Plastic Prestrain Research Articles

Correlation dependences between hardening in terms of yield strength and microhardness for austenitic and ferritic-martensitic steels

Instrumental measurements of microhardness were carried out using a Vickers indenter at a constant strain rate. The yield strength values were determined for austenitic chromium-nickel steels 08Kh18N10Т, 10Kh18N9 and 08Kh16N20M2Т in the initial (non-irradiated) state, after neutron irradiation at various regimes, as well as after plastic prestrain. Similar measurements were carried out for chromium stainless steels of the ferritic-martensitic class 07Kh12NMFB and 16Kh12MVSFBR (EP-823) in the initial (non-irradiated) state and after heat treatment, leading to hardening of the material and also after neutron irradiation. The relationships between microhardness and yield strength were determined for all studied states of all steels studied. A unified correlation has been established between hardening in terms of the yield strength and hardening in terms of Vickers micro-hardness, independently of the nature of the strengthening factor and the class of steel.

Effect of pre-strain on springback behavior after bending in AA 6016-T4: Experiments and crystal plasticity modeling

Modeling springback in sheet materials is challenging in aluminum alloys, especially when a complex strain path is applied. This paper presents results from pure bending experiments on AA 6016-T4 sheet material, where various plastic pre-strains were first applied prior to bending. A crystal plasticity based elasto-plastic self-consistent (EPSC) model that includes the effect of backstress in the hardening law was used to predict final part shape after unloading. The backstress term in the model was calibrated using geometrically necessary dislocation (GND) content, measured experimentally by high resolution electron backscattered diffraction (HREBSD). The EPSC model predicted springforward angles for unstrained 1 mm AA 6016-T4 sheet with an error of 0.4% (0.3°) in the worst case, while the J2 plasticity isotropic model overpredicted springforward angles by as much as 2.4% (2°). For cases where uniaxial, plane-strain, and biaxial pre-strains were first imparted to the sheets before bending, the EPSC model with backstress accurately predicted the transition from springforward to springback, while the EPSC model without backstress did not. Backstress influence on model accuracy, which increased with greater pre-strain levels, appears to be correlated to the statistically stored dislocation (SSD) density computed by the model at the end of each pre-strain step.

The role of cryogenic pre-strain on the hydrogen embrittlement of FeNiCoCrMn high-entropy alloys

The effect of cryogenic plastic pre-strain (15 %, 30 %, and 45 % true strain) on the hydrogen embrittlement (HE) of an equimolar FeNiCoCrMn high-entropy alloy was studied through slow strain rate tension (SSRT) at 1 × 10−5 s−1. As the pre-strain increased, ductility loss under harsh electrochemical hydrogen charging showed a downward trend accompanied by the promotion of hydrogen adsorption. This work provided an effective strategy for achieving the synergy of high strength and improved HE resistance. Data availability statementThe raw/processed data required to reproduce these findings cannot be shared at this time as the data is related to an ongoing study.

Hydrogen diffusion kinetics in dual-phase (DP 980) steel: The role of pre-strain and tensile stress

The current study investigates the influence of pre-strain (un-strained, 3 %, and 6 % engineering strain) and in-situ tensile stress (50 %, 110 %, and 125 % of the yield stress (YS)) on the hydrogen diffusion and trapping behaviour in DP 980 steel. Electrochemical hydrogen permeation tests were conducted using a custom-built Devanathan-Stachurski (DS) cell integrated with a horizontal tensometer. The results showed that the value of apparent diffusion coefficient (Dapp) decreased from 5.4 ± 0.1 to 3.8 ± 0.029 × 10−7 cm2 s−1, while trap density (NT) and sub-surface hydrogen concentration (Capp) increased from 1.08 ± 0.01 to 1.58 ± 0.012 × 1021 cm−3, 2.70 ± 0.13 to 3.27 ± 0.10 × 10−4 mol H cm−3, respectively for unstrained and 6 % pre-strain conditions. This is due to increased dislocation density in ferrite and martensite. Hydrogen permeation tests under tensile stress (in-situ) revealed that the applied elastic stress of 50 % of YS showed a slight increase in Dapp and a decrease in NT values. This is due to an increase in the interstitial space between the atoms at applied elastic load favouring hydrogen diffusion. The plastic tensile stress of 110 % of YS showed the highest value of steady-state permeation current density iss (169 ± 10.6 µA cm−2). It could be due to an expanded lattice of martensite accommodating more hydrogen because of its elastic deformation. The tensile stress of 125 % of YS results in a significant reduction in Dapp (2.86 × 10−7 cm2 s−1) and an increase in Capp (5.52 × 10−4 mol H cm−3) and NT (2.11 × 1021 cm−3) as a result of plastic deformation in ferrite and martensite. The decay transient studies revealed that the applied elastic tensile stress enhances the rate of desorption of diffusible hydrogen, increasing lattice diffusion coefficient DL. The plastic pre-strain and plastic tensile stress resulted in slower desorption, lower DL, and an increase in reversible trap density.

Insight into microstructure, microhardness and corrosion performance of 2205 duplex stainless steel: Effect of plastic pre-strain

This study explored the microstructure evolution of 2205 duplex stainless steel (DSS) and further probed its effects on microhardness and corrosion performance. The results show that the dislocation multiplication, strain-induced martensite generation and grain refinement resulted from pre-stain increase the microhardness of DSS. Compared with the degradation of corrosion resistance at 7% pre-strain, the improved corrosion resistance at 14% and 21% pre-strain is related to the smaller surface energy difference, more strain-induced martensite and more low-angle grain boundaries. The changed pitting nucleation sites with the rise of pre-strain strongly depend on the increased dislocation density and strain-induced martensite.

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels.

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.

A new constitutive model to describe evolving elastoplastic behaviours of hexagonal close-packed sheet metals

This study develops a new phenomenological constitutive model to capture the unique evolving cyclic elastoplastic behaviours of hexagonal close-packed (HCP) sheet metals. This new constitutive model is developed by adopting the concepts of multiple-yield surface approaches. Four phenomenological deformation modes, including Monotonic Compression (MC), Monotonic Tension (MT), Reverse Compression (RC), and Reverse Tension (RT), are considered to represent the hardening evolution of the materials, including the twining/untwining behaviours. Reference flow stress equations are introduced, and a Cazacu-Barlat 2004 (CB2004) type yield surface is employed to each deformation mode. In addition, the RT hardening parameters are defined as functions of plastic pre-strains to mitigate the interpolation error caused by parameter determination processes of existing models. For validation, the calculated stress–strain curves of AZ31B magnesium alloy are compared with experimental curves available from literature. Moreover, to show the accuracy of the proposed analytical model, the reproduced stress–strain curves are compared with those of an existing model—the modified homogeneous anisotropic hardening (HAH) model. The obtained results show that the new constitutive model can successfully reproduce experimental Tension–Compression-Tension (TCT) and Compression-Tension–Compression (CTC) stress–strain curves of HCP sheet metals with considerably less percentage errors.

Characterization and Modelling of Sheet Material with Graded Strength for more Accurate Finite Element Analysis

Many sheet materials do not exhibit constant properties over the sheet thickness. On the one hand, this can be caused by a rolling process with a pass reduction of 1-2%, the so-called skin pass rolling. This is mainly used for deep drawing steel grades to eliminate the pronounced yield strength of the base material. Another possibility is abrasive blasting or shot peening of sheet metal. This causes plastic deformation of the sheet surface and material strengthening. The grading can be used to locally strengthen components or locally adapt the roughness. Since many production processes today are designed numerically, the mapping of such a grading is necessary but currently not implemented in FE code. Aim of this research is, to correlate the material hardness to a plastic pre-strain with a newly developed characterization method. For this purpose, graded material properties are generated by means of abrasive blasting as an example process. The resulitng locally varying work hardening over the sheet thickness is further integrated in finite element analysis by the assignment of a starting condition of a 2d-shell element formulation. With this approach, it is possible to map the graded material properties easily in FE simulations with a very good accuracy.

Significant Bauschinger effect and back stress strengthening in an ultrafine grained pure aluminum fabricated by severe plastic deformation process

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation.

Damage Assessment for Steel Structures Subjected to cyclic pre-strain

Plastic deformation provoked by unexpected events, such as earthquakes, can deteriorate the steel fracture toughness, and it can also induce premature failure of a structure and accidents. This study tries to formulate new equations to quantify the fracture toughness degradation of steel that was cyclic pre-strained. Four point bending tests were used to carry out the cyclic plastic pre-strain and Crack Tip Opening Displacement (CTOD) tests were conducted to compare pre-strained specimens with the steel without plastic deformation (as-received). The cumulative damage law was employed to account for the difference between each specimen, associated with Weibull stresses and critical CTOD values. Furthermore, additional Finite Element Method (FEM) simulations were conducted, using the new approach for the establishment an accurate Weibull Stress criterion, which can represent different cleavage fracture resistance according to the last pre-strain cycle. In the end, a new CTOD equation was also formulated using macro parameters, and it can account for the fracture toughness degradation of material that was subjected to cyclic plastic deformation.

Overview of an Experimental Program for Development of Yield Surfaces Tracing Method

This paper develops an experimental technique to evaluate the initial yield surfaces of metallic materials, as well as to study their evolution during plastic flow. The experimental tracing of yield surfaces is necessary for deriving and calibrating more robust phenomenological models of directional distortional hardening. Such models can be used to characterize the behavior of structures experiencing complicated and demanding loading modes, such as multiaxial ratcheting. The experimental technique developed in this work uses thin-walled tubular specimens, along with a servo-hydraulic machine, under various modes of tension/compression and torque. Identification of the onset of plastic flow is based on a small proof equivalent plastic strain evaluated from the outputs of a contact biaxial extensometer firmly attached to a specimen surface. This allows for evaluation of both the initial yield surface, as well as theevolved yield surface after a plastic prestrain. Throughout a test, continuous and fully automatized evaluation of elastic moduli and proof plastic strain is assured through algorithms written in C# language. The current technique is shown to provide promising results to effectively capture the yield surfaces of conventional metallic materials.

Influence of plastic prestraining on the fatigue crack propagation rate of S355MC and S460MC structural steels

AbstractCold forming induces significant plastic deformation at corner locations of structural steel members, resulting in both material strain hardening and the development of residual stresses, which influence the fatigue resistance of the steel members. Reliable fatigue assessment at such critical locations requires knowledge of the influence of pre‐existing plastic deformation on material behavior. The effect of plastic prestraining on the fatigue crack growth resistance of S355MC and S460MC structural steels has been evaluated experimentally. A comparative study, including fatigue crack growth and fracture resistance tests for both reference (non‐prestrained) and prestrained materials, has been performed. The experimental results, which are correlated to examined microstructural features and fracture characteristics of the materials revealed that fatigue crack growth rates of the prestrained material are higher than the reference material in the low Δκ region. Also, the S460MC steel exhibits higher crack growth rates compared to S355MC steel within the examined Δκ range.

Elemental segregation to lattice defects in the CrMnFeCoNi high-entropy alloy during high temperature exposures

The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2<110>-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a <110>-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2<110> dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing.

On the effect of plastic pre-straining on the corrosion behaviour of a duplex stainless steel and how the emergence of slip steps affects the hydrogen evolution reaction kinetics

The effect of increasing pre-straining on the corrosion behaviour of UNS S32304 duplex stainless steel has been studied using potentiodynamic tests in a highly acidic chloride-rich solution. The emergence of slip steps in the austenitic phase has been particularly addressed showing that the hence modified topographies had a significant impact on the kinetics of the Hydrogen Evolution Reaction (HER) that, in turn, tuned a non-monotonic behaviour of the corrosion rate. An attempt is made to link the variation of the Electron Work Function (EWF) of the surface with the electrochemical results by quantifying slip steps using Atomic Force Microscopy.

Effect of plastic pre-strain on the fatigue properties of welded joints of X80 LSAW pipes

The fatigue properties of the original, 1%, 2%, and 3% plastic pre-strained welded joints of X80 pipeline steels were tested to find the effect of plastic deformation on the service of pipelines. Results showed that the fatigue cracks growth period remarkably decreased with the increasing plastic pre-straining extent, reducing the safety of pipeline transportation. The decrease was caused by the cleavage fractures generated by dislocation synthesis reactions during the fatigue process. The results reveal the effects of dislocation distribution on fatigue property of the welded pipe after plastic deformation and provide a reference for the safety design of pipelines.

Dependency of the Young's modulus to plastic strain in DP steels: A consequence of heterogeneity?

The accurate springback prediction of dual phase (DP) steels has been reported as a major challenge. It was demonstrated that this was due to the lack of understanding of their nonlinear unloading behavior and especially the dependency of their unloading moduli on the plastic prestrain. An improved compartmentalized finite element model was developed. In this model, each element was assigned a unique linear elastic J2 plastic behavior without hardening. The model's novelty lied in the fact that:(i)a statistical distribution was discretized in a deterministic way and used to assign yield stresses to structures called compartments,(ii)those compartments were randomly associated with the elements through a random compartment element mapping (CEM).Multiple CEM were simulated in parallel to investigate the intrinsic randomness of the model. The model was confronted with experimental data extracted from the literature and it was demonstrated that the model was able to reproduce the dependence of the apparent moduli on the tensile prestrain. It was also observed that the evolution of the apparent moduli was predicted even if it was not an explicit input of the experimental dataset used to identify the input parameters of the model. It was then deduced that the shape of the hardening and the dependency of moduli on the prestrain were two manifestations of a single cause: the heterogeneous yield stress in DP steels.

Analysis of the Bauschinger Effect in Cold Drawn Pearlitic Steels

Prestressing steel wires usually undergo cyclic loading in service. Therefore, it is of interest to analyse certain features of their mechanical behaviour under this type of loading, such as the Bauschinger effect (BE) or the hardening rule, that fit the real mechanical behaviour appropriately. In this study, different samples of high strength pearlitic steel wires were subjected to cyclic tension-compression load exceeding the material yield strength, thus generating plastic strains. From the experimental results, various parameters were obtained revealing that analysed steels exhibited the so-called Masing type BE. In addition, the variation of the BE characteristics (of the effective and internal stresses) with the applied plastic pre-strain indicated that the studied materials followed a mixed strain hardening rule with the domination of the kinematic component.

Influence of Texture and Hydrogenation on the Mechanical Properties and Character of Fracture of Pipe Steel

As a result of tension of cylindrical specimens cut out in different directions relative to the axis of a pipe of gas main after long-term operation, we establish the differences between the mechanical properties of steel depending on the orientation of the specimen. As an important factor of influence on the mechanical behavior of the ferrite-pearlitic steel, we can mention its susceptibility to delamination in the direction of rolling. This trend is especially well manifested under the joint action of plastic prestraining and hydrogenation.

Establishment of damage estimation rules for brittle fracture after cyclic plastic prestrain in steel

The microprocess of brittle fracture in steel is suggested to occur because microcracks appearing in the brittle layer at the grain boundary propagate driven by increased piled-up dislocation. Thus, loading plastic deformation, equivalent to the increase in dislocation, is directly connected to the increase in brittle fracture risk. Because of these mechanisms, prestraining means approaching the limits of the material, which could lead to deterioration of fracture toughness. For a lifetime evaluation of steel structure, it is important to generalize the effect of various cyclic prestrains.In this paper, material toughness changes when tensile and compression prestrain occur were investigated. A general three-point bending test was employed to evaluate fracture. Synthetic analysis using Conventional Mechanism-based Strain Gradient Plasticity (CMSGP) theory, in which dislocation density is used to determine the amount of material damage, was carried out as well as employing conventional macroscopic material damage consideration rules. The consistency of the simulation and the experimental results were supported by the Electron Back Scattered Diffraction Pattern (EBSD) observation. Additionally, critical stress was calculated, and the change in critical stress for prestrain conditions was investigated. Then, the mechanism of critical stress changes could be expressed by the yield point increase and dislocation density.

Effects of Hardening Model and Variation of Elastic Modulus on Springback Prediction in Roll Forming

In this paper, the uniaxial loading–unloading–reloading (LUR) tensile test was conducted to determine the elastic modulus depending on the plastic pre-strain. To obtain the material parameters and parameter of Yoshida-Uemori’s kinematic hardening models, tension–compression experiments were carried out. The experimental results of the cyclic loading tests together with the numerically predicted response of the plastic behavior were utilized to determine the parameters using the Ls-opt optimization tool. The springback phenomenon is a critical issue in industrial sheet metal forming processes, which could affect the quality of the product. Therefore, it is necessary to represent a method to predict the springback. To achieve this aim, the calibrated plasticity models based on appropriate tests (cyclic loading) were implemented in commercial finite element (FE) code Ls-dyna to predict the springback in the roll forming process. Moreover, appropriate experimental tests were performed to validate the numerical results, which were obtained by the proposed model. The results showed that the hardening models and the variation of elastic modulus have significant impact on springback accuracy. The Yoshida-Uemori’s hardening represents more accurate prediction of the springback during the roll forming process when compared to isotropic hardening. Using the chord modulus to determine the reduction in elastic modulus gave more accurate results to predict springback when compared with the unloading and loading modulus to both hardening models.