Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.     

Fatigue behavior and dislocation substructures for 6063 aluminum alloy under nonproportional loadings

In this paper, the fatigue behavior and dislocation substructures of 6063 aluminum alloy were studied under several nonproportional path loadings, which were circle, ellipse, rectangle and square paths. After fatigue test the micro-structure especially the dislocation substructures of the failure materials was carefully observed with the transmission electron microscope (TEM) method. Under the same 93 MPa equivalent stress amplitude loading, the alloy has the shortest life and the most severe cyclic additional hardening with circle path loading among all the loading paths. This attributes to the complicated dislocation substructures and severe stress concentration of the alloy during the cycling process. While under the ellipse path loading, the alloy has a comparably long life and light cyclic additional hardening. The deformation of the alloy and the morphology of the dislocation substructures determine the fatigue behavior of 6063 alloy under the same equivalent stress amplitude loading. Under the circle path loading, the fatigue life decreases while the cyclic strain increases as the loading stress amplitude increases from 47 MPa to 163 MPa. The dislocation evolution of 6063 alloy during the cycling process under circle path loading was examined with TEM. It was found that the dislocation merges with each other and changes from single lines to crossed bands. The movability of dislocation reduces and the stress concentration degree rises during the cycling process.     

Cyclic deformation behavior and fatigue lives of ultrafine-grained Ti-6AL-4V ELI alloy for medical use

It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3 × 10^?4 and 5 × 10^?3 and stress amplitudes ranging from 550 to 670 MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure.     

Effect of strain amplitude on the low-cycle fatigue behavior of a new Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy

The low-cycle fatigue (LCF) properties and post-fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy were investigated under an axial strain control mode with total strain amplitudes, Δε_t/2, ranging from 2.5 × 10⁻³ to 2 × 10⁻². The fatigue resistance of the alloy was described by Coffin–Manson’s and Basquin’s relationships, and the corresponding fatigue parameters were evaluated. In addition, the Masing behavior, which is associated with a constant deformation mode during fatigue, was revealed at the examined strain amplitudes. Microstructural observations of the fatigue fractured samples showed that the strain induced ε-martensitic transformation accompanied by a planar slip of the Shockley partial dislocations in the austenite is the main deformation mode controlling the fatigue behavior of the studied alloy at Δε_t/2 < 2 × 10⁻². However, at Δε_t/2 = 2 × 10⁻², the formation of a cell structure was found in the austenite in addition to ε-martensitic transformation. The LCF resistance of the alloy was compared with conventional Cr–Ni austenitic stainless steels, ferrous base TRIP and TWIP steels and low yield point damping steels. It was found that at the studied strain amplitudes the alloy possessed a higher LCF resistance compared to conventional Fe-base alloys and steels. Remarkably, the fatigue ductility coefficient, ε_f′, of the studied alloy is 1.3–6 times higher than that of the stainless steels because of a cyclic deformation-induced ε-martensitic transformation. The results showed that the ε-martensitic transformation that occurred in the studied alloy during LCF is the main reason for the improved LCF resistance.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes

In order to investigate the effect of twinning–detwinning on the mechanical properties of AZ31 extruded magnesium alloy pre-compression and pre-stretch deformation were conducted along extrusion direction (ED) at 1%, 3%, 5% strain levels. After pre-strain, the strain-path was inverted by performing tensile or compressive tests at room temperature. Results showed that the detwinning behavior occurred during the inverse tension after the pre-compression. Although due to the aforementioned effect the tensile yield strength decreased, by increasing the pre-compressive levels both fracture elongation and peak strength improved. In the inverse compressive tests after pre-stretch the {1 0 −1 2} twinning was restrained and the volume fraction of twins decreased, leading to the improvement of yield strength by increasing in pre-stretching levels.     

Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300 °C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20 °C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20 °C are faster than for 300 °C. Crack initiation however occurs earlier at 300 °C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300 °C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300 °C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

Crystal plasticity modeling of damage accumulation in dissimilar Mg alloy bi-crystals under high-cycle fatigue

Damage accumulation in Mg AZ31–AZ80 alloy bi-crystals under fatigue loading at room temperature is studied using a modified version of the crystal plasticity finite element model of Abdolvand and Daymond. The model accounts for strain accommodation by both slip and tensile twinning, and is first shown to reasonably describe monotonic single crystal Mg experimental data from the literature. The high cycle fatigue behavior was then investigated in misoriented dissimilar alloy bi-crystals through stress-controlled simulations up to 1000 cycles. Nine different orientation combinations were simulated and the fatigue damage evolution, defined as the cumulative shear strain amplitude, were compared and analyzed. The bi-crystal geometry was used to simulate possible microstructure combinations occurring, for instance within an idealized friction stir weld. Findings suggest that when either of the alloy bi-crystal grains is oriented for basal slip, poor fatigue performance can occur by twinning or slip localization depending upon the neighboring orientation.     

Cyclic behavior of 9–12% Cr steel under different control modes in low cycle regime: A comparative study

Cyclic behavior of 9–12% Cr steel under both stress and strain control modes was investigated at 873 K. Significant asymmetric deformation and cyclic softening were observed for both modes. Under the strain-controlled fatigue, a strain level-independent softening factor (SF) was observed. The SF was dependent on applied stress under the stress-controlled fatigue. For the strain-controlled fatigue, the magnitude of cyclic asymmetry decreases with the increase of strain amplitude, while under the stress-controlled fatigue, the asymmetry increases with the increase of stress amplitude. The anomalous ratcheting strain resulted from the asymmetry under the stress-controlled mode has a detrimental effect on fatigue life.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

Torsional fatigue with axial constant stress for Sn–3Ag–0.5Cu lead-free solder

A series of multiaxial ratcheting–fatigue interaction tests have been carried out on Sn–3Ag–0.5Cu lead-free solder specimens. All tests were conducted under cyclic shear strain with the constant axial stress at the room temperature with the shear strain rate of 5 × 10⁻³ s⁻¹. It was found that the ratcheting strain increased with increasing axial stress and shear strain amplitude while the fatigue life decreased at the same time. The ratcheting strain rate was linear with axial stress in double logarithmic coordinate. The Ohno–Wang II constitutive model was employed to simulate the stress–strain responses. Several fatigue life prediction models were applied to predict the multiaxial ratcheting–fatigue life of the Sn–3Ag–0.5Cu lead-free solder. The Gao–Chen model which adopted the maximum shear strain and the ratcheting strain rate as the damage parameter predicted the multiaxial ratcheting fatigue life well.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

Ultrafine grain structure features in spray-formed AZ31 magnesium alloy

The ultrafine grain structure was developed in spray-formed AZ31 magnesium alloy by optimizing delivery tube orifice diameter. A significant refinement of grain size ∼1 μm in ultrafine level was achieved by using 2.25 mm delivery tube orifice diameter. The tensile strength value was increased from 145 MPa of as-cast alloy to 250 MPa of spray-formed alloy, registering an enhancement of ∼72%. On the other hand, elongation was increased from 6% to 13% using 2.25 mm orifice, registering more than onefold increase in elongation. Also, hardness enhancement of ∼49% was observed in spray-formed AZ31 alloy compared to as-cast alloy. The fracture surface of spray-formed AZ31 Mg alloy evidences the mixed type of ductile and brittle fracture.     

The fatigue behavior of irradiated Reactor Pressure Vessel steel

Fatigue specimens of A508-3 steel were irradiated in the swimming-pool test reactor in China Institute of Atomic Energy, the fluence was 3 × 10¹⁹ n/cm² at 300 °C, then low-cycle fatigue tests were carried out at ambient temperature, with the fatigue strain range is 0.32–1.8%. The results indicate that, irradiated A508-3 specimens exhibit cyclic softening and instability behavior during the test, and the cyclic softening rate increased with strain range increased; fatigue life decreased from 1.7 × 10⁵ to about 5 × 10², as the strain range increased from 0.32% to 1.8%, the fatigue life of A508-3 steel increased after the neutron irradiation; fatigue fracture initiated at the surface of specimen, and more individual cracks formed on the specimens of higher strain range compared with the specimens of lower strain range.     

Lifetime behaviour of unidirectionally wire reinforced lightweight metal matrix composites

In the field of lightweight construction for transportation means, hybrid structures composed of high-strength and low-density materials exhibit a high application potential. The current work shows that a spring steel wire reinforcement content of 11.1 vol.% leads to a significant increase in lifetime of the magnesium alloy AZ31 under stress controlled fatigue loading. Based on the knowledge of the quasi-static behaviour of the single components and the fatigue behaviour of the single matrix material and on the understanding of mechanisms and mechanics of constituent deformation and damage evolution a new model is deduced from the quantitative strain response in order to predict the lifetime for different unidirectionally reinforced material systems.     

Effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE)

Three different mold structures were designed by changing the parameters of mold cavity to study the effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE) with 2 interim dies. Microstructure and texture evolution of AZ31 magnesium alloy after CVCDE were studied by electron backscatter diffraction (EBSD). Mechanical properties were determined by uniaxial tensile tests along extrusion direction (ED) at room temperature. Due to the differences of local strains among the three schemes, the microstructure of Scheme 1 was the most uniform and the average grain size of Scheme 1 was the smallest. Meanwhile, tensile strength and elongation of Scheme 1 were the highest. Different textures had been formed in the three schemes. Lots of extension twins {10–12} (86°< 1–210 >) occurred in the products of the three schemes. The main deformation modes of Scheme 1 and Scheme 2 were slip and twinning. However, slip was dominant in Scheme 3. The deformation modes provided an essential basic for the design of CVCDE mold structure with more interim dies.     

Localized microtwin formation and failure during out-of-phase thermomechanical fatigue of a single crystal nickel-based superalloy

The deformation and damage mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated under out-of-phase thermomechanical fatigue (OP TMF) condition. The deformation was highly localized to the area near the crack tip, where multiple groups of parallel twin plates on {1 1 1} planes formed during the high temperature-compressive half cycle. The atomistic a/6 〈1 1 2〉 twinning shear-based approach is presented which explains the origin of twinning. The localized twins provided a preferential path for crack propagation. OP TMF deformation was dominated by partial dislocation movement with {1 1 1}〈1 1 2〉 slip system, resulting in the formation and propagation of deformation twins.     

Effects of specimen aspect ratio on the compressive properties of Mg alloy foam

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. The effects of specimen aspect ratio (the thickness/width ratio, AR) on the compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that the length of stress–strain plateau stage extended and ideality energy absorption efficiency improved with the specimen AR increasing and the yield strength decreased. Specimens with the AR = 1.00 possess good combination of yield strength, plateau stage length and compressive stability when compressed under the experiment conditions.