Low Plastic Strain Amplitudes Research Articles

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.

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.

Influence of mean stress and light water reactor environment on fatigue life and dislocation microstructures of 316L austenitic steel

Influence of mean stress on fatigue life of the austenitic stainless steel 316 L in air and light water environments (boiling water reactor/hydrogen water chemistry) at 288 °C was determined with a series of tests carried out in load-control mode. Fatigue life was found to increase with application of compressive and tensile mean stress in air and light water reactor environments. Secondary hardening was regarded as the main reason for this behavior. A modified Smith-Watson-Topper (SWT) model was considered to account for mean stress and was shown to predict fatigue life accurately in air and water environments. The reduction of fatigue life in water environment, determined with the SWT curves, was about 2.5. Observations of the end-of-life dislocation arrangements by transmission electron microscopy showed that the dislocation microstructure depends essentially on plastic strain amplitude, which in turn is strongly correlated to stress amplitude and mean stress. The microstructures were found consistent with those usually observed after strain-controlled experiments. At rather low plastic strain amplitudes, corduroy structure consisting of small dislocation loops was observed. Acting as significant obstacle to dislocation motion, corduroy structure affects overall dislocation mobility therefore contributing to notable secondary cyclic hardening.

Influence of loading frequency and role of surface micro-defects on fatigue behavior of metastable austenitic stainless steel AISI 304

High frequency fatigue testing always provokes questions about whether the testing conditions are representative for the real application conditions. Therefore, any influences coming from the testing procedure must be known and understood before relying on the fatigue data based on high frequency testing for component design and validation phases. For this reason and because metastable austenitic steels are well known for their strain rate sensitivity (Müller-Bollenhagen, 2011; Sorich, 2016), the steel AISI 304 and the role of surface micro-defects produced by laser beam cutting were analyzed regarding the influence of load frequency on the cyclic response and fatigue behavior, and the findings of the investigation are thoroughly discussed in this paper. Fatigue tests were performed at load frequencies of 100Hz and 1000Hz using two resonance pulsation test systems, as well as by means of a servo-hydraulic test machine at 1Hz and 50Hz. All fatigue experiments were performed at tensile-tensile loading condition (R=0.1). The cyclic deformation behavior was characterized based on the evaluation of stress-strain hysteresis loops and temperature measurements. The deformation-induced phase transformation from γ-austenite to α′-martensite was globally and locally evaluated by means of magneto-inductive measurements and EBSD analysis, respectively. Furthermore, for the first time it was possible to compare test results generated by a 1000Hz resonance pulsation test system with results from a conventional resonance test stand cycling at around 100Hz. The analyses showed that higher amounts of α′-martensite and lower plastic strain amplitudes are observed when the cyclic experiments are carried out at lower frequency, promoting higher fatigue strengths. Nevertheless, the influence of test frequency for specimens containing surface micro-defects is dominant in the low cycle fatigue (LCF) regime while in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) range the fatigue life determining parameter is the severity of the micro-notches present along the laser cut surface.

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.

Fatigue behaviour and crack growth rate of cryorolled Al 7075 alloy

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life, and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stress controlled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing ΔK) technique. The microstructural characterization of the alloy was carried out by using Field emission scanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloys showed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa, 530 Mpa, 21 J, 24 Mpa m1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppression of dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. The cryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy due to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been found be higher, especially at higher values of applied stress intensity factor ΔK The reasons behind such crack growth retardation is due to diffused crack branching mechanism, interaction between a propagating crack and the increased amount of grain boundaries (GB), and steps developed on the crack plane during crack-precipitate interaction at the GB due to ultrafine grain formation.

Dependence of the distribution of deformation twins on strain amplitudes in an extruded magnesium alloy after cyclic deformation

Cyclic deformation characteristics and the distribution of deformation twins in a recently developed magnesium alloy AM30 after strain-controlled low cycle fatigue tests were studied in the loading direction along the extrusion direction. The alloy was basically cyclically stable at a lower strain amplitude of 0.2% with low stress amplitude and small plastic strain amplitude, while it exhibited strong cyclic hardening at a higher strain amplitude of 0.6%. In addition to the initially high stress amplitude and large plastic strain amplitude, the stress amplitude increased and plastic strain amplitude decreased significantly as cyclic deformation proceeded at the higher strain amplitude. Twinning in compression and detwinning in tension occurred at both strain amplitudes, and the distribution of the residual twins observed near the fracture surface was a consequence of competition between twinning and detwinning, depending on the strain amplitude, cyclic hardening–softening characteristics, and the plastic zone size ahead of the crack tip. In the stable cyclic deformation at the lower strain amplitude, deformation twins appeared mostly in the area prior to the final rapid fracture, while at the higher strain amplitude with cyclic hardening the extensive and well-developed twins were mainly observed in the crack initiation area.

Characterization of dislocation structures in copper single crystals using electron channelling contrast technique in SEM

AbstractThe dislocation structures induced by low‐plastic‐strain‐amplitude cyclic deformation of [111] multiple‐slip‐oriented Cu single crystals were investigated using electron channelling contrast (ECC) technique in scanning electron microscopy (SEM). At a low plastic strain amplitude γpl of 8.8 × 10–5, the saturated dislocation structure is mainly composed of labyrinth‐like vein structure (or irregular labyrinths), and the cyclic hardening behavior at such a low γpl is interpreted as being the result of dislocation multiplication by a Frank‐Read mechanism. As γpl increases to 4.0 × 10–4, the unsaturated dislocation structure exhibits two kinds of distinctive configurations, i.e., dislocation walls and misoriented cells. Interestingly, these misoriented dislocation cells are strictly aligned along the primary slip plane (111), constituting a unique persistent slip band (PSB) structure. Here, these cells are thus called PSB cells. In addition, there is a locally distinctive region comprising some cells having a recrystallization‐like feature in the whole structure of PSB cells. The formation of the structure of PSB cells is discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of fatigue dislocation structures in [233] coplanar double-slip-oriented copper single crystals using SEM electronic channelling contrast

Abstract Dislocation structures in fatigued [] coplanar double-slip-oriented copper single crystals were studied using the electron channelling contrast technique in scanning electron microscopy, especially focusing on the observations of the microstructures of persistent slip bands and deformation bands. It was found that, at low plastic strain amplitudes (γpl ≤ 3.4 × 10– 4), the dislocation structures of the [] copper single crystal comprise a majority of elongated veins and a few elongated cells along the primary slip plane (111). As γpl increases to 6.2 × 10– 4, a special persistent slip band structure consisting of irregular dislocation cells, rather than generally-observed ladders in single-slip crystals, was found to form, which is responsible for the occurrence of a quasi-plateau in the cyclic stress – strain curve of the [] crystal. When γpl ≤ 3.5 × 10– 3, different dislocation microstructures in deformation bands, such as wall-like features, dislocation walls, elongated cells and dense dislocation cells, were found in the cyclically saturated [] crystal. Effects of the crystallographic orientation, the applied plastic strain amplitude, the accumulated plastic strain and the strain localization in the crystal on the microstructure of deformation bands are summarized and discussed.

Cyclic plasticity of single crystal nickel oriented for single slip

The cyclic deformation behavior of single crystal nickel was investigated by performing uniaxial fully reversed constant plastic strain amplitude fatigue experiments at plastic shear strain amplitudes ranging from 1.1 × 10 −4 to 8.8 × 10 −3. Digitally acquired stress–strain hysteresis loops were used to calculate friction and back stresses and to relate the shapes of the loops to the evolving dislocation structures and magnetomechanical effects. The results indicate that the cyclic stress–strain curve exhibits a plateau of 50 MPa between plastic strain amplitudes of 1 × 10 −4 and 7 × 10 −3. Saturation friction stress, calculated using the Cottrell method, is reasonably constant over the entire range of plastic strain amplitudes at a value of 15 MPa. The back stress is 35 MPa within the plateau and increases to 48 MPa at the highest strain amplitude. When cycled at low plastic strain amplitude, magnetomechanical effects account for a significant portion of the measured inelastic strain.

Modeling aspects of low plastic strain amplitude multiaxial cyclic plasticity in conventional and ultrafine grain nickel

Aspects of the cyclically saturated responses of initially annealed, conventional grain size (average grain diameter of approximately 50 μm) and electrodeposited, ultrafine grain (grains from 20 to 500 nm) nickel to reversed proportional and 90° out-of-phase axial-torsional, strain-controlled cycling at a nominally constant equivalent inelastic strain amplitude of approximately 100 μ strain are reported. An anisotropic, axial-torsional subspace version of Abdel-Karim and Ohno’s kinematic hardening model is presented. Within the framework of conventional small strain, rate-independent plasticity, this approach is used to model the responses. An anhysteretic, phenomenolically based, magnetomechanical model is coupled to the rate-independent plasticity model to include the cyclic magnetostriction response. The kinematic hardening parameter determination scheme, using the proportional path responses, is described. The model correlations achieved are presented and the ability of the resulting models to capture the 90° out-of-phase responses is examined. The model parameter sets, as determined from the proportional responses, require small changes to result in more accurate correlation of the 90° out-of-phase responses and the implications of this are discussed. The relative values of the model parameters between the two materials reflect the initial microstructures. Persistent mean stresses associated with mean total strains imposed are successfully modeled for the proportional strain path responses but not for the 90° out-of-phase responses.

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.

Cyclic plasticity of soft oriented NiAl single crystals at 600 K

Cylindrical specimens of stoichiometric NiAl single crystals with axial orientations near [ 2 ¯ 2 3 ] in the stereographic unit triangle were cyclically loaded at 600 K, imposing low plastic strain amplitudes. A main aim was to study the slip phenomena, involving type and persistence of strain localisation, as well as the dislocation patterns occurring after cycling at ɛ pa = 2.5 × 10 −4. Single slip is dominant in the [0 0 1]( 1 ¯ 1 0 ) system on a macroscopic level. Broad cumulative slip bands appear after a sufficiently large number of cycles. Within these bands the strain is considerably concentrated in individual zones of localised slip, although the average strain carried by all of these slip zones is rather small. During a quarter cycle, the slip is highly localised in a small number of thin slip bands. Atomic force microscope (AFM) measurements show that there exists a broad spectrum of local strain amplitudes (LSAs) reaching up to 100%. The mean strain of the LSAs is more than a factor of 150 larger than the average strain of the cumulative zones of localised slip. The cumulative slip bands are not cyclically persistent. The dislocation patterns in a zone of localised slip (ZLS) characteristically differ from those formed in the surrounding matrix. Multiple slip has to be taken into account on a micrometer-scale in order to understand the ZLS dislocation substructure.

Finite element simulation of PSB macroband nucleation and propagation in single crystal nickel cycled at low plastic strain amplitudes

Substructure models for vein matrix and persistent slip band (PSB) structures are extracted from a uniaxial mixtures model that was developed to simulate cyclic loading experiments on nickel single crystals oriented for single slip. Reverse magnetostriction is included as well. These substructure models are implanted in a single crystal plasticity framework with fully anisotropic elasticity. The resulting constitutive models are incorporated in finite element models to simulate the process of PSB macroband formation and propagation. Perturbation elements (PEs), elements assigned with PSB properties, are used as the loci for PSB macroband nucleation. Transition of elements with vein matrix properties to elements with PSB properties is triggered at integration points by a shear stress criterion applied on slip systems. The resulting finite element models successfully demonstrate the process of PSB formation and propagation, and plastic strain amplitude partitioning between vein matrix and PSB macrobands. The effect of model boundary constraints, strain increment dependence, mesh sensitivity, PE distribution, specimen axis misorientation, and PSB volume fraction generated is examined.

A substructure mixtures model for the cyclic plasticity of single slip oriented nickel single crystal at low plastic strain amplitudes

The cyclic plasticity behavior of nickel single crystals oriented for single slip is characterized by uniaxial, symmetric, tension–compression, strain controlled tests carried out at constant plastic strain amplitudes ranging from 5(10 −5) to 1(10 −3). Annealed single crystals are cycled in this manner to post-cyclic saturation and microstructural characterizations, including transmission electron microscopy and optical micrographs of specimen surface replicas are used to verify and evaluate dislocation substructures. Stress–strain and microstructure data are used to construct a mixtures model that couples cyclic plasticity models for three substructures as well as a model for reverse magnetostriction (Villari effect) that is a significant component of inelastic strain at the lower plastic strain amplitudes. The model is used to correlate the stress–plastic strain hysteresis loop responses over the range of plastic strain amplitudes and from cumulative plastic strains from 0.3 to post-cyclic saturation. Complex evolution of substructure plastic strain amplitudes toward their so-called intrinsic values upon the formation of persistent slip bands is modeled. Additionally, bulk Young’s modulus is found to vary significantly with plastic strain amplitude and cumulative plastic strain. A correlation of this behavior is included.

Orientation dependence of dislocation structures in cyclically deformed Cu-16 At. Pct Al alloy single crystals

The dislocation structures induced by the cyclic deformation of Cu-16 at. pct Al alloy single crystals oriented, typically, as [(1) over bar 23] for single slip and [023] and [(1) over bar 17] for double slip, were studied by transmission electron microscopy (TEM) and compared with the results of Cu single crystals. Completely unlike the dislocation structures of Cu single crystals of corresponding orientations, the dislocation structures of these oriented Cu-16 at. pet Al alloy single crystals show a typical planar morphology. As the applied-plastic-stain amplitude increases, the dislocation configuration changes, on the whole, from multipolar arrays to dislocation tangles in the primary slip plane and front low-density planar slip bands to well-developed persistent Luder's bands (PLBs) in the planes normal to the primary slip plane, respectively. Secondary slips can be clearly observed to activate from very low plastic-strain amplitudes in all three Cu-16 at. pet Al single crystals investigated. Interestingly, the crystallographic orientation has almost no effect on the dislocation structure of Cu-16 at. pct Al single crystals.

Influence of Fully Reversed Stress/Strain-Controlled Cycling on the Transformation Behavior of Ti-49.6Ni Shape Memory Alloy

The transformation behavior of Ti-49.6Ni shape memory alloy was investigated under fully reversed stress/strain cycling using electrical resistance measurement. The results showed, that there was no R-transformation in the original alloy and that R-transformation occurred after stress/strain cycling. The R-phase was induced by a rhombohedral distortion of the austenite in the transition process from the high-temperature phase, also called parent phase, to the low-temperature phase, the martensite, and was characterized well by the sharp increase of the electrical resistivity. The austenite start- and finish-transformation temperatures A(s) and A(f) could be observed clearly in the original alloy, whereas they appeared less clearly after stress/strain cycling. Under plastic strain-amplitude controlled cycling with Deltaepsilon(p) = 0.05%, the transformation temperature M-s remained stable between 10(3) to 10(4) cycles. Finally we assumed that low plastic strain-amplitude controlled cycling itself, which could not do any damage to the material, might be used as a better kind of mechanical training method in comparison with the thermo-mechanical training for a stabilization of the transformation temperatures for Ti-49.6Ni shape memory alloy.

Cyclic plasticity of nickel at low plastic strain amplitude: hysteresis loop shape analysis

The cyclic plasticity of nickel was studied by accomplishing room temperature fully-reversed fatigue experiments at constant plastic strain amplitudes of 1.0×10 −4 and 2.5×10 −4 on single crystal and polycrystal nickel with 290 μm grain size. The cyclic plasticity behavior within a hysteresis loop was analyzed by measuring shape characteristics of the loop. Parameters that were evaluated include the loop shape parameter, friction stress, back stress, and second derivative of stress with respect to total strain. Results indicate that these parameters correlate well with the development of dislocation substructures. In addition, at low plastic strain amplitude, polycrystal and single crystal hysteresis loops exhibit a distinct constriction. The constriction is more pronounced in single crystals cycled at the lower plastic strain amplitude.

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.

Cyclic plasticity of polycrystalline nickel at low plastic strain amplitude: constricted hysteresis loops

Cyclic plasticity of polycrystalline nickel at low plastic strain amplitude: constricted hysteresis loops