Ratcheting Strain Rate Research Articles

Numerical study on the ratcheting performance of heavy haul rails in curved tracks

The ratcheting performance of heavy haul rails in curved tracks under different in-service loading conditions was numerically evaluated by using a recently developed approach. The wheel–high rail cyclic rolling contact was simulated by translating the non-Hertzian contact pressure distribution and the estimated longitudinal tangential traction repeatedly on the running surface of the high rail. The maximum ratcheting strain rate of the rail target section was obtained in each loading cycle. Based on the known ductility limit of the rail steel and the obtained maximum stabilized ratcheting strain rate, the crack initiation life of the rail was predicted in each case. The results indicate that the crack initiation life for all the three rail steel grades investigated decreases with the increase of the tangential traction coefficient, the friction coefficient, the ratio of lateral/vertical load and the axle load. Under the same in-service loading conditions, the ratcheting performance of the rails in curved tracks is worse than the one in tangent tracks. For instance, the crack initiation life of the low alloy heat-treated steel installed in the high rails is reduced by approximately 80% with the axle load of 35 t, the lateral/vertical load ratio of 0.3, the friction coefficient of 0.4 and the tangential traction coefficient of 0.5. The hypereutectoid steel rail with a lower carbon content always shows the best ratcheting performance and is likely to be the most reliable choice for the high rails in curved tracks. Moreover, the crack is predicted to initiate around 1 mm beneath from the top initial contact point. As the traction conditions become more severe, the location of crack initiation will shift from the subsurface to the running surface. The results can provide valuable information to assist rail operators in the selection of rail steel grades and the development of rail maintenance strategies.

Ratcheting assessment of pressurized pipelines under cyclic axial loading: Experimental and numerical investigations

The present study aimed to study ratcheting strains of pressurized stainless steel (UNS S32750) pipes with cyclic axial loading conditions. To this aim, some experiments were performed for the tubes involving cyclic axial loading under internal pressure. All specimens were analyzed with a finite element code. Ohno-Wang hardening model was employed in numerical analysis to calculate the ratcheting of the tubes. By using this hardening model, all predicted ratcheting results were consistent with those of the experimental ones. In this study, the effect of pre-compressed strain, internal pressure and stress amplitude were evaluated experimentally and the numerical results were compared with the results. Finally, the tubes collapsed at about the same strain level as it did under monotonic loading. The number of cycles to collapse and the ratcheting strain rate of the pressurized pipelines depend on the different parameters such as cyclic loading parameters, pre-strain and internal pressure. The results show that an increase of pre-strain, internal pressure and cyclic loading parameters leads to increase ratcheting strain rate of pipelines in both axial and circumferential directions.

Ratchetting in Cold-Drawn Pearlitic Steel Wires

The microscopic mechanisms that accommodate uniaxial ratchetting in cold-drawn pearlitic steel wires were explored. A two-stage evolution of ratchetting strain as a function of cycle numbers was observed. The initial sudden increase of plastic strain leads to a rapid decomposition of cementite, followed by a constant ratchetting strain rate with critical role of decomposed carbon atoms played in blocking dislocation motion. The dislocation configuration transforms from low-density lines and tangles to high-density cells and sub-grains with increasing strain. A possible mechanism of cementite decomposition is discussed in terms of carbon-dislocation interactions and an unfavorable cementite surface-to-volume ratio.

Experimental and numerical investigation on compressive fatigue strength of lattice structures of AlSi7Mg manufactured by SLM

Micro-lattice materials represent nowadays a great opportunity for developing new ultra-lightweight materials. Design flexibility and multi-functional properties make them attractive for several applications in automotive, medical, space, aerospace and process industries. Predict fatigue resistance of these micro-structures is a key issue. In this work three different unit cells printed by a Selective Laser Melting process with an AlSi7Mg powder were analysed. Static properties and fatigue strength in compression have been studied by means of different experimental and numerical techniques: experimental tests, Digital Image Correlation, Micro-Computed Tomography and Finite Element analysis. Total fatigue life has been divided in three stages: ratcheting, damage initiation and propagation in one or few struts leading to final failure. The experimental results show that the initial ratcheting strain rate is significantly correlated to the life of each sample. The damage maps show that, even at fatigue limit, crack propagation in one or few struts is expected to occur. The effect of defects and local irregularities on the fatigue strength has been studied by means of finite elements models based on the as-manufactured geometry of the samples and on the definition of an equivalent stress amplitude based on a multiaxial high cycle fatigue criterion. A correlation between broken struts and maximum values of local equivalent fatigue stress amplitude has been demonstrated by post-test tomographies.

Uniaxial ratchetting and low-cycle fatigue failure behaviors of adhesively bonded butt-joints under cyclic tension deformation

Ratcheting and low-cycle fatigue failure behaviors of the adhesively bonded hollow cylindrical butt-joints has been experimentally investigated. A series of uniaxial cyclic tension experiments were carried out under stress-controlled mode. The effects of stress amplitude, mean stress and cycle time on the uniaxial ratcheting response, fatigue damage variable evolution and fatigue life of the adhesively bonded butt-joints were analyzed. The results show that the ratcheting strain, ratcheting strain rate and fatigue damage variable all increase with the increase of stress amplitude and mean stress. The shorter cycle time results in the increase of fatigue damage variable and the degradation of the stiffness of the adhesive material. It is also found that the increase of stress amplitude and mean stress can reduce the low-cycle fatigue life. Meanwhile, the fatigue life increases with the increase of cycle time for the adhesively bonded butt-joints.

Low cycle fatigue and ratcheting failure behavior of AH32 steel under uniaxial cyclic loading

In this paper, the low cycle fatigue failure and ratcheting behavior, as well as their interaction of AH32 steel were experimentally investigated under uniaxial cyclic loading. The effects of mean stress, stress amplitude and stress ratio on the low cycle fatigue life and ratcheting strain were discussed. It was found that the ratcheting strain increased while the fatigue life decreased with the increase of mean stress and stress amplitude, and the increasing stress ratio would result in smaller ratcheting and larger fatigue life. Two kinds of failure modes, i.e. low cycle fatigue failure due to crack propagates and ratcheting failure due to large plastic strain will take place respectively. Based on the experimental results, considered the effect of ratcheting on fatigue life, a model with the maximum stress and ratcheting strain rate was proposed. Comparison with the experimental result showed that the new model provided a good prediction for AH32 steel.

Effects of creep and creep-recovery on ratcheting strain of articular cartilage under cyclic compression

Since the accumulations of ratcheting strain combined with creep deformation, which are produced in normal activities, can accelerate the fatigue damage of cartilage in joint, the creep-ratcheting and creep-recovery-ratcheting behaviors of articular cartilage are experimentally investigated under creep-fatigue loads. The effect of pre-creep on ratcheting behavior of cartilage was probed firstly and it is found that the initial ratcheting strain of cartilage presents the larger value (30% and 35%) due to its pre-creep deformation in spite of the short pre-creep time applied. With the increasing pre-creep time the ratcheting strain of sample increases while the ratcheting strain rate decreases. The effects of pre-creep and recovery on ratcheting behavior of cartilage were also investigated and it is noted that the strain of cartilage increases fast at first, decreases partly and then changes periodically with cyclic stress. The ratcheting strain evolutions of different layers are not coincident for cartilage sample after the pre-creep or pre-creep-recovery and the ratcheting strains of different layers with pre-creep are larger than them with pre-creep-recovery. Finally the creep-ratcheting strain of cartilage with different peak-holding time during cyclic compression was studied and it is found that the creep-ratcheting strain with peak-holding time is significantly greater than that without peak-holding time. The creep-ratcheting strain increases with increase of peak-holding time or stress amplitude, while it reduces with rising of stress rate. The creep-ratcheting behavior of cartilage is dependent on its depth. These findings point out that the accumulated deformations, including creep deformation and ratcheting deformation, can accelerate the cartilage's damage.

Uniaxial and biaxial ratcheting behavior of ultra-high molecular weight polyethylene

Experimental studies were conducted to investigate the uniaxial and biaxial ratcheting behaviors of Ultra-high molecular weight polyethylene (UHMWPE). The effects of stress amplitude, stress rate and hydroxyapatite content on uniaxial ratcheting behavior were studied firstly. It is found that the ratcheting strain and its rate increase as stress amplitude increases. However, the ratcheting strain and its rate decrease with rising of stress rate. Meanwhile, it is found that the ratcheting strain decreases with increase of hydroxyapatite content. The ratcheting strain rates with different hydroxyapatite contents are not obviously different. The modified ratcheting strain accumulative model was constructed to predict the uniaxial ratcheting behavior of UHMWPE with different stress amplitudes, stress rates and hydroxyapatite contents. It is seen that the predictions agree with the experimental results very well. The effects of different loading paths on biaxial ratcheting behavior of UHMWPE were studied. Both ratcheting strain and ratcheting strain rate are strongly influenced by the loading path. It is found that the uniaxial loading path gives the highest ratcheting strain and its rate while the proportional loading path gives the lowest ratcheting strain and its rate.

Compressive ratcheting and creep of non-asbestos fibre composite considering temperature effect

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.

Influence of stress level on uniaxial ratcheting effect and ratcheting strain rate in austenitic stainless steel Z2CND18.12N

Uniaxial ratcheting behavior of Z2CND18.12N austenitic stainless steel used nuclear power plant piping material was studied. The results indicated that ratcheting strain increased with increasing of stress amplitude under the same mean stress and different stress amplitude, ratcheting strain increased with increasing of mean stress under the same stress amplitude and different mean stress. Based on least square method, a suitable method to arrest ratcheting by loading the materials was proposed, namely determined method of zero ratcheting strain rate. Zero ratcheting strain rate occur under specified mean stress and stress amplitudes. Moreover, three dimensional ratcheting boundary surface graph was established with stress amplitude, mean stress and ratcheting strain rate. This represents a graphical surface zone to study the ratcheting strain rates for various mean stress and stress amplitude combinations. The graph showed the ratcheting behavior under various combinations of mean and amplitude stresses. The graph was also expressed with the help of experimental results of certain sets of mean and stress amplitude conditions. Further, experimentation cost and time can be saved.

Rate-dependent low cycle fatigue and ratcheting of 25Cr2MoVA steel under cyclic pulsating tension

ABSTRACTLow 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.

Ratcheting response of nylon fiber reinforced natural rubber/styrene butadiene rubber composites under uniaxial stress cycles: Experimental studies

AbstractThe present study intends to study the ratcheting response of nylon fiber reinforced natural rubber (NR)‐styrene butadiene rubber (SBR) composite samples under asymmetric stress cycles. Uniaxial tests conducted on composite samples have shown how influential the weight fraction of short nylon fibers on the stress‐strain curves/loops under monotonic and cyclic loads is. NR/SBR composite samples with various fiber contents of 0, 10, 20, 30, and 40 per hundred rubber (phr) were tested under asymmetric stress cycles. In these tests, stress‐strain hysteresis loops were progressively shifted over stress cycles resulting in progressive plastic strain accumulation. Over stress cycles, ratcheting strain progressed within the first few cycles with a relatively high rate, and as the number of cycles increased, this rate decayed resulting in a plateau in strain accumulation (shakedown). The ratcheting strain rate and magnitude resulting in shakedown were highly affected by the nylon fiber content. The experimental observations showed that this plateau (shakedown) occurred after a number of cycles in NR/SBR composite samples where the widths of hysteresis loops stayed unchanged. Samples with no fiber and that with 10 phr fiber content possessed high ratcheting rates leading samples to failure after a few stress cycles. Fracture surfaces in these samples were further analyzed through SEM investigation.

Compressive ratcheting effect of expanded PTFE considering multiple load paths

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.

Ratcheting behavior of zirconium alloy tubes under combined cyclic axial load and internal pressure at 350 °C

Zirconium alloy is used as nuclear fuel cladding in nuclear reactors because it offers the merits of low neutron absorption cross-section, high corrosion resistance, excellent mechanical properties, and satisfactory creep resistance under operating conditions. However, the cladding experiences uniaxial and multiaxial stresses, which can lead to material failure due to the ratcheting effect. In this context, in this study, a series of uniaxial and multiaxial ratcheting tests were conducted on zirconium alloy tubes at 350 °C. The experimental results show that the temperature significantly affects the properties of zirconium alloy, and its ductility at 350 °C is poorer than that at room temperature. The axial ratcheting strain rate greatly increases with increase in the mean stress, stress amplitude, and peak stress holding time. In addition, the loading history also greatly affects the uniaxial and multiaxial ratcheting of zirconium tubes. A lower stress level after a loading history with higher stress level leads to shakedown of the ratcheting, while a higher mean stress after a loading history with lower mean stress brings down the ratcheting strain rate. The existence of a constant internal pressure significantly retards the failure of the zirconium tubes.

Quasi-static and ratcheting properties of trabecular bone under uniaxial and cyclic compression

The quasi-static and ratcheting properties of trabecular bone were investigated by experiments and theoretical predictions. The creep tests with different stress levels were completed and it is found that both the creep strain and creep compliance increase rapidly at first and then increase slowly as the creep time goes by. With increase of compressive stress the creep strain increases and the creep compliance decreases. The uniaxial compressive tests show that the applied stress rate makes remarkable influence on the compressive behaviors of trabecular bone. The Young's modulus of trabecular bone increases with increase of stress rate. The stress-strain hysteresis loops of trabecular bone under cyclic load change from sparse to dense with increase of number of cycles, which agrees with the change trend of ratcheting strain. The ratcheting strain rate rapidly decreases at first, and then exhibits a relatively stable and small value after 50cycles. Both the ratcheting strain and ratcheting strain rate increase with increase of stress amplitude or with decrease of stress rate. The creep model and the nonlinear viscoelastic constitutive model of trabecular bone were proposed and used to predict its creep property and rate-dependent compressive property. The results show that there are good agreements between the experimental data and predictions.

Determination of Steady Ratcheting Strain Rate Under Biaxial Loading Based on Cycle Nonproportionality Coefficient

An approach to the determination of steady ratcheting strain rate under biaxial loading depending on the loading cycle geometry is proposed. Based on the data analysis, the microstructural investigation results, as well as the similarity detected between the microstructural formations after deformation at the stress- and strain-controlled modes of loading, it is proposed to use the cycle nonproportionality parameter to predict the ratcheting rate. It is shown that the geometric cycle parameters and basic experiments allow one to adequately predict the ratcheting rate for complex cyclic paths.

Mean stress and ratcheting corrections in fatigue life prediction of metals

AbstractRatcheting 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.

Time-dependent ratcheting of 35CrMo structural steel at elevated temperature considering stress rates

Cyclic uniaxial stress-controlled ratcheting behaviors of 35CrMo structural steel were investigated at 500 °C. The effects of stressing rates and stress amplitudes were discussed in detail. Results present that the accumulated ratcheting strain increases with the increment of stress amplitude, while decreases with the reduction of stress rate at elevated temperature. Moreover, the ratcheting evolution curves with increasing number of cycles are similar to traditional static creep curves, and can be mainly divided into three stages. Especially, the ratcheting strain rate during the steady stage can be easily obtained to estimate the ratcheting or shakedown behavior. Furthermore, the steady ratcheting strain rate significantly decreases with increasing the stressing rate when the stressing rate is relatively slow. However, when the applied stressing rate is greater than a critical value, such as ≥ 2·5 MPa s−1 for = 200 MPa and ≥ 25 MPa s−1 for σmax = 500 MPa, the effect of stressing rate has little effect on the cyclic ratcheting strain rate.

Analysis of partially pulsating fatigue process on carbon steel with microstructural observation

Pulsating low-cycle fatigue processes, up to the present, have been divided into three states: the transient state, steady state, and accelerating state of ratcheting. In our previous work, we suggested that fatigue behavior of pulsating fatigue process should be classified into five stages in which the plastic strain amplitude and the ratcheting strain rate are plotted on the X and Y axis, respectively. In this study, at the condition of R=−0.3 (partially pulsating fatigue), the change in the plastic strain amplitude and ratcheting strain rate for each cycle to failure was examined on AISI 1025 carbon steel. The dislocation substructure was examined using transmission electron microscopy (TEM) for each stage, except for stage I. It was also demonstrated that the fatigue process can be divided into five stages: stage I corresponds to the un-pinning of dislocations from the Cottrell atmosphere and propagation of the Luders band. Stage II corresponds to the restriction of dislocation movement by dislocation tangles. Stage III corresponds to the formation of dislocation cells. Stage IV corresponds to the promotion of the to-and-fro (back-and-forth) motion of dislocations by a re-arrangement of the dislocations in the cells. Stage V corresponds to the release of dislocation movement by the collapse of dislocation cells.

Ratchetting behavior of 20MnMoNi55 reactor pressure vessel steel

Uniaxial ratchetting characteristics of 20MnMoNi55 steel at room temperature were studied experimentally. The effects of stress amplitude and mean stress on the ratchetting behavior of the steel were analyzed. It can be concluded that the ratchetting strain amplitude and ratchetting strain rate of 20MnMoNi55 steel increases with increasing stress amplitude or mean stress correspondingly. The steel exhibited large accumulation of ratchetting strain which is attributed to its cyclic softening characteristic. Variation of ratchetting strain and strain rate with cycle number displayed three stage behavior.