Low Cycle Fatigue Experiments Research Articles

Ductile steel knee brace with built-in comb-shaped seismic damper

The attachment of supplemental knee braces is an efficient seismic retrofitting measure for existing steel buildings. A new ductile knee brace with a built-in comb-shaped damper is proposed in this study. The proposed ductile knee brace is composed of a wide-flange section with multiple slot holes in its web plate, which is designed to plasticize earlier at shear-yielding struts in the comb-shaped web. The isolated knee brace components and subassemblages reinforced by the proposed knee brace were subjected to cyclic loading tests to investigate the cyclic behavior of the knee brace. The test results showed that the knee brace exhibited sufficient deformation capacity. The overall strength and cyclic behavior of the knee brace was estimated using a simple mechanical model. Additionally, it was shown that the experimental low-cycle fatigue life of the knee brace can be predicted using the fatigue curve of notched specimen of the steel and Miner's rule.

Experimental study on the cumulative damage constitutive model of high-performance steel Q345GJ under cyclic loading

GJ series steel is a new type of high-performance structural steel, which has emerged in China steel construction, and it has been widely utilized in steel-framed building structures. To investigate the low cycle fatigue performance and cumulative damage behaviors and find a suitable constitutive relationship of GJ series steel under cyclic loading, a total of eighteen Q345GJ steel low cycle fatigue experiments have been carried out. The mechanical behavior of Q345GJ steel were discussed, including monotonic loading behavior, hysteresis loading behaviors and energy dissipation capacity. Based on the cyclic behavior of GJ series steel, a modified cumulative damage model was recommended. Then, with comparison of the Q345GJ tests data (including the data in public papers) under various loading systems, the model proposed in this paper is proved correct and can be used to simulate the cyclic behavior of Q345GJ members.

Effect of constitutive model on residual stress development in the pressure tube rolled joint

Being one of the most important joints in a pressurized heavy water reactor (PHWR), pressure tube rolled joint is critical during the construction and operation. This joint is formed through the roll expansion process that leads to an interference fit between the Zr 2.5%Nb pressure tube and stainless-steel end fitting. For integrity of this joint, a sufficient pull-out strength and leak tightness must be maintained, while having lower undesirable residual stresses, especially in the transition zone. The roll expansion process parameters are controlled to achieve optimum contact pressure and limit the tensile residual stress. The parameters of this roll expansion process are designed either through experiments or through numerical simulations. One important aspect that is often ignored in the numerical simulations is the effect of material constitutive behavior of the Zr 2.5%Nb alloy, particularly the anisotropic yield behavior and the cyclic stress-strain response. In this paper, these two aspects of constitutive behavior are investigated, using three-dimensional finite element simulations, to understand their influence on the residual stress development in the roll expansion process. The cyclic behavior was modelled using Chaboche nonlinear kinematic hardening model. The model parameters were extracted from low cycle fatigue experiments with the strain amplitude equivalent to that seen in roll expansion process. The results show that constitutive behavior, especially the cyclic behavior, has a significant impact on the residual stress. Anisotropic yielding of the pressure tube was also seen to affect the tensile residual stress and the contact pressure, although to a lesser extent. Finally, after comparison between four combinations of constitutive behavior, a simple isotropic yield with isotropic hardening model is suggested to predict the tensile residual hoop stress in the rolled joint, conservatively.

Assessment of Residual Service Life of Cast Bodies of Control Valves of 220 MW Power Units

In the regulatory documents of the Ministry of Energy and Coal Industry of Ukraine, the beyond-design operating life of the high-energy equipment of 220 MW power units is limited to the operating life of 220 thousand hours and 800 start-ups. To date, the high-temperature cast bodies of the control valves for the high- and intermediate-pressure cylinders of the K-200-130 200 MW steam turbines of DTEK Lugansk TPP have operated about 305–330 thousand hours with the total number of start-ups from 1438 to 1704, which exceeded the beyond-design service life characteristics. Therefore, it is necessary to assess the residual operating life of the control valve bodies of the high- and intermediate-pressure cylinders of K-200-130 steam turbines in order to determine the possibility of their further operation. These calculations were carried out on the basis of our earlier studies of the thermal and stress-strain states of cast turbine equipment. This paper establishes the values of stress intensity amplitudes, the values having been reduced to a symmetric loading cycle for the most typical variable operating modes. Using the experimental low-cycle fatigue curves for the 15Kh1M1FL steel, we established the values of the permissible number of start-ups and the cyclic damage accumulated in the base metal. We also determined the value of the static damage accumulated in the course of stationary operating modes according to our previously obtained experimental data on the long-term strength of the 15Kh1M1FL steel. The calculations showed that the total damage to the control valve bodies of the K-200-130 steam turbine of power unit 15 of DTEK Lugansk TPP is 97 and 98%. The residual operating life of the metal of the control valves of high-pressure cylinders is practically exhausted, being equal to 10 thousand hours. The residual life of the control valves of intermediate- pressure cylinders is 7 thousand hours, i.e. it is also practically exhausted, with safety factors for the number of cycles and strains at the level of 5 and 1.5, as well as the permissible 370,000 operating hours of the metal. With an increase in the permissible operating life of the metal to 470 thousand hours, according to experimental studies of Igor Sikorsky KPI, the total damage to the metal of cast valve bodies is reduced to 80%, and the residual metal life increases to 79,000 h and 75,000 h for the control valves of the high- and intermediate-pressure cylinders, respectively.

Low-cycle fatigue of fillet-welded steel plate connections

The present study focuses on the mechanical behavior of fillet-welded steel plated joints and the development of a strain-based fatigue assessment procedure, motivated by the seismic response of unanchored steel tanks that exhibit repeated uplifting, leading to low-cycle fatigue failure of the base plate connection with the tank shell, due to cyclic inelastic deformation. Low-cycle fatigue experiments on small-scale fillet-welded joints are performed, representing the welded connection the tank base plate with the tank shell, aimed at determining the relationship between the strain range developed at the welded vicinity and the corresponding number of cycles to failure. Prior to fatigue experiments, material tests have been conducted, to determine the mechanical properties of the base plate material, and the weld has been examined with stereo optical microscopy and micro-hardness measurements. In addition to experiments, numerical simulations are also performed, to elucidate special features of joint behavior, and calculate accurately the local strain at the weld region, complementing the experimental results and observations. The numerical simulations employ an advanced cyclic-plasticity model, based on the bounding-surface concept, implemented into the finite element model using an in-house material user subroutine. The experimental and numerical result enable the development of a strain-based fatigue curve, which can be used for the seismic design and assessment of liquid storage tanks. It is further concluded that the fillet-welded connections under consideration are capable of sustaining substantial strain levels for a significant number of cycles, until low-cycle fatigue failure occurs.

The effect of the 18R-LPSO phase on the fatigue behavior of extruded Mg/LPSO two-phase alloy through a comparative experimental-numerical study

Abstract The fatigue behavior of four extruded Mg-Y-Zn alloys containing different volume fractions of long-period stacking ordered (LPSO) grains was investigated through a comparative study combining experiments and crystal plasticity finite element simulations. Strain controlled low-cycle fatigue experiments were conducted at different strain amplitudes and revealed a limited cyclic hardening in Mg89Zn4Y7 alloy or softening in Mg99.2Zn0.2Y0.6 and Mg97Zn1Y2 alloys. A decrease in the fatigue life against the plastic strain with the increase in LPSO phase volume fraction was observed and was related the limited ductility of extruded LPSO grains. Stress-strain hysteresis curves were used to calibrate and validate a crystal plasticity model taking into account twinning and detwinning. The interaction of the different phases on the distribution of local micro-mechanical fields at the grain scale was then analyzed on synthetic microstructures under strain-controlled conditions. Deformation twinning activity was predicted in coarse unrecrystallized grains and tended to disappear with the increase in the LPSO phase volume fraction. Cleavage-like facets observed in LPSO grains were related to high tensile stress, especially at the Mg/LPSO interface, due to the limited number of deformation mechanisms in LPSO crystal to accommodate out-of-basal plane strain. The increase of the fatigue limit with the increase in LPSO phase volume fraction was finally associated with the decreasing presence of coarse unrecrystallized α-Mg grains due to a higher dynamic recrystallization activity during the extrusion process.

The investigation of primary creep regeneration for 10%Cr martensitic steel: Unified constitutive modelling

An elastic-viscoplastic constitutive material model is developed for the representation of the creep response of a 10%Cr steel under cyclic loading conditions. It has been shown that the model is able to describe primary creep regeneration (PCR), i.e. the incidence of a period of high creep strain rate following a stress reversal. The developed model is a variant of the well-known Chaboche viscoplastic constitutive model, and employs a bi-term power-law equation to represent the stress-regime dependence of the viscoplastic strain rate response. Back stress and drag stress are used to describe the kinematic and isotropic hardening/softening behaviour of the material, respectively. The evolutions of back stress and drag stress consider contributions from strain hardening, dynamic softening, static recovery and cyclic hardening/softening.The effectiveness of the developed model for describing the sensitivity of the PCR behaviour to different loading parameters (e.g. reverse-loading magnitude and duration) and to represent the effect of PCR activation on the overall strain accumulation behaviour of the material is discussed. Furthermore, the predictive capability of the model is demonstrated for describing the response of the material during two independent benchmark tests; a stress-varying creep and a low-cycle fatigue experiments.

Stress-controlled LCF experiments and ratcheting behaviour simulation of a nickel-based single crystal superalloy with [001] orientation

Uniaxial ratcheting behaviour and low cycle fatigue (LCF) failure mechanism of nickel-based single crystal superalloy DD6 with [001] orientation are investigated through the stress-controlled LCF tests with stress ratio of −1. Then the deformation behaviour during the whole-lifetime from the beginning of the experiment to the fracture of the specimen, as well as the fractographic/metallographic morphology, are compared with the strain-controlled LCF experimental results. Through the scanning electron microscope (SEM) observations, it is shown that the failure characteristics under stress-controlled LCF loading are similar with those under strain-controlled loading. Nevertheless, unlike strain-controlled LCF loading, even under fully reversed cycle loading for stress-controlled LCF, DD6 shows significant ratcheting behaviour due to the tension-compression asymmetry. In addition, the LCF lifetimes under stress control are significantly shorter than the LCF lifetimes under strain control, and the culprit might be the detrimental effect of ratcheting strain on LCF lifetime. Based on these phenomena, an improved crystal plasticity constitutive model on the basis of slip-based Walker constitutive model is developed through modifying the kinematic hardening rule in order to overcome the inaccurate prediction of decelerating stage and stable stage of ratcheting behaviour. Furthermore, combining the continuum damage mechanics, a damage-coupled crystal plasticity constitutive model is proposed to reflect the damage behaviour of DD6 and the accelerating stage of ratcheting behaviour. The simulation results for the stress-controlled LCF deformation behaviour including the whole-lifetime ratcheting behaviour show good agreement with the experimental data.

Assessment of the chemical compatibility between EUROFER and ceramic breeder with respect to fatigue lifetime

Abstract In advanced designs of DEMO breeding blankets with ceramic breeder, the structural material EUROFER is in direct contact with the pebbles of the ceramic breeder (Li4SiO4 + Li2TiO3) that results in a loss of constituent elements in the ceramic and the formation of a corrosion layer on EUROFER under operation relevant conditions. The impact of this corrosion attack on damage mechanisms which are significantly influenced by microstructural changes at the surface, in particular fatigue, needs to be considered in the design of affected components and is therefore investigated within the work reported here. Fatigue specimens made out of EUROFER in unconstrained contact to ceramic breeder pebbles were exposed to purge gas conditions for different durations. The chemical surface attack observed after the exposure is characterized by round stains, partly even round dimples, consisting of concentric diffusion zones which meet in case of the long exposure time forming an almost closed corrosion multi-layer. The impact of this attack on the fatigue behavior was subsequently investigated by performing strain-controlled low cycle fatigue (LCF) tests at 550 °C. To cover the scatter expected for the fatigue lifetime, a sufficient number of LCF experiments were conducted. The results show significant reduction of the fatigue lifetime, which quantitatively depends on the level of the attack/exposure time as well as the level of the applied load/strain amplitude.

Statistical Analysis of Grain-Scale Effects of Twinning Deformation for Magnesium Alloys under Cyclic Strain Loading.

To reveal the relationship between grain size and twinning deformation of magnesium alloys under cyclic strain, this study carried out a group of strain-controlled low-cycle fatigue experiments and statistical analysis of microstructures. Experimental results show that the shape of the hysteresis loop exhibits significant asymmetry at different strain amplitudes, and the accumulation of residual twins plays an important role in subsequent cyclic deformation. For the different strain amplitudes, the statistical distribution of the grain size of magnesium alloy approximately follows the Weibull probability function distribution, while the statistical distribution of twin thickness is closer to that of Gaussian probability function. The twin nucleation number (TNN) increases with the increase of grain size, but there is no obvious function relationship between twin thickness and grain size. Twin volume fraction (TVF) increases with the increase of grain size, which is mainly due to the increase of TNN. This work can provide experimental evidence for a more accurate description of the twinning deformation mechanism.

Characterization of strain amplitude-dependent behavior of hardness and indentation size effect of SS400 structural steel

In this paper, the continuous stiffness measurement (CSM) indentation is employed to investigate fatigue mechanical properties of structural steel under cyclic loading. For this purpose, several representative analytical approaches were introduced to estimate the basic mechanical properties including Young’s modulus and indentation hardness from the characteristics of the loading/unloading curves. Several experiments including CSM nanoindentation, low-cycle fatigue experiment for four strain amplitude levels, optical microscope (OM), and transmission electron microscopy (TEM) examinations were conducted to observe the variation characteristics of mechanical properties at the microscale and their micro-mechanisms. The microstructural evolution of the specimens deformed by the low-cycle fatigue was observed using the OM and TEM examinations. The standard nanoindentation experiments were then performed at different strain rate levels to characterize the influences of strain rate indentation on hardness of the material. The micro-mechanisms established based on the microstructural evolution and strain gradient plasticity theory were introduced to be responsible for the variation of indentation hardness under cyclic loading. Finally, the indentation size effect (ISE) phenomenon in SS400 structural steel was investigated and explained through the strain gradient plasticity theory regarding geometrically necessary dislocations underneath the indenter tip. The experimental results can be used for practical designs as well as for understanding the fatigue behavior of SS400 structural steel.
 Keywords:
 cyclic loading; fatigue; nanoindentation; indentation size effect; strain rate sensitivity; structural steel.

Experiment and modelling on the effect of microstructural morphology on fatigue life of a Ni-based superalloy

A directionally solidified Ni-based superalloy was pre-treated under high temperature with/without the applied stress to obtain different microstructural morphology. Then, strain-controlled low cycle fatigue experiments were carried out under 850°C with a 0 strain ratio to measure the reduction in the fatigue resistance. The scanning electron microscopy (SEM) was employed to characterize the microstructures of the alloy with different treatments. The micro-deformation and dislocation movement were investigated via transmission electron microscopy (TEM). The results show that both coarsening and rafting morphology have negative effects on the fatigue properties, which is closely related to the deformation mechanism of the alloys with different γ' morphology. The coarsening morphology decreases the shear resistance of the γ' precipitates, resulting in shearing of γ' particles. More plastic deformation introduces during the cyclic process, leading in the reduction of fatigue life. On the other hand, the wide γ channel promoted the piling up and movement of dislocations in the matrix, also resulting in reduction of plastic deformation resistance and lifespan. Finally, a fatigue life prediction model was proposed based on the continuum damage mechanics (CDM) by introducing the variation of matrix channels’ width, showing a good competency compared with the experimental results.

Investigation on fatigue performance of adhesively bonded butt-joints and multiaxial life estimation using stress-based failure models

Uniaxial and multiaxial low-cycle fatigue experiments were conducted on the automobile sealant adhesive bonding hollow cylindrical butt-joints under stress-controlled mode, and the effects of equivalent stress amplitude, equivalent mean stress and loading frequency on the fatigue life of specimens were analyzed. The experimental results showed that the fatigue life of uniaxial tensile cycle was shorter than that one of uniaxial torsion cycle. The fatigue life of the automobile sealant adhesive bonding butt-joints decreased with the increase of equivalent stress amplitude, mean stress, loading frequency. The fatigue life of the non-proportional loading path was shorter than that of the proportional loading path. In addition, a modified stress-based fatigue life prediction model was proposed that collectively considered the effect of loading path, equivalent stress amplitude, equivalent mean stress and loading frequency on the multiaxial fatigue life. The comparisons of the predicted results and the experiment life indicated that the modified fatigue life prediction model had good prediction ability for the multiaxial fatigue life of the sealant adhesive bonding butt-joints.

Prediction of static and low-cycle durability of porous cellular structures with positive and negative Poisson's ratios

The static and low-cycle durability of three planar cellular structures, hexagonal, auxetic and auxetic-chiral, have been compared. The three structures have the same critical cross-section and are made from an aluminium alloy Al7075-T651. The reference region of each structure is represented by a matrix of nine elementary shaped cells (3 rows by 3 columns). For each structure static and low-cycle fatigue experiments at different loading amplitudes were made. Numerical simulations were then performed for the same boundary conditions to predict the static and low-cycle fatigue durability. For this purpose a continuum damage mechanics approach with element removal was used in explicit dynamic simulations. The results of static simulations were also checked using the eXtended Finite Element Method (XFEM). All the numerical simulations were carried out using Abaqus. Good agreement was observed between the simulated and measured results for each of the three cellular structures.

Shear cracks of Zircaloy tube during cold pilgering: experimental investigation and numerical modeling

Cold pilgering process is a seamless tube forming technology, in which the metal is subjected to a series of small incremental deformations. It has been extensively applied for the manufacturing of cladding tube in nuclear reactors, due to high dimensional accuracy and favorable texture evolution. Notwithstanding, the pilgered tubes showed a kind of shear cracks similar to those obtained during low-cycle shear fatigue experiments. To confirm it, a novel low-cycle shear fatigue experiment was designed, in which the shear stress amplitude was obtained from a tracking point in the inner surface of the pilgered tube through finite element model (FEM). Additionally, a theoretical model was established based on the experimental results to predict fatigue life during the cold pilgering process. Through this model, the effects of Q value, side relief, and friction coefficient on the safety factor during cold pilgering were investigated. The results showed that with the increase of Q value, and the decrease of the difference between the friction coefficients of the inner and outer surfaces, the safety of cold pilgering tube could be improved by decreasing the tendency of shear cracks.

An investigation of short crack propagation in a single crystal Ni-based superalloy using crystal plasticity and the extended finite element method

Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates.

Correlating Effect of Temperature on Cyclic Plastic Deformation Behavior with Substructural Developments for Austenitic Stainless Steel

Low-cycle fatigue experiments have been carried out at elevated and sub-zero temperatures. Corresponding effect on cyclic plasticity characterizing parameters such as cyclic hardening/softening and Masing behavior is compared for different loading conditions. Disparities in the fatigue life as well as the cyclic plastic behavior have been attributed to the phase transformations that largely obstruct the dislocation motion. Further, the changes in strains in the materials matrix have been quantified through misorientation studies, wherein clear demarcation in strain distributions due to fatigue loading at different temperatures was obtained and further correlated with the substructural alterations observed through transmission electron microscopy.

Lifetime assessment of the process-dependent material properties of additive manufactured AlSi10Mg under low-cycle fatigue loading

Systematic low-cycle fatigue (LCF) experiments are carried out on additive manufactured AlSi10Mg specimens for several material conditions with varying layer thickness, heat treatment, building direction and surface quality. The deformation behaviour depends significantly on the heat treatment. It is outlined that the process control and heat treatment can produce fatigue properties comparable with the cast material, whereby an as-built specimen surface leads to a lifetime reduction in all cases. The experiments are accompanied with detailed metallo- and fractographic investigations. For all tested LCF specimens, the defect type and the failure origin defect size are characterized in terms of the √area parameter by using scanning electron microscopy. The failure of the specimen is mostly caused by lack of fusion surface or near-surface defects, whereby the defect size is determined by the SLM process parameters, such as building direction, surface quality and layer thickness. On the basis of the experimental data and the observed defects, a mechanism-based, deterministic lifetime model is developed and adapted to the specific damage mechanisms of the additive manufactured AlSi10Mg alloy.

Experimental low cycle fatigue testing of circular bars with radial through-holes subjected to tension-compression loading

Experimental low cycle fatigue testing of circular bars with radial through-holes subjected to tension-compression loading

Probabilistic Lifetime Assessment Approach of 2%-Cr Steel Considering Material and Loading Profile Scatter

AbstractIn regions with high intermittent renewable energy share, thermal plants are forced to operate with greater flexibility beyond their original design intent. Decreasing energy prices and capacity factors will further force these plants to more transient operation with steeper load gradients. Older steam turbine (ST) protections systems on site are often not designed for such flexible operation and do not properly supervise the resulting impact on lifetime consumption. Therefore, precise lifetime management concepts are required to increase plant reliability and flexibility, and to mitigate risks for new implemented operation modes. Several lifetime assessment methods were developed to quantify the damage evolution and the residual lifetime for ST components. Usually, these methods require both input about representative or operated loading profiles and characteristic material curves. These characteristic curves are determined by a number of standardized material tests. Due to the material scatter and other different sources of uncertainties, each test is a realization of a stochastic process. Hence, the corresponding characteristic material curves inherit these uncertainties and do not represent an absolute limit. The analyses of different loading profiles even for the same plant and same start-up class reveal that the consideration of statistically evaluated specific start-up distributions and further transient events are of major importance. Probabilistic methods are able to quantify all of these uncertainties and compute the probability of failure for a given lifetime or vice versa. Within this paper, at first an extensive and systematic operational profile analysis is carried out and discussed, which acts as an input for a probabilistic lifetime assessment approach. For that, a developed probabilistic workflow is presented to quantify the uncertainties and for lifetime prediction using the generalized damage accumulation rule with focus on creep-fatigue loading. To quantify the characteristic material curves, existing experimental data of a 2%-Cr forged steel (23CrMoNiWV8-8) is used. A probabilistic representation of the Wilshire–Scharning equation characterizes the creep rupture behavior. The maximum-likelihood method is used for parameter estimation and to take still running long term creep experiments into account. The end of life in low cycle fatigue experiments is characterized by a macroscopic crack initiation, and the Manson–Coffin–Basquin equation is utilized to represent the characteristic material curve. A temperature-modified version of the Manson–Coffin–Basquin equation is used to represent the experimental data. The parameter estimation is done by using the linear regression analysis followed by a comprehensive regression diagnostic. Taking both the material and the load scatter into account, a reliability analysis is carried out to compute the probability of crack initiation. Finally, different load cases are considered and evaluated against each other.