Plastic Strain Amplitude Research Articles

Low-cycle fatigue behavior of Silafont®-36 automotive aluminum alloy: Effect of negative strain ratio

The aim of this study was to determine cyclic deformation behavior of a high-pressure die-cast Silafont®-36 automotive alloy with heterogeneous microstructures, focusing on the effect of strain ratio (Rε) ranging from negative infinity (-∞) to +0.5 at a constant strain amplitude of 0.4%. Pronounced cyclic hardening behavior was observed from the stress-strain hysteresis loops at different strain ratios. The degree of cyclic hardening, evaluated via both the traditional method and a newly-proposed slope method, increased with decreasing strain ratio. Fatigue life of the alloy was slightly lower at positive strain ratios, with its peak emerging at zero mean strain (or Rε = −1). In the negative range of strain ratios (Rε<−1), the fatigue life increased slightly with increasing strain ratio, which was mainly related to the plastic strain amplitude. A new power-law based equation was proposed to describe the change of mean stress relaxation at negative strain ratios (Rε<−1), along with a polynomial equation for depicting the situation at zero and positive strain ratios. Strain-energy density analyses corroborated that the intrinsic fatigue toughness was a material constant since it was not affected by strain ratio.

Experimental and modelling study of cyclic plasticity of high strength steel Q690

High strength steels (HSSs) have received attention in the research community over the past few years owing to their high strength-to-weight ratio. This study examined the cyclic plastic behaviour of HSS Q690 under various cyclic loading protocols via experimental investigation, and a constitutive model was subsequently proposed. The strain amplitude and mean strain were the key parameters for constructing the cyclic loading protocols. The test results indicated that the peak stress of each cycle decreased with increasing equivalent plastic strain, which confirmed the cyclic softening behaviour. The peak stress evolution was also observed to be independent of the maximum strain amplitude, but dependent on the current strain amplitude, which proved the absence of the strain range memory effect. Furthermore, the cyclic plastic response of Q690 steel was comprehensively examined within the framework of metallic cyclic plasticity, including kinematic hardening, isotropic hardening/softening, and elastic modulus evolution. The kinematic hardening of Q690 steel was proven to be dependent on the equivalent plastic strain and the current plastic strain amplitude, whereas its isotropic hardening/softening was determined only by the equivalent plastic strain. It was also observed that the elastic modulus decreased to a stable value with increasing equivalent plastic strain. A new constitutive model was proposed to quantify these cyclic plastic features. The proposed model provided an excellent prediction of the cyclic response of Q690 steel under all loading protocols. The relative error between the simulated results and the test results was within a narrow band between -5% and 5%.

Influence of Plastic Strain Control on Martensite Evolution and Fatigue Life of Metastable Austenitic Stainless Steel

Metastable austenitic stainless steel was investigated during fatigue tests under strain control with either constant total or constant plastic strain amplitude. Two different material conditions with coarse-grained and ultrafine-grained microstructure were in focus. The influence of plastic strain control of the fatigue test on both the martensitic phase transformation as well as on the fatigue lives is discussed. In addition, an approach for calculating the Coffin–Manson–Basquin parameters to estimate fatigue lives based on strain-controlled tests at constant total strain amplitudes is proposed for materials undergoing a strong secondary hardening due to martensitic phase transformation.

The influence of hydrogen on the low cycle fatigue behavior of strain-hardened 316L stainless steel

The present study focuses on the low cycle fatigue (LCF) behavior of strain-hardened 316L stainless steel under plastic strain control, aiming to understand the influence of internal hydrogen on fatigue performance and cyclic deformation behavior for energy-related technologies. The strain-hardened 316L specimens tested at plastic strain amplitudes between 0.1% and 0.7% with and without hydrogen displayed continuous cyclic softening. Internal hydrogen increased the cyclic strength of this steel, with a greater difference in strength at low plastic strain amplitudes. A Bauschinger analysis revealed that effective stresses represented the major contribution to the flow stress for all material conditions and amplitudes. At low plastic strain amplitudes, effective stresses were responsible for differences in cyclic strength between hydrogen precharged and non-charged specimens. In contrast, back stresses became more significant at high amplitudes, and more similar deformation structures were observed. Although internal hydrogen reduced the total LCF lifetime at all amplitudes, this degradation was more significant in the low amplitude regime. This result is attributed to high effective stresses that led to the onset of multiple slip at lower cumulative plastic strains in the hydrogen precharged condition compared to the non-charged condition at low amplitudes. In addition, the evolution of the apparent elastic response of the material suggests that the primary crack(s) nucleated at a smaller percentage of the total lifetime and propagated in fewer number of cycles in the hydrogen precharged than in the non-charged condition. Fracture surface and gage section observations revealed a transgranular crack path and planar slip traces in both material conditions, with internal hydrogen promoting multiple slip at lower values of cumulative plastic strain than in the non-charged condition.

Comparison of Novel Low-Carbon Martensitic Steel to Maraging Steel in Low-Cycle Fatigue Behavior

The study systematically compares the low-cycle fatigue (LCF) behaviors of novel martensitic steel (22MnSi2CrMoNi) and maraging steel (00Ni18Co9Mo4Ti). Results show that the two types of tested steel have a similar cyclic deformation behavior. The cyclic softening resistance of 22MnSi2CrMoNi steel is slightly inferior to that of 00Ni18Co9Mo4Ti steel at a low total strain amplitude. However, the gap gradually disappears with the increase of the total strain amplitude. At the same plastic strain amplitude, the LCF lifetime of 22MnSi2CrMoNi steel is higher than that of 00Ni18Co9Mo4Ti steel. The retained austenite film between martensite lath and the existence of precipitated phase in matrix can effectively improve the fatigue lifetime of the two types of tested steel.

Intergranular fatigue crack initiation in polycrystalline copper

Polycrystalline copper specimens were fatigued in strain-controlled regime with high and low strain amplitude. Stress amplitude, plastic strain amplitude and hysteresis loops were recorded. The loop shape parameter reflecting the onset and evolution of the localization of cyclic plastic deformation was evaluated. Mechanism of intergranular fatigue crack initiation was studied on the electrolytically polished specimen surfaces using scanning electron microscopy coupled with focused ion beam. Intergranular cracks had initiated already at 4% of the fatigue life. Corrugated surface of grain boundary cracks close to the surface of the grains was closely related to the occurrence of the extrusions and intrusions produced on the grain boundaries. The observations give further support to the novel mechanism of grain boundary crack initiation based on the emergence of extrusions and intrusions on the grain boundary surface. Factors affecting the initiation of grain boundary cracks were identified.

Monotonic Tensile and Cyclic Deformation Behaviors of Carbide‐Free Bainitic Steel Subjected to Austempering and Tempering After Air Cooling

The monotonic tensile and tension–compression cyclic deformation behaviors of carbide‐free bainitic steel with two microstructures obtained by isothermal treatment (ISO) and air cooling followed by tempering (ACT) are investigated. Tensile results demonstrate that the ISO sample exhibits a lower yield strength but longer elongation due to more and bigger blocky retained austenite than that of the ACT sample, which can lead to a higher plastic deformation capacity for the former sample. Fatigue results indicate that the ACT sample shows a higher cyclic hardening capacity but lower cyclic softening capacity at lower strain amplitudes compared to the ISO sample, whereas an opposite phenomenon can be found at higher strain amplitudes. At given total strain amplitudes, the fatigue life of the ISO sample is shorter, which is mainly attributed to its lower yield strength and more brittle martensite. In comparison with the ACT sample, the ISO sample exhibits similar or slightly longer fatigue life at lower plastic strain amplitudes but shows shorter fatigue life at higher plastic strain amplitudes. Accordingly, a bilinear Coffin––Manson relationship is revealed for the ISO sample, which is closely associated with significantly more transformation at higher strain amplitudes compared to the lower strain amplitudes.

Achieving 1 GPa fatigue strength in nanocrystalline 316L steel through recovery annealing

Nanostructured metals offer an improved fatigue limit and high cycle fatigue performance compared to their microcrystalline counterparts. However, even the small cyclic plastic strain amplitudes present at these cyclic loading regimes can induce grain growth, a prerequisite for fatigue crack initiation. Retarding cyclically induced grain growth, for instance by alloying, should thus further improve the fatigue performance. Here, we followed a different strategy and investigate the role of recovery heat treatments, known to shift the onset of plasticity to higher stress levels. Indeed, we observe for the recovered specimens an unprecedented increase of the fatigue limit, reaching for 316L steel values of almost 1 GPa at a stress ratio of R=-1. First experiments further indicate that this strategy could also be used to improve the fatigue limit of conventionally cold worked austenitic structures.

Cyclic hardening/softening and deformation mechanisms of a twip steel under reversed loading

Push-pull tests at fixed plastic strain amplitude or fixed stress amplitude were run on a TWIP steel, and followed by TEM observations, to analyze its cyclic behavior in relation with the deformation mechanisms. The kinematic and isotropic components of the flow stress were measured throughout the whole cyclic hardening/softening stages, and their evolution with the cumulated plastic strain was compared to those measured in tension. The rise of the internal stress was found responsible for the initial cyclic hardening, but this stress reached at most 50% of the flow stress, as compared to nearly 70% in tension. Special constitutive equations were identified to capture these evolutions, as well as the transition from hardening to softening. Both components of the flow stress slightly decrease during the softening stage, whose origin is discussed, based on TEM observations at peak stress amplitude, or after softening. The present measurements and TEM observations, combined with those from a previous study of twinning/detwinning kinetics in push-pull on the same steel [29], suggest that under cyclic loading, mechanical twinning cannot be responsible for significant kinematic hardening of intragranular nature (or “dynamic Hall-Petch effect”), but rather contributes to a back stress of intergranular origin.

Deformation mechanism and cyclic stress response of Zircaloy-4 alloy cladding tube during low cycle fatigue at room temperature

This work systematically investigated the deformation mechanism and cyclic stress response behavior of Zircaloy-4 alloy cladding tube at room temperature. Low cyclic fatigue tests revealed that cyclic softening and hardening strongly depend on the applied strain amplitude. Besides, prismatic < a > slip was the primary cyclic deformation mode when the value of plastic strain amplitude less than that of elastic strain amplitude, otherwise prismatic < a > slip and {101¯2} <1¯011 > twinning dominated the deformation. Due to the combination effect of grain rotation, dislocation accumulation and twinning, cyclic softening, saturation and hardening behaviors were presented in Zircaloy-4 alloy.

Cyclic deformation and dislocation structure evolution of a copper bicrystal with components rotating gradually along the grain boundary

Abstract The push –pull cyclic deformation of a copper bicrystal with components rotating gradually along the grain boundary (GB) was carried out under constant plastic strain amplitude at room temperature in air. The evolution of slip morphology and dislocation structure was observed by scanning electron microscopy electron channelling contrast technique. It was shown that slip morphology and dislocation structure in one of the grains changed gradually along the GB. This phenomenon can be understood based on the local irreversible rotation of crystals, which exists during the symmetrical push– pull loading. When the secondary slip was localized to form the secondary persistent slip bands, which interacted with the regular walls in the second type of deformation band, then the cell structure was formed. The second type of deformation band is one kind of sites for cell formation.

Influence of laser surface hardening on low cycle fatigue performance of homogeneous-structure super austenitic stainless sheets by laser beam welding

In this investigation, low cycle fatigue (LCF) studies were carried out on the super austenite stainless steel (SASS) sheets by laser surface hardened (LSH) method using laser beam welding process. Microstructure and LCF studies were conducted at the fusion zone. Parameters such as laser beam of 1200 W, welding speed of 200 mm/min and focus distance of 10 mm were used in laser beam welding process to achieve the narrow weld profile and increased mechanical properties of the materials. Two different types of heat inputs were applied in the LSH method and high-power diode laser of 600–700 W and focal plane position of 18 mm were applied. The heat inputs of laser beam welding reached full depth of penetration of the materials. Moreover α’ prior austenite structure was converted into β shapes and widmanstatten structure. The heat inputs were the deciding factor of grain boundaries growth. Further, more amount of α ferrite structures were transformed into basket twin boundaries in fusion zone and LSH process. Hardness profile indicated that the hardness values of LSH showed 6% improvement compared to fusion zone. Heat inputs were the main problem making of dropping the hardness value of fusion zone. Tensile sample displayed the failure in the fusion zone and a higher tensile strength of 850 MPa was observed due to the plastic strain amplitude which depended on the grain boundaries growth and α’martensite structures. Microstructure studies indicated no major holes in the fusion zone; however, LCF life was significantly improved after LSH process. Potential dynamic polarization studies were conducted in the fusion zone and SASS material processed with LSH & observed that high corrosion resistance was achieved without the formation of CrN and Cr2N.

Cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser-beam powder bed fusion

This paper studies the cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser-beam powder bed fusion for different processing states, namely in the as-built, stress-relieved, and T6 conditions. Low-cycle fatigue tests (Rε = -1) are performed using the single step test method, at room temperature, with a constant strain rate, under strain control mode for strain amplitudes in the range 0.2-1.5%. The main results show that the processing route clearly affects the cyclic stress-strain response of the tested alloy. The strain energy density per cycle of the stress-relived and T6 conditions is much smaller than that of the as-built sate, which is explained by a reduction of the linear portion of hysteresis loops. For lower lives, stress-relieved and T6 conditions led to higher fatigue lives at the same strain amplitude. In addition, on log-log scales, a bi-linear relationship between the plastic strain amplitude and the number of reversals to failure was found for the as-built and stress-relief conditions. On the contrary, for the T6 condition, the data were fitted via a linear function.

極低サイクル疲労下における荷重条件を考慮したき裂進展条件式の評価

The validity of the crack propagation criterion under extremely low cycle fatigue due to earthquakes is still under discussed. Existing evaluation parameters are difficult to apply because they change under the constraint of restraining effects due to large plastic deformation. In a previous study, the crack propagation criterion was proposed and evaluated using equivalent plastic strain amplitude and stress triaxiality at the center of the plate thickness. Those parameters are physical quantities that are difficult to obtain from fracture experiments. In this study, the equivalent plastic strain amplitude and stress triaxiality at the crack front were analyzed under three different loading conditions to evaluate the applicable range in the direction of the plate thickness other than the center of the plate thickness and to improve the equation to be applicable near the plate thickness surface. As a result of the evaluation of the range of applicability in the thickness direction, it was found that the crack does not propagate near the surface in some cases.

Improved description of low-cycle fatigue behaviour of 316L steel under axial, torsional and combined loading using plastic J-integral

Low-cycle fatigue behaviour and fatigue crack kinetics of the 316L austenitic stainless steel were studied under cyclic axial, torsional and in-phase combined loading using hollow cylindrical (tubular) specimens with a small hole for crack initiation. The concept of plastic.J-integral was used, which was shown in previous studies to unify the crack growth rate data for several different materials. Dependencies of Jp on crack length were determined by extensive finite element modelling considering non-linear material behaviour according to the cyclic stress–strain curve. Locally deflected cracks were modelled in accordance with amplitudes of the axial and torsional components of combined loading. The measured crack growth rate diagrams for all types of loading and for various loading amplitudes were unified using amplitude of Jp. Fatigue lives under torsional loading were much longer than under axial loading for the same equivalent plastic strain amplitude, which was explained by higher crack driving forces in terms of Jp under axial loading than under torsional loading. Fatigue lives estimated by crack propagation based on a master curve in terms of Jp,a were in a good agreement with those obtained experimentally under all types of loading. The used concept can reduce the experimental program to obtaining of material data only for axial loading, which can then be used for prediction of behaviour under in-phase multiaxial loading. The von Mises formula for multiaxial low-cycle fatigue loading εeq,p2 = εp2 + γp2 / 3 was modified so that the fatigue lives under axial, torsional and combined loading were characterized in a matching way. Using the formula εp,Nf2 = εp2 + γp2 / 25, the fatigue life data fell on a single Coffin-Manson curve.

Low-cycle fatigue properties and unified fatigue life prediction equation of hot-rolled twin-roll-cast AZ31 sheets with different thicknesses

This study investigates the low-cycle fatigue properties of AZ31 sheets with different thicknesses of 1, 1.5, 2, and 3 mm—which are fabricated by twin-roll casting and subsequent hot rolling—through fully reversed strain-controlled fatigue tests. As the thickness of the sheets decreases, their average grain size decreases, texture intensity increases, and tensile yield strength and elongation gradually increase. At strain amplitudes of greater than or equal to 0.6%, {10–12} twinning in compression and detwinning and subsequent slip in tension occur repeatedly, which forms asymmetric hysteresis loops. The different grain sizes of the sheets result in different compressive peak stresses during the fatigue tests. However, the overall cyclic deformation behavior is similar in all the sheets, and consequently, their fatigue lives exhibit an insignificant difference. The loading direction also has a negligible influence on both the cyclic deformation behavior and the fatigue life, which implies that the sheets exhibit in-plane isotropic fatigue properties. The stress amplitude and plastic strain amplitude vary considerably during the fatigue test. In contrast, the variation in total strain energy density is insignificant over the entire fatigue life, and therefore, it is a proper fatigue damage parameter for predicting fatigue life. A unified fatigue life prediction equation using the total strain energy density is established, and the fatigue lives predicted using the equation are found to be in good agreement with the experimentally determined values, regardless of the strain amplitude, sheet thickness, and loading direction.

A plastic strain energy method exploration between machined surface integrity evolution and torsion fatigue behaviour of low alloy steel

To explore the evolution mechanism of multistage machining processes and torsional fatigue behaviour based on strain energy for the first time and provide process optimization of axis parts of low alloy steel for service performance, four multistage machining processes were applied to the 45CrNiMoVA steel, including the Rough Turning process (RT), RT+ the Finish Turning process (FRT), FRT+ the Grinding process (GFRT) and RT+ the Finish Turning process on dry cutting condition (FRT0). The result showed that the FRT process’s average low-cycle torsional fatigue life increased by 50% when it evolved from the RT process. The lower surface roughness of Ra 1.3 μm caused the total strain energy to increase by 163.8 Pa mm/mm instead of the unchanged strain energy density, and the crack feature evolved from some specific bulges to flat shear plane characteristics. When the GFRT process evolved from the FRT process, its average fatigue life increased by 1.45 times, compared with the RT process. Plastic strain amplitude decreased by 21%, and the strain energy density decreased by 4% due to more considerable compressive residual stress (−249 MPa). Plastic deformation layer depth had a consistent tendency with surface roughness. In this paper, surface integrity evolutions on cyclic characteristics and fatigue behaviour have also been explained. A fatigue life prediction model based on the energy method for machined surface integrity is proposed.

Low cycle fatigue performance of DP900 steel under cyclic shear paths

AbstractIn this study, a sheet fatigue shear test device is designed and applied to the low‐cycle fatigue testing of DP900 with varying strain amplitudes within the range of 0.5%–6.0%. The microstructure is analyzed by using electron backscatter diffraction, and fracture surfaces are examined via scanning electron microscopy. Results indicate that the material exhibits cyclic softening behavior after the first two cycles of hardening, with a stable softening rate and a high damage evolution rate related to loading amplitude. The variation of the hysteresis curve in the cyclic process is shown. Total plastic strain energy absorbed increases as loading amplitude decreases and it reaches the peak at approximately 1%. The life prediction model based on plastic strain energy density and strain amplitude is verified to be suitable for the cyclic shear path. The influence of microinhomogeneity on the distribution of stress and strain, especially the deformation of martensite, is closely related to the bi‐linear region of fatigue life curve.

A Comparison of Amplitude-and Time-Dependent Cyclic Deformation Behavior for Fully-Austenite Stainless Steel 316L and Duplex Stainless Steel 2205.

Austenite and duplex stainless steels are widely used in engineering, and the latter exhibits a more excellent combination of mechanical properties and corrosion resistance due to the coexistence of austenite and ferrite and higher nitrogen. However, fatigue failure still threatens their structural integrity. A comprehensive comparison of their cyclic deformation behavior is a major foundation to understand the role of duplex-phase microstructure and nitrogen in the safety assessment of engineering components. Thus, in this paper, the cyclic deformation behavior of fully-austenitic stainless steel 316L and duplex stainless steel 2205 was studied by a series of low cycle fatigue tests with various strain amplitudes, loading rates and tensile holding. A theoretical mechanism diagram of the interaction between nitrogen and dislocation movements during cyclic loads was proposed. Results show that the cyclic stress response of 2205 was the primary cyclic hardening, followed by a long-term cyclic softening regardless of strain amplitudes and rates, while an additional secondary hardening was observed for 316L at greater strain amplitudes. Cyclic softening of 2205 was restrained under slower strain rates or tensile holding due to the interaction between nitrogen and dislocations. The cyclic plasticity of 2205 started within the austenite, and gradually translated into the ferrite with the elevation of the cyclic amplitude, which lead to a decreased hardening ratio with the increase in amplitude and a shorter fatigue life for a given smaller plastic strain amplitude.

Cyclic deformation and fatigue behavior of 7075-T651 Al alloy with a gradient structure

Fully reversed stress-controlled tension–compression fatigue experiments (R = −1) were conducted to study the cyclic deformation and fatigue behavior of 7075-T651 aluminum alloys with and without a surface mechanical rolling treatment (SMRT). The strain amplitude, ratcheting effect, and asymmetric behavior of base metal (BM) and SMRT specimens were analyzed. SMRT specimens showed more remarkable ratcheting effects and tension–compression asymmetry under high stresses. Fatigue tests showed that the fatigue strengths of 7075 Al alloys were increased significantly by the SMRT. Both BM and SMRT specimens presented a failure-mode transition from the intrusions and extrusions failure modes to the Al18Ti2Mg3 constituent phase. The hysteresis loop was utilized to obtain the plastic-strain amplitude. Furthermore, according to the cyclic-deformation behavior combined with the Manson–Coffin law, strain–life evaluation of BM and SMRT specimens under the stress-control is given.