Ratcheting Strain Research Articles

Roles of microstructures in high-cycle fatigue behaviors of 42CrMo high-strength steel under near-yield mean stress

In this study, high-cycle S-N characteristics and fatigue cracking behaviors under near-yield mean stress of 42CrMo steel with two different fine/coarse-grain tempered sorbite/bainite microstructures (FGM/CGM) were investigated. The results show that fatigue strength is greater for FGM than for CGM as expected, but fatigue crack growth (FCG) threshold is higher for the latter. Compared to FGM with a single crack source, CGM has multiple fatigue sources together with higher-degree ratchetting-fatigue damage, i.e., plastic-strain, dislocation cells and sub-grains, thus accelerating fatigue crack initiation. In the near-threshold regime, roughness-induced crack closure in coarse-microstructures of CGM leads to FCG retardation behavior, which increases FCG threshold. In the Paris regime, CGM presents a similar FCG rate to FGM. Microstructure barriers (i.e., crystal interfaces, carbides and interfaces with different orientations), secondary cracks, crack-path fluctuations, ratchetting and plastic strain jointly result in severer crack-tip stress shielding effects in CGM than in FGM. However, fatigue crack tends to propagate through the high Schmid factor region with {110}<111> slip system with lower slip resistance in FGM but not in CGM, which facilitates fatigue cracking of the former. Under the combined effects of the above factors, similar FCG rates are thus exhibited in both steels with different tensile strengths.

Fatigue Behavior of the FGH96 Superalloy under High-Temperature Cyclic Loading.

Strain-controlled low-cycle fatigue (LCF) tests and stress-controlled creep-fatigue interaction (CFI) tests on the FGH96 superalloy were carried out at 550 °C to obtain the cyclic softening/hardening characteristics at different strain amplitudes and ratcheting strain characteristics under different hold time. The failure mechanism of the FGH96 superalloy under different loading conditions was analyzed through fracture observations. The results show that the FGH96 superalloy exhibits different cyclic softening/hardening characteristics at different strain amplitudes, and the introduction of the hold time at peak stress exacerbates the ratcheting strain of the FGH96 superalloy under asymmetric stress cycles. Fracture observations show that the magnitude of the strain amplitude, high-temperature oxidation, and the introduction of the hold time will affect the mechanical properties of the FGH96 superalloy and change its fracture mode.

Uniaxial ratcheting behavior and microstructure evolution of 316H stainless steel under the random cyclic loads

In this study, the Computational Fluid Dynamics (CFD)-Finite Element Method (CFD-FEM) were used to simulate the transient stress response of the structure surface under thermal striping environment, and a uniaxial random cyclic loading (RCL) method was used to reveal the mechanical properties and initial microstructure evolution of 316H austenitic stainless steel (SS) under random stress induced by thermal striping. The experimental results showed that 316H exhibited a lower saturation ratcheting strain rate at 334.44 MPa, along with stronger cyclic hardening effects and resistance to plastic accumulation at loading rate of 100 Hz. Random loading tests with different cycles were performed at σpeak=334.44MPa and fRCL=100Hz, and ratcheted specimens were characterized to investigate the effect of the micro-mechanism on cyclic softening/hardening. The geometrically necessary dislocation (GND) density increases and then decreases with cyclic loading, which is mainly attributed to the deformation twin (DT) and multi-dislocation slip mechanisms. The deformation mechanism during the ratcheting cycle from 1000 to 3000 cycles is mainly dominated by the twinning reduction, where the dislocation migration accelerated the transformation from dislocation walls to subgrain boundaries; the twinning increase dominates from 3000 to 9000 cycles, where the subgrain boundary grew into boundaries with high angle misorientations.

Experimental study of fatigue behaviour of Q690qENH bridge weathering high-strength steel welded joints

The Q690qENH weathering high-strength steel has a considerable potential for bridge engineering due to its superior mechanical strength, toughness, and corrosion resistance. To evaluate its performance under fatigue loads, the high-cycle fatigue behaviour of Q690qENH base metal and its welded joints is studied. A total of 68 specimens are tested to obtain their stress–fatigue life (S−N) curves. Results show that the fatigue strengths of the base metal, butt-welded joint, and cross-welded joint are greater than those specified in the ANSI/AISC 360-16, Eurocode 3-2005, and BS 7608-2014 design standards. The most suitable S−N curve for evaluating the fatigue properties of the Q690qENH base metal, butt-welded joint, and cross-welded joint was deemed to be that given by ANSI/AISC 360-16. New S−N curves were proposed for the base metal and butt-welded joint, whereas the S−N curve for the cross-welded joint remained unchanged as per the existing codes. Additionally, the cumulative fatigue damage behaviour of the butt-welded and cross-welded joints was analysed, and a safety threshold based on the ratcheting strain rate was established to provide early warning at 85%–95% of fatigue life. Through digital image correlation and fatigue fracture analysis, the cause of fatigue fracture in the welded joints was determined to be caused by stress concentration at the weld toe.

Phase-field theory based finite element analysis for ratchetting behavior of medium-manganese steel

A phase field model for martensitic transformation considering crystal plasticity was developed to investigate the microscopic mechanism of the transformation-induced plasticity in response to ratchetting of medium-manganese steel with a ferritic matrix containing retained austenite. This model introduced various inelastic deformation mechanisms, i.e., martensitic transformation, the austenite, ferrite and martensite plasticity caused by dislocation slip. Combining phase field model with finite element method, the ratchetting behavior of medium-manganese steel under asymmetric load was simulated by COMSOL software, and the martensitic transformation and its effect on ratchetting was discussed. The simulated results show that the ratchetting strains evolutions of the medium-manganese steel are well described by the developed phase field model. Ratchetting strain increases with increasing stress amplitude, mean stress and stress ratio. The martensite content increases with increasing stress amplitude and decreases with increasing mean stress and stress ratio.When the plastic strain exceeds a certain value, the plastic strain induced by martensitic transformation will increase the ratchetting strain of medium-manganese steel, which indicates that the critical importance of the martensitic transformation for the overall ratchetting behavior of medium-manganese steel. The plastic deformation of medium- manganese steel is mainly composed of dislocation slip of austenite and ferrite, where the former is the main part. Because of martensitic transformation, the non-uniformity of micro strain field in medium-manganese steel is aggravated, and the equivalent plastic strain increases.

Experimental and numerical investigation of ratcheting behavior of SS316L seamless straight pipe under cyclic internal pressure and in-plane cyclic bending

In this paper, the ratcheting behavior of SS316L seamless straight pipes under cyclic internal pressure and in-plane cyclic bending is investigated experimentally and numerically. In order to applying the simultaneous cyclic bending and cyclic internal pressure, a special fixture was designed that can be mounted on the gripers of fatigue machine and so ability to apply cyclic pressure to inside pipe specimen and apply in-plane cyclic bending on pipe specimen simultaneously. According to experimental fatigue data, increasing the cyclic bending load and cyclic pressure, increases the accumulation of ratcheting strain in hoop and axial directions. So axial ratcheting strain creates due to cyclic bending and hoop ratcheting strain creates due to cyclic pressure inside pipe specimen. Finally, the finite element analysis was performed by ABAQUS software base on the nonlinear kinematic/isotropic hardening model and the experimental fatigue data were compared with the numerical results and it was found that the numerical results are in good agreement with experimental data.

Ratcheting assessment of pressurized elbow pipes using the visco-plastic Chen-Jiao-Kim hardening rule frameworks

In this study, the ratcheting responses of pressurized long and short radius 90° elbow pipes made of 304L austenitic stainless steel under displacement-controlled cyclic loading is evaluated mainly using the Chen-Jiao-Kim (CJK) kinematic hardening rule at room temperature. Visco-plastic flow rule and isotropic hardening are introduced into the CJK model to better reflect the time-dependent behavior and cyclic hardening characteristics of the 304L stainless steel material. The model is implemented employing the user material subroutine (UMAT) in the ABAQUS finite element software. The predictions show that the maximum ratcheting strain occurs at the flank of the long and short radius 90° elbow pipes. The influence of internal pressure, cyclic loading, and bending radius on the ratcheting responses of the elbow pipes is respectively investigated. It is found that the predictions of the CJK model are very nearly the same as the test data. In addition, the ratcheting responses of two long radius elbow pipe specimens under force-controlled cyclic bending are evaluated. Further, the C-TDF (The Committee of Three Dimensional Finite Element Stress Evaluation) combined with CJK model and Linear Matching Method (LMM) is respectively used to determine the shakedown and ratcheting boundaries of the pressurized long and short radius 90° elbow pipes under displacement-controlled and force-controlled bending. The comparison with the test data in this study shows that the shakedown and ratcheting boundaries determined by LMM, divides the shakedown and ratcheting zone well.

Ratcheting-fatigue behavior and life prediction of Z2CN18.10 austenitic stainless steel elbow

The ratcheting fatigue behavior of Z2CN18.10 austenitic stainless steel elbow under internal pressure and cyclic loading was studied. The evolution of axial and hoop ratcheting strains at four different positions, respectively at intrados (0°), 45°, crown (90°) and extrados (180°) on the outer surface of the elbow was discussed. It is shown that the hoop ratcheting strain of the elbow is always greater than the axial ratcheting strain. Under cyclic test with force control, the order of ultimate hoop ratcheting strain is intrados (0°)>crown (90°)> 45°> extrados (180°). Under cyclic test with displacement control, the order of ultimate hoop ratcheting strain is 45°> intrados (0°)> crown (90°)> extrados (180°). Under cyclic tests with both force and displacement control, the crown (90°) of the elbow leaked first, which is the critical point to evaluate ratcheting fatigue life. Afterward, the ratcheting fatigue life is predicted based on ASME code, and it is found that the prediction results tend to be non-conservative. Then, a modified Manson-Coffin equation considering ratcheting strain is proposed to predict fatigue life of elbow well.

Experimental and numerical investigation of ratcheting behavior in seamless carbon steel 90° elbow pipe with small dimensions under constant internal pressure and in-plane cyclic bending

The purpose of this study is to identify the effects of mean fatigue load, amplitude fatigue load, internal pressure and loading history on ratcheting behavior of seamless A234 WPB steel elbow pipe. So, in this paper the ratcheting behavior of seamless 90° elbow pipe of A234 WPB steel under constant internal pressure and in-plane cyclic bending is investigated experimentally and numerically. At first, in order to the confirm of elbow pipe material, a series of experimental tests were performed on elbow specimen, including: chemical composition analysis, metallography testing, hardness testing and uniaxial tensile testing. In the next step, the fatigue tests were performed on elbow pipe specimens at ambient temperature (25 °C) under asymmetric cyclic loading with load-control conditions. According to the experimental fatigue data, concluded that the increasing of mean load, amplitude load and internal pressure, increases the accumulation of ratcheting strain in axial direction at intrados and extrados locations, and also in hoop direction at crown location. So that the increasing of mean load or amplitude load has the greatest effect on the ratcheting strain at intrados and extrados locations. Also, it was found that the loading history affected to ratcheting behavior and premature failure of pressurized elbow pipes. Finally, the finite element simulation was carried out by ANSYS software based on the nonlinear kinematic/isotropic hardening model, and the FE simulation results were compared with experimental fatigue data and it was found that the numerical results are in good agreement with experimental fatigue data.

The ratcheting effect on the SS316L U-shaped pipe under internal pressure and bending cycle

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.

Compressive fatigue behavior of graded tantalum scaffolds produced by electron beam powder bed fusion

In this study, graded tantalum scaffolds were manufactured by electron beam powder bed fusion (EB-PBF). Compression-compression fatigue testing was conducted on the EB-PBF built graded tantalum scaffolds. The compressive fatigue behavior and compressive fatigue strength of graded tantalum scaffolds with vertical gradient (VG) and parallel gradient (PG) states were investigated. In the fatigue testing, the failure of PG graded scaffold was concentrated on the lowest strength constituent, resulting in higher cyclic ratcheting strain and fatigue damage strain. In terms of the VG graded scaffold, due to the simultaneous uniform deformation of each structure, the strain accumulation in a single cycle was small, resulting in the low cyclic ratcheting strain. In addition, the propagation of fatigue cracks in pore structure with higher strength can be retarded by the stress redistribution, resulting in the low fatigue damage strain. Therefore, the design idea of graded pore structure under identical strain condition is beneficial to obtain porous tantalum scaffold with high compressive fatigue strength.

Rate-dependent ratcheting characteristics of high density polyethylene subjected to cyclic pulsating compression

Monotonic and pulsating cyclic compression tests were performed systematically based on high density polyethylene (HDPE). The compressive creep behaviors were further implemented to compare with the ratcheting deformation under the same peak stress level. Monotonic compression experiments reveal that HDPE shows obvious four-stage characteristics which are sensitive to temperature, but independent of the stress rate when the temperature is greater than 80 °C. Interestingly, the strain range during the stage III always starts at 7.4% and ends at 62.6% under various temperatures and stress rates. Moreover, the compressive ratcheting behavior of HDPE significantly depends on temperature T, peak stress σp, stress rate σ˙, stress amplitude Δσ and stress ratio R, especially when T ≥ 80 °C, σp ≥15 MPa, σ˙ ≤0.1 MPa/s and R ≥ 0.8. Additionally, decreasing the stress rate or increasing the temperature will decrease the compressive modulus of HDPE. An approximate linear relationship between the creep strain rate and the ratcheting strain rate under different temperatures and stress ratios was observed. The compressive deformation characteristics of HDPE were compared with that of some other gasket materials, a universal phenomenon is found that the compressive ratcheting evolution with time can be characterized by creep approximately under the same loading peak stress, stress rate and temperature for various gasket materials when the stress ratio is greater than zero, which indicates that the ratcheting deformation for gaskets under cyclic loads may be estimated by the corresponding static creep strain in practical engineering.

Influence of Microalloying on the Microstructures and Properties of Spalling-Resistant Wheel Steel

Microalloyed steels have emerged to replace conventional plain-carbon steels to achieve longer wheel life on Chinese railroads. In this work, with the aim of preventing spalling, a mechanism that consists of ratcheting and shakedown theory correlated with steel properties is systematically investigated. Mechanical and ratcheting tests were carried out for microalloyed wheel steel to which vanadium was added in the range of 0-0.15 wt.% and the results were compared with that obtained for conventional plain-carbon wheel steel. The microstructure and precipitation were characterized via microscopy. As a result, the grain size was not obviously refined, and the pearlite lamellar spacing decreased from 148 nm to 131 nm in microalloyed wheel steel. Moreover, an increase in the number of vanadium carbide precipitates was observed, which were mainly dispersed and uneven, and precipitated in the pro-eutectoid ferrite region, in contrast to the observation of lower precipitation in the pearlite. It has been found that vanadium addition can lead to an increase in yield strength by precipitation strengthening, with no reduction or increase in tensile strength, elongation or hardness. The ratcheting strain rate for microalloyed wheel steel was determined to be lower than that for plain-carbon wheel steel via asymmetrical cyclic stressing tests. An increase in the pro-eutectoid ferrite content leads to beneficial wear, which can diminish spalling and surface-initiated RCF.

A ratcheting mechanism-based numerical model to predict damage initiation in twin-disc tests of premium rail steels

This work presents a ratcheting mechanism-based numerical model to study the initiation location and initiation life of rolling contact fatigue (RCF) or wear damage and the initial wear rate of premium rail steels under the laboratory twin-disc test conditions. Twin-disc tests are widely used in the studies of rail steels due to their ability to reproduce critical aspects of full-scale wheel-on-rail contact under controlled conditions and a relatively short test period compared to field tests. This study introduces a computational model to simulate the cyclic rolling contact for three premium rail steels (HE1, HE2, and LAHT) against wheel steel AAR Class-C under twin-disc test conditions. The cyclic rolling contact is achieved by repeatedly moving a non-Hertzian distribution of contact pressure and the calculated longitudinal surface traction upon a segment of the circumferential surface of the lower disc until a stabilized maximum ratcheting strain rate is reached. The RCF or wear damage initiation location is determined by the location showing the stabilized maximum ratcheting strain rate, and the damage initiation life is estimated by applying the ratcheting failure mechanism. Wear and RCF damages are distinguished by the depth of the damage initiation location. Wear damage is dominant when the location is near the surface. Otherwise, the damage will be RCF-dominated. The initial wear rate due to wear-dominated damage is estimated by identifying a critical profile and affected volume of worn material within the ratcheting strain rate field. The predicted initial wear rate is compared with the experimental result of the examined contact pair. This research can assist in a more profound understanding of the experiment results of a twin-disc test and provide a numerical basis for an experimental twin-disc test design. Furthermore, it may provide significant references for choosing rail steels.

An improved kinematic hardening rule describing the effect of loading history on plastic modulus and ratcheting strain

An improved kinematic hardening rule describing the effect of loading history on plastic modulus and ratcheting strain

In-situ observation of strain evolution and ratchetting of growing fatigue cracks

Crack-tip strain localization and accumulation has long been recognized while the mechanics and physics of fatigue are yet to be understood for complicated cracking modes, microstructural damage, and cross-scale fatigue crack growth modeling. In this work, near-tip strain was characterized at high-resolution and statistically analyzed to develop a strain-based yielding criterion for crack-tip plasticity evaluation which was found applicable to both tensile and fatigue loadings. The near-tip strain of face centered cubic 304 stainless steel satisfied Lognormal distribution whereas body centered cubic 25Cr steel obeyed Gauss distribution, and was found evolving Logistic-Gauss-Lognormal with fatigue crack growing and plasticity increasing. The relative strain accumulation was proposed and found well representing branch or stable growth mode, and drove fatigue crack growth when relative strain accumulation was beyond 1.3. The cracking speed parameter in terms of strain ratchetting per crack extension was established, which indicated that different cracking at macro-scale behaved similarly at the atomic scale for a given material. The microstructural damage analysis near crack-tip illustrated that fatigue crack growth of 25Cr steel was governed by breakdown of lathy martensite and formation of fine grains while the cracking of 304ss was accompanied with austenite-to-martensite transformation that was occurred at a near-tip strain of approximately 13%.

Dynamic strain aging-mediated temperature dependence of ratcheting behavior in a 316LN austenitic stainless steel

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

Designing Cladded Components for High Temperature Nuclear Service: Part I—Analysis Methods

AbstractUse of corrosion-resistant cladding can greatly extend the design life of structural components in many advanced reactor systems. However, there are currently no ASME design rules for cladded components to guard against creep-fatigue failure and ratcheting strain accumulation in elevated temperature nuclear service. This paper, presented in two parts, addresses this gap by proposing a design strategy for cladded components that do not require long-term testing of clad materials. The proposed approach relies on approximate design analysis methods for two types of clad materials—a soft clad that creeps much faster than and has lower yield stress than the class A base material and a hard clad that creeps much slower than and has higher yield stress than the class A base material. Part I discusses the approximate analysis strategies for the clad materials—treat a soft clad as perfectly compliant and a hard clad as linear elastic—and Part II develops a complete set of design rules for each of the two types of cladded components. Finite element analyses of representative high temperature reactor components show that the proposed design analysis methods can bound the design quantities in soft cladded components and approximate the design quantities in hard cladded components.

Improving fatigue properties of 18CrNiMo7-6 steel by surface strengthening

• A gradient layer about 300 μm is formed on the surface after USRP. • Tensile properties and fatigue resistance are improved after USRP. • USRP reduces the ratcheting effect, prolongs fatigue life. • Transition of crack initiation mode is a reason for fatigue strengthening. Surface gradient structures with small to large grain sizes were obtained on 18CrNiMo7-6 steel by ultrasonic surface rolling processing (USRP). Both tensile properties and fatigue resistance have been enhanced. Based on the gradient layer generated after USRP, the fatigue strengthening mechanism was discussed. After USRP, the reduction of the ratcheting strain reduced the fatigue damage of the material under the same stress level and prolonged the fatigue life. Meanwhile, USRP changed the grain orientation of the surface layer, the initiation mode of the fatigue crack was transformed from the surface to the interior at a low stress level.

Study on the ratchetting behavior of glass fiber-reinforced epoxy composites: Experiment and theory

To discuss the ratchetting behavior of short glass fiber-reinforced epoxy polymer composites, experimental and theoretical studies were carried out. The effects of the fiber content and peak stress on the ratchetting behavior of the composite materials were discussed. The results show that the ratchetting strain decreases with increasing fiber content and decreasing peak stress. It was also found that the residual strain after unloading partly recovers with time, which indicates that the ratchetting strain consists of viscoelastic strain and plastic strain. Based on the experimental results, a viscoelastic-viscoplastic constitutive model is proposed, which can well simulate the ratchetting behavior and strain recovery of composites with different fiber contents.