Plastic Strain Localization Research Articles

Effect of pitting degradation on ductile fracture initiation of steel butt-welded joints

Monotonic tensile experiments and corresponding finite element (FE) analyses including a ductile fracture damage criterion have been conducted to investigate the ductility degradation behaviours of steel butt-welded joints with corrosion pits. The results showed that the ultimate elongation of welded joint samples rapidly decreased because the existence of pits led to the earlier initiation of ductile cracks after necking. Pit location was the main factor affecting ductility degradation, pits located in the weakest region of sectional stiffness were the biggest threat to the overall deformability of welded joints. In addition, both pit depth and aspect ratio had a significant effect on local ductile crack initiation and propagation, the local stress triaxiality and plastic strain accumulation increased steadily with the increase of pit size, resulting in the decrease of equivalent plastic fracture strain. Furthermore, based on the parametric numerical analyses, equivalent ductile fracture criterion considering pitting parameters was proposed to replicate the effects of actual pits on ductility degradation.

Interaction between ductile damage and texture evolution in finite polycrystalline elastoplasticity

In this paper, a multiscale model of ductile damage and its effects on the inelastic behavior of face centered cubic polycrystalline metallic materials, such as the evolution of their crystallographic textures, are investigated. The constitutive equations are written in the framework of rate-dependent polycrystalline plasticity at the microscopic scale. Plasticity and damage are coupled through a ductile damage variable introduced at the scale of the crystallographic slip systems of each grain. When homogenized to the macro-scale, this becomes an approximate phenomenological measure of the macroscopic ductile damage which can describe the material degradation by initiation, growth, and coalescence of micro-defects. In this paper, thermally activated intergranular (or creep) damage is not taken into account. Both theoretical and numerical aspects of the model are presented. The model is implemented into a general-purpose finite element code in order to analyze the effects of texture evolution and ductile damage initiation in the grains with favorably oriented slip systems. The capability of the proposed model to predict the plastic strain localization and the induced textural evolution, as well as the effects of the ductile damage and its evolution up to the final macroscopic failure are studied for a classical tensile loading path, applied to a representative volume element and to a 3D tensile specimen on which a parametric study has been carried out.

Localized plastic deformation and hardening in laser shock peened Inconel alloy 718SPF

Localized plastic deformation and hardening was studied in laser shock peened (LSP) Inconel 718 SPF (Super plastic forming), a fine grained Ni-Base superalloy, using electron backscatter diffraction (EBSD), nanoindentation, and micro pillar compression testing. 718SPF samples were LSP-treated with two different laser power densities. After LSP, the plastic deformation is manifested as increase in the fraction of low angle misorientations and the formation of sub-grain boundaries. EBSD was used to study plastic deformation as a function of distance from the peened surface for different laser intensities. Different approaches like image quality mapping and local misorientations analysis were used to study the in-depth distribution of the plastic strain. Results show that EBSD is effective in the qualitative characterization of localized plastic strain of the materials subjected to LSP.

Microstructural evolution and strain hardening behavior of AISI 316L type austenitic stainless steel

Abstract The microstructural evolution of AISI 316L austenitic stainless steel subjected to interrupted uniaxial tensile testing is investigated in relation to the strain hardening behavior. The microstructural evolution is characterized by means of scanning electron microscopy equipped with electron backscattered diffraction, optical profilometry and atomic force microscopy. The experimental results show that the increase in level of deformation causes increased surface roughness, extrusion height, plastic strain value, fraction of low-angle grain boundaries and geometrically necessary dislocations. The tensile work hardening analysis reveals a three-stage behavior, where the deformation mechanism is dominated by localization of plastic strain in slip bands due to dislocation–dislocation interactions in stage II and by rotation of austenite grains and dynamic recovery which is due to dislocation annihilations in stage III.

A thermal-mechanical finite element workflow for directed energy deposition additive manufacturing process modeling

This work proposes a finite element (FE) analysis workflow to simulate directed energy deposition (DED) additive manufacturing at a macroscopic length scale (i.e. part length scale) and to predict thermal conditions during manufacturing, as well as distortions, strength and residual stresses at the completion of manufacturing. The proposed analysis method incorporates a multi-step FE workflow to elucidate the thermal and mechanical responses in laser engineered net shaping (LENS) manufacturing. For each time step, a thermal element activation scheme captures the material deposition process. Then, activated elements and their associated geometry are analyzed first thermally for heat flow due to radiation, convection, and conduction, and then mechanically for the resulting stresses, displacements, and material property evolution. Simulations agree with experimentally measured in situ thermal measurements for simple cylindrical build geometries, as well as general trends of local hardness distribution and plastic strain accumulation (represented by relative distribution of geometrically necessary dislocations).

Anisotropy and Strain Localization in Dynamic Impact Experiments of Tantalum Single Crystals

Deformation mechanisms in bcc metals, especially in dynamic regimes, show unusual complexity, which complicates their use in high-reliability applications. Here, we employ novel, high-velocity cylinder impact experiments to explore plastic anisotropy in single crystal specimens under high-rate loading. The bcc tantalum single crystals exhibit unusually high deformation localization and strong plastic anisotropy when compared to polycrystalline samples. Several impact orientations - [100], [110], [111] and [bar{1}49] - are characterized over a range of impact velocities to examine orientation-dependent mechanical behavior versus strain rate. Moreover, the anisotropy and localized plastic strain seen in the recovered cylinders exhibit strong axial symmetries which differed according to lattice orientation. Two-, three-, and four-fold symmetries are observed. We propose a simple crystallographic argument, based on the Schmid law, to understand the observed symmetries. These tests are the first to explore the role of single-crystal orientation in Taylor impact tests and they clearly demonstrate the importance of crystallography in high strain rate and temperature deformation regimes. These results provide critical data to allow dramatically improved high-rate crystal plasticity models and will spur renewed interest in the role of crystallography to deformation in dynamics regimes.

Characterization of deformation in primary α phase and crack initiation and propagation of TC21 alloy using in-situ SEM experiments

In present work, the localized deformation, crack initiation and propagation behavior of Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) titanium alloy was investigated through the in-situ scanning electron microscope (SEM) test and electron backscatter diffraction (EBSD). The results indicate that deformation in α phase is mostly accommodated by the prismatic slip. The localized deformation is concentrated at the interface of primary α lath. Some parallel slip bands with prismatic and pyramidal slip are prevalent in primary α lath, while the multiple slip bands with prismatic slip are dominant in equiaxed primary α phase. Higher ability to coordinate local plastic strain (nearly to 79.5%) is induced in equiaxed α than the primary α lath (7.9%). Most of the cracks easily initiate at the shear bands in primary α lath, which is a fatal site for the tensile deformation. The main crack propagates quickly along the shear bands or interfaces of α lath and then penetrates into the βtrans matrix and also transfers across the equiaxed α phase along the slip bands. The results indicate that the high compatibility of deformation in equiaxed primary α phase delays the initial crack nucleation. Whereas, it does not show satisfied resistance against crack propagation in front of the main crack tip due to the abundant slip bands.

Numerical analysis of the effect of surface roughness on mechanical fields in polycrystalline aggregates

This paper is dedicated to the study of the influence of surface roughness on local stress and strain fields in polycrystalline aggregates. Finite element computations are performed with a crystal plasticity model on a 316L stainless steel polycrystalline material element with different roughness states on its free surface. The subsequent analysis of the plastic strain localization patterns shows that surface roughness strongly affects the plastic strain localization induced by crystallography. Nevertheless, this effect mainly takes place at the surface and vanishes under the first layer of grains, which implies the existence of a critical perturbed depth. A statistical analysis based on the plastic strain distribution obtained for different roughness levels provides a simple rule to define the size of the affected zone depending on the rough surface parameters.

On a novel strain indicator based on uncorrelated misorientation angles for correlating dislocation density to local strength

We present a new method based on uncorrelated misorientation measurements by Electron Backscattered Diffraction (EBSD) to characterize the dislocation density of site-specific areas selected on a bulk material. Gold samples submitted to different degrees of pre-straining are analyzed. A new scalar misorientation parameter called the Characteristic Misorientation Angle (CMA) is derived from uncorrelated misorientation data and compared to the more conventional parameters Grain Average Misorientation (GAM) and Grain Orientation Spread (GOS). We show that CMA is nearly independent of the scan step size and is more sensitive to plastic deformation than GAM and GOS. A coupled effect of local plastic strain and area size is observed on the measured values of CMA. Based on that, values of dislocation density are determined for site-specific areas whose strengths, as defined by the hardness at first pop-in, are subsequently measured by spherical nanoindentation. Results show that the site-specific strength of gold decreases with increasing initial dislocation density. While previous studies have suggested the same trend, the present work offers a new approach to more quantitatively correlate local dislocation densities to the onset of plasticity, without the need for destructive TEM investigations or micro-sample fabrication.

Micromechanics of multiaxial plasticity of DP600: Experiments and microstructural deformation modeling

Multiaxial plasticity behavior of Dual Phase (DP) 600 steel was evaluated through macroscopic testing and predicted through both continuum and micromechanics simulations. The stress state dependent mechanical properties of the material were examined through multiaxial loading under five stress states. The proposed continuum plasticity model was found to accurately predict the multiaxial mechanical response of the material through macroscopic simulations. The microstructure of DP600 contains a relatively soft ferrite matrix and relatively hard martensite particles, suggesting a more complex deformation behavior at the microscale compared to single phase materials. To understand the deformation mechanisms, a representative volume element (RVE) model based on the observed microstructures was built, which was able to predict the macroscopic multiaxial plasticity behavior of the material from the microstructural level. The local deformation inhomogeneity and stress states were also examined through this RVE model. The simulation results showed that in the DP steel studied, closely situated martensite particles increased the local plastic strain and stress triaxiality of the ferrite between them, with no trend for Lode angle parameter.

A NUMERICAL STUDY OF THE MICROSCALE PLASTIC STRAIN LOCALIZATION IN FRICTION STIR WELD ZONES

A crystal plasticity approach was used to study the effects of grain shape and texture on the deformation behavior of friction stir weld (FSW) microregions. The explicit stress-strain analysis was performed for two representative grain structures with equiaxed and extended grains. Grain orientations were assigned to simulate no texture or a weak or strong cubic texture. Calculations have shown that the texture gave rise to earlier plastic strain localization on a larger scale. The highest stresses were found to develop in a non-textured specimen with equiaxed grains where the grain boundaries served as a barrier to dislocation motion. In both equiaxed and extended grain structures with a strong cubic texture no pronounced strain localization was seen on the grain scale but mesoscale shear bands appeared early in the deformation process. The calculations have shown that the microstructure-based simulation is a reasonable tool to study the deformation behavior of FSW materials, which is difficult to be predicted within macroscopic models alone.

A coupled dislocation dynamics-continuum barrier field model with application to irradiated materials

A new computational methodology for 3-dimensional (3D) Discrete Dislocation Dynamics (DDD) in barrier-strengthened materials is developed. We couple the discrete 3D DDD framework with the Finite Element Method (FEM) solution of a continuum field equation for the evolution of dispersed barriers. Coupled model parameters are obtained from detailed statistical analysis of 3D DDD simulations of single dislocation-barrier interactions. We develop a crystal lattice based continuum material point arrangement method to precisely distribute localized plastic strain as a result of dislocation-barrier interactions, enabling the method to be crystal-structure sensitive. The model is demonstrated by an application to the study of the physics of dislocation channel formation and plastic instability phenomena in irradiated materials. The results are shown to be in agreement with experiments on the magnitude of radiation hardening and the onset of plastic instability. Plastic flow localization in irradiated materials was shown to be more prevalent at high irradiation dose. The method enables future studies of the plastic deformation in precipitation-strengthened alloys, hydrogen-embrittled materials, and the plasticity of irradiated materials.

Special Features of Strain Localization and Nanodipoles of Partial Disclinations in the Region of Elastic Distortions

In the zones of strain localization in the region of elastic distortions and nanodipoles of partial disclinations representing the defects of elastically deformed medium, a theoretical analysis of the elastically stressed state and the energy of these defects, including the cases of their transformation into more complex ensembles of interrelated disclinations, is performed. Using the analytical results, the mechanisms of strain localization are discussed in the stages of nucleation and propagation of the bands of elastic and plastic strain localization formed in these zones (including the cases of nanocrystalline structure formation).

Strain localization and fabric development in polycrystalline anorthite\xa0+\xa0melt by water diffusion in an axial deformation experiment

We performed two axial deformation experiments on synthetic polycrystalline anorthite samples with a grain size of ~ 3 μm and 5 vol% Si–Al-rich glass at 900 °C, a confining pressure of 1.0 GPa, and a strain rate of 10−4.8 s−1. One sample was deformed as-is (dry); in the other sample, two half-cut samples (two cores) with 0.15 wt% water at the boundary were put together in the apparatus. The mechanical data for both samples were essentially identical with a yield strength of ~ 700 MPa and strain weakening of ~ 500 MPa by 20% strain. The dry sample appears to have been deformed by distributed fracturing. Meanwhile, the water-added sample shows plastic strain localization in addition to fracturing and reaction products composed of zoisite grains and SiO2 materials along the boundary between the two sample cores. Infrared spectra of the water-added sample showed dominant water bands of zoisite. The maximum water content was 1500 wt ppm H2O at the two-core boundary, which is the same as the added amount. The water contents gradually decreased from the boundaries to the sample interior, and the gradient fitted well with the solution of the one-dimensional diffusion equation. The determined diffusion coefficient was 7.4 × 10−13 m2/s, which agrees with previous data for the grain boundary diffusion of water. The anorthite grains in the water-added sample showed no crystallographic preferred orientation. Textural observations and water diffusion indicate that water promotes the plastic deformation of polycrystalline anorthite by grain-size-sensitive creep as well as simultaneous reactions. We calculated the strain rate evolution controlled by water diffusion in feldspar aggregates surrounded by a water source. We assumed water diffusion in a dry rock mass with variable sizes. Diffused water weakens a rock mass with time under compressive stress. The calculated strain rate decreased from 10−10 to 10−15 s−1 with an increase in the rock mass size to which water is supplied from < 1 m to 1 km and an increase in the time of water diffusion from < 1 to ~ 10,000 years. This indicates a decrease in the strain rate in a rock mass with increasing deformation via water diffusion.

Исследование локализации пластической деформации и разрушения при динамическом нагружении методом инфракрасной термографии

Исследование локализации пластической деформации и разрушения при динамическом нагружении методом инфракрасной термографии

Кинетика развития паттернов макролокализации пластического течения металлов

AbstractThe features of the macroscopic inhomogeneity of plastic deformation in the form of autowaves with a pulsating amplitude are analyzed, and data on the localization of sources of acoustic emission at different stages of plastic flow in the stretching of fcc mono- and polycrystals are presented. The relationship between the local components of the plastic distortion tensor in the strain localization zone is traced. The role of acoustic phenomena accompanying the localization of plastic strain in the development of the process of plastic deformation is considered.

Environmental Degradation Effect of High-Temperature Water and Hydrogen on the Fracture Behavior of Low-Alloy Reactor Pressure Vessel Steels

Structural integrity of reactor pressure vessel (RPV) in light water reactors (LWR) is of highest importance regarding operation safety and lifetime. The fracture behaviour of low-alloy RPV steels with different dynamic strain aging (DSA) & environmental assisted cracking (EAC) susceptibilities in simulated LWR environments was evaluated by elastic plastic fracture mechanics tests (EPFM) and by metallo- and fractographic post-test analysis. Exposure to high temperature water (HTW) environments at LWR temperatures revealed only moderated reductions in the fracture initiation and tearing resistance of low alloy RPV steels with high DSA or EAC susceptibility, accompanied with a moderate, but clear change in fracture morphology, which indicates the potential synergies of hydrogen/HTW embrittlement with DSA and EAC under suitable conditions. The most pronounced degradation effects occurred in a) RPV steels with high DSA susceptibility, where the fracture initiation and tearing resistance reduction increased with decreasing loading rate and were most pronounced in hydrogenated HTW and b) high sulphur steels with high EAC susceptibility in aggressive occluded crevice environment and with preceding fast EAC crack growth in oxygenated HTW. The moderate effects are due to the low hydrogen availability in HTW together with high density of fine-dispersed hydrogen traps in RPV steels. Stable ductile transgranular tearing by microvoid coalescence was the dominant failure mechanism in all environments with additional varying few % of secondary cracks, macrovoids and quasi-cleavage in HTW. The observed behavior suggests a combination of plastic strain localisation by the Hydrogen-enhanced Local Plasticity (HELP) mechanism, in synergy with DSA, and Hydrogen-enhanced Strain-induced Vacancies (HESIV) mechanism with additional minor contributions of Hydrogen-enhanced Decohesion Embrittlement (HEDE) mechanism.

Strain Localization in Titanium with a Modified Surface Layer

Plastic flow localization in commercially pure titanium (VT1-0 according to the Russian classification) with the surface modified by low-energy high current electron beams has been numerically studied. The structure and mechanical properties of the modified surface layer and titanium substrate correspond to the experimentally observed ones and are taken into account explicitly as initial data of a dynamic boundary value problem. The tension of titanium structures with a modified surface layer is simulated by the finite difference method in a plane strain formulation. The dependence of the plastic strain localization parameters on the mechanical properties of structural elements in the titanium substrate has been determined.

Residual stress and dislocation density development in single track deep rolled AISI 4140H steel

Plastic deformations induced by the rolling contact in the deep rolling process are the origin of the formation of residual stress fields and increased dislocation density in the material. The near surface material modification achieved through the process, like a compressive residual stress field, improved hardness and reduced surface roughness, give rise to improved component performance during operation. For a general understanding of the generated surface-near properties, an analysis of single tracks contributing to surface integrity is required. In the present study, a detailed investigation of the achieved residual stress state distribution as well as the local dislocation density of single processed tracks was performed through high spatial resolution X-ray diffraction analyses. Using two spherical deep rolling tools of 6 mm and 13 mm diameter, single tracks were processed with different parameters on samples of steel AISI 4140H. It could be observed that residual stress fields at the surface reach much larger lateral expansion than the plastically deformed track which can be determined by optical measurements, while a correlation with the dislocation density distribution was found. For the maximum dislocation density in the tracks, a correlation with the theoretical maximum equivalent stress was determined. Furthermore, a method to connect the measured dislocation density to the localized plastic strains during processing was applied to exemplary distributions.

Damage characterization of aluminum 2024 thin sheet for different stress triaxialities

Due to its attractive mechanical properties, aluminum 2024 is widely used in aircraft manufacturing industries, especially as fiber metal laminates, such as GLARE. In the present work, a series of experiments for different stress triaxialities are used to study the ductile damage of Al 2024 considering continuum damage mechanics (CDM). Stress triaxiality is produced using notched specimens. The main objective of the present study is to predict the local equivalent plastic strain to fracture and introducing a relation which describes the effect of stress triaxiality factor (TF) on it in the medium range of stress triaxiality. Hence, a nonlinear damage model is utilized for Al 2024 and its parameters are determined by an experimental/numerical/optimization procedure using tensile test on plain specimens. The experiments showed that for large notch specimens (Al-NL) and medium notch samples (Al-NM) fracture started from the center of the notch root of the specimens, whereas for small notched specimens (Al-NS) the failure initiated from the notch root surface. Finite element simulations are performed using the presented nonlinear damage model and are compared with the experimental data. Results show that the proposed damage model can predict the damage evolution for different stress triaxialities.