Higher Total Strain Amplitudes Research Articles

The fatigue properties, microstructural evolution and crack behaviors of low-carbon carbide-free bainitic steel during low-cycle fatigue

The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.

Low-cycle fatigue properties of CoCrFeMnNi high-entropy alloy compared with its conventional counterparts

The behavior of CrCoFeMnNi high-entropy alloy (Cantor) during cyclic deformation at room temperature was compared with those of a twinning-induced plasticity (TWIP) steel and 304 stainless steel (SS304). The three materials with similar grain sizes (~65 μm) were made by controlled heat treatment and tested to evaluate tensile and low-cycle fatigue (LCF) properties. The tensile strength and ductility of Cantor were much lower than those of TWIP steel and SS304. Cantor and TWIP steel revealed cyclic hardening and then softening; SS304 showed secondary hardening at the latest stage of cyclic deformation due to formation of α′-martensite, which led to a significant decrease in its LCF life. At high total strain amplitudes >0.4% LCF life was longer in Cantor than in SS304, but shorter than in TWIP steel. However, at total strain amplitudes ≤0.4%, Cantor showed the shortest LCF life. It was found that the Coffin-Manson plot of Cantor followed the trend of TWIP steel rather than that of SS304. Microstructural analysis of the Cantor with increasing cycles revealed the formation of twins and well-defined cell structures, the latter of which is the evidence of wavy slip.

Cyclic deformation behavior of friction-stir-welded dissimilar AA5083-to-AA2024 joints: Effect of microstructure and loading history

The microstructural evolution across the friction-stir-welded dissimilar AA5083-H112 to AA2024-T351 aluminum alloy joints was characterized via electron backscatter diffraction (EBSD), aiming to identify the effect of inhomogeneous microstructures on the tensile properties and cyclic deformation behavior along with the influence of loading history. Results show that the top region of the stir zone (SZ) mainly consisted of AA2024 which was initially positioned on the retreating side during welding. On the AA5083 side, the lowest density of the geometrically-necessary dislocations (GNDs) assessed from the local misorientations of 0°–2° appeared in the SZ, while it occurred in the heat-affected zone (HAZ) of the AA2024 side. The fractions of recrystallized grains in the AA5083 SZ and AA2024 SZ were ~73% and ~34%, respectively. Strain localization and the resultant failure occurred in the lowest hardness zone of the AA5083 side. The extent of cyclic hardening increased as the stress amplitude increased. Both plastic strain amplitude and plastic strain energy density decreased with increasing number of cycles at higher total strain amplitudes. In the stepwise cyclic deformation tests in the form of ascending-descending loading, the prior cyclic deformation process tended to generate more stabilized structure and performance.

Influence of the surface morphology on the cyclic deformation behavior of HSD® 600 steel

The presented research work investigates the fatigue properties in the low cycle fatigue (LCF) regime of the high manganese metastable austenitic High Strength and Ductility (HSD®) 600 TWIP steel dependent on its surface morphology. The steel features, according to its chemical composition following the alloying concept Mn-Al-Si-C and heat treatment, a fully austenitic microstructure that shows deformation induced twinning at ambient temperature. Due to this microstructural deformation mechanism, HSD® 600 steel has an outstanding combination of strength and formability. Besides monotonic deformation behavior, characterized by tensile tests, cyclic deformation behavior was investigated with varying the surface morphology of fatigue specimens. In order to create different surface morphologies, flat fatigue specimens were excised from larger sheets by waterjet-cutting. Depending on the surface morphology, further climb milling or up-climb milling in the gauge length was performed. The three investigated morphologies (asreceived with rolling skin, climb milled and up-climb milled) differed in roughness, initial residual stresses and initial phase compositions. For all variants, total strain controlled fatigue tests with stepwise increasing load amplitudes as well as total strain controlled single step tests were performed in the low cycle fatigue regime with a load ratio of Rε = -1 and a frequency of f = 0.2 Hz. Beside stress-strain hystereses, the changes in temperature ΔT and the magnetic properties ξ were measured. The magnetic properties directly correlate with the transformation from paramagnetic γ-austenite to ferromagnetic α’-martensite. The cyclic deformation behavior of the HSD® 600 steel in the LCF regime was characterized by cyclic softening until fracture at low total strain amplitudes but changed with increasing total strain amplitudes into initial cyclic hardening followed by cyclic softening. This initial cyclic hardening became more pronounced when the total strain amplitude increased. Furthermore, single step tests at lower total strain amplitudes showed a saturation state before fracture. A comparison between the monotonic and cyclic deformation behavior showed a significant difference of the stress levels at the same amounts of plastic deformation respectively. Nevertheless, the different surface morphologies led to different lifetimes at high total strain amplitudes but to similar lifetimes at lower total strain amplitudes.

Low-cycle fatigue behavior of a cast Mg–Y–Nd–Zr alloy by T6 heat treatment

Strain controlled low-cycle fatigue (LCF) tests of a cast Mg-3.5Y-2.4Nd-0.5Zr alloy by two different T6 heat treatment (T60: solution at 525°C for 4h and then aging at 225°C for 34h; T61: conditional industrial T6 heat treatment - solution at 520°C for 8h and aging at 225°C for 16h) were studied at room temperature under different total strain amplitudes. And theirs fatigue parameters, fatigue lives and failure behaviors after cyclic loading were evaluated. At relatively low total strain amplitudes (0.3–0.45%), the T60 alloy presented basically cyclic stabilization; and the T61 alloy was increasing hardening until failure. While at higher total strain amplitudes (0.45–0.9%), both them showed hardening through the entire LCF testing. Fatigue cracks of the alloy initiated from the specimen surface, the fracture surfaces became more rough and concavo-convex as the strain amplitude increased. Due to the fine and dense precipitates, the T60 alloy showed the super fatigue properties. And the alloy after T60 heat treatment obtained a distinctly longer fatigue life than that of the alloy treated under T61 condition at any given total strain amplitude.

Low cycle fatigue properties of linear friction welded joint of TC11 and TC17 titanium alloys

In this works, we investigated the microstructures and low cycle fatigue (LCF) performance of linear friction welded (LFWed) joint of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) and Ti-4Mo-4Cr-5Al-2Sn-2Zr (TC17) titanium alloys. The results show a large number of fine lath martensite precipitations α′ with three variants occur on the welded zone (WZ) of TC11 side, and coarsened β phases with a small amount of precipitations appear on the WZ of TC17 side. The orientation relationship of matrix and precipitation is 〈112¯0〉p//〈111〉m and {0001}p//{110}m. The stress amplitudes and plastic strain amplitudes exhibit cyclic softening characteristics at higher total strain amplitudes (0.8–1.2%) for the damaging dislocation tangles and obstacles with cyclic deformation process. The increasing rate of softening with the increasing strain amplitudes is due to more slip systems starting and complete shearing of martensites. Elastic moduli of the joint during the loading and unloading phases are constant at lower total strain amplitudes, which decrease with increasing strain amplitudes and increasing number of cycles at the higher total strain amplitudes. Meanwhile, as the applied total strain amplitude decreases, the number of cycles to failure increases, and the fatigue lifetime follows the Basquin's equation and Coffin–Manson relationships. Finally, the fragment of the joint locates at the WZ of TC17 side and propagates along the flow direction of plasticized materials.

Low cycle fatigue behavior of casting A319 alloy under two different aging conditions

To determine the influence of peak- and over-aging on low-cycle fatigue (LCF) behavior of casting A319 alloy, the strain controlled low-cycle fatigue tests was studied at room temperature under different total strain amplitudes. At lower total strain amplitudes (0.2–0.3%), the A319 alloy presented initial cyclic hardening and then, steady stage and cyclic softening; while at higher total strain amplitudes (0.35–0.4%), it presented continuous hardening in the peak-aged specimens and constant softening in the over-aged specimens. At a given total strain amplitude, the hysteresis loops in the peak-aged specimens were relatively narrower than those in the over-aged specimens. The fatigue life of the peak-aged samples was higher than that of the over-aged samples at any given total strain amplitude. Fractographic morphology indicated that the size of the fatigue crack propagation zone at peak-aged condition was larger than that at over-aged treatment. The crack characteristic in the propagation region and the fast fracture zone also showed some differences between two aging conditions.

Low cycle fatigue behavior of T4-treated Al–Zn–Mg–Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting (GC) and squeeze casting (SC) T4-treated Al–7.0Zn–2.5Mg–2.1Cu alloys were employed to investigate the microstructures, mechanical properties and low cycle fatigue (LCF) behavior. The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing (SDAS). Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy. GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes (less than 0.4%) and cyclic hardening at higher total strain amplitudes. The degree of cyclic hardening of SC samples is greater than that of GC samples. Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them, while the crack initiation sites for SC samples are slip bands, eutectic phases and inclusions at or near the free surface.

热处理对挤压变形Mg-7%Zn-0.6%Zr-0.5%Y合金低周疲劳行为的影响 *

对热挤压、时效处理（T5态）及固溶＋时效处理（T6态）Mg-7%Zn-0.6%Zr-0.5%Y合金（质量分数）分别进行了低周疲劳实验,探讨了热处理对合金低周疲劳变形行为的影响.结果表明：时效处理和固溶＋时效处理可提高热挤压Mg-7%Zn-0.6%Zr-0.5%Y合金的循环变形抗力;时效处理降低了合金的疲劳寿命,固溶＋时效处理可以提高合金在较高外加总应变幅下的疲劳寿命,但降低合金在较低外加总应变幅下的低周疲劳寿命;不同状态合金的弹性应变幅和塑性应变幅与载荷反向周次的关系可分别用Basquin和Coffin-Manson公式来描述;时效及固溶＋时效处理过程中形成的长周期堆垛有序结构（LPSO相）是合金的循环变形抗力大幅提高的主要原因,而疲劳变形过程中形成的孪晶可能是时效态合金疲劳寿命降低的原因.

Effect of cooling rate during hot stamping on low cyclic fatigue of boron steel sheet

Boron steel is widely used throughout the automobile industry due to its high tensile strength and hardenability. When boron steel is used for body parts, only high strength is required for crashworthiness. However, when boron steel is used for chassis parts, a high fatigue life is needed. The microstructure of boron steel is mainly affected by the cooling rate during hot stamping. Therefore, this study investigated the low cyclic fatigue life according to the cooling rate. The fatigue life increased at a low strain amplitude when the cooling rate was fast. However, at a high strain amplitude, the fatigue life decreased, due to the low ductility and fracture toughness of the martensite formed by rapid cooling. Martensite formed by a fast cooling rate shows excellent fatigue life at a low total strain amplitude; however, a multiphase microstructure formed by a slow cooling rate is recommended if the parts experience high and low total strain amplitudes alternately. In addition, the cooling rate has little effect on the distribution of solute boron and boron precipitations, so it is expected that boron rarely affects low cyclic fatigue.

Low-Cycle Fatigue Behavior of Permanent-Mold Cast and Die-Cast Al-Si-Cu-Mg-Sc Aluminum Alloys

The low-cycle fatigue behaviors of permanent-mold cast and die-cast Al-Si-Cu-Mg-Sc alloys at room temperature were investigated. The results show that at the higher total strain amplitudes, both permanent-mould cast and die-cast Al-Si-Cu-Mg-Sc alloys exhibit the cyclic strain hardening. However, at the lowest total strain amplitude, the cyclic strain hardening occurs in the initial and middle stages of fatigue deformation and the stable cyclic stress response is noted in the later stage of fatigue deformation for the permanent-mould cast alloy, while the cyclic strain hardening phenomenon is observed during whole fatigue deformation for the die-cast alloy. At the same total strain amplitude, the die-cast alloy shows the higher cyclic deformation resistance than the permanent-mold cast alloy. Compared with the permanent-mold cast alloy, the die-cast alloy possesses the longer fatigue life at the lower total strain amplitudes. The relationship between both elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior for both permanent-mold cast and die-cast Al-Si-Cu-Mg-Sc alloys

Low Cycle Fatigue Behavior of AZ91D Magnesium Alloy Containing Rare-Earth Ce Element

Low cycle fatigue test of AZ91D+0.3%Ce alloy was carried out in tension-compression symmetric cycle at room temperature, with triangular waveform-loading frequence in 1Hz and equivalent strain amplitude varied from 0.2% to 1.2% .The fatigue fracture morphology of the alloy was observed by SEM. The results indicate that the σ-ɛ hysteresis loops of AZ91D+0.3%Ce magnesium alloy exhibit some behaviors such as inflexion, asymmetry of tension and compression and serrated flow deformation at the higher total strain amplitudes; Fatigue life of AZ91D+Ce alloy is decreased with the increase of the equivalent strain amplitude, confirming to the equation of the Manson-Coffin;The behaviors of cyclic stress response and stress-strain of magnesium alloy mainly show cyclic strain hardening; Fatigue fracture of all the magnesium alloys exists several fatigue source zones, as the strain amplitude increases, the fracture surface becomes more rough and concavo-convex.

Texture Evolution in Thermomechanically Control Processed Multiphase Medium Carbon Microalloyed Steel: Forged, Rolled and Low Cycle Fatigued Microstructures

A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium carbon microalloyed steel through two routes, namely, low temperature finish forging or rolling, followed by a two step cooling and annealing. The texture formed in control forged and rolled material after two step cooling followed by annealing (TSCA) was examined. Texture investigation was also carried out after low cycle fatigue testing at low and high total strain amplitudes. Transmission electron microscopy was employed to study the microstructural evolution. Fatigue tested F-B-M microstructure obtained through the rolling route was stable up to a total strain amplitude of 0.6%. This paper reports the evolution of texture and microstructure in two-step cooled F-B-M microstructure and their stability during fatigue loading.

Application of Flat-Clad Optical Fiber Bragg Grating Sensor in Characterization of Asymmetric Fatigue Deformation of Extruded Magnesium Alloy

We report the application of a novel flat-cladding optical fiber Bragg grating sensor array in the characterization of low cycle fatigue deformation behavior of an extruded AZ31 magnesium alloy. The results obtained from optical fiber sensors showed strong asymmetry of stress-strain hysteresis loops, especially at high strain amplitudes, due to the occurrence of twinning in compression and detwinning in tension. At a given total strain amplitude, the degree of asymmetry of the hysteresis loops decreased with increasing number of cycles. The alloy also exhibited strong cyclic hardening at higher total strain amplitudes. All of these observations were in good agreement with those measured from an extensometer, and with those reported in the literature. In particular, the plastic strain amplitudes evaluated from optical fiber sensors demonstrated a nearly perfect agreement with those obtained from the extensometer, further corroborating the suitable use of the optical fiber sensors as an important tool in the evaluation of cyclic defromation behavior of materials.

Low cycle fatigue properties and cyclic deformation behavior of as-extruded AZ31 magnesium alloy

The low cycle fatigue(LCF) properties of as-extruded AZ31 Mg alloy were investigated under total strain amplitudes in the range of 0.4%–1.2% with strain rate of 1×10 −2 s −1. Due to the twinning effect in compression during loading and the detwinning effect during unloading, the alloy showed an asymmetric hysteresis loop. The cyclic stress response exhibited cyclic hardening at high total strain amplitudes. The cyclic deformation behaviors were discussed using the Coffin-Manson plot, which divided the plastic strain amplitudes into the tension side and the compression side. Through the LCF tests that were started from either tension or compression under a total strain amplitude of 1.0%, the interaction between the twinning effect and dislocation was analyzed. The twinning effect during the LCF test and the variation of the dislocation density were investigated using optical microscopy and transmission electron microscopy, respectively.

Low cycle fatigue behaviour of hot extruded Al–4Mg–1Nd alloy

The low cycle fatigue behaviour of the hot extruded Al–4Mg–1Nd alloy has been investigated under those total strain amplitudes ranged from 0·3 to 1·2% at room temperature. The results of the low cycle fatigue experiments show that during the fatigue deformation, the significant cyclic strain hardening and stable cyclic stress response can be noted for the hot extruded Al–4Mg–1Nd alloy. The relationship between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behaviour, and can be described by the Basquin and Coffin–Manson equations respectively. It is also observed that at the higher total strain amplitudes of 0·8 to 1·2%, the so called dynamic strain aging takes place during the fatigue deformation of hot extruded Al–4Mg–1Nd alloy. In addition, the observations on the fracture surfaces reveal that under the low cycle fatigue loading condition, the fatigue cracks initiate transgranularly at the free surface of fatigue specimens and propagate in a transgranular mode.

Low-Cycle Fatigue Behavior of an As-Extruded AM50 Magnesium Alloy

The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-e hysteresis loop in the compressive direction is greater than that in the tensile direction. At the total strain amplitude of 1.5 pct, a serrated flow can be observed in both tensile and compressive directions of the σ-e hysteresis loop. This means that dynamic strain aging takes place during fatigue deformation. The relation between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior and can be well described by the Basquin and Coffin–Manson equations, respectively. In addition, crack initiation and propagation modes are suggested, based on scanning electron microscopy observations on the fracture surfaces of fatigued specimens.

Failure behaviour of EUROFER 97 in the low-cycle fatigue region under multi-step loading

For verification of high temperature creep–fatigue interaction rules for EUROFER 97, different LCF multi-step experiments (2- and 3-step tests) were performed at 450 and 550 °C. The load conditions were varied by changing the total strain amplitude during an experiment. In 2-step tests the load was changed after a given number of cycle from a high level to a low level or vice versa and afterwards the experiments were continued until fracture (high–low, or low–high loading). In 3-step tests, the load was changed in the first and second steps after given numbers of cycles. In the third step, it was switched back to the load applied in the first step and the experiments were continued until fracture (high–low–high, or low–high–low loading). As expected, the results show a non-linear damage accumulation. In 2-step experiments, it was found that beginning the tests with a low total strain amplitude resulted in longer normalised total lifetimes than in the case of high total strain amplitudes at the beginning. In 3-step tests, one can detect a similar behaviour. But here, additionally it is also observed that the number of cycles in the first step influences the lifetime. A comparison of the results with the standard design curve shows that there is more than enough space available to be on the safe side for calculations of constructions.

Transmission electron microscopy studies of thermomechanically control processed multiphase medium-carbon microalloyed steel: Forged, rolled, and low-cycle fatigued microstructures

A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium-carbon microalloyed (MA) steel through two routes, namely, low-temperature finish forging and rolling, followed by a two-step cooling (TSC) and annealing. Transmission electron microscopy (TEM) was employed to study the microstructural evolution in control forged and rolled material after TSC followed by annealing (TSCA). A TEM investigation was also carried out on samples low-cycle fatigue (LCF) tested at low and high total strain amplitudes of 0.4 and 0.7 pct in case of the forged steel (F-B-M(F)TSCA) and 0.55 and 0.8 pct for the rolled steel (F-B-M(R)TSCA), respectively. Microstructural changes accompanying the LCF testing were identified. The two-step cooled microstructure processed through forging (F-B-M(F)TSC) as well as rolling (F-B-M(R)TSC) revealed a complex multiphase microstructure, along with films and blocks of retained austenite. In both microstructural conditions, vanadium carbide precipitates were too fine to be identified after the TSC treatment. Annealing after TSC produced a stress-free microstructure. The F-B-M(F)TSCA microstructure predominantly consisted of granular/lower bainite, lath martensite, and polygonal ferrite with interlath films as well as blocks of retained austenite, while the F-B-M(R)TSCA microstructure predominantly consisted of lath martensite, granular/lower bainite, and polygonal ferrite with interlath strips/films of retained austenite. Lath martensite content was higher in the F-B-M(R)TSCA condition than in the F-B-M(R)TSCA condition. In both conditions, vanadium carbide precipitates could be seen after annealing. Fatigue-tested F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.4 pct and F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.55 pct were stable. Lath martensite did not undergo deformation and in both microstructural conditions dislocation cell structures were not observed in the ferrite or bainite regions. The interlath retained austenite strips/films played a significant role in preventing the softening during fatigue loading. First, it was stable up to a total strain amplitude of 0.4 and 0.55 pct in the respective microstructures. Second, it underwent heavy deformation during fatigue loading at high total strain amplitudes, thereby accommodating the strain. Fatigue-tested F-B-M(F)TSCA microstructure at a total strain amplitude of 0.7 pct and F-B-M(R)TSCA microstructure at a total strain amplitude of 0.8 pct revealed deformed bainite/martensite laths, dislocation cells, and slip bands in the ferrite regions, which are characteristic features of cyclic softening. The retained austenite transformed to martensite through a strain-induced transformation mechanism and, at that stage, the microstructure contained in addition dislocation-rich bainite and ferrite.

Low-cycle fatigue behavior of Al-based composites containing in situ TiB 2, Al 2O 3 and Al 3Ti reinforcements

Aluminum-based composites reinforced with in situ TiB 2 and Al 2O 3 particles were prepared through reactive hot pressing of TiO 2, Al and B powders. Brittle Al 3Ti blocks were also formed in situ when the B/TiO 2 molecular ratio is smaller than 2. The low-cycle fatigue behavior of in situ composites under total strain-controlled conditions at room temperature was investigated. The results showed that the composite reinforced with in situ TiB 2 and Al 2O 3 particles exhibits a relatively stable cyclic response at low total strain amplitudes. At higher total strain amplitudes, cyclic softening from the onset of deformation was observed. However, the presence of Al 3Ti blocks led to a very slight cyclic hardening followed by softening at total strain amplitude of 0.4%. Moreover, the intermetallic Al 3Ti blocks reduced the fatigue life of in situ composites as they promoted microscopic cracking during cyclic deformation. Finally, the fatigue life data of in situ composites can be described by the Coffin–Manson relationship.