Plastic Strain Localization Research Articles

Strength-ductility balance of AZ31 magnesium alloy via accumulated extrusion bonding combined with two-stage artificial cooling

AZ31 Mg alloy with heterogeneous bimodal grain structure (smaller grain size of 5–20 µm and coarser grain size of 100–200 µm) was subjected to accumulated extrusion bonding (AEB) at 250 ℃ combined with two-stage artificial cooling in this work, viz. local water cooling and artificial cooling. The microstructure developed consecutively as a result of discontinuous dynamic recrystallization (DDRX) for the AEBed samples. {10–12} tensile twinning also played an important role for the AEB with local water cooling at the initial extrusion stage in the container. Local water cooling could further reduce the DRXed grain size to ~ 2.1 µm comparing that without water cooling. And the grain growth rate was reduced by artificial cooling out of extrusion die. Under the combination of two-stage cooling, the fine DRXed grains at sizing band were almost retained with average grain size of ~ 2.3 µm after the sheet out of extrusion die, and the unDRXed grains with high residual dislocation density accumulation were also reserved. The tensile tests results indicated that a good strength-ductility balance with a high ultimate tensile strength (319 MPa vs. 412 MPa) and fracture elongation (19.9 % vs. 30.3%) were obtained. The strength enhancement was mainly owing to the grain refinement and local residual plastic strain reserved by the artificial cooling. The excellent ductility originated from fine DRXed microstructure and ED-tilt double peak texture.

In-situ SEM-EBSD investigation of the low-cycle fatigue deformation behavior of Inconel 718 at grain-scale

The low-cycle fatigue (LCF) deformation behavior of the Inconel 718 (IN718) specimen with micro-notch was investigated by an in-situ fatigue test inside a scanning electron microscope (SEM) combined with electron backscattered diffraction (EBSD). The initiation condition and propagation path of cracks were observed by SEM. The essential reasons for crack initiation, propagation and fatigue fracture are elaborately characterized by crystallographic information obtained from EBSD. Meanwhile, the strain evolution during fatigue deformation simulated by the crystal plastic finite element (CPFE) method is consistent with the experimental content, which further verifies and supplements the experimental results. Two LCF deformation mechanisms (location-related) of the IN718 specimen are proposed at grain-scale based on the direct observation of crack characteristics and in-depth analysis. Near the edge micro-notch region, the deformation mechanism is mainly related to crack initiation and propagation: the plastic strain localization along the most easily activated slip band leads to the initiation of fatigue microcracks inside the grain. The crack propagation is carried out by microcrack coalescence in different grains. Away from the edge micro-notch region, the deformation mechanism is mainly related to the instantaneous fatigue fracture: fatigue fracture comes from the intergranular plastic strain accumulation and heterogenous deformation at grain-scale. The result also deeply explains the fatigue crack propagation in metals (two consecutive modes).

A procedure to obtain correlation curve between local misorientation and plastic strain

Numerous studies have assessed the degree of plastic strain induced in polycrystalline materials using crystal orientation mapping data obtained by electron backscatter diffraction. However, it was pointed out that the local misorientation, defined as the averaged misorientation between adjacent measurement points, was difficult to use for a quantitative assessment because the magnitude of the local misorientation was affected by various measurement conditions including the step size, that is the spacing of the crystal orientation measurement points. To quantitatively assess the local misorientation, this study proposed a procedure for estimating the correlation curve between the local misorientation and the degree of plastic strain, which is referred to as the master curve. The procedure to determine the master curve was proposed using Type 316 stainless steel specimens of various degrees of plastic strain. The procedure was based on the characteristics of the change in the misorientation parameters due to the measurement conditions that were investigated using the crystal orientations obtained for various conditions and specimens. It was shown that similar local plastic strain distributions were obtained regardless of the measurement conditions, including especially the step size. From the findings, it was recommended that a smoothing filter should be applied when making a quantitative assessment of the local misorientation.

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

This study deals with microstructural aspects of the deformation behavior of as-cast homogenized CoTi intermetallic by using in-situ tensile testing coupled with EBSD analysis. CoTi exhibits excellent wear resistance and noticeable room temperature ductility. Therefore, it is commonly used as coating material or a reinforcing phase in superalloys. In-situ study of this intermetallic is pertinent as it will provide information about the microstructural evolution during deformation under tensile loading. During the initial stage of deformation, the plastic strain localization was observed to occur at softer grains due to the activation of {011}⟨100⟩ slip systems, which resulted in the accumulation of GNDs and increase in fLAGBs (fraction of low angle grain boundaries). However, fLAGBs did not increase significantly at higher strains. In addition, the contribution of texture towards deformation behavior was found to be weak. The intermetallic exhibited a mixed-mode failure, where both transgranular and intergranular features were observed on the fracture surface. A slightly higher strain to failure in the intermetallic has been attributed to both the increase in the GNDs density at the initial stage of deformation and increase in SSDs at the later stage of deformation, leading to strain hardening.

Evolution of Poisson’s Ratio in the Tension Process of Low-Carbon Hot-Rolled Steel with Discontinuous Yielding

Low-carbon hot-rolled steel generally undergoes a deformation process composed of four phases, i.e., elastic deformation, discontinuous yielding, work hardening, and macroscopic plastic-strain localization in a tension test. The evolution of the Poisson’s ratio in terms of the average Poisson’s ratio and the local Poisson’s ratio in the deformation process from the non-load state to the onset point of specimen necking was investigated. The main results are as follows: (1) the average Poisson’s ratio cannot accurately represent the local Poisson’s ratio in the discontinuous-yielding phase; (2) the Poisson’s ratio varied significantly within a plastic band in the discontinuous-yielding phase, and the maximum Poisson’s ratio was reached within the plastic band; and (3) the strain rate greatly increased the Poisson’s ratio.

Very high cycle fatigue behavior of laser powder bed fusion additively manufactured Ti6Al4V alloy at elevated temperature

The crack initiation mechanism of laser powder bed fusion (LPBF) Ti6Al4V was investigated at elevated temperature up to very high cycle fatigue (VHCF) regime. The competition concerning defect location is elaborated using the stress intensity factor range and Z-parameter model. Additionally, localized high stress near the defects is responsible for plastic strain localization in the non-prior α' region, following the grain refinement and fragmentation in the larger grains nearby, which is strongly associated with the formation of microcracks and fine granular area. Meanwhile, the process is facilitated by reducing dislocation resistance and activating multiple slip systems due to high temperature.

Characterizing Pure Polymers under High Speed Compression for the Micromechanical Prediction of Unidirectional Composites

The nonlinear behaviour of fibre-reinforced polymer composites (FRPC) in transverse loading is mainly induced by the constituent polymer matrix. The thermoset and thermoplastic matrices are typically rate- and temperature-dependent, complicating the dynamic material characterization process. Under dynamic compression, the microstructure of the FRPC develops local strains and local strain rates whose values can be much higher than those applied at macroscopic level. The correlation between the local (microscopic) values and the measurable (macroscopic) ones still present challenges when applying the strain rate in the range 10-3-103 s-1. This paper presents an in-house uniaxial compression test setup to provide robust stress-strain measurements applying strain rates up to 100 s-1. A semi-crystalline thermoplastic polyetheretherketone (PEEK) and a toughened thermoset epoxy PR520 are assessed and characterized. The thermomechanical response of the polymers is further modelled using an advanced glassy polymer model, naturally capturing the isothermal to adiabatic transition. A micromechanical model of a unidirectional composite undergoing dynamic compression is developed by using both validated polymers as matrices reinforced by carbon fibres (CF) using Representative Volume Element (RVE) models. These RVEs are used to analyse the correlation between the micro- and macroscopic thermomechanical response of the CF/PR520 and CF/PEEK systems investigated at intermediate to high strain rates. Both systems experience an excessive strain localization with local plastic strain about 19% when a macroscopic strain of 3.5% is applied. The comparison of using a thermoplastic and a thermoset as a matrix in composites is discussed with regard to the rate-dependence, the interface debonding and the self-heating effect.

Stored energy density research on the fatigue crack initiation at twin boundary and life prediction of Inconel718 superalloy

The fatigue crack initiation at twin boundary (TB) and the life prediction of IN718 alloys in the high cycle fatigue (HCF) regime and very high cycle fatigue (VHCF) regime were investigated through crystal plasticity finite element (CPFE) simulations and experiments. The fatigue damage parameter, considering the evolution of geometrically necessary dislocation (GND) density, slip and back stress, known as the back-stress-based stored energy density in this study, Gba, was proposed to precisely characterize the plastic strain distribution and localization, which in turn was engaged in locating fatigue crack initiation site and predicting the fatigue life. The element analysis around TB and the CPFE simulations indicated the coupling of slip activation parallel to the TB and the absence of primarily strengthening phase (γ'') on the both side near the TB was responsible for crack initiation. To predict fatigue life, the varying pattern of the Gba at fracture with fatigue life was studied based on the experimental results in the HCF and VHCF regimes, and the life prediction method was obtained accordingly. Finally, the fatigue life evaluations of the IN718 alloy at three load ratios (R = −0.1, 0.1, and 0.5) were conducted and results showed that the life estimates were basically within ±3 scatted bands.

Finite element assisted self-consistent simulations to capture texture heterogeneity during hot compression

Abstract In the present work, finite element simulations were performed to capture the influence of friction and temperature gradient on the local strain distribution across the cross-section of a hot compressed specimen. The finite element predicted deformation histories for different regions of the hot compressed sample were exported to the viscoplastic self-consistent model to perform texture simulations in the variable velocity gradient mode and to capture the texture heterogeneity. Differences in texture intensity as well as characteristics were observed depending upon the distance from the central region of the hot compressed sample. It was observed that a double fiber texture with {110} and {100} parallel to the compression direction evolved in the central region of the specimen, which accommodates the highest amount of local plastic strain. On the other hand, only {110} fiber evolved in the region close to the dead zone, which accommodates the least amount of local plastic strain. Furthermore, volume fraction of the {100} fiber was observed to be sensitive towards strain rate sensitivity, where a higher rate sensitivity enhances the shear rates on the non-octahedral slip systems resulting in strengthening of {100} texture fiber.

Coupled effects of crystallographic orientation and void shape on ductile failure initiation using a CPFE framework

Ductile failure reveals to be an anisotropic phenomenon, for which the proper mechanism has not been clearly addressed yet in the literature. In this paper, the effects of some key anisotropy factors on ductile failure initiation, detected by void coalescence and plastic strain localization, are investigated using unit-cell computations based on crystal plasticity finite element method. The studied anisotropic effects are induced by the combination of initial crystallographic orientations and void shapes. Therefore, single crystals with three different initial orientations and polycrystalline aggregates with three different initial crystallographic textures are respectively considered. A single void with either spherical, prolate or oblate shape is assumed to be preexisting at the center of each unit cell. By contrast to previous analyses in the literature, plastic strain localization is predicted in the present study on the basis of bifurcation theory. To cover a wide range of stress states, the simulations are performed under two macroscopic loading configurations: proportional triaxial stressing, characterized by constant stress triaxiality and Lode parameter, and proportional in-plane straining, specified by constant strain-path ratio. The obtained results show that the combined anisotropic effects play an important role in the occurrence of void coalescence and plastic strain localization, as well as in the competition between them.

Prediction of Fatigue Crack Initiation of 7075 Aluminum Alloy by Crystal Plasticity Simulation.

The 7075 aluminum alloy is a promising material for the aerospace industry due to its combination of light weight and high strength. This study proposed a method for predicting fatigue crack initiation of the 7075 aluminum alloy by crystal plasticity finite element analysis considering microstructures. In order to accurately predict the total fatigue life, it is necessary to calculate the number of cycles for fatigue crack initiation, small crack growth, and long crack growth. The long crack growth life can be estimated by the Paris law, but fatigue crack initiation and small crack growth are sensitive to the microstructures and have been difficult to predict. In this work, the microstructure of 7075 aluminum alloy was reconstructed based on experimental observations in the literature and crystal plasticity simulations were performed to calculate the elasto-plastic deformation behavior in the reconstructed polycrystalline model under cyclic deformation. The calculated local plastic strain was introduced into the crack initiation criterion (Tanaka and Mura, 1981) to predict fatigue crack initiation life. The predicted crack initiation life and crack morphology were in good agreement with the experimental results, indicating that the proposed method is effective in predicting fatigue crack initiation in aluminum alloys. From the obtained results, future issues regarding the prediction of fatigue crack initiation were discussed.

A microstructure-based fatigue model for additively manufactured Ti-6Al-4V, including the role of prior [formula omitted] boundaries

Microstructure-based models of additive manufactured (AM) Ti-6Al-4V should faithfully represent the unique microstructural features of these materials to provide a more thorough understanding of their role in the mechanical performance of the material. For AM Ti-6Al-4V, the prior β boundaries are likely sites of microscopic plastic strain localization, often leading to fatigue crack initiation. Within the context of crystal plasticity finite element methods, there is an existing gap in the current literature for creating synthetic microstructures of Ti-6Al-4V that capture both the prior β boundaries and α laths. This work focuses on generating such statistically equivalent microstructures, where the prior β boundaries and α laths are explicitly modeled. The framework can generate synthetic microstructures that consider one prior β grain or multiple β grains (and thus prior β grain boundaries) and their associated α laths forming a Widmanstätten microstructure, which are used as inputs for fatigue simulations. Within the fatigue model, the critical accumulated plastic strain energy density (APSED) is a metric used to predict the location and number of cycles of crack initiation. From a statistical average perspective, the presence of prior β grain boundaries was detrimental to the fatigue performance. The effectiveness of the prior β grain boundary in impeding slip, and therefore localizing APSED, is dependent on the loading conditions, prior β orientations, and α variants present in neighboring prior β grains. A higher percentage of failures occurred at the prior β grain boundary in the case of lower applied stress amplitudes, where the applied loads were below the percolation limit of the material and microplasticity was localized to a few locations relative to the microstructure. Additionally, for the case of crack initiation associated with the α variants within a given prior β grain, a combination of soft-hard α lath orientations resulted in sites of APSED localization and predicted fatigue crack initiation. The framework generated in this paper to create representative synthetic microstructures of the Widmanstätten microstructures associated with AM Ti-6Al-4V and the associated fatigue modeling enables trade-off studies between microstructural features and fatigue performance.

Seismic performance of a low-damage rocking column base joint along weak axis

This paper presents an experimental and analytical study on seismic performance of a low-damage rocking column base joint equipped with an asymmetric friction connection (AFC). Cyclic loading tests were conducted along weak axis of the column, including INITIAL, AFTERSHOCK and REPAIR cases. Compared with the INITIAL case, the joint exhibited a 2.5% decrease in average moment resistance and a 24.9% decrease in rotational stiffness in the AFTERSHOCK case without any repair taken. By re-installing and tightening the AFC bolts, the moment resistance was completely restored and the rotational stiffness could reach 84.1% of the INITIAL case, achieving resilience and reparability of the joint. Internal forces of the column were effectively limited by frictional sliding of the AFC, and the maximum local equivalent plastic strain at the column base was only 1.0% at a drift ratio of 3%. Axial force, as the main source of restoring forces, provides a favorable tendency for self-centering of the joint. In addition, a finite element (FE) model was developed to quantitatively evaluate damage and sliding behaviors of the joint, and the low-damage characteristics were verified. Finally, a simplified analytical model was proposed to guide the design of this low-damage joint.

Numerical investigation of plastic strain localization for rock-like materials in the framework of fractional plasticity

The plastic strain localization phenomenon is of critical importance for rock-like materials when undergoing non-homogeneous deformation. This paper presents a fractional plastic framework which is applied to take into account the plastic strain localization of rock-like materials. The novelty of the presented framework lies in the non-coaxial plastic flow that is controlled by the RiemannLiouville (RL) fractional derivative without additional plastic potential. The hardening response is described by a function of the generalized plastic shear strain. The capability of the fractional constitutive model to predict the main mechanical behaviors of rock-like materials is assessed by the conventional triaxial compression test data of Vosges sandstones from the literature. With the help of a MATLAB-based finite element method, several numerical examples (i.e., a plate, a plate with a hole, and a plate with a crack) are implemented, compared and analyzed to demonstrate the effectiveness of the proposed framework and the influence of the fractional order on the strain localization. It can be found from the numerical application of the twin-tunnel that the fractional model is promising to flexibly capture the strain localization zone for rock-like materials.

Role of Persistent Slip Bands and Persistent Slip Markings in Fatigue Crack Initiation in Polycrystals

The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno.

Laser-Assisted Robotic Roller Forming of Ultrahigh-Strength Steel QP1180 with High Precision.

Laser-assisted forming provides a perfect solution that overcomes the formability of low-ductility materials. In this study, laser-assisted robotic roller forming (LRRF) was applied to bend ultrahigh-strength steel sheet (a quenching and partitioning steel with a strength grade of 1180 MPa), and the effects of laser power density on the bending forces, springback, and bending radius of the final parts were investigated. The results show that LRRF is capable of reducing bending forces by 43%, and a compact profile with high precision (i.e., a springback angle smaller than 1° and a radius-to-thickness ratio of ~1.2) was finally achieved at a laser power density of 10 J/mm2. A higher forming temperature, at which a significant decrease in strength is observed, is responsible for the decrease of forming forces with a laser power density of higher than 7.5 J/mm2; another reason could be the heating-to-austenitization temperature and subsequent forming at a temperature above martensitic-transformation temperature. Forming takes place at a higher temperature with lower stresses, and unloading occurs at a relatively lower temperature with the recovery of Young's modulus; both facilitate the reduction of springback angles. In addition, the sharp bending radius is considered to be attributed to localized deformation and large plastic strains at the heating area.

The study of mechanism of localized shear fracture in AlMg6 alloy under dynamic loading on a split Hopkinson pressure bar

This work is devoted to the substantiation of one of the mechanisms of plastic strain localization under conditions of high-rate dynamic loading associated with jump-like processes in the defect structure of materials. The specimen behavior under dynamic loading was investigated using a split Hopkinson pressure bar. In order to identify the characteristic stages of strain localization, we performed in-situ thermodynamical studies of the deformation process by recording temperature fields with the CEDIP Silver 450M high-speed infrared camera. The temperature measurements made in the zone of strain localization do not support the wide-spread idea that the strain localization mechanism is caused by thermoplastic instability. Dynamic tests were carried out for specimens, which were specially designed to study plastic strain localization in AlMg6 alloy under conditions of dynamic loading on the split Hopkinson pressure bar using the StrainMaster strain measurement system. The specimens after the experiment were subjected to microstructural analysis using an optical interferometer-profile meter and a scanning electron microscope. The structural analysis revealed the correlated behavior of defect ensembles, which can be classified as a structural transition facilitating strain localization. Experimental data, examination of deformed specimen structure and data of numerical simulation with consideration for kinetics of microdefect accumulation in the material allow us to infer that one of the mechanisms of plastic strain localization at high loading rates is caused by jump-like processes in the defect ensembles.

A study of macroscopic plastic strain localization during creep of polycrystalline aluminum

This paper studies the deformation behavior of commercially pure aluminum 1050A during creep tests at room temperature. The plastic flow parameters at the stage of steady-state creep at different stress levels are determined. The kinetics of localized plasticity zones in the creep mode is studied using the digital image correlation method. The stress dependence of the spatial period of plastic strain localization is established.

Plastic strain localization mechanism in AlMg alloys as a result of jumpwise processes in the defect structure of materials

Plastic strain localization mechanism in AlMg alloys as a result of jumpwise processes in the defect structure of materials

Micromechanical aspects of the effect of temperature and local plastic strain magnitude on the fracture toughness of ferrite steels

The paper shows that the influence of plastic strain and temperature on the rate of crack nuclei (CN) formation is a crucial factor controlling the shape of the temperature dependence of fracture toughness and its scatter limits. Within the framework of the microscopic model described, it is explained that the dependence of the incompatibility of microplastic deformation at grain boundaries or interfaces on the value of plastic strain and temperature is the reason for the effect of these factors on the rate of CN formation. The dependencies of the CN bulk density on temperature and plastic strain are given. In the latter case, a non-monotonic change in CN density is observed. The maximum intensity of CN formation is observed when the critical value of plastic strain is reached. For ferritic structural steels, this strain is about 2%. Using reactor pressure vessel steel and cast manganese steel as examples, it is shown that not taking these effects into account in the local fracture approach leads to considerable errors in the prediction of the temperature dependence of the fracture toughness and its scatter limits.