Cyclic Plastic Response Research Articles

Cyclic plastic response of rotary swaged Fe-14Cr-10Al-4Y2O3 ODS alloy at 1000–1200 °C

This study is dedicated to the investigation of the cyclic stress-strain response of a newly developed ferritic steel reinforced with a high-volume fraction of oxide nanoparticles. The mechanically alloyed powder was consolidated by hot rotary swaging into cylindrical rods with diameters of 15 mm and 12 mm. Subsequently, the rods were annealed at 1200 °C for 4 h to achieve a fully recrystallized coarse-grained microstructure optimal for high-temperature applications. The cyclic tests were carried out in controlled deformation mode in a symmetrical push-pull cycle with triangular waveform at 1000 °C and 1200 °C in air. The Fe-14Cr-10Al-4Y2O3 ODS steel exhibits extraordinary cyclic strength of up to 140 MPa at temperature of 1200 °C. The results show that rotary swaging to a smaller diameter does not affect the resulting stress-strain response. The coarsening of the Y2O3 nanoparticles occurs only very slowly even at 1200 °C. In areas with significant plastic deformation, a slight rotation of the coarse grains from the ideal [111] direction and partial dynamic recrystallization was observed.

Experimental investigation and crystal plasticity simulation for the fatigue crack initiation of the equiaxed Ti-6Al-4V alloy in the very high cycle regime

This work investigated the fatigue crack initiation mechanism of the equiaxed Ti-6Al-4V alloy in the very high cycle fatigue (VCHF) regime. Ultrasonic fatigue experiments were carried out on Ti-6Al-4V specimens and a detailed analysis of the failure mechanism was conducted by means of scanning electron microscopy (SEM). The localized cyclic plastic response of the alloy was investigated using the crystal plasticity finite element (CPFE) method to further clarify the fatigue crack initiation process in the VHCF regime, and the CPFE models with varying transformed β(βt) phase volume fractions were presented to investigate the impact of βt phase on the VHCF behavior. The simulation results indicate that the proposed CPFE model can effectively captures the microstructural plastic deformation mechanisms, such as stress concentration near the phase boundaries and plastic accumulation. And the βt phase significantly affects the VHCF behavior of the alloy.

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%.

A dislocation-based model for cyclic plastic response of lath martensitic steels

A dislocation-based model for cyclic plastic response of lath martensitic steels

Numerical study of deformation behavior of rolled AZ31B plate under cyclic loading in different material orientations based on the EVPSC-TDT model

The EVPSC-TDT model is employed to mimic the stabilized stress-strain hysteresis loops of the rolled AZ31B plate under cyclic loading in different material orientations. The predicted stabilized stress-strain hysteresis loops for different material orientations are generally in good agreement with the corresponding experimental data. The key features associated with twinning and detwinning are interpreted in terms of activities of deformation modes. To appropriately mimic the cyclic plastic response of the rolled AZ31B plate, cyclic loading experiments should also be accounted for in determining the Voce-type hardening law parameters in the current model. The equation proposed in the literature is used in the current model to describe the evolution of residual twinning during cyclic loading, and a threshold equation with a new parameter is proposed to constrain fresh twinning and retwining. The effects of parameters in the two equations on cyclic plastic response are investigated. To approximately describe the apparent easier reverse motion of twin boundaries, the initial CRSS of detwinning is set to be half of that of twinning, and for simplicity the other voce-type hardening law parameters are kept the same for both twinning and detwinning. The effect of initial CRSS of detwinning on plastic response during reverse loading is also investigated. The inelastic behavior is predicted to occur at the very end of the unloading process for the 90° (ND) material orientation, which is ascribed to detwinning. It is assumed that the magnitude of internal tensile stress upon unloading together with that of the activation stress for detwinning determine when or whether detwinning will occur during unloading. The discrepancies between numerical simulations and experiments of the pseudo-elasticity behavior might be because the current mean field model based on self-consistent schemes might not properly describe some microstructure related properties of the AZ31B plate and so underestimates the internal tensile stresses of twinned grains before unloading, which deserves further investigation.

Characterisation of LCF performance of X100 weld-joints: Mechanistic yield strength modelling, finite element analyses and DIC testing

This paper is concerned with the effect of welding on the fatigue behaviour of X100 material for steel catenary risers. The methodology includes both modelling and experimental characterisation. The modelling combines (i) a physically-based yield strength model to capture the thermally-induced microstructural heterogeneity and associated spatial variations in relative contributions of the key strengthening mechanisms due to welding, and (ii) a five-material cyclic plasticity model with a Coffin-Manson strain-life fatigue model for prediction of cross-weld heterogeneity in cyclic plasticity and fatigue response. The combined non-linear isotropic-kinematic cyclic plasticity behaviour of the five weld joint constituent materials (PM, weld metal (WM) and heat-affected zone (HAZ) subregions) is implemented via a user material (UMAT) subroutine, including Kocks-Mecking monotonic-cyclic evolution of yield stress. The experimental methodology consists of tensile tests with digital image correlation (DIC) for X100 PM and cross-weld samples. The results indicate that the primary phenomenon driving the detrimental effect of welding on fatigue is the evolution of cyclic strain localisation in the inter-critical heat-affected zone (ICHAZ), leading to predicted ICHAZ failure.

Effect of welding on microstructure and mechanical response of X100Q bainitic steel through nanoindentation, tensile, cyclic plasticity and fatigue characterisation

This paper presents an experimental characterisation of fatigue at welded connections for the next-generation high-strength low-alloy offshore riser steel, X100Q. An instrumented girth weld is conducted with a parallel programme of physical-thermal simulation (Gleeble) to develop heat affected zone (HAZ) test specimens. X100Q is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. Significant differences are demonstrated between the parent material and simulated HAZ in terms of hardness, monotonic strength and cyclic plasticity response, which can be related to the observed microstructural transformations: the refined grain and bainitic block size in the fine-grained HAZ are shown to give a harder and stronger response than parent material, whereas the coarsened bainitic lath structure in the intercritical HAZ gives a softer and weaker response. The simulated HAZ materials exhibit superior fatigue performance to the parent material and weld metal. A significant reduction in life is shown for cross-weld specimens, indicating susceptibility to failure due to HAZ softening for matched or over-matched X100Q welds.

Comparative assessment of backstress models using high-energy X-ray diffraction microscopy experiments and crystal plasticity finite element simulations

Crystal plasticity (CP) models have been evolving since their inception. Advanced experimental characterization methods have contributed significantly to assess the performance and subsequent improvement of many empirical relations in CP, which were directly adopted from classical plasticity theories of solids at the macro-scale. In this research, high energy X-ray diffraction microscopy (HEDM) has been used to track the stress-state of individual grains within a polycrystalline aggregate of a Nickel-base superalloy subjected to cyclic loading. Using path-dependent, mesoscopic stress-states from the HEDM experiment, the performance of two kinematic hardening models, in the context of CP, has been assessed. One of the models is an empirical Armstrong-Frederick equation, and the other is a geometrically necessary dislocation (GND)-based phenomenological model. The results suggest that the GND-based model is capable of capturing the cyclic crystal plasticity response. The present validation efforts are expected to take CP models one step closer towards their implementation in modern engineering workflow.

Characterization of the fatigue behavior of additive friction stir-deposition AA2219

This work characterizes aluminum alloy (AA) 2219 created by use of the solid-state additive friction stir-deposition (AFS-D) process. Specifically, it determines the “goodness” for use in particular circumstances of as-deposited material created by varied process parameter sets as a function of microhardness and grain size measurements. It presents ideal process parameters for AFS-D AA2219 and characterizes the local optimal as-deposited material by cyclic plastic response, monotonic tensile response, microhardness, grain size, and precipitate existence. Ultimately, this work provides the foundation for the AFS-D technology to be used in construction of NASA’s Space Launch System (SLS) vehicle.

High Temperature Cyclic Plastic Response of New-Generation ODS Alloy

The very recently developed coarse-grained new-generation oxide dispersion strengthened (ODS) alloys containing 5 vol.% homogeneously distributed yttrium nano-precipitates seems to be a promising oxidation-resistant structural material for applications at temperatures above 1000 °C. The primary aim of the present paper is the introduction of the new-generation oxide dispersion strengthened (ODS) alloy and the first testing of its high temperature fatigue properties at 800 °C, concurrently demonstrating a novel and very efficient methodology by using an incremental fatigue step test. The successful application of the methodology motivates the authors to test the fatigue properties of new generation ODS alloys at 1000–1200 °C in the near future.

Elasto-Viscoplastic Material Model of a Directly-Cast Low-Carbon Steel at High Temperatures.

A model-based process control of material production processes demands realistic material models describing the local evolution of the thermal and mechanical state variables, i.e., temperature, stress, strain, or plastic strain, for the relevant microstructure state. In the present work, a material model for the specific microstructure in a continuously cast strand shell, viable for reproducing cyclic viscoplastic effects, was developed for a 0.17 wt.% C steel. Experimental data was generated using directly-cast samples and a well-controllable testing facility to apply representative loading conditions. Displacement- and force-controlled experiments in the temperature range of 700–1100 °C were conducted, with a special focus on the relevant strain rates documented for the straightening operation. A temperature-dependent constitutive material model combining elastic, plastic, and viscoplastic effects was parameterized to fit the whole set of experimentally-determined material response curves. In order to account for the cyclic plastic material response, a combination of isotropic and kinematic hardening was considered. The material model sets a new standard for the material description of a continuously cast strand shell, and it can be applied in elaborate continuous casting simulations.

Cyclic plastic response and damage mechanisms in superaustenitic steel Sanicro 25 in high temperature cycling – Effect of tensile dwells and thermomechanical cycling

The cyclic plastic response and damage evolution in isothermal high temperature constant strain rate cycling and during in-phase thermomechanical fatigue in superaustenitic Sanicro 25 steel have been studied both with and without introduction of dwells at maximum strain. Fatigue hardening/softening curves and fatigue life curves are reported for all types of fatigue tests. Rapid hardening and a tendency to saturation has been found primarily in isothermal cycling with dwells and in thermomechanical cycling. Study of the surface damage evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries perpendicular to the stress axis. Introduction of dwells in maximum tension leads to the enlargement of the plastic strain amplitude and to additional creep damage in the form of cavities along grain boundaries and internal cracks. Fatigue hardening rate in thermomechanical cycling is higher than in constant strain rate cycling and fatigue life decreases substantially in in-phase thermomechanical cycling. These findings are discussed in the perspective of effectiveness of dominant damage mechanisms relevant to individual types of cyclic straining.

The microstructure, mechanical, and fatigue behaviours of MAG welded G20Mn5 cast steel

AbstractThe microstructure, mechanical strength, and fatigue response of metal active gas (MAG) butt‐welded G20Mn5 cast steel was thoroughly investigated for exploring the service safety and reliability of new‐generation railway bogie frames. The fatigue properties of the matrix and welded joints were determined by both low‐ and high‐cycle service regimes. On the basis of nanoindentation testing, the fatigue crack growth (FCG) was derived by correlating with cyclic plastic response of microdomain materials across the MAG joint. The results show that the MAG induces considerable changes in microstructures and hardness of the G20Mn5 matrix and resultantly produces an overmatching welded joint but show comparatively low‐ and high‐cycle fatigue properties to as‐received material. The calculated threshold FCG range based on the Murakami model indicates that the maximum 1.5‐mm defect might be the cracking site subjected to fatigue loading from the structural integrity viewpoint.

Influence of Mn addition on cyclic deformation behaviour of bainitic rail steels

The relationship between the Mn content and the cyclic deformation behaviour in bainitic rail steels is not yet clearly understood. The aim of this paper is, therefore, to study the cyclic plastic response of bainitic rail steels with different Mn contents. In order to meet these goals, low-cycle fatigue tests under fully-reversed strain-controlled conditions are performed at strain amplitudes in the range of 0.6 to 1.0%. The microstructure is analysed by TEM, SEM and XRD, and fracture surfaces are examined by SEM. The results show that the cyclic stress-strain response is significantly affected by the Mn addition. The steel without Mn initially strain hardens and then strain softens until final failure, while the steels with Mn content have an initial stage of strain hardening followed by saturation. Concerning the fatigue response, the addition of Mn is detrimental under higher strain amplitudes but, on the contrary, leads to longer lives under lower strain amplitudes. The fatigue response is explained by the different microstructure morphologies associated with the Mn alloying element.

Cyclic Plastic Deformation Response of Nanocrystalline BCC Iron

Cyclic plastic deformation behavior of nanocrystalline body-centered cubic (BCC) iron is investigated using molecular dynamics simulation. The formation of X-junction and Y-junction during deformation causes the faster mobility of dislocation, which results in softening behavior of the material. In addition, the generation of vacancy defects behind the moved perfect screw dislocation represents the jerky deformation behavior of nanocrystalline BCC iron. The study also highlights the distribution of atomic strain. The grain boundary sliding may result in local atomic strain distribution at the grain boundary. Whereas the region of maximum defects activity shows a higher distribution of atomic strain. The vivid story of crystalline defects in BCC iron helps to develop a deep understanding of cyclic plastic deformation response in the atomic scale.

Microstructure and martensitic transformation in 316L austenitic steel during multiaxial low cycle fatigue at room temperature

Austenitic stainless 316L steel in cold worked condition was subjected to torsional cyclic loading with equivalent strain amplitudes of 0.3% and 0.4% and multiaxial (in-phase and out-of-phase) cyclic loading with equivalent strain amplitude of 0.35% at room temperature. Cyclic plastic response has been measured and analyzed. Internal structure in the as-received state was compared with the deformation microstructures that developed during cyclic straining with various strain histories. In all three investigated cases the volumes with dislocation structures corresponding to cyclic strain localization (persistent slip bands, irregular wall and channel structures) were found. In addition, also islands of deformation induced α′-martensite were observed. Transmission electron microscopy, selected area electron diffraction and ferritscope measurements allowed to estimate the fraction and distribution of α′-martensite islands. The shape of fatigue hardening/softening curves has been discussed in relation with the localization of cyclic plastic strain in the low energy dislocation structures and its blockade by the islands of α′-martensite.

A process-structure-property model for welding of 9Cr power plant components: The influence of welding process temperatures on in-service cyclic plasticity response

A process-structure-property methodology is presented for welding of 9Cr steels under representative flexible operating conditions for a typical thermal power plant girth-welded pipe. The welding-induced evolution of microstructural variables is represented via (i) a solid-state phase transformation model for martensite-austenite transformation and (ii) empirical equations for prior austenite grain size, martensite lath width, hardness and M23C6 precipitate diameter and area fraction, calibrated from published heat treatment data. The temperature-dependent, physically-based, unified viscoplastic constitutive model, which includes a fatigue damage initiation criterion, is based on dislocation density evolution and is validated against high temperature cyclic plasticity data at a range of relevant temperatures for parent material P91, including combined isotropic-kinematic hardening effects. This model is shown to successfully predict weld-life reduction factor for cross-weld tests. The effects of key welding process variables on the microstructure gradient in the heat-affected zone, and associated thermo-mechanical, cyclic plasticity response are assessed. The inter-critical and fine-grained heat-affected zones are identified as the critical regions, consistent with observed plant experience. Increasing post-weld heat-treatment temperature from 760 °C to 780 °C is predicted to be detrimental due to increased precipitate coarsening. In contrast, increasing preheat and interpass temperature from 350 °C to 400 °C is predicted to be beneficial due to increased hardness in the critical regions.

Cyclic plastic response and damage in superaustenitic steel in high temperature cycling with dwells and in thermomechanical cycling

The cyclic plastic response and damage evolution in high temperature cycling with dwells and during thermomechanical fatigue of superaustenitic steel Sanicro 25 has been studied. Fatigue hardening/softening curves in cycling with dwells have been compared with those in constant strain rate cycling. Rapid hardening and early saturation has been found in cycling with dwells. Study of the surface evolution using SEM observations and FIB cutting revealed the preferential oxidation of grain boundaries which were perpendicular to the stress axis. Introduction of dwells in maximum tension lead to the opening of the early cracks, their sliding and formation wedge cracks. Fatigue hardening/softening curves in thermomechanical in-phase cycling do not exhibit saturation and the hardening is higher than in constant strain rate cycling. Damage mechanism was similar to that in isothermal cycling and even more damaging as found by plotting Manson-Coffin curves.

Fatigue Analysis of A356-TiB2 (5wt%) in-situ Nano composites

Abstract In the present work, Low-cycle fatigue (LCF) tests were performed on A356 Aluminium alloy and A356-TiB2(5wt%) in-situ nano-composites to estimate their fatigue performance. The tests were performed by strain controlled with a strain ratio of Re = -1. The cyclic stress–strain curves were determined for each imposed strain levels. The LCF and their characterization in the cyclic plastic response of both the alloy and composites were analysed. It is interesting to note that the type of cyclic deformation behaviour in Al-Mg-Si alloys seems to be influenced by the dispersed phases and TiB2 particles. The LCF values for the A356-TiB2(5wt%) composites were found to be higher than those of the A356 alloy. The separation fatigue life’s of the A356-TiB2(5wt%) in-situ nano-composite and A356 alloy was found to be 15652 and 12350 cycles respectively. This is attributed due to the presence of minute TiB2 particles which obviously affected the microstructure, hardness, tensile strength and fatigue life of the composites compared to the base alloy. It is further noted that the LCF values of the present composites were found to be higher than those of the alloys reported in the literature.

Cyclic plasticity and low cycle fatigue damage characterisation of thermally simulated X100Q heat affected zone

This paper presents the cyclic plasticity and low cycle fatigue (LCF) damage characterisation of thermally simulated heat affected zone (HAZ) for API 5L X100Q weldments. Microstructures representative of the HAZ for two cooling rates are generated using a Gleeble thermomechanical simulator for manufacture of strain-controlled cyclic plasticity test specimens. The simulated HAZ specimens are subjected to a strain controlled test programme which examines the cyclic effects of strain-range and the tensile response at room temperature. A modified version of the Chaboche rate independent plasticity model, which accounts for early stage damage is implemented to characterise the cyclic plasticity response, including isotropic and kinematic hardening effects. The constitutive parameters are fitted to experimental data using an optimisation procedure developed within a MATLAB code. The measured response of the simulated HAZ specimens is compared to that of the X100Q parent material (PM), and the simulated HAZ is shown to share the early stage fatigue damage behaviour of the PM, but exhibits significantly a higher yield and cyclic strength.