Different Levels Of Plastic Strain Research Articles

Investigation of microstructure evolution and martensite transformation developed in austenitic stainless steel subjected to a plastic strain gradient: A combination study of Mirco-XRD, EBSD, and ECCI techniques

The evolution of the microstructure and deformation mechanism at different levels of plastic strain are investigated for 304 L austenitic steel with a combination of micro X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI). A plastic strain gradient is developed along the longitudinal rolling direction in a wedge-shaped 304 L austenitic steel sample. The graded deformed microstructure includes various amounts of deformation bands, ε- and α′-martensites, and their intersections form during the plastic deformation. ECCI observations reveal several deformation mechanisms of the formation of partial dislocations, dislocations, and interactions. This study suggests geometrically necessary deformation bands (GNDBs) are introduced and stored instead of geometrically necessary dislocations (GNDs) in low stacking fault energy (SFE) materials such as austenite steels. Consequently, increasing the strain gradient leads to an increase in the geometrically necessary martensitic transformation (GNMT); this is the result of the deformation-induced martensite in these materials. In addition to the statistically stored dislocations (SSDs), GNDs are generated at the grain boundaries of the fragmented grains to preserve the continuity of the grains. Accordingly, the strain hardening of the austenite steel includes multiple interactions of the deformation bands, SSDs, GNDs, GNDBs, and GNMT. From the viewpoint of microstructure design, our study provides quantitative information about the relationship between the amount of plastic deformation and the extent of microstructure evolution, in a continues design space.

Dynamic Strength of Copper at High Pressures Using Pressure Shear Plate Experiments

Measurement of strength at high pressures is critical to understanding the behavior of materials subjected to extreme loading conditions. Recent developments have extended the pressure shear plate impact (PSPI) technique to the high-pressure regime. Such experiments provide a unique opportunity to extract the complete stress–strain behavior of materials at relatively high pressures. The modified technique includes a new fiber-optic heterodyne transverse velocity interferometer system and new analysis methods to account for the inelastic response of the anvil plates. In this study, PSPI experiments are conducted on oxygen-free high conductivity (OFHC) copper at pressures ranging from 10 to 43 GPa and at strain rates of ~ 105 s−1. Complete stress–strain curves of copper are obtained using a hybrid methodology that relies on numerical simulations and particle velocity records gathered in these experiments. Experiments are conducted at similar pressures and strain rates using two types of anvils that undergo different levels of plastic strains: tool steel and tungsten carbide, to check the robustness of the hybrid method. The behavior of copper at these high pressures and strain rates is compared with previous literature data, and the effect of pressure and strain rate on the strength behavior of copper are discussed. It was observed that the rate of change in yield strength with pressure was 2.5 times the rate of change of shear modulus with pressure. The strength models that predict the response of copper at these extreme conditions are also examined and validated.

Nonlinear Rayleigh waves to evaluate plasticity damage in X52 pipeline material

This research uses nonlinear Rayleigh waves to examine different levels of plastic strain in samples made from legacy X52 pipeline material. Here the plastic strain is used as a surrogate for the damage associated with a pipeline dent. Tension specimens are subjected to a static, mechanical load. Nonlinear Rayleigh waves are used to measure the relative acoustic nonlinearity parameter, β′ in these specimens as a function of the accumulated plastic strain in the specimens. These results are then compared to previous β′ measurements made in A36 steel specimens, and a scenario based on the formation of dislocations is proposed to explain the increase in β′ observed. These measurements demonstrate the sensitivity of β′ to the microstructural changes caused by plastic deformation in X52 pipeline steel.

Damage assessment in Q690 high strength structural steel using nonlinear Lamb waves

The nonlinear ultrasonic technique has been used to assess the plastic damage in High strength Q690 steel. The Q690 steel was stretched to different levels of plastic strain so as to obtain the specimen with different degrees of plastic damage. When strain was increased from 1% to 9%, the nonlinear parameter shows a general upward trend, and in the subsequent strain process, the nonlinear response drop off instead of going up. Meanwhile, metallographic examination indicates that, within the strain 9%, the growth of the void as well as the variation of dislocation structure and density are the main damage characteristic in Q690 steel. Many dislocation structures such as the dislocation cell and dislocation wall have formed in the tensile specimen, which is contribute to raise of the nonlinear effect. The results indicate that the nonlinear ultrasonic technique can be utilized to characterize the plastic damage in Q690 structural steel at the early stage.

Effect of Plastic Deformation on Pitting Mechanism of SS304

Localized corrosion induced by plastic deformation has been reported for many alloys used in structural and functional applications. To understand the interplay of plastic deformation and pitting corrosion, samples of stainless steel 304 were subjected to different levels of plastic tensile strain and their corrosion behavior was studied. Plastic deformation is believed to alter materials’ surface condition, hence deteriorating the pitting resistance of materials. Results from this study suggest that in situ plastic deformation from 0.1 to 9 pct decreases the pitting potential of SS 304 to a similar level. Upon releasing the stress, slip steps formed at the metal surface due to plastic deformation contribute to the decrease in the pitting resistance of SS 304. Residual strain hinders pit repassivation to a minor extent. Results from electrochemical tests on samples with different levels of plastic strain are discussed in this paper.

Representative Stress Correlation of Global Scratch Quantities at Scratch Testing: Experimental Verification

Evaluation and correlation of global quantities, i.e., normal and tangential hardness, at scratch testing in the context of a representative stress description was investigated. In particular, verification based on experimental results is at issue. It has been shown previously that within the framework of classical von Mises elasto-plasticity and quasi-static conditions, correlation can be achieved by using a combination of stresses at different levels of plastic strains to define representative quantities at scratching, accounting for the difference in mechanical behavior at elasto-plastic and rigid plastic scratching. However, verification from experimental results is required, which has been attempted in this study. Predictions based on previous theoretical results were compared with experimental findings for polymeric materials, as well as for different metals. Good agreement was found between the two sets of results, particularly so for the case of polymers modelled by von Mises elasto-plasticity. Accordingly, these results are of direct practical, accurate, and novel relevance for semi-crystalline polymers where viscoelastic effects are negligible.

Effects of Hydroforming Process on Fatigue Life of Reinforced S-Shaped Bellows

Reinforced s-shaped bellows, which can withstand high pressure, is a kind of typical reinforced metal bellows. The reinforced s-shaped bellows mainly uses the hydroforming process, and the forming process is a severe plastic deformation process. The hydroforming process and its effects on the fatigue life of reinforced s-shaped bellows were discussed in the present study. Different levels of plastic strain and wall thickness thinning were detected in the hydroforming process. The maximum plastic strain can reached 32%, while the maximum wall thickness thinning ratio is 20%, which occurs on the wave peak. Mechanical characteristics of reinforced s-shaped bellows were discussed considering the effects of hydroforming process. The maximum stress appears on the upper and lower ends, which is the weak part of the structure. Fatigue life of the reinforced s-shaped bellows was analyzed based on the modified Manson-Coffin method. Mechanical properties of related materials, which can be more accurate consideration the effects of hydroforming process, were tested under the pre-plastic deformation. Fatigue life analysis of reinforced s-shaped bellows was carried out and the effects of hydroforming process were discussed. The hydroforming process will lead to a decline in fatigue life, which needs to be considered well in the structural design and analysis. Keywords: Reinforced s-shaped bellows, Hydroforming process, Fatigue life, Mechanical characteristics.

Continuum damage mechanics-based ductile behavior and fatigue life estimation of low carbon steels: AISI 1020 and AISI 1030

This paper presents an experimental estimation of the ductile behavior and low-cycle fatigue life for widely used structural steels AISI 1020 and AISI 1030 based on continuum damage mechanics approach. This method identifies the deterioration in stiffness of a material arising from micromechanisms of formation, growth, and coalescence of microvoids. This helps the characterization of the ductile flow behavior of metals through a damage variable D, evaluated via load–unload cyclic tensile test. The influence of strain hardening exponent, commonly treated as a constant in ductile flow characterization, is also explored in the current investigation. Its determination uses the Hollomon constitutive relation. Estimated D at different strain levels defines the corresponding effective stress. Application of this stress to the strain equivalence theory then enables the prediction of the stress–strain curve. The model-based results closely approximate the experimental stress–strain curve up to the onset of necking. The agreement of experimental results for fatigue life of the materials from low-cycle fatigue tests with damage-based low-cycle fatigue model demonstrates the correctness of the experimental findings. The damage-based model additionally helps in the prediction of microcrack nucleation and crack propagation life separately. Fractographic examinations of test specimen exhibit usually observed morphology of involved failure mechanisms. The present study emphasizes the experimental means of damage-based ductile flow assessment involving strain hardening exponent term and also the low-cycle fatigue life estimation. The significance of varying strain hardening exponent is further expressed in terms of the corresponding damage magnitude. The material data obtained from this study depicts the damage state at different levels of plastic strain that may serve as a useful information for metal-forming process design.

Determination of plasticity following deformation and welding of austenitic stainless steel

Intergranular strain has been associated with high-temperature cracking of welded pipework in 316H austenitic stainless steel material used in nuclear power plant heat exchangers. In this study, neutron diffraction has been used to study the development of intergranular strains in plastically-deformed and welded 316H stainless steel. Measurements have been made of the intergranular strain evolution with increasing plastic strain in base material, and correlated with further measurements made in samples extracted from welded pipes, where the pipes were welded following plastic deformation to different levels of plastic strain. Strong tensile strain evolution was seen on the compliant 200 grain family. The results were correlated with various proxy measures of plastic strain, including hardness and diffraction peak width, and excellent agreement was obtained.

Effect of pre-strain on susceptibility of Indian Reduced Activation Ferritic Martensitic Steel to hydrogen embrittlement

The role of pre-strain on hydrogen embrittlement susceptibility of Indian Reduced Activation Ferritic Martensitic Steel was investigated using constant nominal strain-rate tension test. The samples were pre-strained to different levels of plastic strain and their mechanical behavior and mode of fracture under the influence of hydrogen was studied. The effect of plastic pre-strain in the range of 0.5–2% on the ductility of the samples was prominent. Compared to samples without any pre-straining, effect of hydrogen was more pronounced on pre-strained samples. Prior deformation reduced the material ductility under the influence of hydrogen. Up to 35% reduction in the total strain was observed under the influence of hydrogen in pre-strained samples. Hydrogen charging resulted in increased occurrence of brittle zones on the fracture surface. Hydrogen Enhanced Decohesion (HEDE) was found to be the dominant mechanism of fracture.

Hardness, electrical resistivity, and modeling of in situ Cu–Nb microcomposites

This paper reports our investigation on the evolution of microstructure, hardness, and electrical conductivity of in situ Cu–16.5vol.%Nb microcomposites as a function of drawing strain. Both interface area density and lattice distortion were taken into account for analyzing the hardness and resistivity at different levels of plastic strain. A model was developed for describing microstructure dimensions including both spacing and curl of Nb ribbons. Based on this model, a modified Hall–Petch formula was derived. Our results suggest that interface area density and lattice distortion have significant impacts on both hardness and electrical resistivity.

On representative stress correlation of global scratch quantities at scratch testing of elastoplastic materials

Scratch testing is studied aiming at correlation of global quantities in the context of a representative stress description. Previous investigations concerning this matter have shown that good accuracy predictions for different material classes, within the framework of classical Mises elastoplasticity, can be achieved by using a combination of stresses at different levels of plastic strains to define representative quantities at scratching. It is important to emphasize though that the correlation must account for the difference in mechanical behavior at elastoplastic and rigid plastic scratching and that pertinent results have only been presented for the normal scratch hardness. It is therefore the aim presently to investigate if such a correlation can be extended to apply also for global scratch quantities in general. From a practical point of view the results are valuable both when describing the mechanical behavior at scratching and at material characterization. Predictions based on the present study can within the framework of classical Mises elastoplasticity and quasi-static conditions, be made for a wide range of materials ranging from polymers and ceramics to metals.

Evaluation of plastic damage for metallic materials under tensile load using nonlinear longitudinal waves

An experimental method based on nonlinear ultrasonic technique is presented for evaluating material damage due to plastic deformation in a metal plate. In this approach, specimens made from AZ31 magnesium–aluminum alloy are loaded to produce different levels of plastic strains. The ultrasonic harmonics generated in AZ31 magnesium–aluminum alloy, as a result of the interaction of ultrasonic waves with substructural changes, are measured precisely and the acoustic nonlinearity parameter (ANP) based on the fundamental and second harmonics is determined. The results show that the ANP increases monotonically with the level of plasticity (or the tensile stress). This research also shows the coincidence between the ANPs based on the microplasticity model and the measured results.

Effect of the forming process on the microstructure of aluminium parts for collective transports

The topic of the current work is an investigation of the microstructure of 6082 Aluminium alloy in artificial aged condition (T6 condition) before and after cold forging test. Aluminium is an essential choice when working on the lightening of metallic structures. Different levels of plastic strain have been applied to correlate the forging parameters with the microstructure changes. EBSD in combination with EDS analysis have been performed to well describe the microstructure properties. It was found that the local misorientation inside the grains increases with the nominal plastic strain. The formation of low angle grain boundaries (2∘ ) takes place both along initial grain boundaries (θ > 15∘ ) and within the original grains. This result is an indication of the local deformation of grains during cold forging.

On the correlation of scratch testing using separated elastoplastic and rigid plastic descriptions of the representative stress

The correlation of hardness values at scratch testing is studied within the framework of a differentiated description of representative stress values. The analysis is based on hardness values determined from a comprehensive finite element study of scratching of classical elastoplastic materials using a conical indenter. Large deformations are accounted for and the strain-hardening behaviour is varied in order to achieve generality of results. It is shown that good accuracy predictions for different material classes can be achieved by using a combination of stresses at different levels of plastic strains to define representative quantities at scratching. It is also shown that this correlation must account for the difference in mechanical behaviour at elastoplastic and rigid plastic scratching. From a practical point of view the results are valuable both when describing the mechanical behaviour at scratching and at material characterization. From the generality of the analysis, both elastoplastic and rigid plastic behaviour are accounted for, predictions based on the present study can be made for scratching of highly elastic materials such as polymers and ceramics but also for scratching of metals when plasticity dominates.

Effect of Plastic Deformation on the Corrosion Behavior of a Super-Duplex Stainless Steel

The role of plastic deformation on the corrosion behavior of a 25Cr-7Ni super-duplex stainless steel (SDSS) in a 3.5 wt.% sodium chloride solution at 90 °C was investigated. Different levels of plastic strain between 4 and 16% were applied to solution annealed tensile specimens and the effect on the pitting potential measured using potentiodynamic electrochemical techniques. A nonlinear relationship between the pitting potential and the plastic strain was recorded, with 8 and 16% causing a significant reduction in average E p, but 4 and 12% causing no significant change when compared with the solution-annealed specimens. The corrosion morphology revealed galvanic interaction between the anodic ferrite and the cathodic austenite causing preferential dissolution of the ferrite. Mixed potential theory and the changing surface areas of the two phases caused by the plastic deformation structures explain the reductions in pitting potential at certain critical plastic strain levels. End-users and manufacturers should evaluate the corrosion behavior of specific cold-worked duplex and SDSSs using their as-produced surface finishes assessing in-service corrosion performance.

A nonlinear-guided wave technique for evaluating plasticity-driven material damage in a metal plate

This research develops an experimental method for evaluating material damage due to plastic deformation in a metal plate using nonlinear-guided waves. An improved version of a previously proposed measurement technique is used. The material nonlinearities of aluminum specimens loaded to produce different levels of plastic strain are measured with Lamb waves. A pair of wedge transducers is used to generate and detect the fundamental (s1) and second harmonic (s2) Lamb waves. The weak amplitude of the second harmonic wave is extracted from the spectrogram of a received signal. The measured acoustic nonlinearity increases monotonically with the level of plasticity (plastic strain), which is similar to the behavior of longitudinal and Rayleigh waves. This result indicates that Lamb waves can be used to assess plasticity-driven material damage in the established framework of the second harmonic generation technique. The effects and the importance of signal processing in determining the nonlinearity parameter are also discussed.

Densification of sintered molybdenum during hot upsetting: experiments and modelling

The densification behaviour of sintered molybdenum is investigated experimentally and by modelling using a pressure dependent plasticity model. Therefore the yield condition of Gurson, extended by Tvergaard is used. The uniaxial compression test is applied to determine the evolution of the density as well as the stress–strain curves for the porous metal. Powder metallurgical molybdenum exhibits closed porosity after consolidation due to sintering with nearly spherical shaped pores. The experimental results show that the densification, especially during the first stage of deformation, is different from that of powder compacts or partially consolidated powder materials with open porosity. During hot upsetting, the pores change their size and shape. This behaviour strongly affects the densification rate. For an accurate prediction of the evolution of the density using Gurson’s model, the parameters q1 and q2 introduced by Tvergaard, will be defined as internal variables. The use of internal variables is justified by the fact that the pores change their shape during deformation, although the link between the internal variables and the pore shape is not explicitly established in this paper. If the loading is proportional (which means that the ratio of the stress-components does not change with plastic strain), the pore shape can be associated with the applied plastic strain. With this association the parameters qi can be defined as a function from the invariant quantity equivalent plastic strain, which can be used as the internal variable in the finite element simulation. The influence of the porosity on the flow stress at different levels of plastic strain will also be investigated and is used as a second information to fit both parameters q1 and q2.

The effect of plastic deformation on Al2CuLi (T 1) precipitation

The enhancement ofT1 precipitation in Al-Li-Cu alloys by plastic deformation prior to aging (that is, cold work) and the subsequent increase in alloy strength is investigated. The increased understanding of the role of matrix dislocations in the nucleation and growth ofT1 plates, discussed in the previous paper,[1] permits a detailed study of the phenomenon. In this paper, the effect of different levels of plastic strain on theT1 particle distributions as a function of aging time at 190 °C is quantified, and the subsequent influence on tensile properties is thereby described. The effect of plastic deformation is shown to decrease theT1 plate length and thickness, increase the number density by almost two orders of magnitude, increase the yield strength by 100 MPa, while simultaneously reaching peak strength in 20 pct of the time required without plastic deformation.