position of micro-cavity. [ABSTRACT FROM AUTHOR]

Creep-ratcheting deformation behavior of columnar nanocrystalline (NC) Al varying in grain sizes have been studied using molecular dynamics simulations at three different temperatures (300 K, 467 K, and 653 K). The underlying deformation mechanisms of columnar NC Al for two stress ratios of ratcheting are also evaluated in this study. The highest strain accumulation and rapid hysteresis loop proliferation are attained at 653 K. The dislocation density is low at 653 K in contrast to the other two deformation temperatures. The perfect dislocations support the creep-deformation process followed by Shockley partial dislocations. It is observed that the cyclic hardening phenomenon at 300 K and the cyclic softening phenomenon at 653 K dominate during creep-ratcheting loading. The specimen fails earlier at higher temperatures owing to the change in the shape of hysteresis loops. The predominant grain boundary-based deformation mechanisms of columnar NC Al specimens are grain boundary (GB) migration, GB diffusion, GB widening, GB sliding, and GBs merging for all the deformation temperatures. The variation in the extent of deformation with respect to temperatures depends on strain accumulation in the plastic region under creep-ratcheting loading. [Display omitted] • GB diffusion process in the columnar NC specimen during creep-ratcheting process. • The highest strain accumulation and rapid hysteresis loops propagation at 653 K. • Grain boundary-based deformation mechanisms involved in columnar NC Al specimen. • Cyclic hardening phenomenon at 300 K and the cyclic softening phenomenon at 653 K. [ABSTRACT FROM AUTHOR]

In this paper the framework of a combined Chaboche type kinematic and isotropic hardening model is explained for large deformation with Henky strain description to evaluate strain ratcheting of interference fitted double shear lap joints near the hole based on the works of Montáns et al. [doi: 10.1016/j.mechrescom.2012.03.001 ] and Henann and Anand [doi: 10.1016/j.ijplas.2008.11.008 ]. The described elasto-plasticity formulation is in the context of hyper-elasticity and based on the dual multiplicative decomposition of the deformation gradient into elastic and plastic parts. The decomposition of plastic part into energetic and dissipative part is considered and a new rheological model is suggested for extension of one term Armestrong–Frederick (A–F) model to multi-component A–F (Caboche) model in finite regime. A brief and simple numerical integration scheme is suggested for integrating the presented kinematic hardening. The suggested integration algorithm reduces the complexity of large deformation and for convenience, allows embedding small strain subroutines. To solve the system of nonlinear equations of the constitutive model, the successive substitution method is suggested for simplifying the solution and reducing the computational cost appreciably. In order to avoid non-smooth interaction between the pin and the hole, and also unrealistic plastic deformation, a well-established and an accurate frictional mortar contact formulation is used and combined with the material nonlinearity. The whole numerical formulations are coded in 3D finite element code in FORTRAN programming language and the code is optimized so that it can be efficient for ratcheting simulation over cycles. The approach shows stability and robustness in convergence due to consistent linearization of all contact and internal forces even in the presence of contact, geometry and high material nonlinearity. To evaluate the efficiency and accuracy of the presented formulations, the ratcheting data from simulation is compared with the experimental ratcheting data from strain gauges attached to the studied double shear lap joints. [ABSTRACT FROM AUTHOR]

The present investigation looks into the effect of forward deformation mode, monotonic and cyclic, but asymmetric, on the Bauschinger phenomenon of annealed polycrystalline copper in terms of different Bauschinger parameters for 2.5–18% unidirectional tensile strain. Apart from the amount of deformation, the Bauschinger phenomenon has also been looked into as a function of stress rate used for forward deformation and also the grain size. It is found that Bauschinger effect is less pronounced when the forward deformation is carried out through asymmetric cyclic loading as compared to monotonic loading. This difference in the Bauschinger effect is attributed to lower back stress developed during asymmetric cyclic loading as compared to monotonic deformation. In the present investigation very negligible influence of stress rate on Bauschinger effect has been observed. On the other hand, for both monotonic and asymmetric cyclic deformation it is observed that Bauschinger effect is more in case of fine grain size. [ABSTRACT FROM AUTHOR]

Plastic zones and associated deformations ahead of a fatigue crack are well established nowadays. In-depth plane strain elasto-plastic finite element analysis is conducted in this investigation to understand the nature of cyclic plastic deformation and damage around soft and hard elliptical inclusions. Similar to fatigue crack tip, cyclic/reverse plastic zone and monotonic plastic zone are visible for soft elliptical inclusion. In the cyclic plastic zone, low cycle fatigue is the dominant cyclic deformation mode during symmetric load cycling, while ratcheting is dominant during asymmetric load cycling. The size of cyclic plastic zone depends upon the amplitude of remote stress while, the size of monotonic plastic zone depends upon the maximum remote stress. The size of monotonic plastic zone is equal to cyclic plastic zone during symmetric load cycling. The shape and size of plastic zones also depend upon the orientation of the soft inclusion. Cyclic plastic damage progression in the cyclic plastic zone for soft (MnS) inclusion is significant, while no cyclic plastic zone is visible for hard inclusion (Al 2 O 3 ). [ABSTRACT FROM AUTHOR]

To investigate the material behavior of a stainless steel grade AISI 316L, subjected to loading conditions typical for the primary cooling circuit of a light water nuclear reactor, a set of uniaxial, torsional and multiaxial low cycle fatigue and strain-controlled ratcheting tests has been carried out. The main goal of these experiments is to analyze how the cyclic deformation response is influenced by strain amplitudes, loading paths and ratcheting, and to estimate how fatigue life is affected by these elements. Finally, the performance of three advanced multiaxial fatigue criteria (i.e. Smith-Watson-Topper, Fatemi-Socie and Jiang) has been critically evaluated for all the considered loading cases. [ABSTRACT FROM AUTHOR]

Performance-based design differs from conventional design in that target performance is clearly stated and explicitly checked. A faithful application of this design approach requires refined analysis methods to quantify performance and explicit consideration of all relevant uncertainties. The paper presents a design procedure that satisfies these requirements. The procedure is illustrated with reference to flexible earth-retaining diaphragm walls, which represent a challenging test for such a procedure given the influence of dynamic soil–structure interaction on the response in terms of internal forces and deformations. In order to remain affordable and feasible in practice, yet accounting for the aforementioned sources of uncertainties and the important physical aspects of the system response, the procedure is based on small-sample Monte Carlo simulation and a recently developed nonlinear dynamic model of such soil–wall systems. The latter strikes a balance between model setup and analysis time, on one hand, and accuracy on the other, being capable of re-producing the cumulative build-up of soil pressure in cyclic deformation and the accumulation of permanent displacement of the wall, with an overall modeling and analysis effort in the order of few tens of minutes (on a standard consumer-grade laptop). Performance is measured in terms of mean annual rate of exceedance curves from which design values are obtained. [This paper is an extended version of the contribution presented at the mini-symposium on Performance-based design held at the ICOSSAR conference in June 2013, NY.] [ABSTRACT FROM AUTHOR]

Abstract: Ratcheting behavior of sintered nanosilver joints was examined by cyclic shearing tests at different temperatures. Fatigue lives of sintered nanosilver joints were compared with those of soldered SAC305 joints. An improved non-contact displacement detecting system combining infrared heating was developed to measure the deformation of the joints. Effects of temperature, mean stress, and stress amplitude on the ratcheting behavior of both kinds of the joints were analyzed. Shear strength of the sintered nanosilver joints is larger than that of the soldered SAC305 ones. The ratcheting displacement and its rate in both kinds of the joints increase with increase in temperature, mean stress, and stress amplitude. The fatigue life of sintered nanosilver joint is much longer than that of soldered SAC305 one under the same loading conditions. It is concluded that sintered nonosilver joint has demonstrated a longer fatigue life and better response to shearing and cyclic loading than SAC305 joint, especially at high temperatures. An improved temperature-dependent modified Goodman model is proposed and is proved to be able to accurately predict the fatigue life of the sintered nanosilver joints and the soldered SAC305 joints by introducing the temperature-dependent fatigue strength exponent and the fatigue strength coefficient. [Copyright &y& Elsevier]

Abstract: A model to predict the ratcheting behavior of the near-surface material layer under high traction coefficients has been developed. The model explicitly takes into account the longitudinal, lateral and spin creep in determining the displacement increment at the surface due to plastic shear deformation. The model is computationally efficient and allows to perform plasticity calculations based on results from multi-body system simulations. The approximate model has been validated with results from finite element simulations and calibrated for R260 rail steel with data from twin disc tests. In addition, a parametric study has been performed to show the model behavior as a function of creep, in which the interaction with wear as an independent mechanism is taken into account. Application of the model to results of multi-body system simulations of the metro system in Vienna shows that the prediction of the crack pattern at the rail surface is feasible with the chosen approach. [Copyright &y& Elsevier]

Highlights: [•] Modeling of cyclic plastic and ratcheting deformation behavior of AISI 316L. [•] Capability to reproduce the effect of strain amplitude, temperature and ratcheting. [•] Internal variable dependent Chaboche-type model. [•] Calibration procedure characterized by reduced complexity allowing the application in an industrial context. [•] Straightforward implementation in a commercial finite element code. [Copyright &y& Elsevier]

Highlights: [•] Ratcheting experiments conducted for the first time involving twinning–detwinning in Mg. [•] Long-term ratcheting rate decay identified to follow a power law relationship. [•] Three types of cyclic deformation of Mg identified. [•] Twinning–detwinning found to be a key cyclic hardening mechanism in Mg. [ABSTRACT FROM AUTHOR]

Abstract: Deformation of homogeneous elastic-perfectly plastic (EPP) and elastic-linear kinematic hardening plastic (ELKP) half-spaces due to repeated adhesive sliding contact is examined with a two-dimensional finite element model. Interfacial adhesion is modeled by nonlinear springs with a force-distance constitutive relation derived from the Lennard–Jones potential. Deformation behavior is interpreted in terms of dimensionless parameters, such as the Maugis parameter, interaction distance (interfacial gap), and plasticity parameter. Numerical results provide insight into the stress–strain response and the evolution of subsurface plasticity. The effects of the interaction distance, plasticity parameter, interfacial adhesion, and Maugis parameter on the friction and normal forces and the accumulation of plastic strain are analyzed in terms of the number of sliding cycles. Deformation maps providing information about the steady-state mode of deformation are presented for both EPP and ELKP material behaviors. Results of this study illustrate the importance of elastic–plastic material behavior and interfacial adhesion in contact deformation due to repeated surface sliding. [Copyright &y& Elsevier]

Abstract: Ratcheting behavior of 316LN austenitic stainless steel (SS) was investigated at 823K as a function of mean stress (σ m ) and stress amplitude (σ a ). Test results obtained under different combinations of σ m –σ a were analyzed in terms of total accumulated strain and cyclic life in order to understand the manifestations of dynamic strain aging (DSA) during ratcheting. The results obtained under specific combinations of σ m and σ a were correlated with the surface relief, crack density distribution, fracture behavior and substructural changes. Deformation behavior was found to vary with the loading conditions. DSA increased the fatigue life of 316LN SS. [Copyright &y& Elsevier]

Abstract: Experimental data are reported on isotactic polypropylene in uniaxial multiple-step cyclic tests at room temperature. The deformation programs involve (i) one- and two-step cyclic tests with a stress-controlled program (ratcheting with various maximum stresses and the zero minimum stress), (ii) short-term creep tests on specimens subjected to cyclic deformation, (iii) cyclic tests with a stress-controlled program on specimens subjected to creep flow, and (iv) triple-step tests (ratcheting with various maximum stresses interrupted by creep). A pronounced difference is observed in damage accumulation under creep and cyclic loading. Applicability is discussed of these observations for lifetime prediction under fatigue conditions. [Copyright &y& Elsevier]

Abstract: Ratcheting deformation is studied on elbow pipe made of Z2CND18.12N stainless steel with local wall thinning subjected to constant internal pressure and reversed in-plane bending under load control. The local wall thinning is located at extrados and crown of elbow. It is shown that local wall thinning has a very pronounced effect on the ratcheting behavior of elbows, as compared to the ratcheting behavior of sound elbow specimen. Three-dimensional elastic-plastic analyses with ANSYS in which Chaboche and Chen–Jiao–Kim (CJK) kinematic hardening model are carried out to evaluate structural ratcheting behaviors. A reasonable agreement has been found between the experimental and the simulated results with CJK model for the ratcheting of the elbows. The ratcheting boundary is determined by evaluating variations in the plastic strain increment with CJK model. The effects of the depth and location of local wall thinning on the ratcheting response are discussed by CJK model. [Copyright &y& Elsevier]

Abstract: The present paper is concerned with the experimental investigation of the progressive accumulation of deformation observed in cyclic tension tests performed on an epoxy polymer. The strain rate dependency is almost negligible and the material behaviour is elastic-plastic rather than elastic-viscoplastic or viscoelastic. Under cyclic loading and unloading, this polymer exhibits hysteresis (a phase lag), which leads to dissipation of mechanical energy and to the accumulation of cyclic plastic deformation. It is verified experimentally that this phenomenon is essentially due to kinematic hardening. Preliminary ideas for an elastic-plastic phenomenological model able to describe the plastic cyclic behaviour of this epoxy polymer are also presented. [Copyright &y& Elsevier]

Abstract: To determine the nature of damage acting on a fatigue crack-tip, it is mandatory to know the detailed cyclic plastic deformation response. In this work, cyclic plastic deformation response has been studied on a stationary CT specimen crack-tip in an elasto-plastic finite element frame work at selected R-ratios and ΔK. Chaboche kinematic hardening model has been used to describe material behavior. Monotonic and cyclic/reverse plastic zone identified during plane strain finite element analysis. It is noticed that progressive accumulation of permanent strain i.e. ratcheting occurs in the cyclic plastic zone ahead of a fatigue crack-tip. [Copyright &y& Elsevier]

The influence of dynamic strain aging (DSA) on the ratcheting behaviour of 316LN stainless steel was investigated at 823 K as a function of mean stress (σm) and stress amplitude (σa). Test results obtained under different combinations of σm - σa were analysed in order to arrive at a map delineating different deformation regimes viz. ratcheting, strain burst and elastic shakedown. It was shown that the synergistic effect of σm and σa can be described by the ratio of mean stress and stress amplitude (σm/σa). A critical value of this ratio was identified to mark the transition between ratcheting and elastic shakedown and the same was used to predict deformation behaviour of the material in DSA under asymmetrical loading. [ABSTRACT FROM AUTHOR]

ABSTRACT This study intends to formulate ratcheting strain evolution in steel alloys of 42CrMo, 20CS, SA333 Gr. 6 C-Mn and OFHC copper over uniaxial stress cycles. Stages of ratcheting deformation were related to stress cycles, lifespan, mechanical properties and amplitude and mean stress components by means of linear and nonlinear functions. Terms of mechanical properties in the ratcheting formulation enabled to characterize ratcheting response of various materials over life cycles. These terms were further employed to interpret the influence of softening/hardening response of materials on ratcheting deformation. Ratcheting data for 42CrMo, 20CS, SA333 steels and OFHC copper reported in the literature were employed to evaluate the proposed ratcheting formulation. The predicted ratcheting strain values based on the proposed equation were found in good agreements as compared with the experimental data. [ABSTRACT FROM AUTHOR]

This paper investigates the response of granular materials to large numbers of shear stress cycles applied after monotonic loading to various levels of mobilized shear strength. Such stress conditions occur underneath the foundations of structures loaded by non-seismic oscillating forces, e.g. those due to wind and sea waves in tall buildings and offshore structures or induced by vibrating machines. Large numbers of load cycles can induce severe irreversible settlements of foundations and occasionally collapse, even when large safety factors are adopted with respect to bearing capacity in static conditions. Dense Toyoura sand has been used throughout the experimental investigation. Monotonic and cyclic tests have been carried out on specimens loaded in triaxial compression and extension, both in drained and undrained conditions. Specimens have undergone a large number of shear stress cycles and the conditions which cause accumulation of large deformations and even collapse have been analysed. An empirical approach for evaluating the accumulated irreversible strains is also presented. [ABSTRACT FROM PUBLISHER]

Abstract: The ratcheting effect greatly challenges the design of components of the nuclear power plant. Not only the mean stress and the stress amplitude, but the temperature also affects the ratcheting strain. In this study, the tensile properties of Z2CND18.12 stainless steel are studied. The relationship between the Young’s modulus and the temperature was studied experimentally. A series of ratcheting experiments under bending loading for Z2CND18.12 stainless steel were carried out on a dynamic mechanical analyzer. The characteristic of progressive inelastic deformation of the material in the different temperatures and different loadings were investigated. The difference of the domestic and imported stainless steel on the tensile properties and bending ratcheting properties was studied. The experiment results have shown that the imported Z2CND18.12 is better than the domestic one on the tensile properties. The Young’s modulus is linear with the temperature change, and gradually decreased with the increase of temperature. The temperature and the bending loading affect the ratcheting greatly. In addition, the imported one is better than the domestic one on the resistance to the ratcheting deformation at various temperatures. The ratcheting boundary of the imported one is defined based on the ratcheting experiments. [Copyright &y& Elsevier]

Abstract: Cyclic plastic deformation phenomena include the Bauschinger effect, cyclic hardening/softening, strain range effect, loading history memory, ratcheting, mean stress dependent hardening, mean stress relaxation and non-proportional hardening. In this work, different cyclic plastic deformation responses of piping materials (SA333 C–Mn steel and 304LN stainless steel) are experimentally explored. Cyclic hardening/softening is depends upon loading types (i.e. stress/strain controlled), previous loading history and strain/stress range. Pre-straining followed by LCF and mean stress relaxation shows similar kind of material response. Substantial amount of non proportional hardening is observed in SA333 C–Mn steel during 90° out of phase tension-torsion loading. During ratcheting, large amount of permanent strain is accumulated with progression of cycles. Permanent strain accumulation in a particular direction causes cross-sectional area reduction and which results uncontrollable alteration of true stress in engineering stress controlled ratcheting test. In this work, true stress control ratcheting on piping materials has been carried out in laboratory environment. Effects of stress amplitude and mean stress on the ratcheting behaviors are analyzed. A comparison has also been drawn in between the true and engineering stress controlled tests, and massive difference in ratcheting life and strain accumulation is found. [Copyright &y& Elsevier]

Abstract: Part I presented a set of experiments in which pressurized tubes were cycled axially under stress control about a compressive mean stress. This loading history causes biaxial ratcheting involving compressive axial strain and expansion of the diameter of the tube. The compressive strain in turn induces the initiation and growth of axisymmetric wrinkles. Persistent cycling resulted in localization of the wrinkles and collapse. In Part II the problem is first modeled as a shell with initial axisymmetric imperfections while the biaxial ratcheting of the material is modeled using the Dafalias–Popov two-surface nonlinear kinematic hardening model. It is demonstrated that when suitably calibrated this modeling framework reproduces the prevalent ratcheting deformations and the evolution of wrinkling including the conditions at collapse accurately for all experiments. The calibrated model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories experienced by axially restrained pipelines. [Copyright &y& Elsevier]

Abstract: Low cycle fatigue, ratcheting and dependence of their deformation response on previous deformation histories are investigated at room temperature laboratory environment for SA333 C–Mn steel. It is noticed that cyclic hardening/softening response of low cycle fatigue is considerably influenced by applied strain amplitude, monotonic pre-straining and high to low step loading sequences. Results indicate that ratcheting under high to low step loading has significant effect on its subsequent ratcheting response. However, ratcheting under low to high step loading has very little effect on successive ratcheting responses. [Copyright &y& Elsevier]

ABSTRACT Engineering stress-controlled ratcheting tests under different sets of stress amplitudes and mean stresses show that ratcheting deformation in polycrystalline OFHC copper occurs in three different stages. A plateau region with almost no accumulation of inelastic strain follows general ratcheting deformation during initial loading cycles. With breakdown of the plateau region inelastic ratcheting deformation occurs at an increasingly rapid rate. The effect of the stress amplitude on the ratcheting process is found to be more than mean stress effect. Reconstruction of the ratcheting curves clearly separates the conditions for stress-controlled low cycle fatigue with zero mean stress and ratcheting with tensile mean stress. [ABSTRACT FROM AUTHOR]

Abstract: 304LN stainless steel samples are tested in room temperature under low cycle fatigue (LCF), ratcheting, pre-ratcheting followed by LCF and tensile, and pre-LCF followed by tensile conditions to understand the deformation behavior and damage evolution during cyclic plastic deformation. It is found that pre-ratcheting have massive detrimental effect on its subsequent LCF life. Systematic alterations in tensile properties are noticed with cyclic plastic damage evolution in this work. Orderly alteration of tensile yield stress with cyclic hardening/softening of the material is also observed. [Copyright &y& Elsevier]

local constitutive model for anisotropic granular materials is introduced and applied to isobaric (homogeneous) axial-symmetric deformation. The simplified model (in the coordinate system of the bi-axial box) involves only scalar values for hydrostatic and shear stresses, for the volumetric and shear strains as well as for the new ingredient, the anisotropy modulus. The non-linear constitutive evolution equations that relate stress and anisotropy to strain are inspired by observations from discrete element method (DEM) simulations. For the sake of simplicity, parameters like the bulk and shear modulus are set to constants, while the shear stress ratio and the anisotropy evolve with different rates to their critical state limit values when shear deformations become large. When applied to isobaric deformation in the bi-axial geometry, the model shows ratcheting under cyclic loading. Fast and slow evolution of the anisotropy modulus with strain. Lead to dilatancy and contractancy, respectively. Furthermore, anisotropy acts such that it works 'against' the strain/stress, e.g., a compressive strain builds up anisotropy that creates additional stress acting against further compression. [ABSTRACT FROM AUTHOR]

The present study aims to evaluate the capabilities of a class of models that take into account non-linear isotropic and kinematic hardening. The purpose is to predict the behaviour of a 'Bi-tube' structure when submitted to a cyclic thermo-mechanical loading. The first part consists of an overview of the methods of describing different kinematic hardening variables. In the second part, the studied structure and the results of numerical simulation of the models used are presented, with an emphasis on the sensitivity of the analysis with regard to the choice of the element type, using a FE software 'Code-Aster', developed by Electricité de France. This study constitutes a first step in the validation of the models of cyclic behaviour with respect to tests on structures, limited in this first case to Bi-tube structure. The comparison between numerical predictions and the experimental results demonstrates that the model of Taheri seems to be the best adapted to the study of 'Bi-tube' structures because it leads to stabilisation around 1%, which is comparable to the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this report is to examine the nature of accumulation of ratcheting strain (ɛ r) vis-à-vis in situ substructural and microstructural variations in 304LN stainless steel under varied asymmetric cyclic loading (ACL) conditions. For this purpose, a series of ACL experiments have been carried out on cylindrical tensile specimens under different combinations of mean stress (σ m) and stress amplitude (σ a). In one set of experiments, σ m has been varied between 50 and 150MPa keeping σ a constant, while in a second set σ m has been kept constant and σ a is varied between 300 and 400MPa. From the results it can be inferred that ɛ r increases from cycle to cycle for ACL tests and the phenomenon is considered to be governed by the associated mechanics and mechanisms of deformation. Fatigue life of a specimen can either increase or decrease depending upon the test condition, which induces different variations in dislocation density, sub-grain size or micro-damage formation. [ABSTRACT FROM AUTHOR]

Abstract: Low cycle fatigue experiments have been conducted on 304LN stainless steel in ambient air at room temperature. Uniaxial ratcheting behavior has also been studied on this material and in both engineering and true stress controlling modes. It is shown that material’s cyclic hardening/softening behavior in low cycle fatigue and in ratcheting is dependent not only on material but also on the loading condition. Improvement of ratcheting life and mean stress dependent hardening are observed in the presence of mean stress. A method based on the strain energy density (SED) is used to represent cyclic hardening/softening behavior of the material in this work. The decrease of SED with cycles is an indication that the life in low cycle fatigue and in ratcheting is improved. The SED represents the area of the hysteresis loops. [ABSTRACT FROM AUTHOR]

Abstract: Cyclic plasticity deals with non-linear stress–strain response of materials subjected to external repetitive loading. An effort has been made to describe cyclic plastic deformation behavior of the SA333 C–Mn steel by finite element based plasticity model. The model has been developed on the framework using a yield surface together with Armstrong–Frederick type kinematic hardening model. No isotropic hardening is considered and yield stress is assumed to be a constant in the material. Kinematic hardening coefficient and kinematic hardening exponent reduces exponentially with plastic strain in the proposed model. The proposed model has been validated through experimental results. [Copyright &y& Elsevier]

Abstract: A series of uniaxial ratcheting experiments has been carried out on cold compaction polytetrafluoroethylene (PTFE) specimens. All the tests were performed under stress control at elevated temperature. The effects of mean stress, stress amplitude, applied temperature and their histories on the ratcheting behavior of PTFE were studied. It is shown that, as the applied temperature was raised, the elastic modulus of PTFE declined rapidly. The ratcheting strain increased as the mean stress, stress amplitude and temperature increased. Especially, when the temperature was over 100°C, the ratcheting strain accumulated rapidly. Furthermore, the loading histories also play an important role in the progress of ratcheting. Previous cycling with higher mean stress and stress amplitude greatly restrains ratcheting strain of subsequent cycling at lower ones. Such a phenomenon is due to the enhancement of the material deformation resistance caused by the previous loadings. As the applied temperature changes, the ratcheting strain still accumulates along the direction of mean stress. [Copyright &y& Elsevier]

Abstract: The size of the deformation and damage fields produced by fretting are of the same scale as the microstructure. Therefore, a microstructure-sensitive modeling approach is needed to study the influence of the microstructure on the fretting behavior. This study is aimed at evaluating the dependence of normal force and tangential force amplitude on the drivers for fatigue crack formation. Previously, fretting maps were developed for Ti–6Al–4V using a two-dimensional crystal plasticity model to identify the conditions leading to elastic shakedown, surface plasticity, and subsurface plasticity in the partial slip fretting regime. In this study, more realistic three-dimensional crystal plasticity constitutive relations are used to describe the deformation behaviors of the primary α and α/β lamellar phases of Ti–6Al–4V at room temperature. In addition, fully three-dimensional finite element simulations are conducted. The new simulations substantiate the results of the previous 2D modeling in that the distribution of plastic strain is highly heterogeneous and ratcheting is the dominant component of the regions within the field where most of the plastic strain accumulates. The relative dependence of the fretting loading parameters on fatigue crack formation is determined using a ratcheting-based critical plane fatigue parameter. [Copyright &y& Elsevier]

Abstract: This work follows a series of experiments carried out earlier at INSA of Rouen (Hassan, T., Taleb, L., Krishna, S., 2008. Influence of non-proportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plast. 24, 1863–1889). It investigates the elastoplastic cyclic behavior of a 304L stainless steel at room temperature. In a first step the cross path effect on ratcheting is confirmed, as well as the crucial role of the loading path non-proportionality. Strain controlled tests are also conducted for different strain amplitudes and loading paths. Cross-hardening effect appears more important when the shearing sequence is followed by the axial one. Moreover for alternating axial and shearing cycles, this phenomenon occurs after each crossing sequence leading to a very significant strain hardening, at least of the same order as the one obtained after a circular strain path. Yet, the magnitude of the observed over hardening does not necessarily seem a function of the cumulated plastic strain. The relative contributions of the isotropic and kinematic parts of the cyclic strain hardening are also very different in the axial direction. The material generally exhibits a transient period of cyclic hardening followed by cyclic softening. Under tension-compression, the importance of the cyclic strain hardening period seems to increase with the strain amplitude. Microstructural investigations have been performed in order to understand the main physical phenomena responsible for this macroscopic behavior. The additional cyclic hardening observed in cross paths may be explained by the high defect density generated: multiple slip systems, intersecting stacking faults and twins, formation of dislocation heterogeneous structures. Furthermore, it is shown that martensite nucleation takes place at the intersections between micro-shear bands or twin faults; the quantity of martensite being estimated through magnetic measurements. After the application of a magnetic field, the material anisotropy induced by the loading path is evaluated; the martensitic needles and platelets seem to develop in the axial direction. [Copyright &y& Elsevier]

The cyclic stress-strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress-strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress-strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation. [ABSTRACT FROM AUTHOR]

Abstract: Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel were observed by the tests under the uniaxial stress-controlled cyclic loading with non-zero mean stress [G.Z. Kang, Y.J. Liu, Mater. Sci. Eng. A 472 (2008) 258–268]. Based on the obtained experimental results, the evolution features of whole-life ratcheting behavior and low-cycle fatigue (LCF) damage of the material were discussed first. Then, in the framework of unified visco-plasticity and continuum damage mechanics, a damage-coupled visco-plastic cyclic constitutive model was proposed to simulate the whole-life ratcheting and predict the fatigue failure life of the material presented in the uniaxial stress cycling with non-zero mean stress. In the proposed model, the damage was divided into two parts, i.e., elastic damage and plastic damage, which were described by the evolution equations with the same form but different constants, since the maximum applied stresses in most of loading cases were lower than the nominal yielding strength of the material. The ratcheting of the material was still described by employing a nonlinear kinematic hardening rule based on the Abdel-Karim–Ohno combined kinematic hardening model [M. Abdel Karim, N. Ohno, Int. J. Plast. 16 (2000) 225–240] but extended by considering the effect of damage. The maximum strain criterion combined with an elastic damage threshold was employed to determine the failure life of the material caused by two different failure modes, i.e., fatigue failure (caused by low-cycle fatigue due to plastic shakedown) and ductile failure (caused by large ratcheting strain). The simulated whole-life ratcheting behavior and predicted failure life of tempered 42CrMo steel are in a fairly good agreement with the experimental ones. [Copyright &y& Elsevier]

Abstract: A series of multiaxial ratcheting experiments have been performed on polyteterafluoroethylene (PTFE) solid cylindrical specimens. All the tests were conducted under cyclic shear strain with a constant axial stress at room temperature. The effects of axial stress, shear strain range, shear strain rate and their histories on the ratcheting behavior of PTFE were studied. It is shown that the ratcheting strain depends on the constant axial stress, cyclic shear strain range and shear strain rate. The ratcheting strain increases more rapidly as the constant axial stress or shear strain range become larger, or the shear strain rate is reduced. Furthermore, the loading histories also play an important role in the progress of ratcheting. The prior cycling with higher axial stress, larger strain range or lower strain rate greatly restrains ratcheting strain of subsequent cycling at lower strains. Such phenomenon is due to the enhancement of the material deformation resistance caused by the prior loadings. [Copyright &y& Elsevier]