Cyclic Stress-strain Response Research Articles

Damage mechanism in austenitic steel during high temperature cyclic loading with dwells

Austenitic stainless Sanicro 25 steel has been subjected to high temperature elasto-plastic cyclic straining at 700 °C. The isothermal cyclic tests without and with a dwell in the loading cycle were implemented. Several strain amplitudes and optional dwells of 10 min in tensile part of the loading cycle were applied to the specimens. Cyclic stress-strain response was recorded, cyclic hardening/softening curves and fatigue life curves were obtained. Scanning electron microscopy combined with focused ion beam allowed observing sections containing secondary cracks. They were used to identify the surface damage evolution. In order to investigate the internal damage the longitudinal cross sections parallel to the stress axis of the fatigued specimens were prepared. The crack paths and their relation to the grain and twin boundaries were studied by means of electron back scatter diffraction. Several distinct mechanisms of the crack initiation were found in both types of cyclic loading. The effect of the dwell in the loading cycles on fatigue life is discussed in relation to the surface and internal damage evolution.

Modeling the cyclic shape memory and superelasticity of selective laser melting fabricated NiTi

Shape memory alloys (SMAs) have found several industrial and biomedical applications due to their superior mechanical and biological properties. Since SMA devices may experience several cyclic loadings during their function, assessment of their cyclic response is of vital importance. In this paper, an existing constitutive model based on small strains is generalized to investigate cyclic response in superelastic and shape memory regimes of additively manufactured NiTi. To do so, Microplane theory is utilized to calculate the elastic and transformation strain tensors while the associated flow rule is used in combination with linear isotropic hardening for plastic strain tensor. In this model, all the material parameters can be obtained using DSC, the stress-strain response in the first loading-unloading cycle, and the accumulated stress-strain curve. To validate the model, superelastic and shape memory NiTi compression samples are fabricated using selective laser melting. All the specimens are tested in stress-controlled cyclic tests at a constant temperature. The obtained cyclic stress-strain response is compared with the numerical results. The model predicts and the experimental results verify that the residual strain, peak strain, and dissipation energy converge to specific values.

Cyclic deformation behaviour and stability of grain-refined 301LN austenitic stainless structure

Low cycle fatigue (LCF) behaviour of metastable austenitic 301LN stainless steel with different grain sizes – coarse-grained (13 μm), fine-grained (1.4 μm) and ultrafine-grained (0.6 μm) – produced by reversion annealing after prior cold rolling was investigated. Fully symmetrical LCF tests with constant total strain amplitudes of 0.5% and 0.6% were performed at room temperature with a low constant strain rate of 2×10-3 s-1. Microstructural changes in different positions within the gauge part of the specimens were examined by optical microscopy (polarized light) and electron backscatter diffraction (EBSD) technique; for quantitative assessment of the volume fraction of deformation induced martensite (DIM) a Feritscope FMP 30 was adopted. The cyclic stress-strain response and specific changes of hysteresis loop shapes in the very early stage of cycling are confronted with the character of DIM formation and its distribution in the whole volume of the material. A possible effect of strain rate (frequency of cycling) on the destabilization of austenitic structure during cyclic straining of materials with different grain sizes is highlighted.

Cyclic stress and strain responses of AZ31 magnesium alloy sheet metal at elevated temperatures

Lightweight materials have been widely and increasingly utilized for the automotive industry to reduce the carbon dioxide (CO2) emissions. To replace the existing ordinary steels, the automotive companies have been investigated the other materials to satisfy the specific strength, stiffness and recyclability. Under these circumstances, some researchers have been paying attention to the magnesium alloy sheets. Although the material have low ductility at room temperature due to small number of slip systems, it is the well-known fact that the formability at the elevated temperatures around 200 ºC is drastically improved. In these days, the local heating techniques have been applied to the press forming systems in order to realize the power saving manufacturing systems in spite of the existing systems. Although the warm stamping has some great advantages to improve the formability, the prediction of the deformed sheet metal is not always enough accurate. In order to investigate the stress strain responses for the material at the elevated temperatures, a testing device which can observe the cyclic stress strain responses was developed in the present research. The magnesium alloy sheets were warmed at around 200 ºC to reduce the critical resolved shear stress of slip systems and improved its deformability. The corresponding stress strain responses by finite element method based on the crystal plasticity were calculated. It was found that the calculations could capture the above-mentioned features very well.

Thermomechanical fatigue – Mechanism-based considerations on the challenge of life assessment

The combination of cyclic mechanical and cyclic thermal loading leads to thermomechanical fatigue (TMF) which is considered to be the primary life-limiting factor for engineering components in many high-temperature applications. Extensive low-cycle fatigue (LCF) data, which is traditionally used for design purposes, has been generated isothermally on various high-temperature materials, and thus, it is tempting to try to predict TMF life based mainly on isothermal LCF data. In this contribution, studies on different metallic structural high-temperature materials, which have mainly been carried out in the author's laboratory, are reviewed addressing the question, in which way and to which extent a reliable, unerring and robust TMF life assessment is possible on the basis of isothermally obtained fatigue life data. It is shown by means of examples that a sound TMF life prediction first of all requires a detailed mechanistic understanding of the isothermal cyclic stress-strain response and the relevant damage mechanisms. Furthermore, the TMF-specific peculiarities in both the non-isothermal cyclic stress-strain behaviour and the non-isothermal damage evolution process must be known. If all these requirements are fulfilled and reflected in the TMF life assessment methodology applied, a reasonable predictive accuracy can be attained.

Asymmetric Cyclic Stress-Strain Response in a Stainless Steel Processed by ECAP

Asymmetric Cyclic Stress-Strain Response in a Stainless Steel Processed by ECAP

Study of springback effect of AHS steels using a microstructure based modeling

In sheet metal forming process of automotive parts, springback effect is crucial, in particular, for advanced high strength (AHS) steels, by which local interactions between different phases play an important role on macroscopic response. Otherwise, most structural components with highly complex shapes required multi–step forming procedures. Better understanding and more precise description of plastic deformation behavior and springback occurrence of such formed steel sheets are thus necessary. In this work, the springback of AHS steel grades 780 and 1000 was investigated, for which tension–compression tests were firstly carried out. Then, finite element (FE) modeling was done on both macroscale and microscale using representative volume elements (RVEs) in order to predict the cyclic stress–strain responses. Hereby, the kinematic hardening Yoshida–Uemori (Y–U) model was applied to consider the elastic recovery of material. The material parameters of the Y–U model were identified for the investigated steels as well as their phase constituents. It was verified that the calculated flow stress curves fairly agreed with the experimental ones. Afterwards, hat–shape stamping tests were performed and the results showed that occurred springback angles determined from the experiments and FE simulations were in accordance. Additionally, RVE simulations were conducted for local area of deformed sheet sample. The plastic deformation of various phases in the microstructure of examined steels under load–unloading behavior during the forming process was characterized. This approach can be further used to incorporate the effects of microstructure on the springback effect of AHS steel sheet.

A kinematic hardening constitutive model for the uniaxial cyclic stress–strain response of magnesium sheet alloys at room temperature

A kinematic hardening constitutive model is presented, in which a modified form of von Mises yield function is adopted, and the initial asymmetric tension and compression yield stresses of magnesium (Mg) alloys at room temperature (RT) are considered. The hardening behavior was classified into slip, twinning, and untwinning deformation modes, and these were described by two forms of back stress to capture the mechanical response of Mg sheet alloys under cyclic loading tests at RT. Experimental values were obtained for AZ31B-O and AZ31B sheet alloys under both tension–compression–tension (T–C–T) and compression–tension (C–T) loadings to calibrate the parameters of back stresses in the proposed model. The predicted parameters of back stresses in the twinning and untwinning modes were expressed as a cubic polynomial. The predicted curves based on these parameters showed good agreement with the tests.

Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2s and 200s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed.

Unexpected cyclic stress-strain response of dual-phase high-entropy alloys induced by partial reversibility of deformation

The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation.

High temperature low cycle fatigue and creep-fatigue behavior of a casting Al-9Si-CuMg alloy used for cylinder heads

The low-cycle fatigue test and the compression creep-fatigue test of the cast Al-9Si-CuMg alloy for cylinder heads were carried out at high temperature. The cyclic mechanical response characteristics of the material under two kinds of test loads were analyzed. The microstructural evolution process and fracture mechanism of the material in the process of failure were studied, and the damage modes of the material under two kinds of loads were summarized. The results showed that, the stress-strain response (hysteresis loop) of the cast Al-9Si-CuMg alloy exhibited cyclic stability under high temperature low-cycle fatigue loading at 200℃, and the crack occurred at the holes and defects. When the temperature was raised to 350℃, the voids and defects in the material would no longer be the crack source, and the cyclic stress-strain response of the material would also show the characteristics of continuous softening. When coupled with the effect of compressive creep, the yielding radius and elastic modulus of the cyclic stress-strain response were almost unchanged, but the yield center moved upward significantly. Microscopic observation indicated that, to some extent, the effect of compressive creep at high temperature could prevent the initiation and propagation of the large-sized crack within the material, and prevent the rapid development of damage under low-cycle fatigue loading. The eutectic silicon particles inside the material firstly cracked under the compressive creep-fatigue load and formed many tiny cracks or cavities. These tiny cracks extended in the aluminum matrix and connected with the neighboring cracks until they broke.

Thermomechanical fatigue behavior of nitrogen enhanced 316LN stainless steel: Effect of cyclic strain

Thermomechanical fatigue (TMF) behavior of nitrogen enhanced 316LN stainless steel (with 0.14wt.% N) is investigated under in-phase (IP) and out-of-phase (OP) conditions at cyclic strain amplitudes of ±0.25 to ±0.8% and with a temperature interval of 623–873K. The study elucidates the differences in cyclic stress-strain response, TMF lives and fracture behavior of the material under IP and OP-TMF in the light of dynamic strain aging, thermal recovery and creep. The manifestations of these factors/phenomena are found to vary along stress-strain hysteresis loops, thereby leading to differences in cyclic strain hardening exponent (n′) and coefficient (K′) between the tensile and compressive branches of hysteresis loops. Irrespective of the imposed cyclic strain, transgranular fatigue failure is observed under OP-cycling in contrast to mixed-mode fracture under IP-cycling. The IP-cycling led to considerably lower cyclic life (i.e. 34–49%) compared to those under OP-TMF, with the life reduction being more significant (∼48.5%) at intermediate mechanical strain amplitudes of ±0.4 and ±0.6%. A comparison of the fatigue design curves under TMF with the corresponding isothermal fatigue curves (as per the RCC-MR design code) revealed a lack of conservatism in the latter.

Effect of metallurgical variables on the austenite stability in fatigued AISI 304 type steels

Low-cycle fatigue behavior, destabilization of austenite and emergence of deformation induced martensite (DIM) was investigated in wrought AISI 304 type (304, 304L, 348) austenitic stainless steels of diverse semi-product forms. The specimens fabricated from cylindrical bars, plates or thin sheets were cyclically strained with high strain amplitudes under total strain control in symmetrical push-pull at room and depressed temperatures to different stages of fatigue life. Quantitative and qualitative data on the formation, morphology and specific distribution of DIM in the volume of the material within the whole gauge part of the specimens are confronted with the respective cyclic stress-strain response and discussed in terms of characteristic chemical heterogeneity. This heterogeneity arises in the form of chemical banding due to the complex solidification behavior of AISI 304 type steels and its contemporary production route. Possible effect of additional solution treatment (various cooling rates due to quenching in different media) on DIM formation is discussed.

Cyclic stress-strain response of directionally solidified polycrystalline Cu-Al-Ni shape memory alloys

Our previous study developed a polycrystalline Cu-Al-Ni alloy by directional solidification, which exhibits excellent superelasticity. To further verify the application feasibility of the Cu-Al-Ni alloy, cyclic superelastic behaviors of the alloy under varying loading frequencies and strain amplitudes were investigated in this study. The results show that the superelasticity of the polycrystalline Cu-Al-Ni alloy has ultra-low dependence on loading frequency in a range of 0.005∼5 Hz. The alloy can sustain over 2000 tensile cycles at the applied total strain of 4%, exhibiting 1750 cycles and 825 cycles for total strain of 6% and 8%, respectively. The normalized critical stress and strain vary only ∼10% and ∼0.03% after 2000 times of loading-unloading with a strain amplitude of 4%. Compared with the commercial polycrystalline Ni-Ti alloy, the directionally solidified Cu-Al-Ni alloys with columnar grains exhibit excellent cyclic stress-strain response and fatigue life, which are promising materials for energy absorption and seismic protection structures or components.

Multiaxial elastoplastic cyclic loading of austenitic 316L steel

Cyclic stress-strain response and fatigue damage character has been investigated in austenitic stainless steel 316L. Hollow cylindrical specimens have been cyclically deformed in combined tension-compression and torsion under constant strain rate condition and different constant strain and shear strain amplitudes. In-phase and 90° out-of-phase cyclic straining was applied and the stress response has been monitored. Cyclic hardening/softening curves were assessed in both channels. Cyclic softening followed for higher strain amplitudes by long-term cyclic hardening was observed. Cyclic stress-strain curves were determined. Study of the surface damage in fractured specimens revealed the types and directions of principal cracks and the sources of fatigue crack initiation in slip bands.

Effect of alumina-silica-zirconia eutectic ceramic thermal barrier coating on the low cycle fatigue behaviour of cast polycrystalline nickel-based superalloy at 900 °C

Cylindrical specimens of nickel-based superalloy Inconel 713LC in an as-cast condition and coated with a thermal barrier coating (TBC) system consisting of a CoNiCrAlY bond coat and an eutectic ceramic alumina-silica-zirconia top coat were cyclically loaded under strain control at 900°C in air. TBC was deposited using atmospheric plasma spray and water-stabilised plasma spray techniques for the bond coat and the top coat, respectively. The microstructure of the substrate material and the TBC was characterised and the hardness of TBC was measured. The cyclic stress-strain response and fatigue life of both materials were assessed. The stress response of the TBC-coated superalloy is lower than the uncoated superalloy. The detrimental effect of TBC on the Basquin curve is documented. The specimen section and fracture surface observations revealed fatigue damage mechanisms in both the TBC-coated and uncoated specimens, and that finding helps explain the differences in the fatigue behaviour of both materials.

Influence of local fatigue damage evolution on crack initiation behavior in a friction stir welded Al-Mg-Sc alloy

Axial loading fatigue tests were carried out in a friction stir welded Al-Mg-Sc alloy. Local strain evolution under fatigue loading was monitored using small strain gages. The weld joint had very complex heterogeneous microstructure and hardness distributions. The cyclic stress-strain responses were strongly dependent on the local hardness distribution. The cyclic plastic strain was observed intensively on the advancing side in the soft stir zone. Transgranular fatigue cracks initiated in the stir zone on the top surface, where high plastic strain was observed. To the contrary, intergranular crack initiations were observed on the bottom surface. The crack initiation was caused mainly by the cyclic plastic straining on the top side. However on the bottom side, the residual initial oxide layer, which was not sufficiently stirred, as well as the local cyclic plastic straining could be attributed to the intergranular crack initiations.

Tension-Compression Tests for Springback Prediction of AHS Steel Using the Yoshida-Uemori Model

In sheet metal formingprocess of automotive parts, springback effect is crucial, in particular, foradvanced high strength (AHS) steels. Most structural components of new vehiclesshow very complex shapes that require multi–step forming procedures.Therefore, finite element (FE)simulation has been often used to describe plasticdeformation behavior and springback occurrence of formed metal sheets.Recently, the kinematic hardening Yoshida–Uemorimodel has showed great capability for predicting elastic recovery of material. In this work, the AHSsteel grade JSC780Y wasinvestigated, in which tension–compressiontests were carried out. From resulted cyclic stress–strainresponses, material parameters were identified using different fitting methods.Determined model parameters were firstly verified by using simulations of 1–elementmodel. The most appropriate parameter set was thenobtained. Finally, a Hat-Shape forming test was performed and springback waspredicted and compared with experimental results.

Low-Cycle Fatigue Behaviors of Pre-Corroded Q345R Steel under Wet H2S Environments

Low cycle fatigue (LCF) behaviors of pre-corroded Q345R steel under different corrosion environments were investigated where both concentration of H2S solution and pre-corrosion time served as two environmental factors. Low cycle fatigue tests were conducted at room temperature in air after the specimens were pre-corroded in H2S solution. The low cycle fatigue behaviours of pre-corrosion test material, such as strain-life relationships and cyclic stress-strain responses were investigated and compared. Results show that environmental factors had little effect on the cyclic stress response while it had much effect on the fatigue life. And pre-corrosion time had more significant effect on low cycle fatigue life of test material than the solution concentration of H2S. Furthermore, the fracture surfaces of the fatigue samples were characterized by scanning electron microscope (SEM) and the fracture mechanisms were discussed under different environmental conditions.

Influence of surface morphology on fatigue behavior of metastable austenitic stainless steel AISI 347 at ambient temperature and 300°C

The effect of surface modification by cryogenic turning on fatigue behavior of metastable austenitic stainless steel AISI 347 was investigated in stress-controlled fatigue tests at ambient temperature (AT) and 300 °C in air. Five different surface morphologies were manufactured by the variation of turning parameters – with and without cryogenic CO2 snow cooling and feed velocity as well as by the application of polishing for reference surfaces with a very small surface roughness. For a comprehensive characterization of the surface and near surface morphology, X-ray diffraction investigations were performed. Three phases (γ-austenite, α-martensite and ε-martensite) were detected in the near-surface microstructure after cryogenic turning while after turning without cryogenic cooling the only microstructural constituent was γ-austenite. Moreover, residual stress state, micro hardness and surface roughness play an important role in surface morphology. The experimental data on the cyclic deformation behavior and stress-strain response of all surface morphologies are reported. Reference specimens with purely austenitic surface microstructure show the highest plastic strain amplitude during cyclic loading at both AT and 300°C. At elevated temperature these specimens achieved the shortest fatigue life. Martensitic surface layers induced by cryogenic turning result in the reduction of plastic strain amplitude during cyclic loading and significantly enhance fatigue life at both tested temperatures.