Cyclic Hardening Materials Research Articles

Low-Cycle Fatigue Behavior and the Combined Cyclic Hardening Material Model of Plate-Shaped Zn-22Al Alloy for Seismic Dampers.

This study investigates the potential of the plate-shaped Zn-22 wt.% Al (Zn-22Al) alloy as an innovative energy dissipation material for seismic damping devices, since plate-shaped material is more suitable to fabricate large-scale devices for building structures. The research begins with the synthesis of Zn-22Al alloy, given its unavailability in the commercial market. Monotonic tensile tests and low-cycle fatigue tests are performed to analyze material properties and fatigue performance of plate-shaped specimens. Monotonic tensile curves and cyclic stress-strain curves, along with SEM micrographs for microstructure and fracture surface analysis, are acquired. The combined cyclic hardening material model is calibrated to facilitate finite element analysis. Experimental results reveal exceptional ductility in Zn-22Al alloy, achieving a fracture strain of 200.37% (1.11 fracture strain). Fatigue life ranges from 1126 to 189 cycles with increasing strain amplitude (±0.8% to ±2.5%), surpassing mild steel by at least 6 times. The cyclic strain-life relationships align well with the Basquin-Coffin-Manson relationship. The combined kinematic/isotropic hardening model in ABAQUS accurately predicts the hysteretic behavior of the material, showcasing the promising potential of Zn-22Al alloy for seismic damping applications.

Low-Cycle Fatigue Life of Continuously Cyclic-Hardening Material

Abstract A general fatigue life equation is derived by modifying the Tanaka-Mura-Wu dislocation pile-up model for variable strain-amplitude fatigue processes, where the fatigue crack nucleation life is expressed in terms of the root-mean-square of plastic strain range. Low-cycle fatigue tests were conducted on an austenitic stainless steel. At 400 °C and 600 °C, the material exhibits continuously cyclic-hardening behavior. The root-mean-square of plastic strain ranges is evaluated from the experimental data for each test condition at strain rates ranging from 0.0002/s to 0.02/s. The variable-amplitude Tanaka-Mura-Wu model is found to be in good agreement with the low-cycle fatigue (LCF) data, which effectively proves Miner's rule on the stored plastic strain energy basis.

Fatigue behavior, microstructural evolution, and fatigue life model based on dislocation annihilation of an Fe-Ni-Cr alloy at 700 °C

Dislocation annihilation behavior of Sanicro 25 alloy was investigated based on various total strain amplitudes (Δεt/2). The slopes of cyclic stress response curves during stage ΙΙ were compared and showed that obvious stable cyclic behavior occurred at Δεt/2=0.3%. Microstructure analysis reveals that the number of slip plane increased with increasing Δεt/2. When Δεt/2=0.3%, dislocations can slip on the slip plane (111¯). When Δεt/2 increased to 0.4%, the slip planes included (1¯11) and (111¯). The fraction of substructured grains under Δεt/2=0.3% was obviously larger than that under Δεt/2=0.4%, which was attributed to the obvious dislocation annihilation. The deformed grains were dominant at Δεt/2=0.4%. The annihilation mechanisms of dislocations were analyzed during tensile and compressive loading based on the cyclic stress-time curves. The results showed that when Δεt/2=0.3%, during stage ΙΙ, dislocation annihilation mainly occurred during compressive loading. Dislocation annihilation included annihilation caused by cross-slip of screw dislocations and annihilation of adjacent edge dislocation dipoles. Furthermore, considering the critical annihilation distance between two adjacent edge dislocation dipoles, a modified dislocation annihilation model was presented. The dislocation annihilation model was verified based on the actual annihilation rate of dislocations. Further, the relationship of microstructure and fatigue properties was discussed. Based on the dislocation annihilation mechanisms, the fatigue life model considering dislocation annihilation was established. When the ratio of ρ¯− to ρ¯ increased, the fatigue life increased. Therefore, for cyclic hardening materials, obvious dislocation annihilation can increase fatigue life.

Determination of crack growth for 6082-T6 aluminium subjected to periodic single and block overloads and underloads using a two dimensional finite element model

The estimation of crack growth under variable amplitude loading is complex due to interaction effects such as plasticity, crack tip blunting, residual stresses, crack tip closure and crack tip branching. Crack closure has been identified to be one of the main interaction effects. In order to study the effect of crack closure the authors have previously carried out experimental testing to obtain more accurate measurements of crack opening and closure (Aguilar-Espinosa, 2009, Aguilar-Espinosa et al., 2013). They have also developed two dimensional plane stress Finite Element models utilising high mesh density whilst maintaining the ability to measure crack growth over long crack lengths (Aguilar Espinosa et al. 2017). This initial work has been extended in this paper to examine the effects of single and block overloads and random spectrum loading on crack growth. The crack length distance that is affected by overloads and underloads measured experimentally and predicted numerically are shown to be very close when using cyclic hardening material properties and kinematic hardening. In addition the comparison of experimental and numerical crack growth versus crack length graphs shows good correlation of the crack growth acceleration and retardation after the applied overload which has not been seen previously. These comparisons seem to be a very useful tool to validate numerical models.

Study of cycle life of a material in conditions of low-cycle fatigue and long-term strength

Experimental studies shows that the main parts - the shaft and the disks of gas turbine engines, working in the conditions of low-cycle fatigue and durability, have a durability far exceeding the resource identified by calculation based on the hypothesis of linear summation of damage. Therefore, further clarification is needed calculation methods, allowing greater use of the reserves of strength parts. The performed research studies the influence on the durability of the increasing duration of zero cycle of loading samples with a period of T=200 s. and the restraint at the maximum and minimum levels: Dtmax=120 s. and Dtmin=60 s. The studies were performed on a standard round specimens with the diameter of the working area of 7.5mm. Shown that when you restrain a loading there is in the material proceeds relaxation process, combined with a deforming cyclic hardening and increasing of the plastic deformation. It was found that the hypothesis of a linear summation of static and cyclic components of the damage in the conditions of low-cycle fatigue and long-term strength is not confirmed. The increasing of the restraint in the loading cycle under the load to Dtmax=20 s. or more leads to a significant increase - up to 94 % of the static component of damage. The research results obtained during cyclic and repeated static loading, helped to confirm and substantiate the existence of a previously unknown Effect of cyclic hardening materials, due to the duration of the maximum load in the cycle of loading.

Ratcheting-based microstructure-sensitive modeling of the cyclic hardening response of case-hardened bearing steels subject to Rolling Contact Fatigue

Rolling Contact Fatigue (RCF) is a commonly observed phenomenon in bearings that imposes a highly localized subsurface volume to a complex triaxial and non-proportional fatigue loading. Case-hardened M50-NiL steel, a widely used aerospace bearing material, when subjected to RCF loading shows a definite increase in hardness within the localized RCF-affected zone as a function of cycles. To simulate such cyclic hardening material response during RCF using finite element (FE) modeling, knowledge of localized cyclic hardening properties of bearing steels is required. However, these properties are not available for through-hardened and case hardened steels nor are they readily measurable from the conventional bulk specimen testing methods used in high cycle fatigue. To overcome this challenge, an alternative method to evaluate the material-specific localized cyclic hardening properties as a function of case-layer depth is presented. This method uses the experimentally measured increase in micro-hardness within the RCF-affected zone, in conjunction with elastic–plastic FE simulation of RCF cyclic hardening, to extract localized cyclic constitutive properties, and investigate the underlying physics associated with the local hardness increase. Case hardened bearing steels are complex composites having a heterogeneous graded microstructure with carbide precipitates ranging from nanometer to micrometer length scales that are largely responsible for the material hardening. Micro-plastic strain accumulation via ratcheting at the scale of the carbide microstructure is proposed to be the underlying mechanism for cyclic hardening. This manuscript presents for the first time a procedure to estimate microstructure-sensitive isotropic and kinematic cyclic hardening parameters, from a judicious combination of numerical and experimental methods. Methods presented are applicable to both through-hardened and case hardened bearing steels.

Prediction of Cyclic Residual Stress Relaxation by Modeling Approach

A new method for prediction of residual stress cyclic relaxation has been developed and implemented in the finite element software Abaqus. The calculated profiles were validated by experimental measurements using X-ray diffraction method. This validated approach is afterward used to investigate the effect of loading path and initial residual stress characteristics on the kinetics of relaxation and the stabilized profile for both cyclic softening and cyclic hardening materials.

Finite Element Simulation of Bending Fretting Fatigue Considering Ratchetting and Cyclic Hardening

Based on a simplified equivalent 2-D plane strain finite element model, the bending fretting fatigue process of 316L stainless steel is simulated numerically by ABAQUS code. In this simulation, the effect of ratchetting on the fretting fatigue process is discussed by implementing an advanced cyclic elasto-plastic constitutive model for cyclic hardening materials into ABAQUS code as a user material subroutine (UMAT). From the numerical simulation, the effect of bending loads on the bending fretting fatigue of 316L stainless steel is addressed, and then the failure lives are predicted by using Smith-Watson-Topper critical plane criteria. Comparison with the corresponding experiments shows that the predicted results are in good agreement with the experimental ones.

Case study of the applicability of cyclic hardening material descriptions in finite element simulation of cyclic thermal shocks

Finite element simulation of cyclic thermal loading induced fatigue can be used to predict the service time of components in industrial environments. Various damage criteria for low cycle fatigue have been developed, which require a temperature-dependent description of the cyclic plastic hardening of the material. In these descriptions the shape of the stress-strain loop is coupled to its cyclic evolution, which leads to problems under specific loading conditions and/or for materials not exhibiting a quick cyclic stabilization. The applicability of a basic, nonlinear kinetic-isotropic cyclic hardening description combined with an approach in which its parameters depend on the accumulated plastic strain and temperature is evaluated in a case study mimicking boundary conditions that occur in the primary cooling circuit of nuclear power plants. The model is calibrated with experimental data for a material that exhibits cyclic softening, requiring a modification of the hardening description.

A computational fatigue analysis of cyclic thermal shock in notched specimens

Thermal shock induced fatigue plays a role in the assessment of the lifetime of different components in the primary cooling circuit of a nuclear plant. In spite of the implementation of substantial and costly safety factors, a few, unexpected cases of fatigue failure have occurred. Here we report on a laboratory experiment which mimics the thermal loading observed in such components. A finite element thermal stress analysis using a calibrated, elasto-plastic, combined kinematic–isotropic cyclic hardening material model is presented. The distribution of transient stresses and strains in the specimens subjected to cyclic thermal shock, are used to predict the number of cycles to crack initiation with a fatigue curve that has been calibrated experimentally with data from equivalent specimens under pure mechanical fatigue. Our results indicate that cyclic thermal shock induced ratcheting occurs locally near the tip of the notch in the specimens, and the potential impact on the number of cycles to crack initiation is explored.

Non-proportionally multiaxial ratcheting of cyclic hardening materials at elevated temperatures: Experiments and simulations

The uniaxial and non-proportionally multiaxial ratcheting behaviors of cyclic hardening materials at room and elevated temperatures were studied by experiments first. As an auxiliary experiment to reveal the cyclic hardening/softening feature of the material and determine the material parameters used in the developed model, the material was also tested under a uniaxial and a non-proportionally multiaxial strain cycling. Simultaneously, the effects of different loading conditions at certain temperature on the strain cyclic deformation and ratcheting were analyzed. It is shown by experiments that, the strain cyclic deformation and ratcheting behavior of the material have a great dependence on the ambient temperatures, especially to a specific range of 400–600 °C, i.e., the material exhibits a markedly dynamic stain aging so that a higher cyclic hardening and lower ratcheting are resulted in, as comparing with the results at other temperatures. Then, based on the aforementioned experimental results, the temperature-dependent ratcheting of the material was simulated by constructing a new visco-plastic constitutive model. In the developed model, the Ohno–Abdel-Karim kinematic hardening model [Ohno, N., Abdel-Karim, M., 2000. Uniaxial ratcheting of 316FR steel at room temperature: II. Constitutive modeling and simulation. ASME J. Eng. Mater. Technol. 122 (1), 35–41] was extended to simulate the temperature-dependence of ratcheting behavior reasonably by introducing some temperature-dependent terms. It is shown that the simulated results agree with the corresponding experimental results well.

Evaluation of fatigue damage at welded tube joint under cyclic pressure using surface hardness measurement

In this paper, fatigue tests were conducted on 1Cr18Ni9Ti stainless steel and its welded joints were subjected to cyclic inner pressure. During the fatigue tests, the HL hardness of base metal (BM), weld metal (WM) and heat affected zone (HAZ) were measured. The results showed that 1Cr18Ni9Ti was a cyclic hardening material and the WM was a cyclic softening material. Moreover, the fatigue damage evolution of BM was a power relationship and the fatigue damage evolution of WM and HAZ were a linear relationship. Applying Chaboche’s nonlinear continuum fatigue damage model and studying on evolution of HL hardness, a damage parameter based on HL hardness and a fatigue damage accumulation model was proposed.

繰返し硬化材の平均応力一定条件下での疲労強度推定(材料の強度と力学的評価)

The fatigue strength of cyclic hardening materials under constant mean stress condition increase when the maximum stress becomes larger than the yield stress and cannot be estimated accurately based on the fatigue limit / strength estimation method proposed previously (e.g. Modified Goodman's curve). In this paper, the fatigue tests of SUS316NG and SM400 steels as cyclic hardening materials are carried out under constant mean stress condition and the relationships between fatigue strength and failure life are obtained. From the test results, it is shown that the difference between modified Goodman's estimation and fatigue test result changes linearly with the increase of maximum stress. Then, the approximate equation for the relationship between the difference and maximum stress is derived. By using the approximate equation for evaluating the difference and the results of modified Goodman's estimation, it is shown that the fatigue strength of cyclic hardening material under constant mean stress condition can be estimated with practically sufficient accuracy.

繰返し硬化材におけるＸ線パラメーターωｙと破壊力学パラメータＫｍａｘに関する研究

Cyclic softening and hardening behaviors are observed in metallic materials which undergo continuous stress. As a typical example, it is known that a cyclic softening behavior will be accepted in the material strengthened by hot treatment etc. and a cyclic hardening behavior will be accepted in the annealing material.In this study, we examined the relation formula between the X-ray parameter and the fracture mechanics parameter which undergo variable loading in the SS400 steel as a cyclic hardening material. In addition, we examined the HW685 steel, a cyclic hardening material having the same carbon content in the similar way for comparison.Furthermore, we summed these data up and examined the relationship between the X-ray parameter and the fracture mechanics parameter when setting the experiment constant value α to 0.12 from the engineering standpoints. As a result, we obtained the significant viewpoints through this study and we will report them.

A two-surface cyclic plasticity model consistent with fundamental stress-strain equations of the power-law type

A plasticity constitutive model which can describe primary features of stress-strain behaviors of cyclically loaded metallic materials under isothermal conditions is developed in the framework of the two-surface plasticity theory. A special plastic hardening modulus function, which is consistent with the power-law type stress-strain expressions frequently used for both monotonic and cyclic stable stress-strain curves, is introduced to represent cyclic hardening as well as saturated behaviors more realistically. The model is validated through its application to the simulation of uniaxial cyclic and biaxial behaviors of cyclic hardening materials. This model is then implemented into a general purpose finite element computer program and applied to the analysis of a notched plate subjected to cyclic loading.

Investigation of diagrams for deformation of metals under static and cyclic loads in relation to their fatigue failure

1. A substantial difference is observed in the initial sections of the deformation diagrams plotted from the results of static and cyclic tests (with the use of the values of the inelastic strains in the stabilization section) of the investigated metals. If it is characterized by the hardening coefficient Kh, equal to the ratio of the stresses corresponding to identical residual deformation tolerance under static and cyclic loads, this difference is most significant with small inelastic strains, and this corresponds to the region of many-cycle fatigue. 2. The hardening coefficient Kh can be evaluated approximately from the ratio of the ultimate tensile strength to the proportionality limit. The hardening coefficient increases with increase in this ratio. As a rule, metals with σb/σpc < 2−3 belong to the cyclic-weakening materials. Metals with a larger value for this ratio, as a rule, belong to the cyclic-hardening materials. 3. A definite relationship was established between the hardening coefficient Kh and the intensity of the decrease in the limited endurance limit with decrease in the test base. With increase in the hardening coefficient the intensity of this decrease falls. However, in this case there is a substantial spread in the results, and further investigations are required in this region.