High Plastic Strain Amplitude Research Articles

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.

Low Cycle Fatigue of an Ultrafine Grained AA5083 Aluminum Alloy Composite Produced by Cryomilling

Low cycle fatigue (LCF) properties were investigated for a novel cryomilled AA5083 aluminum composite with duplex coarse and ultrafine grain sizes and reinforced with boron carbide particulates, referred to as trimodal material. Fully reversed cyclic tests were conducted under plastic strain control at plastic strain amplitudes from 0.15 to 0.6 pct using a constant plastic strain rate in a servo-hydraulic testing system. A nonlinear elastic modulus was used to calculate the elastic contribution to the measured total strain. The LCF performance of this trimodal material is compared to previous results for unreinforced AA5083 aluminum alloy with bimodal grain size (85/15 pct CM/UM) and its coarse-grained wrought counterpart, AA5083-H131. Stress response curves for the trimodal material revealed slow hardening until failure associated with the presence of particulate reinforcements. The very small asymmetry between tension and compression stresses reflects a lack of strain localization beyond the initial cycles. The trimodal and 85/15 pct CM/UM alloys have similar and superior low cycle fatigue strength compared to AA5083-H131. From the Coffin-Manson plot, the trimodal material has a shorter fatigue life than 85/15 pct CM/UM alloy and AA5083-H131 for high plastic strain amplitudes, but nearly identical life at low amplitudes. Microcracks were observed near the dominant crack on trimodal specimen surfaces at failure. Back-scattered images revealed that particulates altered the crack propagation direction; cracks nearly always propagated around particulates.

Influence of dwell times on microstructure, deformation and damage behavior of NiCr22Co12Mo9 under thermomechanical fatigue

Thermomechanical fatigue (TMF) tests under in-phase (IP) and out-of-phase (OP) conditions with continuous and dwell time cycles were conducted on wrought Ni-base alloy NiCr22Co12Mo9 (comparable to Inconel Alloy 617). The temperature range was T = 100–850 °C. In dwell time tests, dwells of 5 min were introduced at Tmax = 850 °C. In continuous cycle tests, the material shows significant cyclic hardening, mainly because finely dispersed carbides of type M23C6 form inside the grains. Introducing dwell times leads to coarsening of these carbides, which reduces cyclic hardening leading to lower stress amplitudes and higher plastic strain amplitudes than in continuous cycle tests. Serrated yielding indicating dynamic strain ageing (DSA) effects occur prominently in dwell time tests and less pronounced in continuous cycle tests. The DSA effects appear to be closely related to formation and growth of the M23C6 carbides. IP and OP loading lead to comparable microstructural evolution and deformation behavior. Damage in form of cracks arises under IP loading predominantly intergranularly and under OP loading mainly transgranularly. Dwell times do not affect the dominant damage form. For both continuous cycle and dwell time tests, IP lifetime is shorter than OP lifetime indicating that intergranular cracks propagate faster than transgranular cracks. Introducing dwells unexpectedly increases the lifetime under IP loading, while OP lifetime is only slightly affected. The lifetime behavior is discussed based on the observed microstructural and damage evolution.

Ultrafine versus coarse grained Al 5083 alloys: From low-cycle to very-high-cycle fatigue

The fatigue performance of coarse and ultrafine-grained (UFG) 5083 Al alloy were compared, from low to very high cycle fatigue. The UFG alloy exhibited cyclic hardening and predominant kinematic hardening. At high plastic strain amplitude (and only in this regime), it showed easier crack initiation and a lower fatigue resistance. Its resistance to HCF was hardly better than that of its coarse grained counterpart until 2.106 cycles, but 43% higher in VHCF, until 5.108 cycles. Beyond that point, internal crack initiation occurred, and the fatigue resistance of the UFG material decreased, which was explained using Fracture Mechanics.

Cyclic deformation behaviors of a high strength carbide-free bainitic steel

The cyclic deformation behaviors of low-temperature bainite, lower bainite and upper bainite obtained on a high-strength carbide-free bainitic steel were examined through low-cycle fatigue testing. The relationship between the bainitic microstructure and fatigue behavior was systematically studied using scanning electron microscopy, transmission electron microscopy, atom probe technology, and electron back-scattered diffraction analyses. The results show that low-cycle fatigue undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. The low-temperature bainite exhibits a long fatigue life under the total strain amplitudes, because of its high strength and larger high-angle misorientation distribution of the packets of bainitic ferrite plates. The low-temperature bainite also presents bilinearity in the Coffin–Manson plots. Coordinate deformation and high-uniform elongation lead to prolonged fatigue life at low plastic strain amplitude, whereas phase component primarily affects the fatigue life at high plastic strain amplitude. Highly stable film-like retained austenite is beneficial to arresting fatigue crack propagation. Blocky type retained austenite easily transforms to martensite, resulting in high compatible deformation capability. The hardening ability of the low-temperature bainite is higher than that of the upper bainite, which is attributed to its high pre-existent density of dislocations transformed from movable to immovable during initial hardening stage.

Ultra‐low cycle torsion fatigue of annealed copper

ABSTRACTTorsion experiments show that pure annealed copper is able to withstand very high plastic strain amplitudes when it is loaded cyclically with less than 30 cycles to failure. Under these ultra‐low cycle fatigue conditions, the performance of copper is significantly better than that of the annealed steels A36 and AISI 304, which were also tested in this study for comparison. The dependence of fatigue life on strain range can be described by a power law. In the case of an initial overloading, fatigue life can be estimated using the Palmgren–Miner rule. The long low cycle fatigue life of copper is explained by a thermally activated softening mechanism which takes place while the material heats up as a result of the cyclically repeated plastic deformation. The softening is accompanied by a change in microstructure. The low cycle fatigue properties of copper can be utilized for designing hysteretic dampers for seismic protection.

Experimental and simulation analysis in edge cracking for cold rolled thin strip

ABSTRACTEdge cracks in rolled metal products occur occasionally, which influence the rolled strip properties and productivity significantly. Previous studies have indicated that the crack initiation at high plastic strain amplitude mainly occurs at the grain boundaries and stress concentration zone. The mechanics of crack propagation is still not fully identified, especially in the cold rolled thin strip. In this research, experimental investigation and associated analysis have been conducted on the edge crack evolution for thin strip in cold rolling. Three‐dimensional finite element numerical simulations have been conducted to assist in better understanding of crack propagation at the rolled strip edge. The impacts of the reduction and the friction coefficients on crack propagation are analysed. Simulation result shows that the thickness reduction and friction influence significantly the propagation of edge cracks. The optimum rolling schedule to obtain crack‐free products in metal forming of thin strip is discussed.

Formation of micro shear bands during cyclic deformation of sub-microcrystalline nickel

Cyclic deformation of sub-microcrystalline nickel at high plastic strain amplitude generated macro shear bands, causing fatal cracking. The macro shear bands consisted of several micro shear bands, each band containing a single layer of elliptical grains that appeared at less than 50° with respect to the loading axis. Micro texture investigations in micro shear bands revealed almost the same texture with similar shear values as in macro shear bands. During cyclic deformation local overlapping of evolving micro shear bands resulted in formation of fatal macro shear bands.

Microstructure of austenitic stainless steels of various phase stabilities after cyclic and tensile deformation

Abstract The microstructures of two metastable high-alloyed CrMnNi cast TRIP steels and a stable AISI 316L austenitic stainless steel were studied in detail after tensile and cyclic deformation. Electron backscattered diffraction was employed to localize the martensitic phase transformation and electron channelling contrast imaging to describe the typical dislocation arrangements. These were complemented by transmission electron microscopy and by scanning transmission electron microscopy performed in a scanning electron microscope. The TRIP steel with the lowest austenite stability shows a more pronounced martensitic phase transformation realized from the austenite via the intermediate formation of ∊-martensite. Martensitic phase transformation also occurred in the stable 316L austenitic stainless steel with a small volume fraction of α′-martensite, but only with cyclic deformation at low temperatures and/or at very high plastic strain amplitudes.

Cyclic behavior of 316L steel predicted by means of finite element computations

The cyclic behavior of 316L steels is predicted based on crystalline elastoplastic constitutive laws. Calculations are per-formed with the finite element software CAST3M, using a polycrystalline mesh where the individual grains are modeled as cubes, having random crystallographic orientations. At the grain scale, the constitutive law parameters are adjusted using single crystal cyclic stress strain curves (CSSCs) from literature. Calculations are performed for different loadingconditions (uniaxial tension-compression, biaxial tension-compression and alternated torsion) and a large range of three remote plastic strain amplitudes. We obtained 3 close macroscopic CSSCs. Somewhat lower stresses are obtained in torsion, particularly at high plastic strain amplitude. Our results are in agreement with all the published experimental data. The mean plastic strain is computed in each grain, yielding a particular polycrystalline mean grain plastic strain distribution for each loading condition and remote plastic strain.The plastic strain scatter increases for decreasing ma-croscopic strains. The number of cycles to the first micro-crack initiation corresponding to the aforesaid plastic strain distributions is then calculated using a surface roughness based initiation criterion. The effect of the different loading conditions is finally discussed.

Stability of austenitic 316L steel against martensite formation during cyclic straining

Solution-annealed AISI 316L steel was fatigued with constant plastic strain amplitudes at room temperature and under various conditions at depressed temperatures down to 113K to reveal its stability against deformation-induced martensite formation. Microstructural changes induced by fatigue were characterized by transmission electron microscopy (TEM), electron channeling contrast imaging (ECCI) and electron backscattering diffraction (EBSD) techniques. Neutron diffraction and magnetic induction method were adopted for quantification of martensite content. Deformation-induced martensite formation in the bulk of material was evidenced for low temperature cyclic straining under various conditions. Room temperature cycling, even with high plastic strain amplitudes, results in a local very limited martensite formation in areas closely linked with the long fatigue crack growth.

Shear banding during cyclic deformation of sub-microcrystalline nickel

Sub-microcrystalline nickel produced by pulsed electrodeposition was investigated during cyclic deformation at high plastic strain amplitude. After a certain number of loading cycles the specimen developed a “macro” shear band at 45° with respect to the tensile axis, which acted as a crack initiator. Electron backscatter diffraction revealed that the shear band consists of relaxed grains elongated along the shear plane, showing a typical shear texture. The shear band texture was reproduced with the viscoplastic self-consistent polycrystal model using {1 1 1}〈1 1 0〉 slip systems.

High Temperature Low-Cycle Fatigue, Deformation Microstructure and Final Fracture Behavior of a Nickel-Base Superalloy

The low-cycle fatigue (LCF) properties of a nickel-base precipitation-strengthened superalloy (GH4145/SQ), obtained at a temperature of 538 o C, were reported and discussed in this paper. The properties investigated include cyclic stress response, fatigue life, deformation microstructure and final fracture features as a function of applied strain amplitude. It was shown that the alloy exhibited a pronounced initial hardening followed by continuous softening to failure at high plastic strain amplitudes ( > 0.2% ap ε ), while at low plastic strain amplitudes ( < 0.2% ap ε ) the initial hardening was followed by a well-defined saturation stage. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2% was observed in the cyclic stress-strain (CSS) and Coffin-Manson (C-M) plots. TEM observations revealed that slip band density increased with increasing total strain amplitude and precipitate degradation resulting from dislocation-precipitate interactions took place with continuous cyclic straining. The change in the microstructure during cycling is thus responsible for the fatigue hardening / softening behavior of the alloy. SEM examinations indicated that at low plastic strain amplitudes ( < 0.2% ap ε ) crack propagation was basically transgranular, while at high plastic strain amplitudes ( > 0.2% ap ε ) crack propagation exhibited intergranular features, as a whole. The variation in both the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain the observed two-stage LCF behavior of the present investigated superalloy.

Role of nitrogen in the cyclic deformation behavior of duplex stainless steels

The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.

Dislocation mechanisms of mean stress effect on cyclic plasticity

Abstract The dislocation mechanisms behind sagging behavior of autosuspension spring steels are far from being understood due to their complicated microstructures. In this study, systematic cyclic loading tests were carried out on two model materials - industrial pure iron and spheroidized 1045 steel – to understand the mechanisms behind sagging. It is found that it is the different dislocation microstructures that control the cyclic creep behavior of different materials. With iron samples with low mean stress values, higher mean stresses do not trigger cell structure formation mainly due to the little requirement for high plastic strain amplitude and hence the lack of necessity for high dislocation density, and a large number of dislocation interactions through large mobile dislocations. In the case of the 1045 steel samples, the collapse of pre-existent substructures and the movement of newly generated mobile dislocations are the main reasons for the cyclic softening observed. The massive reorganization process of dislocation configurations and the competition between softening and hardening both carry on throughout the entire cycling process, but the softening process is found to be the predominant factor.

Fatigue deformation-induced response in a superduplex stainless steel

The cyclic deformation behavior of SAF 2507 superduplex stainless steel (SDSS) was studied under constant plastic-strain amplitudes. The cyclic hardening/softening curves show initial hardening, followed by softening and, finally, saturation behavior. Two regimes can be differentiated in the cyclic stress-strain curve (CSSC) of SDSS. The transition point at which the cyclic strain-hardening rate changes is identified to be ɛ p/2=7 × 10−3. Transmission electron microscopy (TEM) results on dislocation structures suggested that there is a close relationship between the CSSC, hardening/softening curves, and the dislocation substructure evolution. In the low-plastic-strain-amplitude regime of the CSSC, the dislocation activity in the austenite grains is found to be higher than that in the ferrite grains. At higher plastic strain amplitudes, low-energy dislocation structures are found in the ferrite grains, while clusters and bundles of dislocations can be observed in the austenite grains. Strain localization due to formation of these structures resulted in a decrease in the cyclic strain-hardening rate within the high-plastic-strain-amplitude regime. Dislocation substructure evolution is also used to explain the shape of the hardening/softening curve.

Cyclic stress-strain behavior of 9Cr1Mo steel at positive mean stress

Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load controlled cycling with positive mean stresses. The results were compared with data determined under symmetrical cycling. Cyclic hardening followed by softening was found to be inherent to cycling at positive mean stress contrary to the symmetrical loading where only continuous softening was observed. For a given stress amplitude, higher mean stress results in higher plastic strain amplitude. The onset of cyclic softening and the onset of cyclic creep acceleration occur simultaneously. In the region of cyclic softening a unique relation between the instantaneous cyclic creep rate and the instantaneous plastic strain amplitude was found for cycling at constant mean stress.

Cyclic deformation of ferritic stainless steel single crystals showing the stress asymmetry

Fe–20%Cr and Fe–30%Cr alloys single crystals having ( 1 ̄ 2 ̄ 3) maximum resolved shear stressed (mrss) plane were cyclically deformed under constant plastic strain amplitudes ε pl of 2.5×10 −3 and 1.5×10 −4. It was found that the tensile stress amplitude was larger than the stress amplitude in compression at the beginning of cyclic deformation. The stress asymmetry was large at higher cyclic frequency and at higher plastic strain amplitude. During cyclic deformation, the stress asymmetry gradually decreased with increasing number of cycles, especially at ε pl=2.5×10 −3. The changes in internal dislocation process are discussed based on a model that attributes the stress asymmetry to the asymmetric behavior of the screw dislocation.

Cyclic saturation and fatigue fracture of tungsten monofilament-reinforced multicrystalline copper composites

Studies on cyclic saturation and fatigue fracture behavior have been conducted on tungsten monofilament-reinforced multicrystalline copper composites. A model was proposed to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. The results calculated by the model show very good agreement with the experimental data at all applied strain amplitudes by which the composites were fatigued. The fatigue fracture mechanism of the composites highly depends on applied plastic strain amplitudes. Most cracks initiate at grain boundaries in the matrix, and propagate toward and break the fiber at low and intermediate plastic strain amplitudes. At a high plastic strain amplitude, fatigue cracks initiate at the fiber even before surface cracks become pronounced, and propagates from inside outwards at a high growth rate.

Fracture behavior of monofilament-reinforced multicrystalline copper composites

Studies on monotonic tensile fracture and fatigue fracture have been performed on tungsten monofilament-reinforced multicrystalline copper composites and the failure modes are reported. Monotonic tensile fracture is not very sensitive to the initial fiber breaks and grain boundaries in tungsten monofilament-reinforced multicrystalline copper composites. Multiple breaks of the fiber and multiple necking of the matrix near the fiber segment breaks, commonly observed in tensile tests of composites, are reported here. However, the fatigue fracture is very sensitive to a fiber break, and to microstructural features such as grain boundaries and dislocation structures. Once the fiber breaks into two segments, the composite will quickly fail. The fatigue fracture mechanism of the composites depends on applied plastic strain amplitudes. Most cracks initiate at grain boundaries in the matrix at low and intermediate plastic strain amplitudes and the fiber greatly improves the fatigue behavior. At a high plastic strain amplitude, fatigue cracks initiate at the fiber, and the fiber seems ineffective for improving the fatigue life.