Significant Plastic Strain Research Articles

Test-and-FE-based method for obtaining complete stress-strain curves of structural steels including fracture

Ductile fracture is a common limit state of frameworks of steel structures and can be predicted by incorporating models for fracture initiation and propagation in finite element (FE) analyses. However, accurate prediction of fracture relies on unique fracture parameters for a given material, requiring precise predictions of fracture strain and stress states obtained through coupon tests and accompanying FE data analysis. In recent decades, various methods have been proposed to predict ductile fracture initiation, including various design geometries of coupons, test setups and FE analyses, which may lead to inconsistent fracture strains and stress states. Hence, there is a need for proposing a standard procedure for the design, test and accompanying FE-based calibration of fracture parameters, as pursued in this paper. Given that ductile fracture initiates under significant plastic strain, a constitutive model for the full strain range is required, as also proposed in this paper. Moreover, to reduce the experimental and data analytical efforts, an empirical method is proposed that uses only a small number of coupon tests to calibrate the fracture parameters. The method encompasses three levels wherein one, two, or three coupon tests are required. All in all, the paper presents a methodology for determining the full-range true stress-strain curve and the initiation of fracture, including a new post-necking constitutive model and methods for determining the free parameters of the post-necking and fracture initiation models. While applied to steel in this paper, the proposed methodology is potentially also applicable to other metals.

The combined effects of carbides and martensite blocks heterogeneity on the fatigue life scatter in bearing steel

With the mitigation of inclusion-related harm through purifying methods, the fatigue life scatter of bearing steels is now predominantly governed by microstructural heterogeneity. This work employs an integrated experimental-numerical approach to elucidate the microstructure-related localized plastic strain in 52100 bearing steel under cyclic loading. We observed significant localized plastic strain in the coarse grains adjacent to aggregated carbides after unloading, which may act as a precursor to fatigue cracks and contribute to fatigue life scatter. This strain arises from two primary factors: stress concentration due to aggregated carbides and the relatively lower strength of coarse matrix grains, which is usually overlooked. Enhancing the homogeneity of both carbide distribution and matrix grain size will be a strategy to reduce the fatigue life scatter of bearing steels.

The Influence of Cyclic Loading on the Mechanical Properties of Well Cement

The cyclic loading generated by injection and production operations in underground gas storage facilities can lead to fatigue damage to cement sheaths and compromise the integrity of wellbores. To investigate the influence of cyclic loading on the fatigue damage of well cement, uniaxial and triaxial loading tests were conducted at different temperatures, with maximum cyclic loading intensity ranging from 60% to 90% of the ultimate strength. Test results indicate that the compressive strength and elastic modulus of well cement subjected to monotonic loading under high-temperature and high-pressure (HTHP) testing conditions were 14–21% lower than those obtained under ambient testing conditions. The stress–strain curve exhibits stress–strain hysteresis loops during cyclic loading tests, and the plastic deformation capacity is enhanced at HTHP conditions. Notably, a higher intensity of cyclic loading results in more significant plastic strain in oil-well cement, leading to the conversion of more input energy into dissipative energy. Furthermore, the secant modulus of well cement decreased with cycle number, which is especially significant under ambient test conditions with high loading intensity. Within 20 cycles of cyclic loading tests, only the sample tested at a loading intensity of 90% ultimate strength under an ambient environment failed. For samples that remained intact after 20 cycles of cyclic loading, the compressive strength and stress–strain behavior were similar to those obtained before cyclic loading. Only a slight decrease in the elastic modulus is observed in samples cycled with high loading intensity. Overall, oil-well cement has a longer fatigue life when subjected to HTHP testing conditions compared to that tested under ambient conditions. The fatigue life of well cement increases significantly with a decrease in loading intensity and can be predicted based on the plastic strain evolution rate.

Elucidating the deformation characteristics of Ti–Ni–Fe based pseudo-binary B2 intermetallics

The room temperature uniaxial compression behavior of homogenized Ti50Ni50-xFex (5≤x ≤ 45) pseudo-binary B2 intermetallics has been investigated. The compressive stress-strain curves of these intermetallics exhibited three different characteristics (a) intermetallics with low Fe content (i.e., Ti50Ni45Fe5 and Ti50Ni40Fe10): exhibited stress induced martensite (SIM) transformation (B2 → B19 ') (marked by a plateau with non-linear elastic regime) followed by significant plastic deformation up to ∼ 40 % strain marked with significant strain hardening, (b) intermetallics with moderate Fe content (Ti50Ni35Fe15) showed no SIM transformation, but a typical smooth elastic-plastic transition with maximum strain to failure of ⁓ 11–12 % was observed, and (c) intermetallics with high Fe content (≥ 25 at. %) displayed a very small strain to failure (≤ 5 %) without visible plastic deformation. The TEM and XRD analysis of the deformed (15 % and 40 % strain) Ti50Ni45Fe5 and Ti50Ni40Fe10 intermetallics indicated the presence of internally twinned martensite variants (MVs) along with severely dislocated banded B2 matrix. The observed contrast in the deformation characteristics has been attributed to the variation in the values of elastic constants (C' and C44) and Zener anisotropy (A) with Fe content. The SIM transformation in Ti50Ni45Fe5 and Ti50Ni40Fe10 occurred at lower values of C' and C44 (due to elastic softening) and relatively low Zener anisotropy A. The significant plastic strain in these two intermetallics was accommodated by the presence and deformation of super intrinsic stacking faults (SISFs) formed by dissociation of 12⟨111⟩ screw super-partials. In contrast, the deformation in the intermetallics with high Fe content (≥ 25 at. %) was governed by the activation of ⟨100⟩ screw dislocations.

The effect of grain boundaries and precipitates on the mechanical behavior of the refractory compositionally complex alloy NbMoCrTiAl

Refractory compositionally complex alloys (CCAs) have been found to exhibit promising high-temperature properties. However, the brittleness of most CCAs at room temperature limits their application. To understand the influence of microstructure on the deformation behavior of these alloys, we conducted micromechanical experiments, including nanoindentation and micropillar compression tests, on a representative NbMoCrTiAl alloy at room temperature. The indentation at the grain boundaries showed increased shear stress and hardness compared to the matrix (ordered B2 crystal structure) due to the presence of local intermetallic precipitates (C14 and A15 phases). Although the NbMoCrTiAl alloy exhibits no ductility at the millimeter scale as shown in previous studies, the micropillars at the grain boundaries, which were decorated with precipitates, demonstrated higher yield strength and significant plastic strain >30% at room temperature. Numerous slip lines were observed, while cracks and fracture occurred at higher levels of deformation. The grain boundaries with precipitates increased the probability of crack initiation and propagation, but they did not necessarily lead to catastrophic failure of the pillars.

Influence of overlapping friction stir processing on microstructural evolution, texture development, and mechanical performance in Al6061

In the current examination, a large-area stirred zone has been created in Al6061 through the application of overlapping friction stir processing. The investigation focused on examining the microstructural and texture changes in friction stir processed Al6061, using the electron backscattered diffraction technique. Electron backscattered diffraction images provided valuable insights into the recrystallization process, revealing the presence of fine-grained structure with high-angle boundaries in the large-area stirred zone attributed to intense plastic straining and dynamic recrystallization. The use of the tool caused significant plastic straining, resulting in the development of the A- (1 1 1) {1 1 0} texture component. Mechanical testing demonstrated a noteworthy increase in ductility after processing. However, the microhardness and strength were observed to be inferior to the base material due to elevated thermal cycling.

Microstructure manipulation via machining and heat treatments in hybrid manufacturing of 316L stainless steel

Hybrid manufacturing combines additive and subtractive (machining) processes in a single platform to reduce the total time to fabricate a component in its final form. Here we show that if sufficient plastic deformation can be imparted to the material during machining, recrystallization can be triggered during post-fabrication heat treatment. This can enable highly localized microstructure control. We show that stainless steel 316L, when machined without coolant accumulates significant plastic strain compared to when machined with coolant that results in faster recrystallization kinetics during heat treatment. These effects are limited to the surface while the bulk microstructure remains unaffected.

An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint

This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.

Tailoring microstructure and mechanical properties of CP-Ti through combined treatment of pressure and pulsed electric current

In this work, a new processing technology of pressure plus pulsed electric current treatment (PPEC) was proposed to treat CP-Ti to simultaneously enhance its strength and ductility. Specifically, the CP-Ti was subjected to 50 MPa pressure and pulsed electric current (PEC) with variable intensities (1400A, 1600A, and 1900A) using a spark plasma sintering system. Afterwards, the microstructure evolution, texture change, and mechanical properties were characterized by XRD, EBSD, and tensile tests. The increased grain size and noticeable microstructure change occurred with increasing PEC intensity. Additionally, applying 50 MPa pressure leads to the formation of finer α lamellae colonies and higher content of high-angle grain boundaries, which act as barriers for dislocation motion after yielding, resulting in enhanced strain hardening rate and consequently improved mechanical properties. The processed CP-Ti exhibits a yield strength of 350 MPa, an ultimate strength of 702 MPa, and a significant plastic strain of 39.1%, superior to those of CP-Ti processed by severe plastic deformation and pure PEC treatment. The results provide a new strategy to develop Ti materials for demanding structural applications.

Mapping plastic deformation mechanisms in AZ31 magnesium alloy at the nanoscale

By coupling an improved speckle patterning method enabling high resolution digital image correlation (HRDIC) at nanoscale strain resolution (146 nm) with a scanning electron microscope allowing autonomous experimental control and image acquisition during in situ tensile straining, we have mapped the plastic deformation in AZ31 Mg alloy at the grain scale to significant plastic strains for the first time. Complemented by electron backscattered diffraction to reveal the underlying grain orientations this has delineated the activation of intragranular extensional twinning, slip, and intergranular deformation mechanisms. Using a new grain boundary (GB) strain mapping algorithm, displacements tangential to the GBs as large as 140 nm at 2% plastic strain have been recorded. Critically, this occurs not so much by sliding at the boundary but by slip over a mantle region extending 450 nm from the GBs. Within the grain interiors, dislocation activity is much less, taking place mainly by basal slip. Reverse dislocation slip and detwinning were found to occur on unloading emphasising the need for in situ measurements. The proposed methodologies (gold speckle pattern remodelling, automated in situ HRDIC tensile testing and GBS activity analysis) have the potential to characterise the real-time deformation behaviour of a wide range of engineering alloys at the grain scale at room and elevated temperatures.

Study on the crack nucleation mechanism of Ti-2Al-2.5Zr alloy in low cycle fatigue: Quasi in-situ experiments and crystal plasticity simulation

The fatigue crack nucleation mechanism of Ti-2Al-2.5Zr alloy was investigated using the coupling method of quasi in-situ tests with crystal plasticity (CP) simulation. The constitutive parameters in the CP model were validated by digital image correlation based on scanning electron microscope (SEM-DIC). It was found that the transgranular and intergranular cracks nucleated inside grains and at grain boundaries (GBs), respectively, due to slip intrusions/extrusions along prismatic planes with highest SFpris. values, while quasi transgranular cracks were induced by the adjacent microstructure or cracks. The angle Ω between the sample surface and the slip direction of slip planes where the transgranular cracks nucleated was within the range of 20–55°. Three factors for predicting intergranular cracks were identified, and the slip shearing mechanism of GBs was proposed. The CP results revealed that a significant plastic strain difference was generated at GBs where intergranular cracks nucleated. The band averaging method showed better prediction ability than the grain averaging method, and a unified fatigue indicator parameter (FIP) combining the accumulated plastic strain, the strain incompatibility parameter (SIP), and the abovementioned crack prediction factors, was proposed, which improved the unified prediction ability for both transgranular and intergranular cracks.

Analysis of Bearing Safety and Influencing Factors of Supporting Structures of Hydraulic Tunnels in Cold Regions Based on Frost Heave

In order to study the bearing safety and influencing factors of the support structures of hydraulic tunnels in cold regions under the action of low-temperature frost heave, a mechanical model of the support structure and surrounding rock was established. Taking a hydraulic tunnel of a hydropower station in Xinjiang as the research object, a combination of field measurement and a numerical simulation method was adopted to study the bearing safety of the support structure during a period of freezing weather. Based on this model, the effects of different thermal expansion coefficients, temperature differences, and surrounding rock porosity on the bearing safety of the support structure in the low-temperature region were studied. From the calculation results, it was concluded that the simulation results of the numerical model established by using the mechanical model in this paper were in good agreement with the actual measurement results of the project. The circumferential freezing and compressive stresses at the arch waist of the supporting structure of the project were the largest, and significant plastic strain was generated near the arch waist. The displacement at the arch of the supporting structure was the largest, while the weak points were at the arch waist and arch top of the supporting structure. The coefficient of thermal expansion, greater temperature difference, and increased porosity of the surrounding rock all led to an increase in the rock freezing and swelling force to varying degrees, thus reducing the load-bearing safety of the supporting structure. The research results could provide a theoretical basis and a reliable mechanical and numerical simulation model for establishing the bearing safety of tunnels in the cold region.

Rutting Predictions in Flexible Airfield Pavements Using a Newly Modified Drucker–Prager Combined Hardening Model with Progressively Evolving Yield Surface

Granular materials subjected to compressive cyclic loading of constant amplitude when modeled using an isotropic hardening, elastoplastic Drucker-Prager (DP) model do not accumulate significant plastic strain beyond the first loading cycle. This can lead to predictions of permanent deformations in airfield asphalt pavements that do not compare well with experimental observations. Accordingly, this paper proposes a modification to the existing DP model as a means of accounting for this shortcoming. A new parameter, γ, is introduced, which causes the yield surface to progressively evolve and shrink in size during successive load cycles to account for microphysical restructuring of the granular material during constant amplitude cyclic loading. The constitutive model is implemented within the finite element code ABAQUS through the user material subroutine UMAT. The model is subsequently tested via 2D finite element analysis on pavement sections by applying it to the granular base and modeling all other layers as linear elastic. Validation is undertaken by comparing rutting with field experimental values from an accelerated pavement testing program. Increased equivalent plastic strain and vertical plastic strains are observed with this modification, thereby resulting in rutting values that are significantly closer to the experimental data than those obtained with the classical DP model.

Low-Temperature Deformation and Fracture of Cr-Mn-N Stainless Steel: Tensile and Impact Bending Tests

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in <100>-oriented grains; this is suggested to be the reason of brittle cleavage fracture.

Physical Simulation on Weakly Cemented Aquiclude Stability due to Underground Coal Mining

In northwest China, underground mining is frequently conducted in weakly cemented rock environments, including the aquiclude that protects the aquifer from dewatering. In this context, understanding the aquiclude responses to longwall mining is significant for assessing the reliability of water-conserved mining in the weakly cemented rock environment. Taking the Jurassic and Paleogene coal measure geology in Yili Mine in Xinjiang Province, China, as a case study, the paper conducted a laboratorial three-dimensional simulation by configuring a longwall operation and induced groundwater migration. The study analysed the aquiclude depressurisation and revealed the aquiclude stability in response to longwall mining. The results indicated that the aquiclude had a significant plastic strain and self-healing ability in the ground depressurisation condition. The aquiclude experienced tension and then compression, and, accordingly, fracture initiation, propagation, and convergence, during which the aquiclude had significant bending deformation. On the aquiclude horizon, tensile fracturing dominated above the set-up and longwall stop positions. The self-healing behaviour was correlated to the high content of clay minerals and disintegration proneness. The simulation results had a good agreement with field measurements, suggesting that the aquiclude had a satisfactory water-resisting ability and that the simulation results were practically reliable.

Influence of Strain Rates during Severe Plastic Strain Processes on Microstructural and Mechanical Evolution in Pure Zinc.

The study presents an analysis of the influence of the plastic strain rate on the mechanical and structural properties of pure zinc. Thanks to the use of unconventional methods of plastic processing, the process of the equal channel angular pressing (ECAP) and the process of hydrostatic extrusion (HE), the tests were performed in a wide range of plastic strain rates, between 0.04 s−1 and 170 s−1. Plastic strain rate changes were carried out in the course of the significant plastic strain processes, and not on previously deformed samples. All tests were carried out at a constant value of plastic strain rate, ε ~ 2. A strong influence of the plastic strain rate on changes in the microstructure in zinc was observed during the tests. For the rates in the range of 0.04 s−1 to 0.53 s−1 its bimodal nature was observed, and in the range of 7 s−1 to 170 s−1 high homogeneity and evenness of grains related to the processes of continuous dynamic recrystallization was noticed. The effect of the strong homogenization of the microstructure was the increase in mechanical properties, yield point and tensile strength to the maximum values of UTS = 194 MPa, YS = 145 MPa at a strain rate of 170 s−1. Compared to the material with a bimodal microstructure, an over seven-fold increase in the elongation value was observed.

Enhanced Plasticity and Corrosion Resistance in Mg-Zn-Ca-Cu Amorphous Alloy Composite via Plasma Electrolytic Oxidation Treatment

In this study, a dendrite-reinforced Mg-based amorphous alloy composite was prepared through an in situ precipitation strategy. After plasma electrolytic oxidation (PEO) treatment, the Mg85.1Zn12.7Ca2Cu0.2 amorphous alloy composite exhibited enhanced plasticity and corrosion resistance in a simulated body fluid solution (SBF). The PEO-treated composite showed a significant plastic strain of 10.5 ± 1.1%, as well as outstanding strain-hardening behavior. The enhancement of plasticity may be attributed to the in-situ formed coating, which can not only serve as a propagation barrier for shear bands but can also introduce nucleation sites in the bands as a result of stress mismatch and compositional heterogeneity. The corrosion density in the SBF decreased by three orders compared with the composite substrate. The spontaneous formation of apatite on the porous layer demonstrated that the prepared PEO coating has high bioactivity. The current work may provide a fundamental basis for developing biomedical Mg-based alloys with excellent comprehensive mechanical properties and corrosion resistance.

Rapid Plastic Deformation of Cancer Cells Correlates with High Metastatic Potential.

Metastasis plays a crucial role in tumor development, however, lack of quantitative methods to characterize the capability of cells to undergo plastic deformations has hindered the understanding of this important process. Here, a microfluidic system capable of imposing precisely controlled cyclic deformation on cells and therefore probing their viscoelastic and plastic characteristics is developed. Interestingly, it is found that significant plastic strain can accumulate rapidly in highly invasive cancer cell lines and circulating tumor cells (CTCs) from late-stage lung cancer patients with a characteristic time of a few seconds. In constrast, very little irreversible deformation is observed in the less invasive cell lines and CTCs from early-stage lung cancer patients, highlighting the potential of using the plastic response of cells as a novel marker in future cancer study. Furthermore, author showed that the observed irreversible deformation should originate mainly from cytoskeleton damage, rather than plasticity of the cell nucleus.

Influence of twin boundaries and sample dimensions on the mechanical behavior of Ag nanowires

The tensile mechanical properties of silver nanowires produced by the polyol process and containing the characteristic pentatwinned microstructure are compared with single crystal nanowires prepared by template electrodeposition over a range of diameters, d, from 80 to 300 nm. The plastic flow strengths of both sets of nanowires show a significant size effect, ranging from about 400 MPa at d ≈ 300 nm to in excess of 1 GPa with d < 100 nm. The size effect is shown to follow the same empirical scaling law seen with other fcc structured metals. Transmission electron microscopy investigation of the deformed specimens found similar deformation structures in both classes of nanowire studied with relatively low dislocation densities found even after significant plastic strain. The deformed pentatwinned nanowires showed no evidence for any characteristic deformation structures associated with the twin boundaries running parallel to the nanowire axis.

Synergetic toughening mechanism of spherical α-La phases and micron-sized AlLa3 intermetallic in La-based bulk metallic glass composite

A La74Al14Cu6Ni6 bulk metallic glass matrix composite (BMGMC) with in situ spherical α-La phases and micron-sized AlLa3 intermetallic compounds is prepared by adjusting the processing parameters and presents a good combination of high strength and plasticity. Compared with conventional single α-La dendrite-reinforced La74Al14Cu6Ni6 BMGMCs that have a low fracture strength and plasticity, the obtained composite shows a significant plastic strain of 17% and a high fracture strength of 566 MPa. The results show that the formation of ductile bulky spherical α-La phases and hard AlLa3 intermetallic compounds can effectively hinder the rapid propagation of single shear band, and the synergistic effect of the two phases accounts for the significant increase in plasticity. Furthermore, in situ formed AlLa3 intermetallic compounds with a hardness of 238 HV, which has strong interaction with multiple shear bands, dramatically improve the fracture strength of the composite.