Strain-controlled Low Cycle Fatigue Research Articles

Low-temperature fatigue behavior and damage analysis of Al-7Zn-2Mg-1.5Cu alloy based on hysteretic energy

The fatigue performance of Al-7Zn-2Mg-1.5Cu alloy was investigated using strain-controlled low-cycle fatigue (LCF) tests at various low temperatures focusing on the effects of hysteresis energy on fatigue life and the analysis of corresponding parameters. The results show that the alloy exhibits different cyclic hardening ratio depended on the applied total strain amplitude. The cyclic hysteresis energy displays cyclic stabilization at different temperatures. The initiation of fatigue crack occurs on the surface of specimens and many fatigue striations can be found on fatigue crack propagation region. In addition, the hysteresis energy model can be used to evaluate fatigue life at different temperatures. Wherein, intrinsic fatigue toughness (W0) decreases obviously with the decrease of temperature, while damage transition exponent (β) increases slightly. Comparing with low temperature tensile test results, Wo is related to the static toughness and β has a relation with the yield strength ratio. The relationships between the dislocation configuration, fatigue crack propagation mode and two fatigue parameters are discussed from microscopic viewpoint.

An experimental and numerical study on strain controlled fatigue response of TRIP steel under influence of hydrogen

The present work aims to study the influence of hydrogen charging on the transformation induced plasticity steel under strain-controlled low cycle fatigue (LCF). Hydrogen charging had detrimental effect on strain-controlled LCF lives. Cyclic hardening in un-charged specimen due to strain-induced martensitic transformation (SIMT) dominated over cyclic softening. In contrast, generation of large fraction of low angle grain boundaries in hydrogen charged specimen leads to cyclic softening and supresses cyclic hardening due to SIMT. Additionally, Cyclic plasticity modelling using Ohno-Wang kinematic hardening model is validated by simulating cyclic stress–strain hysteresis loop and cyclic softening curve of uncharged and hydrogen charged specimens.

Strain partition induced abnormal low-cycle fatigue behavior in single-phase heterostructured 316L stainless steels

The low-cycle fatigue (LCF) life of conventional materials is largely dependent on their ductility, i.e., the better the ductility, the longer the LCF life. In this work, 316L stainless steel (SS) was subjected to dynamic plastic deformation and subsequent annealing to obtain single-phase heterostructured samples consisting of recrystallized grains and remaining nanostructures. The tensile ductility of the heterostructured 316L SS increases monotonically with increasing volume fraction of recrystallized grains, while the strain-controlled LCF life first decreases and then increases, partially deviating from the monotonically increasing LCF-ductility relation. The reason is that the recrystallized grains with a low volume fraction (≤25 vol.%) in heterostructured 316L SS sustain high cumulative plastic strain due to plastic strain partitioning, leading to cracking in the recrystallized grains at the early stages of LCF and thus reducing the LCF life.

Statistical Characterization of Strain-Controlled Low-Cycle Fatigue Behavior of Structural Steels and Aluminium Material.

Probabilistic evaluation of the resistance to low-cycle deformation and failure of the critical components in the equipment used in the energy, engineering, metallurgy, chemical, shipbuilding, and other industries is of primary importance with the view towards their secure operation, in particular, given the high level of cyclic loading acting on the equipment during its operation. Until recently, systematic probabilistic evaluation has been generally applied to the results of statistical and fatigue investigations. Very few investigations applying this approach to the low-cycle domain. The present study aims to substantiate the use of probabilistic calculation in the low-cycle domain by systematic probabilistic evaluation of the diagrams of cyclic elastoplastic deformation and durability of the materials representing the major types of cyclic properties (hardening, softening, stabilization) and investigation of the correlation relationships between mechanical properties and cyclic deformation and failure parameters. The experimental methodology that includes the calculated design of the probabilistic fatigue curves is also developed and the curves are compared to the results of the experiment. Probabilistic values of mechanical characteristics were determined and calculated low-cycle fatigue curves corresponding to different failure probabilities, to assess them from the probabilistic perspective. A comparison of low-cycle fatigue curves has shown that the durability curves generated for some materials using analytical expressions are not accurate. According to the analysis of the relative values of experimental probabilities of low-cycle fatigue curves, the use of analytical expressions to build the curves can lead to a significant error. The results obtained allow for the revision of the load bearing capacity and life of the structural elements subjected to cyclic elastoplastic loading in view of the potential scattering of mechanical properties and resistance parameters to low-cycle deformation and failure. In addition, the results enable determination of the scatter tolerances, depending on the criticality of the part or structure.

High Temperature Fatigue Behavior and Failure Mechanism of Ti-45Al-4Nb-1Mo-0.15B Alloy

Strain-controlled low cycle fatigue experiments were carried out on the TiAl alloy Ti-45Al-4Nb-1Mo-0.15B at 400 °C and 750 °C to reveal the cyclic mechanical behavior and failure mechanism. The TiAl alloy presents stable cyclic characteristics under fatigue loading at elevated temperatures. No obvious cyclic softening or cyclic hardening was manifested during experiments. The cyclic stress–strain relationship is well described by the Ramberg–Osgood equation. The fatigue lifetime at different temperatures has a log-linear relationship with the total strain ranges. The fracture morphology indicates the main fracture mode of fatigue specimens at 400 °C is a brittle fracture, while there is a ductile fracture at 750 °C. Meanwhile, the trans-lamellar fracture is dominant for the lamellar microstructure and the percentages of the inter-lamellar fracture decreases with the strain amplitude.

Experimental determination of cyclically stabilized material properties of the Mo-based alloy MHC in stress-relieved condition from room temperature to 1400 °C

The aim of the current study was the determination of the time-dependent visco-plastic material behavior of the molybdenum alloy MHC (Molybdenum-Hafnium-Carbon) in stress-relieved condition with cyclic strain-controlled low cycle fatigue experiments at room temperature, 800 °C and 1400 °C. To ensure high data quality, a servohydraulic testing machine modified with a vacuum chamber was utilized to avoid material oxidation. The long-time strain-controlled experiments were conducted with a laser extensometer with high accuracy up to the highest applied test temperature of 1400 °C. The determined data were appropriate to generate cyclic stress-strain curves, strain-Wöhler curves, and their descriptive parameters. Furthermore, the strain rate dependency of the cyclic stress response and the stress relaxation behavior of the cyclically stabilized material were also investigated at the mentioned temperatures. The current study indicates that MHC in stress-relieved condition has a softening ability during cyclic loading, especially at 800 °C and 1400 °C. The strain rate sensitivity of the stress amplitude for the case of cyclic stabilization shows similar behavior as described in the literature for the case of monotonously increasing loading conditions, with a minimum at 800 °C. An interesting effect was identified in relaxation tests with loading at different strain rates. A kind of rebound effect was observed at the highest utilized strain rate of 10−2 s−1, which disappears with decreasing strain rate. Elastic strain proportions can be converted into plastic strain proportions during the application of the slow strain rate.

Low cycle fatigue behavior of micro-grain casting K4169 superalloy at room temperature

The strain-controlled low cycle fatigue tests were carried out at 298 ​K for the newly developed micro-grain casting K4169 superalloy, and the cyclic stress response, deformation and fracture behaviors of the alloy were studied. The results show that the fatigue life of the alloy was increased by more than two times compared with other casting K4169 alloys. This can be partly attributed to grain refinement impeding crack initiation and propagation. When the strain amplitude was in plastic range (Δεt/2 ​≥ ​0.5%), the alloy displayed a short period of initial cyclic stability followed by cyclic softening to fracture. By observing dislocation configurations, only a few dislocations moved around grain boundaries at initial cycles, leading to short cyclic stability. Afterwards, dislocations in the slip bands cut γ" phase continuously leading to cyclic softening. When the strain amplitude was in elastic range (Δεt/2 ​≤ ​0.4%), the alloy exhibited elastic deformation and the peak stress kept stable. In addition, the crack initiation site changed from the MC carbides to the persistent slip bands on surface with increasing strain amplitudes.

EBSD analysis of strain distribution and evolution in ferritic-Pearlitic steel under cyclic deformation at intermediate temperature

Strain accumulation as a result of cyclic loading affects the microstructure and mechanical performance of low-alloy steels under cyclic operation conditition. This manuscript presents a comprehensive analysis of a ferritic-pearlitic SA204 Grade C steel under strain-controlled low-cycle fatigue tests to understand the microstructural deformation and strain localization in the ferritic-pearlitic steel. Low-cycle fatigue tests were performed in a Gleeble® thermo-mechanical simulator at 0.008, 0.01, 0.015, and 0.02 strain amplitudes under an isothermal condition at 250 °C. The microstructure evolution during cyclic loading was evaluated using the samples from interrupted 0.01 strain amplitude tests. The LCF analysis shows strain localization at the ferrite-cementite interfaces and ferrite-pearlite interfaces. The characterization results using OM, SEM, EBSD, and nanoindentation analysis reveals the grain misorientation in the ferritic-pearlitic microstructure due to strain accumulation increases with the strain amplitudes and the loading cycles.

Comparative assessment of strain-controlled fatigue performance of SS 316L at room and low temperatures

In the present study, strain-controlled low cycle fatigue (LCF) behaviour of SS 316L at room temperature (RT) and −80 ℃ were investigated. RT LCF specimens show cyclic softening, whereas −80 ℃ specimens show initial cyclic softening followed by hardening due to strain-induced martensitic transformation (SIMT). Fatigue damage evolution mechanism is proposed for specimens tested at −80 °C, where compressive stresses develop due to SIMT and retards the crack growth. This leads to an increase in fatigue life for −80 °C specimens. An increase in back stress component at the initiation of secondary hardening is noticed for −80 °C specimens.

On the influence of physical vapor deposited thin coatings on the low-cycle fatigue behavior of additively processed Ti-6Al-7Nb alloy

The tensile and fatigue properties of laser powder bed fused Ti-6Al-7Nb are improved by physical vapor deposition (PVD). Thin coatings of titanium nitride (TiN), titanium carbonitride (TiCN), and silver-containing amorphous carbon (a-C:Ag) are deposited by magnetron sputtering. Tensile and strain-controlled low-cycle fatigue tests with various total strain amplitudes are performed. The coatings increase the tensile strength and decrease the breaking elongation. The low-cycle fatigue performance of the PVD-coated conditions, except for the a-C:Ag coating, is improved. The coatings considerably enhance tensile strength and fatigue resistance due to their high residual compressive stresses and increased surface hardness.

In-situ fatigue behavior study of a nickel-based single-crystal superalloy with different orientations

This work studied the relationship between the fatigue behavior and microstructural evolution of a second-generation nickel-based single-crystal superalloy with [001], [102], and [314] orientations at room temperature. Strain-controlled low-cycle fatigue experiments were executed using an in-situ Scanning electron microscope (SEM) fatigue test system. Digital image correlation (DIC) mapping combined with SEM images were used to identify strain concentration and predict areas of crack initiation. Results showed that the deformation of specimens with different orientations was dominated by planar slip and followed Schmid law. Cross-slips were observed in the specimen with the [001] oriented orientation, while a single slip was observed for the other two orientations. Fatigue crack propagation was dominated by one slip system in the [001] and [314] oriented specimens. While in the [102] oriented specimen, it was alternately dominated by two slip systems. The fatigue life of Nickel-based single-crystal superalloys (NBSCs) had a high orientation dependence, and was related to the elastic modulus and crack propagation path. The cyclic softening that occurred during fatigue was caused by dislocation motion.

Life prediction and damage analysis of creep-fatigue combined with high-low cycle loading by using a crystal plasticity-based approach

Many high-temperature rotating components are always subjected to complex loading waveforms, arising the concerns for multi-damage driving crack initiation. In this paper, a series of strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests as well as the novel creep-fatigue combined with high-low cycle (CF-HL) tests were performed in a nickel-based superalloy at 650 ℃. Then, post-test microstructure observations were carried out to reveal the damage mechanisms under the multi-damaged CF-HL loading based on the EBSD-TEM combinative characterizations. In this aspect, the single-slip-dominated deformation mechanism under CFI loadings transformed to double-slip-dominated one under CF-HL loading was revealed. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation was constructed for predicting the CF-HL crack initiation life and quantifying the crack initiation mechanisms. With the help of the developed fatigue and creep indicator parameters represented by accumulated energy dissipation, good agreements between experimental data and simulated results were achieved within a scatter band of ± 2 on life prediction. In addition, the simulation results indicate that the combined effects of grain orientation and multiple slip system activation showed great influence on the CF-HL crack initiation.

Study on the Creep-Fatigue Interaction Behavior of 1Cr11Ni2W2MoV Steel

Abstract In order to investigate the creep-fatigue interaction of 1Cr11Ni2W2MoV steel, strain-controlled low-cycle fatigue tests were performed at various strain ranges with fully reversible cycle of triangular waveform at 500°C. In addition, different hold times were introduced at the maximum tensile strain to investigate the impact of creep damage on fatigue life. Fracture surfaces and axial section were evaluated in terms of crack growth behavior and propagation path. The creep-fatigue life decreased with increasing hold time. It was observed that cyclic softening effect intensified with tensile holding. Crack initiation and growth behavior changed in relation to hold time that exceeded 60 s at a strain range of 1.2 %, which led to premature failure. Under the present test conditions, a good description of fatigue life was provided by the model based on inelastic strain energy density.

A life prediction method and damage assessment for creep-fatigue combined with high-low cyclic loading

Many high-temperature rotating components are always subjected to complex loading waveforms, arising the concerns for various damage modes simultaneously. The aim of the present work is to propose an energy-based model to predict the creep-fatigue combined with high-low cyclic loading (CF-HL) life, wherein the interactions of low cycle fatigue (LCF) damage, high cycle fatigue (HCF) damage and creep damage on the CF-HL behaviors are considered together. In order to validate the prediction capabilities of the proposed model, strain-controlled LCF, creep-fatigue interaction (CFI) tests as well as the newly developed CF-HL tests in Inconel 718 superalloy are performed at 650 ℃. Results show that most of the experimental data points fall into a range within a scatter band of ± 2 on life prediction. In addition, a robust and flexible assessment method within three-dimensional damage interaction diagram is proposed with the extension of traditional creep-fatigue interaction diagram. Finally, a case study of a turbine fir-tree attachment is provided to illustrate the engineering meanings of the developed energy-based model and damage interaction diagram, which is expected to be promoted in many high-temperature components.

Quantitative analysis of the deformation modes and cracking modes during low-cycle fatigue of a rolled AZ31B magnesium alloy: The influence of texture

The anisotropy of mechanical behavior, as extensively reported in wrought Mg alloys, is ultimately associated with the variation of deformation modes. The influences of texture on the slip/twinning-detwinning activity, cracking modes and mechanical behaviors of a hot-rolled AZ31B alloy during strain-controlled low-cycle fatigue were investigated quantitatively and statistically by quasi-in-situ electron back-scattered diffraction (EBSD) analysis and scanning electron microscopy (SEM) observations coupled with slip trace analysis. The fundamental difference in the cracking modes and fatigue behaviors between the 0°, 45°, and 90° samples (i.e., under loading directions of the normal direction (ND), 45°, and rolling direction (RD)) is ascribed to the distinct activation sequences of deformation modes. The cyclic deformation-mode transitions throughout the whole fatigue life can be summarized as: (I) the 0° sample: basal slip + tension twinning→detwinning + basal slip; (II) the 45° sample: basal slip→ basal slip; (III) the 90° sample: tension twinning → detwinning + basal slip. The underlying mechanism for the texture-dependency of deformation modes is discussed. Cracking modes are also closely related to the texture. Intergranular cracking was the primary cracking mode for the 0° and 45° samples, while tension twinning (TTW) cracking was the major cracking mode in the 90° sample. The factors governing the site where microcracks were nucleated are analyzed.

Micromechanical constitutive modeling of tensile and cyclic behaviors of nano-clay reinforced metal matrix nanocomposites

Metal matrix nanocomposites are emerging rapidly among both research and industry communities. Nonetheless, the studies for their use in Low-Cycle Fatigue (LCF) applications are rarely documented. Prior to tailoring the microstructure for the desired LCF performance, it is a fundamental task to understand the structure-property correlations of the nanocomposites. In the present paper, a continuum-based micromechanical model starting from the nanostructure is proposed to fulfill such a task. The proposed model is applied to a piston aluminum alloy reinforced with 1 wt% nano-clay subjected to fully-reversed strain-controlled LCF loading at room temperature. The model, formulated within Eshelby's inclusion framework, considers the active clay/matrix interactions, the intercalation/exfoliation effect via the equivalent stiffness method, and the size effect through the introduction of the interphase thickness as a characteristic length scale. To render the model able to reproduce the cyclic hysteretic behavior of the nanocomposites, the isotropic/kinematic hardening rule is considered to describe the aluminum alloy plasticity. The model results are favorably compared to the experimental observations in terms of monotonic and cyclic elastoplastic stress-strain responses of the aluminum/clay nanocomposite. A parametric study is then carried out in order to understand the separate and synergic effects of clay weight fraction, clay geometrical parameters (e.g., shape and size), and clay structural parameters (e.g., number of silicate layers and interlayer spacing) on the overall monotonic and cyclic responses of the present nanocomposite. The reinforcing effect of each parameter is discussed concerning the micromechanical model.

Low cycle fatigue of a γ'-strengthened Co-based single crystal superalloy at 900 °C

The strain-control low cycle fatigue behaviour of a γ'-strengthened Co-based single-crystal superalloy was investigated at 900 °C. The fatigue life of the alloy was longer than conventional Co-based superalloys due to strengthening provided by the γ'. Additionally, it is found that slip bands consisting of many tangled dislocations and stacking faults continuously cut through the γ' phase with the strain amplitude above elastic limit, which is the main deformation mechanism. The planar slip deformation mode has resulted in crystallographic fracture. In contrast, few dislocations were observed in the sample cycled below elastic limit and severe oxidation at surface introduced Co3W/γ layer and recrystallization leading to mode I crack.

Fatigue strength of additive manufactured Mar-M-509 superalloy

In a recent paper, we have reported successful additive manufacturing of cobalt-based superalloy Mar-M-509® via laser powder bed fusion. The work examined effects of build orientation and subsequent heat treatments on the evolution of microstructure to tailor monotonic strength and ductility of the alloy. In this paper, we present the main results from an experimental investigation into the low cycle fatigue (LCF) and high cycle fatigue (HCF) behavior of the alloy in its optimized condition for strength and ductility. To this end, strain-controlled LCF tests were conducted under strain amplitudes ranging from 1% to 2% without the mean strain component. Stress amplitudes showed small hardening with cycles after an initial transient in rapid cyclic hardening followed by rapid cyclic softening. Some anisotropy was observed in the LCF behavior of the alloy. The amplitudes were higher for the samples tested perpendicular to the build direction (BD) than along the BD, consistent with static strength. In contrast, the LCF life was longer for the samples tested long the BD, consistent with ductility. The measured data was used to delineate the strain-life curves for the alloy per test direction based on the Coffin-Manson model. Additionally, stress-controlled rotary bending HCF fatigue tests were carried out under stress amplitudes ranging from 150 MPa to 750 MPa at room temperature and at 760 °C. The HCF life of the alloy was longer at room temperature than at 760 °C for all stress amplitudes. In contrast to static strength and LCF, the HCF behavior was found isotropic. The observed fatigue characteristics are correlated with the initial microstructure of the alloy.

Transient Phase-Driven Cyclic Deformation in Additively Manufactured 15-5 PH Steel.

The present work extends the examination of selective laser melting (SLM)-fabricated 15-5 PH steel with the 8%-transient-austenite-phase towards fully-reversed strain-controlled low-cycle fatigue (LCF) test. The cyclic-deformation response and microstructural evolution were investigated via in-situ neutron-diffraction measurements. The transient-austenite-phase rapidly transformed into the martensite phase in the initial cyclic-hardening stage, followed by an almost complete martensitic transformation in the cyclic-softening and steady stage. The compressive stress was much greater than the tensile stress at the same strain amplitude. The enhanced martensitic transformation associated with lower dislocation densities under compression predominantly governed such a striking tension-compression asymmetry in the SLM-built 15-5 PH.

Low Cycle Fatigue Performance of Additively Processed and Heat-Treated Ti-6Al-7Nb Alloy for Biomedical Applications

In biomedical engineering, laser powder bed fusion is an advanced manufacturing technology, which enables, for example, the production of patient-customized implants with complex geometries. Ti-6Al-7Nb shows promising improvements, especially regarding biocompatibility, compared with other titanium alloys. The biocompatible features are investigated employing cytocompatibility and antibacterial examinations on Al2O3-blasted and untreated surfaces. The mechanical properties of additively manufactured Ti-6Al-7Nb are evaluated in as-built and heat-treated conditions. Recrystallization annealing (925 °C for 4 h), β annealing (1050 °C for 2 h), as well as stress relieving (600 °C for 4 h) are applied. For microstructural investigation, scanning and transmission electron microscopy are performed. The different microstructures and the mechanical properties are compared. Mechanical behavior is determined based on quasi-static tensile tests and strain-controlled low cycle fatigue tests with total strain amplitudes εA of 0.35%, 0.5%, and 0.8%. The as-built and stress-relieved conditions meet the mechanical demands for the tensile properties of the international standard ISO 5832-11. Based on the Coffin–Manson–Basquin relation, fatigue strength and ductility coefficients, as well as exponents, are determined to examine fatigue life for the different conditions. The stress-relieved condition exhibits, overall, the best properties regarding monotonic tensile and cyclic fatigue behavior.