Increasing Stress Amplitude Research Articles

Effect of surface mechanical attrition treatment on corrosion fatigue behavior of AZ31B magnesium alloy

Surface mechanical attrition treatment (SMAT) is considered as an effective technology to influence fatigue and corrosion resistance as these properties are largely related to surface structure and property. Despite the increasing number of studies that report the effects of SMAT on separate fatigue or corrosion behaviors, the impact of SMAT on fatigue property under corrosive environment is left unexplored. In this paper, the corrosion fatigue properties of AZ31B magnesium alloy before and after SMAT process are investigated. Hydrogen evolution tests and potentiodynamic polarization tests are carried out to obtain the corrosion resistance, and it is found that SMAT promotes corrosion rate. The surface roughness of SMATed specimen also increases, which accelerates corrosion rate. In terms of fatigue properties, the fatigue life of SMATed specimen is found to be higher, in both air and 3% NaCl solution. And the larger the size of stainless steel balls used during processing SMAT specimens, the longer the fatigue life. Under 3% NaCl corrosive environment, the beneficial effects induced by SMAT process gradually emerge with increasing stress amplitude during fatigue tests. Reduced fatigue life of SMATed specimen under corrosive environment is attributed to roughened interface between deformed surface layer and bulk material. Nevertheless, the crack initiation sites for SMAT specimens are the same in both air and NaCl solution, all located at the subsurface area.

Uniaxial ratchetting and low-cycle fatigue failure behaviors of adhesively bonded butt-joints under cyclic tension deformation

Ratcheting and low-cycle fatigue failure behaviors of the adhesively bonded hollow cylindrical butt-joints has been experimentally investigated. A series of uniaxial cyclic tension experiments were carried out under stress-controlled mode. The effects of stress amplitude, mean stress and cycle time on the uniaxial ratcheting response, fatigue damage variable evolution and fatigue life of the adhesively bonded butt-joints were analyzed. The results show that the ratcheting strain, ratcheting strain rate and fatigue damage variable all increase with the increase of stress amplitude and mean stress. The shorter cycle time results in the increase of fatigue damage variable and the degradation of the stiffness of the adhesive material. It is also found that the increase of stress amplitude and mean stress can reduce the low-cycle fatigue life. Meanwhile, the fatigue life increases with the increase of cycle time for the adhesively bonded butt-joints.

Microstructure and ratcheting behavior of 6061 aluminum alloy laser-MIG hybrid welding joint

In this paper, 6061-T6 aluminum alloy was welded by laser-MIG hybrid welding. The microstructures of the upper part of the weld center, the lower part and the edge of the weld near the fusion line in the welded joint were analyzed. The hardness and mechanical properties of welded joints were analyzed in detail. The ratchet strain of 6061 aluminum alloy welded joints was tested, and the asymmetric stress control method was used to test the ratchet strain of 6061 aluminum alloy welded joints. The results show that ratcheting strain increases with the increase of average stress and stress amplitude, and has the characteristics of cyclic softening first and then hardening. The degree of hardening increases with the increase of the maximum cyclic stress. Cyclic hardening is mainly caused by the uninterrupted dislocations in the process of cycling. The accumulation of ratchet strain results in plastic damage of materials. Pre-ratcheting can improve the tensile properties of welded joints by 6.49% and 97.7% yield strength, which is mainly attributed to cyclic hardening caused by ratcheting strain.

Study on dynamic stress-strain response law of frozen saline soil

The subgrade saline soil in a certain area of western Qinghai province was studied, based on the British GDS dynamic triaxial test system, the dynamic triaxial test under laboratory test was carried out to study variation dynamic stress-strain and dynamic shear modulus under the different temperature, frequency and confining pressure. The test results show that the dynamic stress-strain relationship is significantly affected by temperature change under cyclic dynamic load, followed by frequency and confining pressure. Under the same strain amplitude, the lower the temperature, the larger the dynamic shear modulus. When frequency and confining pressure increase, dynamic shear modulus also increases. Whereas, the influence of confining pressure on the dynamic shear modulus is more obvious, and the backbone curve development trend is steeper. Both maximum dynamic shear modulus and final shear stress amplitude increase with the frequency increases when temperature and confining pressure remain unchanged. Meantime, the maximum dynamic shear modulus curve is a convex shape, and the final shear stress amplitude curve is a convex shape at positive temperature,while which a concave shape at negative temperature. At constant frequency and confining pressure, the maximum dynamic shear modulus and final shear stress amplitude increase with the decrease of temperature. Curves of change trend are slower above 0°C, and steeper below 0°C.

Low cycle fatigue and ratcheting failure behavior of AH32 steel under uniaxial cyclic loading

In this paper, the low cycle fatigue failure and ratcheting behavior, as well as their interaction of AH32 steel were experimentally investigated under uniaxial cyclic loading. The effects of mean stress, stress amplitude and stress ratio on the low cycle fatigue life and ratcheting strain were discussed. It was found that the ratcheting strain increased while the fatigue life decreased with the increase of mean stress and stress amplitude, and the increasing stress ratio would result in smaller ratcheting and larger fatigue life. Two kinds of failure modes, i.e. low cycle fatigue failure due to crack propagates and ratcheting failure due to large plastic strain will take place respectively. Based on the experimental results, considered the effect of ratcheting on fatigue life, a model with the maximum stress and ratcheting strain rate was proposed. Comparison with the experimental result showed that the new model provided a good prediction for AH32 steel.

Effect of ratcheting fatigue damage on the evolution of tensile properties of Al–Mg alloy

This work aims to investigate the effect of damage accumulation on the evolution of tensile properties due to prior ratcheting fatigue in as-received and artificially aged conditions of Al–Mg alloy (AA 5754). Uniaxial asymmetric stress controlled tests for various combinations of mean stress and stress amplitude are performed at room temperature. Progressive degradation in the mechanical properties of structures and machines subjected to cyclic loading during service period can affect the structural integrity. In view of this, ratcheting fatigue tests are interrupted at various life fractions for particular mean stress and stress amplitude followed by tensile tests to understand the damage evolution during ratcheting for both conditions of alloy. Also, the effect of mean stress and stress amplitude on the post ratcheting tensile behaviour is analysed by conducting the ratcheting tests up to 150 cycles. Cyclic hardening is exhibited in the initial cycles followed by saturation for both as-received and artificially aged alloy. Ratcheting strain increases with the increase in both mean stress and stress amplitude. Drastic changes in tensile properties are noticed with ratcheting fatigue damage evolution for both conditions of alloy. The evolution of tensile properties due to prior loading history such as ratcheting fatigue can provide fundamental understanding in the design of structural components.

Effect of Ratcheting Strain on Mechanical Properties of Additive Manufactured 4043 Aluminum Alloy

4043 aluminum alloy was printed by cold metal transfer (CMT) additive manufacturing (AM) technology. The microstructure and mechanical properties were analyzed. The effect of ratcheting behavior was analyzed by the tensile test after ratcheting. The results indicate that the dendrite structure of 4043 aluminum alloy has obvious directivity. The binary eutectic structure of α (Al) + Si is mainly distributed at the grain boundaries and the interior of grain is mainly α (Al). The increase of stress amplitude and mean stress leads to ratcheting strain, which can cause plastic damage of AM aluminum alloy. This is related to holes aggregation and dislocation slip caused by ratcheting behavior. Compared to the aluminum alloy of un-ratcheted test, the tensile and yield strength increased and the elongation decreased, but the change of tensile and yield strength are not obvious between the s specimens of different ratcheting.

Creep-ratcheting-fatigue life prediction of bainitic 2.25Cr1MoV steel

Ratcheting fatigue behavior of bainitic 2.25Cr1MoV forged steel was investigated at 455 oC with various hold periods at peak and valley stress. The incorporation of peak stress holding widened stress-strain hysteresis loop, and this enlargement was more significant under peak/valley hold due to the created creep strain. Fatigue life was greatly shortened with the increase of mean stress or stress amplitude, and this reduction was accelerated when stress holding was involved. The extension of stress holding resulted in shorter fatigue life, but the fatigue lives were quite close when the sum of holding period was equal, regardless of the holding direction. Fatigue damage and creep damage in creep-ratcheting fatigue tests were estimated by the time fraction rule, and creep damage became dominant at long hold period, which was closely related to shortened fatigue lives with stress holding. The linear damage summation rule was used for fatigue life prediction, and it gave satisfactory predictions for peak holding mode. A new model was proposed by introducing stress ratio to consider the influence of asymmetric cycling, and estimates for peak/valley holding were greatly improved.

Static and dynamic stiffness measurements with Opalinus Clay

ABSTRACTIn this work, an experimental study was carried out with the aim of reconciling static and dynamic stiffness of Opalinus Clay. The static and dynamic stiffness of core plugs from a shaly and a sandy facies of Opalinus Clay were characterized at two different stress states. The measurements included undrained quasi‐static loading–unloading cycles from which the static stiffness was derived, dynamic stiffness measurement at seismic frequencies (0.5–150 Hz) and ultrasonic velocity measurements (500 kHz) probing the dynamic stiffness at ultrasonic frequencies. The experiments were carried out in a special triaxial low‐frequency cell. The obtained results demonstrate that the difference between static and dynamic stiffness is due to both dispersion and non‐elastic effects: Both sandy and shaly facies of Opalinus Clay exhibit large dispersion, that is, a large frequency dependence of dynamic stiffness and acoustic velocities. Especially dynamic Young's moduli exhibit very high dispersion; between seismic and ultrasonic frequencies they may change by more than a factor 2. P‐wave velocities perpendicular to bedding are by more than 200 m/s higher at ultrasonic frequencies than at seismic frequencies. The static undrained stiffness of both sandy and shaly facies is strongly influenced by non‐elastic effects, resulting in significant softening during both loading and unloading with increasing stress amplitude. The zero‐stress extrapolated static undrained stiffness, however, reflects the purely elastic response and agrees well with the dynamic stiffness at seismic frequency.

Effects of creep and creep-recovery on ratcheting strain of articular cartilage under cyclic compression

Since the accumulations of ratcheting strain combined with creep deformation, which are produced in normal activities, can accelerate the fatigue damage of cartilage in joint, the creep-ratcheting and creep-recovery-ratcheting behaviors of articular cartilage are experimentally investigated under creep-fatigue loads. The effect of pre-creep on ratcheting behavior of cartilage was probed firstly and it is found that the initial ratcheting strain of cartilage presents the larger value (30% and 35%) due to its pre-creep deformation in spite of the short pre-creep time applied. With the increasing pre-creep time the ratcheting strain of sample increases while the ratcheting strain rate decreases. The effects of pre-creep and recovery on ratcheting behavior of cartilage were also investigated and it is noted that the strain of cartilage increases fast at first, decreases partly and then changes periodically with cyclic stress. The ratcheting strain evolutions of different layers are not coincident for cartilage sample after the pre-creep or pre-creep-recovery and the ratcheting strains of different layers with pre-creep are larger than them with pre-creep-recovery. Finally the creep-ratcheting strain of cartilage with different peak-holding time during cyclic compression was studied and it is found that the creep-ratcheting strain with peak-holding time is significantly greater than that without peak-holding time. The creep-ratcheting strain increases with increase of peak-holding time or stress amplitude, while it reduces with rising of stress rate. The creep-ratcheting behavior of cartilage is dependent on its depth. These findings point out that the accumulated deformations, including creep deformation and ratcheting deformation, can accelerate the cartilage's damage.

Ratcheting behavior of UHMWPE reinforced by carbon nanofibers (CNF) and hydroxyapatite (HA): Experiment and simulation

Uniaxial tensile tests were performed to investigate the mechanical properties of the ultra-high molecular weight polyethylene (UHMWPE) with different modification conditions. It was found that the different modification conditions have great influence on the mechanical properties of the UHMWPE. Subsequently, the uniaxial ratcheting behaviors of the UHMWPE/CNF and UHMWPE/HA composite materials were observed under the stress-controlled cyclic tensile condition at room temperature. The dependence of uniaxial ratcheting of composite materials on the mean stress, stress amplitude, stress rate and nano-material content was investigated. The results show that the ratcheting strain and its rate of the two composite materials increase as the mean stress and stress amplitude increase, however, the ratcheting strain and its rate decrease with the increase of the stress rate and nano-material content. Furthermore, it is found that the ratcheting strain of the UHMWPE/HA composite material is more remarkable than that of the UHMWPE/CNF composite material. A new viscoplastic constitutive model is proposed to describe the ratcheting behavior of the UHMWPE composite materials. In this model, a new viscosity function and modified kinematic hardening law were employed. Comparison of simulation and experimental results shows that the simulations are in good agreement with the experimental results.

Slow relaxation dynamics in binary glasses during stress-controlled, tension-compression cyclic loading

The effect of cyclic loading on relaxation dynamics and mechanical properties of metallic glasses is studied using molecular dynamics simulations. We consider the Kob-Andersen three-dimensional binary mixture rapidly cooled across the glass transition and subjected to thousands of tension-compression cycles in the elastic range. It was found that during cyclic loading at constant pressure, the system is relocated to progressively lower levels of the potential energy, thus promoting greater densification and higher strength. Furthermore, with increasing stress amplitude, the average glass density increases and the minimum of the potential energy becomes deeper, while the elastic modulus is reduced. The typical size of clusters of atoms with large nonaffine displacements becomes smaller over consecutive cycles, which correlates with the gradual decrease in the potential energy. These results are important for thermomechanical processing of metallic glasses with improved mechanical properties.

Influence of Hydrogen on Steel Components for Clean Energy

The influence of hydrogen on the mechanical properties of four, medium-strength, commercial, quenched-and-temped steels has been studied using the linearly increasing stress test (LIST) combined with cathodic hydrogen charging. The relationship was established between the equivalent hydrogen pressure and the hydrogen charging overpotential during cathodic hydrogen charging, though the use of electrochemical permeation experiments and thermal desorption spectroscopy. The cathodic hydrogen charging conditions were equivalent to testing in gaseous hydrogen at hydrogen fugacities of over a thousand bar. Under these hydrogen-charging conditions, there was no effect of hydrogen up to the yield stress. There was an influence of hydrogen on the final fracture, which occurred at the same stress as for the steels tested in air. The influence of hydrogen was on the details of the final fracture. In some cases, brittle fractures initiated by hydrogen, or DHF: Decohesive hydrogen fracture, initiated the final fracture of the specimen, which was largely by ductile micro-void coalescence (MVC), but did include some brittle fisheye fractures. Each fisheye was surrounded by MVC. This corresponds to MF: Mixed fracture, wherein a hydrogen microfracture mechanism (i.e., that producing the fisheyes) competed with the ductile MVC fracture. The fisheyes were associated with alumina oxide inclusion, which indicated that these features would be less for a cleaner steel. There was no subcritical crack growth. There was essentially no influence of hydrogen on ductility for the hydrogen conditions studied. At applied stress amplitudes above the threshold stress, fatigue initiation, for low cycle fatigue, occurred at a lower number of cycles with increasing hydrogen fugacity and increasing stress amplitude. This was caused by a decrease in the fatigue initiation period, and by an increase in the crack growth rate. In the presence of hydrogen, there was flat transgranular fracture with vague striations with some intergranular fracture at lower stresses. Mechanical overload occurred when the fatigue crack reached the critical length. There was no significant influence of hydrogen on the final fracture.

The Portevin-Le Châtelier effect of gradient nanostructured 5182 aluminum alloy by surface mechanical attrition treatment

Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment (SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Châtelier (PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.

Uniaxial and biaxial ratcheting behavior of ultra-high molecular weight polyethylene

Experimental studies were conducted to investigate the uniaxial and biaxial ratcheting behaviors of Ultra-high molecular weight polyethylene (UHMWPE). The effects of stress amplitude, stress rate and hydroxyapatite content on uniaxial ratcheting behavior were studied firstly. It is found that the ratcheting strain and its rate increase as stress amplitude increases. However, the ratcheting strain and its rate decrease with rising of stress rate. Meanwhile, it is found that the ratcheting strain decreases with increase of hydroxyapatite content. The ratcheting strain rates with different hydroxyapatite contents are not obviously different. The modified ratcheting strain accumulative model was constructed to predict the uniaxial ratcheting behavior of UHMWPE with different stress amplitudes, stress rates and hydroxyapatite contents. It is seen that the predictions agree with the experimental results very well. The effects of different loading paths on biaxial ratcheting behavior of UHMWPE were studied. Both ratcheting strain and ratcheting strain rate are strongly influenced by the loading path. It is found that the uniaxial loading path gives the highest ratcheting strain and its rate while the proportional loading path gives the lowest ratcheting strain and its rate.

Uniaxial cyclic elasto-plastic deformation and fatigue failure of API-5L X65 steel under various loading conditions

The cyclic elasto-plastic deformation and fatigue failure properties of API-5L X65 steel was characterised under monotonic, high cycle fatigue, strain controlled low cycle fatigue, and stress controlled ratchetting loading regimes. The steel combines high strength, which increases with strain rate under monotonic loading, with high fatigue limit/yield strength ratio. It is found that the steel exhibits non-Masing and cyclic softening behaviours. Low cycle fatigue lifetime decreased with increase in total strain amplitude and plastic strain amplitude. It is shown that, under uniaxial fully reversed loading, the Chaboche model with three backstresses is capable of describing the cyclic hysteresis stress-strain curves of the steel. Ratchetting lifetime of the steel decreased with increase in ratchetting strain and increase in both mean stress and stress amplitude; the stress amplitude however dominates the damage process compared to mean stress.

Study of load history effects on the high cycle fatigue properties of high-strength low-alloy steel from self-heating measurements

In the context of high cycle fatigue (HCF), the experimental characterization of the fatigue properties is often performed by using specimens in a virgin state (i.e., without preliminary loading), and with a constant stress amplitude for each specimen. However, the load history applied to a real structure is more complex and the fatigue life prediction remains a difficult task because of the time dedicated to the classical fatigue tests (i.e., the specimen is loaded until failure) and the dispersion of fatigue lives. The load history effects on the HCF properties is characterized using an alternative method: self-heating measurements under cyclic loadings. This method is based on the observation of the mean steady state temperature evolution of a specimen under a successive series of cyclic loadings with increasing stress amplitude for each loading series. A probabilistic two-scale model was developed from the self-heating method able to predict HCF properties. Some self-heating tests are performed to study the influence of a load history effects. It seems that the plasticity is the most influential factor. So, the evolution of the plasticity is observed at the surface of the material under cyclic loading. There is a significant evolution in function of the plastic pre-strain.

Experimental validation of vibration-excited subsidence model of seasonally frozen soil under cyclic loads

Validation of the vibration-excited subsidence model is limited for seasonally frozen soil. Based on constant-amplitude load tests by a new low-temperature triaxial device under realistic confining pressure, amplitudes of dynamic stress, consolidation and freezing methods, we determined the residual strain behavior and validated the vibration-excited subsidence model of frozen soil during frozen period considering the influence caused by temperatures, loading amplitudes and loading cycles. Results show that the residual strain during the frozen period increased with increasing cycles of loading, but decreased with decreasing temperatures. The residual strain development model indicates that residual strain increased dramatically in the initial loading cycles followed by a gradual increase over time. In most cases, the frozen soils tested are not failing that the developing curves of residual strains under different temperatures develop nearly parallel after certain cycles of loading. Residual strain was fairly sensitive to the negative temperatures at large amplitudes of dynamic stress, and the influence of low temperature was limited after a critical temperature in −5°C to −10°C. The two-parameter exponential model is able to express exactly the change rule of vibration-excited subsidence under different temperature. With the increase of dynamic stress amplitude, parameters A and B increase rapidly, especially when the soil approaches the failure state. In contrast, parameters A and B decrease as temperature decreases, but decrease little as the temperature decreases below the critical temperature. Our test layout overcomes previous testing limitations such as excessively large consolidating pressure and dynamic loading so that the test results are more consistent with the real conditions of seasonally frozen soil. The vibration-excited subsidence of subgrade in seasonally frozen soil regions could be predicted through this model.

Effect of the dynamic evolution of dislocations under cyclic shear stress on damping capacity of AZ61 magnesium alloy

The effect of the dynamic evolution of dislocations on damping capacity of AZ61 magnesium alloy was investigated by using dynamic mechanical analysis and transmission electron microscope. The results indicate that the dynamic evolution of dislocations caused by the increased cyclic shear stress significantly influences the damping capacity, and it exhibits four regions. In the first region, the low damping capacity is caused by forced vibrations of dislocation segments pinned by weak pinning points. The second region is the linear increase of damping capacity, the dislocations breakaway from weak pinning points is responsible for it. The third region is characterized by a non-linear increase of damping capacity, and the growth rate of the damping capacity increases as the increase of shear stress amplitude, dislocation multiplication and dislocation movement in the cyclic shear stress acting plane lead to the non-linear increase of mobile dislocation density, which is responsible for that. Because dislocation pile-up causes the mobility of the dislocation lines decreases, the growth rate of the damping capacity decreases - the fourth region.

True stress-control ratcheting behaviour of cold-rolled interstitial-free steel sheet

True stress-control ratcheting behaviour of 2.3 mm thick titanium stabilised interstitial-free steel has been investigated under constant maximum stress, constant mean stress and constant stress amplitude conditions until complete failure of the specimens. The ratcheting strain to failure, and cycles to failure under different stress conditions have been rationalised by considering the evolution of back stresses vis-a-vis total and net cyclic plastic strain response. It is found that higher stress asymmetry due to increase of stress ratio at constant maximum stress results in lower ratcheting deformation. This happens due to both mean stress assisted hardening and lower back stress amplitude resulting from decreasing stress amplitude. On the other hand, mean stress induced hardening improves ratcheting-fatigue life at constant stress amplitude. The amplification of cyclic plastic strain response with increase of stress amplitude at constant mean stress results in higher ratcheting strain accumulation at higher stress amplitude.