Low Cycle Fatigue Deformation Research Articles

Role of micro‐constituents on the fatigue deformation micro‐mechanisms of IN 713 C alloy

AbstractThe cast IN 713 C alloy is extensively used in the aerospace and automobile industries. Even with improved casting methods, the inhomogeneity in the microstructure is still inevitable and causes strain localizations during cyclic loading. So far, the effect of microstructural variables on the low cycle fatigue (LCF) deformation behavior of IN 713 C alloy has rarely been studied. In this study, IN 713 C alloy is LCF tested at 650°C and 750°C for strain amplitudes of 0.35% to 0.6%. The results showed that the deformation heterogeneity resulted from carbides, γ′ precipitates, and γ–γ′ eutectics distribution, governing the LCF failure micro‐mechanisms. The principle fatigue cracks were seen to be initiated from the precipitate‐free zone and oxidized‐surface carbides. It is corroborated that the higher GNDs were observed at carbide/matrix, eutectic/matrix, γ′/matrix interfaces, and in the interdendritic areas where voids and cracks formed. The higher GNDs at these sites facilitate crack initiation and propagation.

Effect of Al on the low cycle fatigue deformation behavior and microstructure evolution of Fe–22Mn-0.6C TWIP steel

The dependence of low cycle fatigue (LCF) behavior and microstructure evolution of Fe–22Mn-0.6C twinning-induced plasticity (TWIP) steel on the alloying element Al has been investigated under fully reversed total strain amplitude control with a nominal strain rate of 0.006 s−1 and strain amplitudes ranging from 0.002 to 0.010 at room temperature. The results indicate a strong dependence of LCF response and evolved microstructure on the addition of Al, owing to its effect on the stacking fault energy (SFE). At all strain amplitudes, the 3Al steel with a higher SFE exhibits a visibly lower cyclic hardening and a higher cyclic softening, resulting in a lower cyclic yield strength than the 0Al steel. As compared with the 0Al steel, the relatively higher SFE of the 3Al steel causes higher accumulated plastic strain, decreased slip planarity and weaker reversibility of dislocations, thereby decreasing the fatigue life. The optical microstructure is featured by increase in slip band density with increasing strain amplitude for both steels. However, the slip band density of the 3Al steel is higher than that of the 0Al steel. Moreover, cyclic loading promotes the formation of dislocation cells, rough labyrinths as well as planar dislocation arrays. Deformation twins are observed only in the 0Al steel with a visible lower critical twinning stress due to its low SFE as compared to the 3Al steel.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Investigation of the high-temperature fatigue mechanism related to altered local short-range order in AL6XN austenitic stainless steel

An increase in crystal local short-range order (SRO), characterized by obvious diffuse intensity maxima at 1/3{422} in the [111]-axis selected area diffraction pattern, is observed in AL6XN austenitic stainless steel subjected to high-temperature low-cycle fatigue deformation. SRO is an ordered fcc structure with nanometer size, which can be well explained by the ordered arrangement of Cr/Mo atomic layers segregated on the {422} crystal plane in austenite. Many planar slip bands are observed in the fatigue sample, the distribution of SROs is highly related with the distribution of dislocations. In situ high-energy X-ray diffraction (HE-XRD) experiments are performed to investigate the deformation behavior of both as-received and fatigued samples under uniaxial loading. The lattice strain evolution reveals an interesting lattice expansion in the initial elastic stage, which provides direct evidence of a decrease in SRO during further uniaxial tensile deformation, indicating that the deformation modes greatly affect the SRO transition. We believe that the high-temperature fatigue damage originates mainly from the accumulation of planar slip bands, accompanied by a local increase in SRO, which promotes work hardening near the final fatigue fracture.

Low cycle fatigue behaviour of engineering metallic materials: Review on cyclic deformation micro-mechanism

The micro-mechanism of low cycle fatigue deformation behaviour has been summarised, utilising the recent advances in the state-of-the-art facility for the characterisation of deformation microstructure, and micro-texture. Besides, recent development in the approach of numerical simulation of cyclic stress-strain behaviour of polycrystalline metallic materials at multi-scale has been discussed. It has been carried out by reviewing, and re-interpreting the evidences from the investigations carried out over the last few decades on several engineering metallic materials having critical applications in various industries. The understanding of the existing material behaviour will help us in the better characterisation of the low cycle fatigue deformation microstructure of the new generation metallic materials having complex microstructure.

Influence of 0.5% Ag Addition on Low-Cycle Fatigue Behavior of Hot-Extruded Al-5Cu-0.8Mg-0.15Zr-0.2Sc Alloy Subjected to Peak-Aging Treatment

The total strain amplitude controlled low-cycle fatigue tests were performed at room temperature and 200 °C to clarify the influence of 0.5% Ag addition on the low-cycle fatigue behavior of an Al-5Cu-0.8Mg-0.15Zr-0.2Sc (in wt.%) alloy subjected to the peak-aging treatment after hot extrusion and solid solution treatment. The experimental results demonstrate that during low-cycle fatigue deformation, peak-aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag) alloys exhibit cyclic hardening, cyclic stability, or cyclic hardening followed by cyclic stability, depending on the Ag addition, imposed total strain amplitude, and testing temperature. The addition of 0.5% Ag greatly increases the low-cycle fatigue life of peak-aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc alloy, where the maximum rising amplitude is about 126.7% at ambient temperature and approximately 90.1% at 200 °C. Furthermore, it has been discovered that the addition of 0.5% Ag has no effect on the beginning and spreading modes of low-cycle fatigue fractures. For the peak-aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag) alloys subjected to low-cycle fatigue deformation at different total strain amplitudes and testing temperatures used in this investigation, fatigue cracks initiate trans granularly at the free surface of the fatigue specimen and propagate in a trans granular mode.

In-situ SEM-EBSD investigation of the low-cycle fatigue deformation behavior of Inconel 718 at grain-scale

The low-cycle fatigue (LCF) deformation behavior of the Inconel 718 (IN718) specimen with micro-notch was investigated by an in-situ fatigue test inside a scanning electron microscope (SEM) combined with electron backscattered diffraction (EBSD). The initiation condition and propagation path of cracks were observed by SEM. The essential reasons for crack initiation, propagation and fatigue fracture are elaborately characterized by crystallographic information obtained from EBSD. Meanwhile, the strain evolution during fatigue deformation simulated by the crystal plastic finite element (CPFE) method is consistent with the experimental content, which further verifies and supplements the experimental results. Two LCF deformation mechanisms (location-related) of the IN718 specimen are proposed at grain-scale based on the direct observation of crack characteristics and in-depth analysis. Near the edge micro-notch region, the deformation mechanism is mainly related to crack initiation and propagation: the plastic strain localization along the most easily activated slip band leads to the initiation of fatigue microcracks inside the grain. The crack propagation is carried out by microcrack coalescence in different grains. Away from the edge micro-notch region, the deformation mechanism is mainly related to the instantaneous fatigue fracture: fatigue fracture comes from the intergranular plastic strain accumulation and heterogenous deformation at grain-scale. The result also deeply explains the fatigue crack propagation in metals (two consecutive modes).

Low cycle fatigue lifetime and deformation behaviour prediction of nickel-based single crystal superalloy considering thickness debit effect

Through stress-controlled low cycle fatigue (LCF) experiments, LCF performances for thickness debit effect of nickel-based single crystal superalloy DD6 with [001] orientation are investigated. Based on scanning electron microscope (SEM) observation, the formation mechanism of thickness debit effect and the damage mechanism under LCF loads are revealed. Then, considering the thickness debit effect of nickel-based single crystal superalloy DD6, an improved slip-based damage model is developed. Predicted LCF lifetimes are mainly within a scatter band of factor 2, and the predicted laws between LCF lifetimes and wall-thicknesses are in good agreement with the experimental data, which verifies the rationality and accuracy of the thickness-sensitive LCF damage model established in this paper. Furthermore, a damage-coupled crystallographic constitutive model reflecting thickness debit effect is developed. For DD6 specimens with different wall-thicknesses, the predicted results of the stress-controlled LCF deformation behaviours including the whole-lifetime ratcheting behaviours show good agreement with the experimental data.

Low-cycle fatigue behavior and life prediction of TA15 titanium alloy by crystal plasticity-based modelling

It is of significance to reveal the low-cycle fatigue deformation mechanism of titanium alloy with tri-modal microstructure for improving its properties. This work developed a crystal plasticity finite element model based on realistic microstructure to predict the low-cycle fatigue behavior of TA15 titanium alloys with tri-modal microstructure containing primary α (αp), secondary lamellar α (αl) and transformed β (βt). During modelling, plastic strain accumulation (PSA) and energy dissipation were used as the fatigue indicator parameter (FIP) to predict fatigue damage and life. It is found that the fatigue deformation prefers to localize at the interfaces of αp/β, αl/β and at triple junctions, which leads to the initiation of fatigue crack. In addition, the soft geometry of αl will also contribute to the strain localization inside αl grains causing fatigue damage. This is because the local lattice rotation is more prone to occur at these locations due to the tension-compression asymmetry of lattice rotation, which induces the occurrence of the microtexture and aggravates the heterogeneity of the lattice rotation, leading to the localized PSA and the total energy dissipation. And the asymmetry of lattice rotation in each cycle also leads to the tension-compression asymmetry of flow behavior. Both of the two FIPs can effectively predict fatigue life. With the strain amplitude increasing, the PSA-based model shows a relatively higher predictive accuracy of fatigue life than that of the accumulated energy dissipation-based model. The predicted fatigue life is in accordance with the C-M model.

Insight into the low cycle fatigue deformation mechanisms of minor element doping single crystal superalloys at elevated temperature

The roles of minor elements, usually as grain-boundary strengtheners, in the fatigue properties of single crystal (SX) superalloys have not received adequate attention. Our study reveals that both C and B remarkably increased the stress amplitude, while reduced the cyclic hysteresis energy, under the condition of approximate low cycle fatigue (LCF) lifetime at 980 ºC. In terms of the enhanced fatigue stress, it is considered to be associated with the interstitial solid solution strengthening. Based on the analysis of deformation microstructure, the improved fatigue deformation resistance can be ascribed to the formation of slip bands and a<010> superdislocations in C-containing alloys, whereas, B additions triggered the denser dislocation-tangle at the γ/γ' interface and the homogenization of overall dislocation distribution.

Influence of Ti addition on MX precipitation and creep-fatigue properties of RAFM steel for nuclear fusion reactor

As part of the research framework to improve the mechanical properties of the reduced activation ferritic/martensitic (RAFM) steels suitable for higher temperatures, the effects of Ti addition on microstructures and low cycle fatigue (LCF) properties have been investigated. The addition of 0.015 wt% Ti to conventional base RAFM steel (9.3Cr‒0.93W‒0.22V‒0.094Ta‒0.10C‒Fe) increased the volume fraction of MX particles (2.1–4.6%). Atom probe tomography results showed that Ti atoms were rarely partitioned to (V,Ta)-rich MX precipitates with the size of 4‒13 nm, and instead most Ti atoms were involved in the formation of TiC precipitates with the size of 2‒8 nm. The nano-sized TiC particles were homogeneously distributed with the highest population. The LCF property of the Ta/Ti-added RAFM steel was inferior to the base RAFM steel. The cyclic stress reduction rate (softening rate) was higher in the Ta/Ti-added RAFM steel due to a higher density of dislocations at the initial state. The 10 min hold time at the peak tensile strain was imposed on the LCF testing to add creep damage to the cumulative fatigue damage. The creep-fatigue life of the Ta/Ti-added RAFM steel was about 2 times longer than the base RAFM steel. These results indicated that Ti addition-induced nano-sized MX particles are highly resistant to creep damage rather than fatigue damage. The higher fraction of nano-sized TiC and (V,Ta)-rich MX particles was responsible for the substantial retention of fine laths with a high density of dislocations during creep. The cyclic softening mechanism was discussed in terms of microstructure evolution during LCF and creep-fatigue deformation to explain the contradictory creep and fatigue properties.

Fatigue and fracture of additively manufactured polyethylene terephthalate glycol and acrylonitrile butadiene styrene polymers

This paper aims to study and compare the static tensile properties and Low Cycle Fatigue (LCF) deformation behavior of Additively Manufactured (AM) samples made of two commonly used polymers: Polyethylene Terephthalate Glycol (PET-G) and Acrylonitrile Butadiene Styrene (ABS). Such materials are commonly in use in the Material Extrusion (ME) AM technologies – especially in Fused Deposition Modelling (FDM) or Fused Filament Fabrication (FFF). That kind of approach would be useful to understand the fracture phenomena of AM parts obtained by means of FDM/FFF technologies which are entirely different than in the conventionally made material, because of the characteristic layered structure of AM parts. The study has been supplemented by additional fracture analysis of AM samples subjected to LCF tests. The conducted research exposed an increased LCF of PET-G samples (36050 cycles at constant strain equal to ε = 0.40%) in comparison to ABS (24586 cycles at the same strain level). At the same time, PET-G samples indicated a significant material softening during LCF testing. Such a phenomenon was better described by additional analysis with the use of the Morrow equation which indicated quasi-brittle cracking with visible brittle fracture phenomena in PET-G samples.

Low-cycle fatigue behavior and deformation mechanisms of a dual-phase Al0.5CoCrFeMnNi high-entropy alloy

We uncover the low-cycle fatigue response and deformation mechanisms of a dual-phase Al0.5CoCrFeMnNi high-entropy alloy (HEA) by comparing to CoCrFeMnNi HEA. Al0.5CoCrFeMnNi demonstrates higher cyclic stress resistance, meanwhile maintaining comparable cyclic strain resistance at low-to-medium strain amplitudes. Microstructural investigations revealed dislocation slip as the primary deformation mechanism, which changes from planar slip to wavy slip with increasing strain amplitude. The enhanced cyclic stress resistance is related to precipitation hardening and improved solid solution strengthening. Meanwhile, the comparable cyclic strain resistance is ascribed to their similar deformation mode. Lastly, comparisons with Al0.5CoCrFeNi and CoCrNi alloys provide strategies for tailoring HEAs with enhanced fatigue resistance.

Effect of rhenium on low cycle fatigue behaviors of Ni-based single crystal superalloys: a molecular dynamics simulation

Molecular dynamics simulations are carried out to investigate the effects of rhenium (Re) on low cycle fatigue behaviors and deformation mechanisms of Ni-based single crystal superalloys. The low cycle fatigue behaviors and deformation mechanisms at different temperatures are discussed. The results show that Re has a positive effect on the low cycle fatigue behaviors of Ni-based single crystal superalloys and the cyclic deformation mechanism is strongly dependent on temperature, which are in good agreement with the previous experimental results. The addition of Re can increase the saturation stress amplitude, reduce the cyclic hysteresis energy and dislocation density, and improve the plastic deformation resistance of the superalloys. During cyclic loading, Re atoms in γ matrix phase gradually move to the γ/γʹ interface, these enrichment of Re atoms at the γ/γʹ interface have pinning effect on dislocations, improving the microstructure stability. At the later of the saturation cycle, segregations of Re atoms prevent the dislocations from cutting into the γ′ phase by dragging dislocations, thus improving the fatigue resistance of superalloys. The simulation results also show that the deformation mechanism gradually changes from dislocations and stacking faults shearing γʹ precipitate phase to Orowan bypassing and climbing mechanism with temperature increases.

Correlation between microstructure evolution and mechanical response in a moderately low stacking-fault-energy austenitic Fe–Mn–Si–Al alloy during low-cycle fatigue deformation

Through in-depth microstructural and mechanical characterization, a mechanism for low-cycle fatigue (LCF) deformation of an Fe-30.7Mn-4.3Si-1.8Al (in wt%) austenitic transformation-induced-plasticity (TRIP) steel at a total strain range of 2% was investigated. The LCF deformation of the steel was performed through two main deformation modes which were planar slip of Shockley partials, and ε-martensitic transformation and its reverse transformation. The varying significance of each deformation mode as well as the relevance between the two modes determined the microstructures evolved during fatigue deformation and resulted in a four-stage cyclic hardening behavior. With increasing fatigue cycles, the LCF deformation was dominated sequentially by the multiplication of partial dislocations, development of slip bands, reversible ε-martensitic transformation, and formation of more thick and crossed ε-martensite along with enhancement of planar dislocation slip, thus generating the characteristic deformation stages of primary cyclic hardening, cyclic softening, cyclic saturation, and secondary cyclic hardening correspondingly. In general, the superior LCF resistance of the steel was attributed to appreciably high plasticity reversibility during the entire fatigue deformation which was caused by reversible planar dislocation slip and ε-martensitic transformation.

Experimental and analytical investigations on hysteretic behavior of assembled mild steel rod energy dissipaters

Metallic dampers have been widely used in engineering structures to absorb seismic energy and protect primary structure members, while conventional large-scale metallic dampers have the inherent disadvantages of space-occupying and poor post-earthquake replacement performance, resulting in expensive repairing cost and long downtime. The miniature energy dissipaters featuring relatively low bearing capacity and light-weight property seem to offer a feasible way to solve this dilemma. Existing dissipaters failed to form a reasonable anti-buckling mechanism for the inner yielding core, bringing problems as difficult grouting or large buckling deformation. Besides, low-cycle fatigue performance of such dissipaters was not quantitatively investigated in previous research. In this paper, a novel fuse-type miniature damper named assembled steel rod energy dissipater (ASRED) and free of welding and grouting was developed, which uses an equally divided metallic tube to fill the space between the core rod and the restraining tube. Experimental studies were conducted on two types of specimens with different lengths to investigate the low-cycle fatigue performance and deformation ability of the proposed ASRED. The test results showed all specimens exhibited satisfactory deformation performance, and stable hysteretic loops were obtained with no fluctuation in the compressive excursion. The lateral deformation of the inner rods was very small which guarantees an excellent low-cycle fatigue performance for the dissipaters. Besides, a hysteretic model was proposed to reproduce the cyclic behavior of the dissipaters. Finally, the commonly used Manson-Coffin equation was applied to predict the fracture cycle number of ASRED, and the proposed strain amplitude modifying method successfully eliminated the difference in low-cycle fatigue performance between the dissipaters and the mild steel making it.

Cyclic response and dislocation evolution of a nickel-based superalloy under low cycle fatigue deformation

The low cycle fatigue behavior of Inconel 750H superalloy, a newly designed Nickel-based superalloy, was studied in this work. The superalloy exhibited an initial cyclic hardening and a following continuous cyclic softening behavior at the strain amplitudes from 0.35% to 0.6%, while a stress saturation at the strain amplitude of 0.3%. Cyclic response considering cyclic stress, plastic strain, and hysteresis loop area was studied. Four interrupted cycle tests were conducted at the strain amplitude of 0.35% to investigate the microstructure evolution during fatigue deformation. As fatigue cycles increased, the fraction of γ′ phase decreased while the diameter of γ’ phase increased. The planar slip deformation with dislocation pile-ups at half cycles was observed, and finally evolved into simple dislocation lines when the specimen ruptured. The coarsening γʹ precipitation degraded the strengthening effect, resulting in more uneven slip deformation. Compared with chain-shaped M23C6 carbides, a larger dimension of the block like MC carbides (i.e. TiC and NbC) led to higher stress at the interface between MC and matrix, where cavities preferred to nucleate and propagate to a micro-crack.

Low cycle fatigue behavior and deformation mechanism of different microstructures in Ti-5Al-5Mo-5V-3Cr alloy

Two fundamental questions have been addressed on a recently developed high strength β Ti alloy, Ti-5Al-5V-5Mo-3Cr, with respect to two very different types of microstructures, one bimodal and the other fully lamellar, both having similar yield strength. Those are: 1) If and what would be the difference in their LCF response? 2) What would be the sequence of plastic deformation among the phases of different morphology, β, globular α and lath α? TEM based deformation micro mechanism explained the LCF response at various strain amplitudes. Crack path-microstructure interaction explained the difference in life, specifically observed at low strain amplitude.

On the microstructure evolution during low cycle fatigue deformation of wrought ATI 718Plus alloy

In the present work, strain-controlled low cycle fatigue (LCF) tests were performed at 704 °C using 0.25 Hz in cyclic frequency with tension-compression loading on the forged alloy 718Plus to investigate the fatigue performances determined by structures, and the corresponding microstructure evolutions and deformation mechanisms were analyzed through techniques of scanning electron microscope (SEM) and transmission electron microscope (TEM). The fatigue specimens from different locations (core and rim) showed similar response on cycle stress, but smaller grain size contributed to superior fatigue performance. Fatigue deformation induced the generation of η precipitates in grain interior and promoted the growth of grains and η precipitates, but brought insignificant change on the morphology of spherical γ′ phase and slightly increase in particle size and γ′/γ lattice misfit, indicating its excellent mechanical stability at 704 °C. A transition area was observed between the η precipitates and γ matrix, which was determined to be γ phase via high resolution transmission electron microscopy (HRTEM) and elements mapping techniques. Twinning and dislocation planar slip bands were confirmed to be the primary fatigue deformation modes of alloy 718Plus. Dislocations piled up and formed networks at the edges of η precipitates during the fatigue process, inducing the initiation and propagation of micro-cracks. As the cycle accumulated, the η and γ′ precipitates were sheared by gliding dislocations, resulting in weakened resistance to deformation and fatigue softening.

Effect of microstructure and low cycle fatigue deformation on tensile properties of P91 steel

AbstractIsothermal furnace heat treatments were carried out to simulate the microstructures of inter‐critical, fine grain and coarse grain heat‐affected zones of P91 steel weld joint at different soaking temperatures ranging from just above AC1 (837 °C) to well above AC3 (903 °C). Interrupted low cycle fatigue tests were performed on the specimens of P91 steel up to 5 %, 10 %, 30 %, and 50 % of the total fatigue life at the strain amplitude of ±0.6 %, strain rate of 0.003 s−1 and temperatures of 550 °C and 600 °C. Subsequently, tensile tests were conducted on the interrupt tested specimens at the same strain rate and temperatures. Soaking at the inter‐critical temperature region reduces / deteriorates the tensile and yield strengths of base metal compared to fine grain and coarse grain regions. The inter‐critical heat‐affected zone accounted higher damage contribution towards the overall tensile behavior of the actual P91 steel weld joint. Substructural coarsening during strain cycling at elevated temperatures attributes to the rapid reduction in the initial yield strength up to 10 % of fatigue life of P91 steel. A higher amount of plastic strain accumulation during low cycle fatigue deformation resulted in a decrease in fatigue life of the inter‐critical heat‐affected zone of P91 steel.