Amount Of Plastic Strain Research Articles

Anisotropy of plastic flow in Zr-2.5Nb pressure tube material analysed using a viscoplastic self-consistent approach

Plastic anisotropy is observed during plastic flow in samples from the axial and transverse directions of Zr-2.5Nb pressure tubes used in CANDU11CANada Deuterium Uranium - Pressurized Heavy Water power reactors. nuclear reactors tested in uniaxial tension. Plastic anisotropy was also measured in shear by testing in torsion ‘mini’ pressure tubes from the same material. Room temperature results from these experiments were analysed using a visco-plastic self-consistent model that takes into account the crystallographic texture of the material and allows for the single crystal work hardening behaviour to be described by means of a deformation law specific to each slip system.The visco-plastic self-consistent model was used to derive: (i) the evolution with strain of the critical resolved shear stress values consistent with prismatic, basal and pyramidal dislocation glide, (ii) the evolution of slip system activity as a function of strain, and (iii) the values of Hill's plastic anisotropy coefficients that are consistent with the observed anisotropy of yielding and their dependence on accumulated strain. The model also allows for the prediction of the components of the flow stress tensor that cannot be measured experimentally and their dependence on the work hardening behaviour of pressure tube material. Moreover, the yield surface after different amounts of plastic strain was calculated using the visco-plastic self-consistent model, compared to the one that results from using the Hill's anisotropy coefficients. Our work exposes the limitations of the Hill ellipsoid for describing plastic yield when microstructural evolution is present.

Ductile crack growth of high-graded pipeline steels in the presence of Lüders plateau

Strain-based engineering critical assessment methods allows certain amount of plastic strain during installation and in service, for pipelines crossing seismically-active areas. Most strain-based engineering critical assessment methods are established for materials with smooth stress-strain relationship, leaving the so-called Lüders plateau influence on ductile fracture response seldomly explored. This work studied the effect of the Lüders plateau on mode I ductile crack growth with the modified boundary layer (MBL) model under plane strain conditions. The “up-down-up” constitutive model was implemented and the Gurson damage model was selected to simulate crack propagation. Factors including the plateau length and softening modulus in the “up-down-up” constitutive model, strain hardening of the matrix material and the initial void volume fraction parameter were analyzed. Numerical results show that the Lüders plateau modifies the crack tip constraint and the damage evolution, and the plateau length plays the dominant role on ductile crack growth behaviour in the presence of Lüders plateau.

Low plasticity burnishing surface treatment: impact on surface integrity parameters of turned cylindrical Ti- 6Al-4V specimen

ABSTRACT Surface integrity parameters are the major aspect in the extension of the fatigue life of aero- engine components. In this study, a modified burnishing surface treatment (low plasticity burnishing) with reduced cold work was proposed to improve surface integrity characteristics such as surface finish and hardness, and to induce stable, advantageous compressive residual stress in a cylindrical Ti-6Al-4 V specimen (as-received). In this process, a rolling rigid spherical tungsten carbide ball is pressed across a Ti-6Al-4 V specimen at an appropriate fluid pressure generated by the hydraulic unit. The low plasticity ball burnishing tooling system supported by a hydraulic unit of capacity 30 MPa is designed and developed in this study. By using a suitable specimen holding fixture, the ball burnishing with a small amount of plastic strain was carried out on the Ti-6Al-4 V cylindrical specimen. This research examined the effect of burnishing process variables namely tool pass and burnishing fluid pressure on compressive residual stresses, surface roughness, and hardness in Ti-6Al-4 V. The explicit dynamic analysis was carried out by using FE-Analysis software ABAQUS 6.17 as a part of numerical simulation. Hydraulic burnishing pressure and burnishing passes are the most influential parameters in generating favourable compressive residual stress, qualitative surface finish with less cold work in Ti-6Al-4 V turned specimen.

A review on recent advances in accumulative roll bonding of similar, dissimilar and metal matrix composites

Extracting of lightweight metallic materials with ultra-fine grain arrangement passed through severe plastic deformation (SPD) having high strength had a large stretch of applications in automobiles, aerospace, and bio-medical sectors. SPD is a metal forming method in which the material is subjected to a considerable amount of plastic strain, resulting in an ultra-fine grain structure having less than 50 nm. In several (SPD) processes, the accumulative roll bonding (ARB) process is mostly preferred in industries that can be widely used to produce bulky materials continuously. This present keynote review paper discusses the modern improvements in the ARB process, basics and mechanisms of bonding of materials, mechanical properties such as strength, ductility, toughness and it also discuss the microscopic structure of the material, and it draws a contrast between the materials that underwent ARB process compared with normal metal sheets. Moreover, ARB processed materials have the potential for high performance.

Heat Treatment Effects on Pristine and Cold-Worked Thin-Walled Inconel 625

Thin-walled Inconel 625 sheet metal was sectioned into tensile specimens, plastically strained, and then heat treated. Specimens were pulled to a targeted strain, unloaded, and then subjected to one of two heat treatments with the goal of restoring the full ductility and total plastic strain capability of the material. Post-heat treatment tensile testing was performed at room temperature to evaluate the heat treatment efficacy and then followed by hardness and microstructural analysis. The results showed the amount of material recovery was affected by the initial amount of plastic strain imparted to the tensile specimen before heat treatment. Although recrystallization was not observed, grains did elongate in the load direction, and the Kernel average misorientation (KAM) increased with heat treatment. Furthermore, specimens prestrained to 40% and heat treated at 980 °C successfully recovered 88% of pre-heat treatment strain capability prior to fracturing.

Thermo-elasto-plastic analysis of thick-walled cylinder made of functionally graded materials using successive approximation method

In this paper, the thermo-elasto-plastic behavior of a thick walled cylindrical shell made of a Functionally Graded Material (FGM) is analyzed by Successive Approximation Method (SAM). The FGM ingredients include Aluminum and Silicon Carbide in which the effective material properties are estimated using the Modified Rule of Mixture. The shell is subjected to a combination of internal pressure and temperature gradient. After application of equilibrium equations for derivation of governing equations, the Differential Quadrature Method (DQM) is used for numerical solution. It is assumed that the two ends of the cylinder are closed and the plane strain conditions are established. The distribution of elastoplastic stresses and strains along the cylinder's thickness is presented in terms of characteristics of material composition and thermo-mechanical loadings such as volume fraction, in-homogeneous index, internal pressure and thermal loadings. Trueness and accuracy of the present problem is justified through comparison with available results in literature. The results show that by enhancement of ceramic particles percentage in the outer layers of cylinder's wall, amount of plastic strains is reduced at that layers, but at the same time, the amount of plastic strains and range of plastic region, increases at inner layers of thickness.

AFRL Additive Manufacturing Modeling Series: Challenge 4, 3D Reconstruction of an IN625 High-Energy Diffraction Microscopy Sample Using Multi-modal Serial Sectioning

High-energy diffraction microscopy (HEDM) in-situ mechanical testing experiments offer unique insight into the evolving deformation state within polycrystalline materials. These experiments rely on a sophisticated analysis of the diffraction data to instantiate a 3D reconstruction of grains and other microstructural features associated with the test volume. For microstructures of engineering alloys that are highly twinned and contain numerous features around the estimated spatial resolution of HEDM reconstructions, the accuracy of the reconstructed microstructure is not known. In this study, we address this uncertainty by characterizing the same HEDM sample volume using destructive serial sectioning (SS) that has higher spatial resolution. The SS experiment was performed on an Inconel 625 alloy sample that had undergone HEDM in-situ mechanical testing to a small amount of plastic strain (~ 0.7%), which was part of the Air Force Research Laboratory Additive Manufacturing (AM) Modeling Series. A custom-built automated multi-modal SS system was used to characterize the entire test volume, with a spatial resolution of approximately 1 µm. Epi-illumination optical microscopy images, backscattered electron images, and electron backscattered diffraction maps were collected on every section. All three data modes were utilized and custom data fusion protocols were developed for 3D reconstruction of the test volume. The grain data were homogenized and downsampled to 2 µm as input for Challenge 4 of the AM Modeling Series, which is available at the Materials Data Facility repository.

Reduction in Residual Stress during Plastic Deformation of the Aluminum Alloy 7449

Abstract Rectilinear blocks of the very high-strength aerospace aluminum alloy 7449 were solution treated and cold water quenched. They were then stress relieved immediately by cold compression. The amount of cold compression was systematically varied from 0 to 0.3 %. The amount of plastic strain was limited to well below the industrial standard of ~2 %. The blocks then received a stabilizing aging treatment. The residual stress distribution remaining in the blocks was characterized using neutron diffraction, with the aim of tracking how the through thickness 3-D residual strains and stresses change as cold compression progresses. Stress relief was of the order of 30–50 %, but there was no discernible systematic increase in stress relief as the plastic deformation magnitude was increased. It is suggested that this is mainly due to the differences between the samples being less than the experimental uncertainty of the neutron diffraction strain measurement technique.

A novel measure for the material resistance to ductile fracture propagation under shear-dominated deformation

Our engineered metallic materials are being designed to be more and more ductile with an increase in strength. Consequently, the failure mode has changed – from brittle to ductile fracture, which leads to a limited applicability of the previously used parameters that quantify the material resistance developed for brittle fracture. This is owing to that a considerable amount of plastic strain is introduced to the fracture process. Shear-dominated ductile fracture failure is commonly observed in many of the structural metals. In the current work, a novel measure Omega Ω is proposed, which fundamentally describes the material’s ability to motivate relative mass motion in response to Mode Ι shear-dominated fracture propagation. The effects of specimen geometry, fracture tip constraint and deformation modes on Ω were discussed and analysed, which inspired the ideas of normalisation approaches that lead to the development of a dimensionless parameter Normalised Omega Ω¯. Accompanying with this concept, our systematically conducted experimental results (DWTT and SENT) indicated, that Ω¯ is independent to both in-plane and out-plane (specimen thickness) specimen configurations. So that it possesses the characteristic of being an intrinsic material constant for ductile fracture under shear mode.

Understanding strain localisation behaviour in a near-α Ti-alloy during initial loading below the yield stress

Near-α Ti-alloys such as TIMETAL® 834 are known for their superior mechanical properties at high temperature, and as such are found in applications where high strength and improved fatigue performance at elevated temperatures (>450°C) are required. However, these alloys can be susceptible to cold-dwell fatigue; a failure mechanism that is not well understood. The present work investigates the strain localisation behaviour during cold creep and the implications it has in terms of dwell susceptibility for two different bi-modal microstructures. Slip traces and strain distributions have been analysed for different material conditions by employing High-Resolution Digital Image Correlation (HRDIC) in combination with orientation mapping. Using this approach, it was possible to distinguish deformation patterns in primary α grains and transformed-β colonies, loaded incrementally to stress levels of 70%, 80% and 90% of the yield stress. Different prior β-grain morphologies didn’t affect the average strains when stresses are low; but strain distributions have been affected by the β-grain morphology. Material with coarse transformation product accumulated larger amounts of plastic strain compared to material with fine transformation product, at the same relative stress levels. At low stress levels, slip bands have been detected both in primary α, as well in the transformed-β phase, cutting through the lamellae, for the material condition with a coarse transformation product; on the other hand, for the material conditions with a fine transformation product, slip bands are localised only in primary α grains at low stress levels. It was also found for both conditions that at low stress levels slip bands are found in grains that are well oriented for basal slip. Based on these observations it is discussed if b-ligaments are significant obstacles to dislocation movement. Finally, the requirement of crystal-plasticity modelling to take into account differences in crystallographic orientations and the elastic and plastic anisotropy of HCP-titanium will be discussed and considered.

Modeling of the ratcheting behavior in flexible electrodes during cyclic deformation

Many flexible electrode materials with good electrochemical properties show poor sustainability due to the complex stress field inside the electrodes. Here, an analytical model is established to describe the ratcheting behaviors of the flexible electrode during electrochemical cycle with the presence of bending deformation, and is verified by finite element simulation. Under the impact of bending stress, the evolution of plastic strain during cyclic deformation is discussed and the effects of material softening due to the diffusion of Li-ions are considered. It is found that the electrode material accumulates a certain amount of plastic strain during each electrochemical cycle even under small bending deformation. Therefore, the flexible electrode material continuously accumulates plastic strain during cycling and becomes ineffective after several cycle times. Further, a strain hardening model is developed to discuss the possibility of mitigating ratcheting behavior. The increased yield strength in the cycling is found to be effective in suppressing the accumulation of plastic strain. The present work gives the new explanation for the poor sustainability of flexible batteries and provides a theoretical basis for further improvement.

The Influence of Plastic Deformation on the Low-Cycle Fatigue During the Burnishing of Holes in Flat Specimens of D16chT Steel

The paper deals with one of the most important problems of such closely related industries as mechanical engineering and aircraft manufacturing, involving the determination of the service life of components and structures operating under cyclic loading conditions. The presence of local structural and manufacturing stress concentrators greatly complicates the determination of the predicted life of components and structures at their design stage. The burnishing technique, namely, the method of plastic deformation of walls of the hole, aimed at achieving the amount of the residual plastic strain on their surface, is used to strengthen components with holes. The results of low-cycle fatigue tests conducted in repeating tension are presented for flat laboratory specimens of an aircraft aluminum alloy D16chT with a central cylindrical manufacturing hole hardened by burnishing. The burnisher geometry that ensures the required value of the residual plastic strain level (1, 2, and 3%) on the hole surface is calculated. The cyclic tensile loading of specimens at the stresses in a pulsating load cycle was performed in the range of 150 to 270 MPa at a frequency of 3 Hz using the equipment Bi-02-112. The influence of the amount of plastic strain in the burnished hole of the specimen on its cyclic life and fracture as a function of the load value is observed. It is found that due to plastic strain hardening of the surface of manufacturing holes, a local area of compressive residual stresses occur, causing the stress concentration around the manufacturing hole to decrease and the level of limit loads to increase. It is shown that the specimen, having a manufacturing hole hardened by burnishing (up to a plastic strain of 3%) and being cyclically loaded by a tensile stress to 170 MPa, failed not at the hole (the stress concentrator). Both the onset of the microcrack initiation and its further propagation take place in a solid region of the specimen. This is indicative that with a certain amount of residual stresses in the burnished hole, the specimen fracture under low-cycle fatigue becomes insensitive to the presence of this concentrator.

Effect of magnetic field on crystallographic orientation for stainless steel 316L laser-MIG hybrid welds and its strengthening mechanism on fatigue resistance

In order to extend fatigue life of structures, traditional methods involving controlled creation of boundaries and local microstructure discontinuities are used to obstruct dislocation motion and change the crack propagation path during fatigue service. However, such strategies inevitably sacrifice ductility due to the reduced ability to accommodate dislocations. Herein, an external magnetic field is used to assist laser-MIG hybrid welding for 316L austenitic stainless steel. Fatigue crack propagation rate is decreased by 33% at f = 0.1 Hz and 12% at f = 1 Hz in air. The external magnetic field optimizes fatigue resistance of 316L welds from two aspects: diverse orientation distributions of ferrite and non-K-S orientation relationship (OR) between ferrite (δ) and austenite (γ). Diverse orientation distributions of δ induced by magnetic field activate different slip systems, promoting crack deflection in γ grain interior during cyclic deformation. Semi-coherent interface between δ-γ with non- Kurdjumov-Sachs (K-S) OR not only impedes transmission of the dislocation slip but also accommodates a considerable amount of plastic strain by continual loss of coherency. Thus, external magnetic field assisted welding of 316L extends fatigue life by increasing strength and ductility, and promoting crack deflection in grain interior.

Influence of gaseous hydrogen on plastic strain in vicinity of fatigue crack tip in Armco pure iron

A multi-scale characterisation of the crack tip plasticity has been investigated in a fatigue crack propagation under gaseous hydrogen at gas pressure of 35 MPa in a commercially pure iron, Armco iron. The dislocation structure beneath a fracture surface was observed by a Transmission Electron Microscopy(TEM), and the cyclic and monotonic plastic zones were evaluated by an Out-of-Plane Displacement (OPD) measurement. By the TEM observation in a non-accelerated regime (ΔK = 11 MPa×m1/2), a dislocation cell structure was observed even in the brittle intergranular fracture in hydrogen. This result indicates a certain amount of plastic strain is introduced into the grains in front of an intergranular crack in hydrogen, and this may explain the mechanism of hydrogen-induced intergranular fatigue crack propagation. On the other hand, in an accelerated regime (ΔK = 18 and 20 MPa×m1/2), a distribution of scattered dislocation tangles without any cell or vein structure was observed in hydrogen. Besides, the inhibition of the cyclic plasticity near the crack path in hydrogen was confirmed by the OPD measurement. These results are clear evidences of hydrogen-induced localization of cyclic plasticity in the vicinity of a crack tip, which suggests a mechanism model of hydrogen-enhanced fatigue crack growth based on the plasticity localization.

The Effect of Inner Corner Radius of ECAP Die on Strain Distribution and Damage Accumulation in Deformed Sample

In the present research, the effect of inner corner radius of ECAP die on the material flow characteristic and the strain distribution inside sample were analyzed using 2D plain strain finite element simulation. Results showed that increase in inner corner radius results in the formation of smaller corner gap and narrow deformation zone. Consequently, the amount of plastic strain in regions at the bottom side of sample is increased. It is also concluded that the amount of damage factor in the upper regions of sample is higher than bottom regions and therefore cracks may initiate from these regions. In addition, the pressing force was raised by increasing inner corner radius.

Effect of ECAP process on structure and hardness of AlMg3 aluminium alloy

Purpose: In the present study, the effect of ECAP die and number of ECAP pressings on the structure evolution and hardness of AlMg3 aluminium alloy was investigated. Design/methodology/approach: Commercial AlMg3 aluminium alloy in the as-cast condition was processed by Equal Channel Angular Pressing method through route A using two different ECAP dies – conventional and modified with additional twist angle. Samples were processed at ambient temperature up to four passes. The investigation was carried out at ambient temperature. Two different ECAP dies were used to investigate the effect of design modification on the possibility of grain refinement to sub-micrometer size. Findings: The experimental results showed that the modification of ECAP die provides to additional grain refinement and introduces a greater amount of plastic strain into the material which results in greater increase in the properties of the investigated material. Research limitations/implications: The presented investigation results were carried out on samples, not on final products. Practical implications: Current research is moving towards to develop high strength materials with increased mechanical properties and refined microstructure that are known as ultra-fine-grained materials, compared to well-known with coarse-grained microstructure. Originality/value: The paper focuses on the investigation of microstructure evolution using mainly polarised light microscopy that reveals shear, micro-shear and slip bands that refine the microstructure of aluminium alloys. In addition, to evaluate the grain size of as ECAPed specimen, EBSD investigation was carried out.

Effect of Hardness and Thickness of Nonoriented Electrical Steel Sheets on Iron Loss Deterioration by Shearing Process

The influence of the shearing process on the iron loss of nonoriented electrical steels with the thickness of 0.20-0.50 mm and the hardness of HV150-220 was investigated. The deterioration ratio of iron loss was clearly smaller in thinner or harder samples. The deterioration of iron loss and the droop height, reflecting the amount of plastic deformation, displayed a good relationship irrespective of the material thickness and the hardness. This is a well-known statement that an elastic stress affects losses of steel sheets. The effective plastic strain distribution through the thickness of the sheet calculated by the elastic-plastic analysis based on the finite-element method (FEM) coincided with the actual measurement result of hardness increase, reflecting the distribution of plastic deformation. From the evaluation of elastic-plastic deformation by the FEM, it was also shown that the material thickness and hardness influence the elastic strain distribution as a plastic strain, and that the droop height reflects both the amount of plastic strain and the amount of elastic strain.

Effect of Cold Mechanical Process on the Mechanical Properties of 316L Stainless Steel for Medical Implant Application

The objective of this research was to study the mechanical properties of cold-worked 316L stainless steel (SS) after it has undergone the cold mechanical process. A 316L SS sheet was cold-rolled until a reduction in thickness was within the range of 10% to 50%. The sheet was then plastically deformed into a U-bend shape using a universal testing machine equipped with a bending jig. The mechanical properties of the cold-rolled and U-bend steel were analysed using a tensile, 3-point bending, and microhardness tests. It was found that the rolling and bending processes produced such amount of plastic strain that affect the mechanical properties of the 316L SS. The strength of the cold-rolled, U-bend 316L SS increased gradually as the cold reduction was raised above 10%, which was mainly due to the strain hardening effects. The ultimate strength of the U-bend steel increased gradually compared to the cold-rolled steel for each stage of the cold reduction process. On the other hand, the microhardness of the U-bend steel increased rapidly after a cold reduction up to 10%, and the value increased slightly after a cold reduction of 30%. This study indicated that the mechanical properties of 316L SS can be increased by using a combination of the cold rolling and bending processes.

Phase field modeling of fracture in multi-physics problems. Part II. Coupled brittle-to-ductile failure criteria and crack propagation in thermo-elastic–plastic solids

This work presents a generalization of recently developed continuum phase field models from brittle to ductile fracture coupled with thermo-plasticity at finite strains. It uses a geometric approach to the diffusive crack modeling based on the introduction of a balance equation for a regularized crack surface and its modular linkage to a multi-physics bulk response developed in the first part of this work. This evolution equation is governed by a constitutive crack driving force. In this work, we supplement the energetic and stress-based forces for brittle fracture by additional forces for ductile fracture. These are related to state variables associated with the inelastic response, such as the amount of plastic strain and the void volume fraction in metals, or the amount of craze strains in glassy polymers. To this end, we define driving forces based on elastic and plastic work densities, and barrier functions related to critical values of these inelastic state variables. The proposed thermodynamically consistent framework of ductile phase field fracture is embedded into a formulation of gradient thermo-plasticity, that is able to account for material length scales such as the width of shear bands. It is applied to two constitutive model problems. The first is designed for the analysis of brittle-to-ductile failure mode transition in the dynamic failure analysis of metals. The second is constructed for a quasi-static analysis of crazing-induced fracture in glassy polymers. A spectrum of simulations demonstrates that the use of barrier-type crack driving forces in the phase field modeling of fracture, governed by accumulated plastic strains in metals or crazing strains in polymers, provide results in very good agreement with experiments.

On the fracture of small samples under higher order strain gradient plasticity

In this work we perform Finite Element simulations within the framework of large deformation elasto-viscoplasticity on a material that is sensitive to the gradients of plastic strain and incorporates a single intrinsic length scale parameter. Both small scale yielding simulations and those on a finite sized sample show that large stress enhancements can occur at the tip of a notch due to gradient effects. The amount of plastic strain and opening stress that can be expected at the notch tip depends on an interplay between the notch radius, specimen dimensions and boundary conditions. It is shown that cleavage can be the favored criterion for failure in even a ductile material when the notch radius is small compared to the intrinsic length scale. Moreover, for large intrinsic length scales, failure may not always initiate at a notch but may be triggered away from it due to the presence of a boundary impermeable to dislocations.