In the past three decades ratcheting fatigue has attracted lots of research interest. Ratcheting can be defined as the directional progressive accumulation of plastic deformation of a material when ...it is subjected to a primary load along with a secondary cyclic load. The current article addresses the recent progresses made on the experimental front on the ratcheting behavior spanning from the specimen level to the component level and its correlation with microstructural evolution. The experimental aspects of ratcheting include the effect of stress levels, stress rate, temperature, planar anisotropy, previous loading history, and multiaxial loading paths. This work also discusses two test controlling modes engineering and true stress control ratcheting and their comparison, ratcheting-tensile, ratcheting-low cycle fatigue, ratcheting-ratcheting, ratcheting-creep interactions and ratcheting in component level. This summarized information clarifies why ratcheting is presently an important topic of engineering research.
Ratcheting is a common and important failure mode in pressure vessels. ASME VIII-2 2019 edition provides two kinds of ratcheting assessment methods——the elastic stress ratcheting assessment ...(paragraph 5.5.6) applied to the design by elastic stress analysis and the elastic-plastic ratcheting assessment (paragraph 5.5.7) applied to the design by elastic-plastic stress analysis. This paper focuses on the elastic stress ratcheting assessment, which includes again two methods——the elastic ratcheting analysis (paragraph 5.5.6.1), i.e. the 3S criterion, and the thermal stress ratcheting assessment (paragraph 5.5.6.3). This article reviews the development history of 3S criterion and discusses its applicable scope. The study found that the 3S criterion is not a complete elastic ratcheting assessment criterion, when the primary membrane stress range exceeds two-thirds of the cyclic yield stress, i.e. (2/3)Sy, the ratcheting assessment based on 3S criterion is unsafe for some situations. When the general primary membrane stress Pm and the local primary membrane stress PL, which allowable limit exceeds (2/3)Sy, are applied simultaneously, there is still the possibility of ratcheting failure in the structure even if the 3S criterion is met. As an extension of the 3S criterion, a complete and safe criterion on three-dimensional elastic ratcheting assessment is proposed in this paper. In order to avoid the inconvenience of partition assessment, a simplified thermal stress ratcheting assessment method based on double checking surfaces is proposed. It is a simple and conservative assessment method. Finally, elastic ratcheting analysis, thermal stress ratcheting assessment and simplified elastic-plastic analysis in ASME VIII-2 are integrated, a universal elastic stress ratcheting assessment is proposed.
Display omitted
•A complete and safe criterion on three-dimensional elastic ratcheting assessment is proposed.•A simplified thermal stress ratcheting assessment method based on double checking surfaces is presented.•A universal method of elastic stress ratcheting assessment is proposed.•A detailed comparison between 3S criterion and thermal stress ratcheting assessment is made.•Examples of new elastic stress ratcheting assessment methods are presented.
In this paper, deformation behaviors and microstructure evolution of a hot-rolled AZ31B magnesium alloy under cyclic loadings are investigated. The relationship between plastic deformation and ...microstructure evolution and the crack formation mechanisms are discussed. Under a high cyclic stress (90–140 MPa), steady ratcheting effect occurred in the material and the development of ratcheting strain went through three stages: 1) Stage I - initial rapid increase stage; 2) Stage II - steady stage; and 3) Stage III - final abrupt increase stage. Under a low cyclic stress (≤ 90 MPa), inconspicuous ratcheting effect was found in the material, indicating a light damage in the material. When the cyclic stress is below 30 MPa, no ratcheting effect is found and only elastic deformation occurs in the material. The formation of cracks in Stages I & II is mainly due to the activation of the basal slip system. The mean geometrically necessary dislocations (GND) are calculated to analyze the relationship between the basal slip and the ratcheting effect during the cyclic loading. Finally, a new approach is proposed to estimate the AZ31B magnesium alloy’s cyclic strength (at 107 cycles) according to the cyclic stress at which steady ratcheting effect starts to occur in the material.
Display omitted
•Parametric study with Chaboche hardening models is performed for ratcheting evaluation.•Including isotropic hardening is essential to improve prediction of stress-strain relation.•A ...simple but effective method of parameter acquisition is proposed and verified by experiments.•Stress-strain relation and ratcheting are evaluated by using proposed method at the same time.
This research proposes a method to obtain parameters of Chaboche combined hardening model for evaluating ratcheting phenomenon in metallic materials when cyclic loadings are applied. Finite element (FE) model is used for simulating ratcheting by changing Chaboche model parameters. Although Chaboche model with single term can describe the ratcheting characteristics, simulated stress - strain (σ-ε) relation deviates from that of metallic materials when the applied alternative stress σa is smaller than the yield strength σo. Whereas, Chaboche model with dual backstress can simulate not only the ratcheting characteristics but also σ-ε relation even when σa is smaller than σo. Finally, tensile and ratcheting experiments are conducted with bearing steel specimens (AISI 52100) to validate the proposed method for evaluation of ratcheting parameters and σ-ε relations.
In this study, a series of uniaxial ratcheting experiments under different mean stresses with a constant stress amplitude were performed on a 316LN stainless steel at room temperature. Meanwhile, ...molecular dynamics (MD) simulation was carried out to reveal the microstructure evolution mechanism at the initial stage of ratcheting deformation at atomic scale. The results showed that the initial stage of ratcheting deformation could be divided into three stages: the stage of beginning (ratcheting strain growth rate (RSGR, dεr/dN) >0.01%), the stage of decreasing ratcheting rate (RSGR at 0.00001–0.01) and the stage of elastic/plastic shakedown (RSGR <0.00001) based on the changes of ratcheting strain rate. The shakedown ratcheting strain was linearly increased with increase of the mean stress in both MD simulation and experiment. And the ratchet deformation at different stages strongly depended on dislocation evolution. At the stage of beginning, the dislocation originated from coincident site lattice (CSL) boundaries and formed pile-up structures at random angle grain boundaries (RAGBs); while at the stage of decreasing ratcheting rate, the dislocation tangled at CSL grain boundaries or inside the grain, the dislocation density gradually increased and tended to be stable. When the given stress was insufficient to produce more plastic strain, the dislocation density balanced dynamically and the ratchet deformation entered the stage of elastic/plastic shakedown.
The current study has attempted to comprehensively review ratcheting response of materials involving various influential parameters such as loading spectra, thermal cycles, stress levels, stress ...raisers, strain rate, and visco‐plasticity and material types with a focus on pressurized pipes and equipment. The mechanism of deformation, types, and the rate of progress over stages of ratcheting were discussed. Safety design codes and procedures were highlighted for reliable design of pressurized pipes against progressive ratcheting and to safeguard the system against catastrophic failure at which both mechanical and thermal ratcheting were integrated. Boundaries and demarcation of ratcheting‐shakedown zones developed based on Bree's diagram were employed to assess plastic deformation accumulation over stress cycles. Shakedown and ratcheting boundaries were discussed through methods developed on the basis of Melan's theorem over last few decades. Ratcheting response of materials was reviewed through descriptions of parametric models and kinematic hardening rules involving various variables and parameters. Interaction of ratcheting with creep and low‐cycle fatigue has promoted progressive damage in materials. Pressurized pipes subjected to thermal cycles and external bending cycles were evaluated by numerical solutions along with kinematic hardening rules to assess ratcheting over stress cycles.
Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-medium thermocline tanks in solar energy applications. Although thermal ratcheting poses a serious impediment ...to thermocline operation, this failure mode in dual-medium thermocline tanks is not yet well understood. To study the potential for the occurrence of ratcheting, a comprehensive model of a thermocline tank that includes both the heterogeneous filler region as well as the composite tank wall is formulated. The filler region consists of a rock bed with interstitial molten salt, while the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). The model accounts separately for the rock and molten-salt regions in view of their different thermal properties. Various heat loss conditions are applied at the external tank surface to evaluate the effect of energy losses to the surroundings. Hoop stresses, which are governed by the magnitude of temperature fluctuations, are determined through both a detailed finite-element analysis and simple strain relations. The two methods are found to yield almost identical results. Temperature fluctuations are damped by heat losses to the surroundings, leading to a reduction in hoop stresses with increased heat losses. Failure is prevented when the peak hoop stress is less than the material yield strength of the steel shell. To avoid ratcheting without incurring excessive energy loss, insulation between the steel shell and the filler region should be maximized.
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.
•Visco-plastic Chen-Jiao-Kim kinematic hardening rule is implemented into ABAQUS with Umat.•Ratcheting behavior and ovalization is studied under cyclic bending.•Influence of different factors on ratcheting behavior and ovalization is studied.•Ratcheitng boundary is respectively determined by the C-TDF and LMM.
Sometimes, railway wheels cannot be re-turned and have to be discarded far before its time in service due to local defects and severe wear. Laser cladding which deposits a surface layer of a repair ...material may be an approach to repair wheels locally. In this study, cladding materials are deposited at a small section of wheel surfaces in order to repair wheels with local defects. 316L, 410, and 420 stainless steel were used as cladding materials on wheel discs while rail discs are unclad. Twin-disc tests were performed to study wear and rolling contact fatigue (RCF) behaviours of clad wheels. Results indicate that adhesion coefficients do not have notable difference between clad and unclad discs. All three clad wheel discs have slightly lower wear rates compared to the unclad wheel discs. The wear rates decrease with increased hardness of the clad materials. Plastic deformation and ratcheting is found on the surface of unclad samples/zones. Only small surface cracks can be found. Deep cracks are found at the boundary between clad and unclad zones leading to a large chunk of material loss. The cracks may be initiated from where maximum shear stress presents below the surface and further propagate along the boundary due to the difference in deformation between clad and unclad materials.
Display omitted
•Laser cladding was used to repair locally damaged railway wheels.•Three stainless steels were clad on one section of wheel discs.•Claddings showed similar adhesion coefficients but lower wear rates.•Deep and large cracks were found at the boundary between clad and unclad zones.
PDF
