Plastic Strain Research Articles

Quantitative evaluation of local strain distribution induced by deformation twins and triggering effect of deformation twinning on neighboring grains in polycrystalline AZ31 magnesium alloys

{101‾2}<1‾011> deformation twinning commonly occurs in Mg/Mg alloys. In their polycrystals, however, twinning is not always activated at locations with high Schmid factor for {101‾2}<1‾011> twinning. It is believed that twinning behavior in polycrystalline Mg/Mg alloys can be understood with a consideration of interaction between adjacent twins as well as local deformation behavior on a micro- and meso‑structural scale. In this study, we theoretically estimate local plastic strain within twins by transforming coordinates and then compare it with that experimentally obtained from digital image correlation (DIC) analysis for understanding the reason for non-Schmid twinning in polycrystalline-Mg alloys. The theoretically estimated strain of twins was found to be in good agreement with the DIC-strain value. Furthermore, it was observed that the variant selection of twin can be explained by the compatibility parameter m’, rather than Schmid factor, suggesting that twin activation in polycrystalline-Mg is strongly influenced by adjacently appeared twins.

Non-isothermal Fractional Order Two-surface Viscoplastic Model for Stiff Clays

Stiff clay exists widely in the world, but its significant time- and temperature-dependent mechanical features have not been fully modelled. In the context of fractional consistency viscoplasticity and bounding/subloading surface theory, this study proposes a novel non-isothermal fractional order two-surface viscoplastic model for stiff clays. First, with proposing a generalized plastic strain rate, the isotach viscosity is modified and extended to both over-consolidated and non-isothermal conditions that takes into consideration the effects of temperature and OCR on thermal accelerated creep. Then, two strain rate and temperature dependent yield surfaces are proposed with isotropic and progressive hardening rules to consider thermal collapse, strain rate effects, and smooth transition from elastic to viscoplastic behaviors. Next, the stress-fractional operator of the loading surface, according to the principle of the fractional consistency viscoplasticity, is introduced to describe the non-associativity of stiff clays. Finally, the predictive ability of the model is validated through simulating triaxial tests on Boom clay with various stress paths considering the temperature- and time-dependent features of stiff clays.

Atomic-scale insights into the effect of layer thickness ratio on the tensile properties of heterostructured nano-aluminum

Gradient-structured alloys with lamellar microstructures show extraordinary potential in breaking the strength-ductility trade-off dilemma. When partial grain size exceeds the Hall-Petch limit, the deformation mechanism is not fully understood. In this study, the competing relationship between grain boundary (GB) softening and hetero-deformation-induced (HDI) strengthening in laminated nano-grained aluminum was investigated under uniaxial tensile loading. In the soft domains, the high activity of grain boundaries results in pronounced GB softening. However, when the volume fraction of the soft layer exceeds 50 %, the flow stress surpasses the predicted values of the rule of mixtures (ROM), demonstrating a significant HDI strengthening. A detailed analysis of microstructural evolution is conducted, including strain gradients, GB migration, and dislocation distribution. The effects of mechanical incompatibility between soft and hard layers on the distribution of plastic strain and overall material strength are elucidated in gradient structures. The study provides critical theoretical insights into the design and development of high-performance gradient nanocrystalline aluminum alloys, particularly in applications where a balance between strength and ductility is of paramount importance.

Numerical Simulation Study on Evolution Process of Carrying-Soil Effect of Rectangular Pipe Jacking

The carrying-soil effect is an important factor affecting the success or failure of rectangular pipe jacking construction. Because it is difficult to explore the dynamic evolution process of the carrying-soil effect in the field test, this paper relies on the pipe jacking project of a comprehensive pipe gallery in Suzhou City, and uses ABAQUS 2021 finite element software to simulate the jacking process of rectangular pipe jacking. Based on the theory of plastic mechanics of rock and soil and the related research and application status, the equivalent plastic strain index is introduced as the basis for judging the failure boundary of soil in the numerical simulation results, and the boundary between the carrying-soil area and the surrounding soil is determined, that is, the shear fracture surface. According to the principle of plastic potential energy, the development direction of the fracture surface is consistent with the direction of plastic flow, so as to predict the development direction of the boundary of the carrying-soil area. According to the change in the equivalent plastic strain cloud map with the jacking mileage, the evolution process of the carrying-soil effect is clarified, and the dynamic evolution law of the carrying-soil area is revealed, which is of great significance for predicting the disturbance of the surrounding soil during the construction of rectangular pipe jacking and accurately evaluating the environmental effect of the construction.

Finite element analysis of square hollow section steel braces incorporating different infill materials under cyclic loading

Previous researches have extensively explored the use of concrete as an infill material for hollow steel sections, but there is limited reference on other types of infill materials.This study investigates the use of various infills, including wood, glulam, and H-beam, in square hollow steel braces to enhance buckling strength, widen stress concentration in the mid-span, and broaden the accumulation of plastic strain to delay local buckling and prolong the fracture cycle life of the brace. The proposed finite element method was utilized to assess the cyclic behavior of the braces, validated through three types of experimental tests on both unreinforced specimens and brace with infill material. An analysis was then conducted on 21 models to examine important parameters, such as section width-to-thickness ratio, flexural rigidity, filler length, and filler material. The results show that infill materials extend the von-Mises stress distribution at mid-span and reduce the PEEQ ratio. Additionally, the infill strengthens the buckling resistance of the brace; however, the effect of the infill does not significantly increase in models with lower width-to-thickness ratios. The infill also helps to improve the ductility ratio and energy dissipation capacity of the brace. From the fracture point of the hysteresis loop, the specimens with infill wood or glulam, as well as the specimens with shortened infill length, all demonstrate an ability to prolong the cycle life of the braces. Moreover, specimens with larger flexural rigidity of infill H-beam dramatically improve the fracture cycle life, which is also favorable for extending the life of the brace before rupture. However, it is recommended that the flexural stiffness ratio should be kept below a certain threshold to prevent buckling mechanisms in the transition zone.

Deformation mechanism of dislocation slip and kinking behaviour in dynamic compression response of TiZrHfNb alloy fabricated by laser directed energy deposition

ABSTRACT In this study, the dynamic response and deformation mechanism of TiZrHfNb RHEA fabricated by laser directed energy deposition (L-DED) were researched under a dynamic compression test at a nominal strain rate of 3500 s−1. The L-DED TiZrHfNb alloy with columnar grains exhibits exceptional mechanical properties with a uniform dynamic flow stress of 1670 ± 10 MPa and a maximum plastic strain of 0.28 ± 0.03. The dislocation slipping and kinking behaviour dominate uniform plastic deformation. First, the dislocations are primarily confined within dislocation channels due to coplanar slip, and the evolution of them in dynamic compressive deformation process has undergone spacing refinement and crossing. Furthermore, the kinking behaviour induced by the lattice rotation with the T = <011> axis denotes the work hardening effect. Particularly, the second kinking behaviour effectively relieves local stress concentration and delays fracture before the formation of adiabatic shear band (ASB).

NUMERICAL SIMULATION AND OPTIMIZATION OF THE POTATO HARVESTER DIGGING SHOVEL IN HILLY AREAS

The shovel face inclination angle is a crucial parameter of the potato harvester digging shovel. Currently, there is limited research on the relationship between the inclination angles of curved digging shovel surfaces. The purpose of this study was to investigate the impact of the shovel face inclination angle on its soil crushing capacity in hilly terrain. A simulation model for the contact between the digging shovel and the soil was developed. The digging process of the shovel was simulated and analyzed under different shovel face inclination angles (α2). The analysis included comparisons of stress, Plastic Equivalent Strain (PEEQ), and energy. The results show that the larger the angle, the stronger its soil crushing ability; however, if the angle difference is too large, the phenomenon of soil flipping back occurs. When α2 = 16–17 °, the shovel length becomes excessive. When α2 = 18 °, the stress is too small. When α2 = 19 °, the energy is too large, and when α2 = 23–24 °, the PEEQ is too small. Therefore, the optimal shovel face inclination angle α2 is between 20 and 22 °, with fixed angles α1 = 15 ° and α3 = 13 °.

Super‐Strong, Super‐Stiff, and Super‐Tough Fluorescent Alginate Fibers with Outstanding Tolerance to Extreme Conditions

AbstractThe combination of multiple physical properties is of great importance for widening the application scenarios of biomaterials. It remains a great challenge to fabricate biomolecules‐based fibers gaining both mechanical strength and toughness which are comparable to natural spider dragline silks. Here, by mimicking the structure of dragline silks, a high‐performance fluorescent fiber Alg‐TPEA‐PEG is designed by non‐covalently cross‐linking the polysaccharide chains of alginate with AIEgen‐based surfactant molecules as the flexible contact points. The non‐covalent cross‐linking network provides sufficient energy‐dissipating slippage between polysaccharide chains, leading to Alg‐TPEA‐PEG with highly improved mechanical performances from the plastic strain stage. By successfully transferring the extraordinary mechanical performances of polysaccharide chains to macroscopic fibers, Alg‐TPEA‐PEG exhibits an outstanding breaking strength of 1.27 GPa, Young's modulus of 34.13 GPa, and toughness of 150.48 MJ m−3, which are comparable to those of dragline silk and outperforming other artificial materials. More importantly, both fluorescent and mechanical properties of Alg‐TPEA‐PEG can be well preserved under various harsh conditions, and the fluorescence and biocompatibility facilitate its biological and biomedical applications. This study affords a new biomimetic designing strategy for gaining super‐strong, super‐stiff, and super‐tough fluorescent biomaterials.

Cryogenic tribological behavior of coarse, ultrafine grained and heterogeneous Fe-18Cr-8Ni austenitic stainless steel

Commonly used austenitic stainless steels (ASSs) have some limitation in sliding wear conditions due to their relatively low yield strength and hardness. To improve the wear resistance, three kinds of microstructure (coarse grain (CG), heterogeneous structure (HS), and ultrafine grain (UFG)) are prepared, to investigate the grain size on the dry sliding tribological behavior, as well as wear mechanisms of Fe-18Cr-8Ni ASSs at room temperature (RT) and cryogenic temperature (−120 °C). The results indicate that at RT the UFG specimen exhibits the lowest wear rate during the wear tests, where the wear mechanisms are mainly oxidation wear and abrasive wear. While, when tested at −120 °C, the CG specimen exhibits the best wear resistance compared with that of the other two specimens due to its superior plastic deformation ability and strain hardening ability. Moreover, the HS specimen exhibits the lowest coefficient of friction (CoF), which is due to the abrasive particles generated on the contact surface provide a certain level of friction reduction, while the wear rate increases as these particles serve as third-party abrasives, which further removing material.

Experimental study on the deformation and failure characteristics of cement stone under true triaxial stress state

The cement stone is widely utilized to fill the gap between the formation and the casing in wells. A clear understanding of the deformation and failure mode of cement stone under true triaxial stress state is necessary to ensure the safety and stability of wells. To analyze the three-dimensional deformation and failure characteristics of cement stone, the loading paths with constant Lode angle were employed in conducting true triaxial tests on cement stone. The stress-strain curve and failure mode of cement stone are first analyzed. Then, the three-dimensional deformation characteristics are discussed in meridian and deviatoric planes. Both peak strength and deformability increase with minimum principal stress, but show a tendency to increase and then decrease with intermediate principal stress coefficient. With the increase of minimum principal stress and intermediate principal stress coefficient, the failure mode of cement stone transforms from brittle to ductile behavior. Under elastic stage, the strain path in the meridian plane is a straight line, and is under constant Lode angle within the deviatoric plane. Due to the existence of plastic strain, the strain path in the meridian plane displays a downward convex shape. The strain path in the deviatoric plane deviates from the reference of constant Lode angle for stress, directly reflecting the Lode angle dependence of deformation. Both deformation and failure characteristics of cement stone are dominated by the three-dimensional stress state. Besides, deformation of cement stone is closely related to the failure characteristics of cement stone.

In situ study of microstructure evolution and α → ω phase transition in annealed and pre-deformed Zr under hydrostatic loading

The detailed study of the effect of the initial microstructure on its evolution under hydrostatic compression before, during, and after the irreversible α→ω phase transformation and during pressure release in Zr using in situ x-ray diffraction is presented. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α→ω initiates at lower pressure for a pre-deformed sample but for a volume fraction of ω Zr, c&amp;gt;0.7, a larger volume fraction is observed for the annealed sample. This implies that the proportionality between the athermal resistance to the transformation and the yield strength in the continuum phase transformation theory is invalid; an advanced version of the theory is outlined. Phenomenological plasticity theory under hydrostatic loading is outlined in terms of microstructural parameters, and plastic strain is estimated. During transformation, the first rule is suggested, i.e., the average domain size, microstrain, and dislocation density in ω Zr for c&amp;lt;0.8 are functions of the volume fraction, c of ω Zr only, which are independent of the plastic strain tensor prior to transformation and pressure. The microstructure is not inherited during phase transformation. Surprisingly, for the annealed sample, the final dislocation density and the average microstrain after pressure release in the ω phase are larger than for the severely pre-deformed sample. The results suggest that an extended experimental basis is required for the predictive models for the combined pressure-induced phase transformations and microstructure evolutions.

A comparative study of low cyclic loading effects on plastic strain, nonlinear damping, and strength softening in fiber-reinforced recycled aggregate concrete

The incorporation of fibers significantly enhances the properties and structural performance of recycled aggregate concrete. This study introduces Fiber-Reinforced Recycled Aggregate Concrete (FRAC) by integrating steel fibers (SF) and polypropylene fibers (PPF) into the recycled aggregate concrete (RAC) matrix. A comprehensive series of cyclic loading tests, including reload and unload sequences, were conducted to meticulously analyze the deterioration of strength and energy dissipation capabilities of SF/PPF-reinforced RAC. The research examines the effects of fiber characteristics, volume ratios, lengths, and diameters on strength degradation after the peak stress along the stress-strain curve. Degradation models based on plastic strain were proposed for both SF-reinforced RAC and PPF-reinforced RAC. The study further investigates the fluctuation of strain energy in FRAC under low cyclic loading, revealing the correlation between plastic strain energy and equivalent viscous damping ratio. A nonlinear viscous damping model that accounts for the influence of the reinforcing index (RI) is proposed. This model effectively predicts the degradation of strength, the evolution of plastic strain, and the dynamics of nonlinear viscous damping in composite materials with varying fiber content. Ultimately, this research provides essential technical insights for the practical engineering application of RAC.

Experiment and prediction of the strain-range dependent ultra-low-cycle fatigue based on micromechanical cyclic void growth model

Under severe seismic action, the local stress concentration region in steel structures is prone to the ultra-low-cycle fatigue (ULCF) under high triaxiality and irregular cyclic deformation with intensively varying strain amplitudes. It emphasizes the necessity of the jointed effects of the stress triaxiality and strain range incorporated in the ULCF life prediction. Given the limited relevant studies in terms of the underlying micro-mechanisms, this paper focuses on experimental and modeling studies of the combined effects of stress triaxiality and strain range on the ULCF failure, based on Gurson-Tvergaard-Needleman (GTN) microvoid evolution theories. Monotonic tensile tests on three types of notched round bars suggest that fracture strain decreases with increasing triaxiality. Cyclic tests on two types of notched bars under various strain ranges confirm the combined effects of stress triaxiality and strain range. Given a specific triaxiality, the ULCF damage accumulation rate increases linearly as the tensile plastic strain range ramps up. For the same tensile plastic strain range, higher triaxiality results in a greater damage accumulation rate. In the modeling studies, the consistency of the GTN and uncoupled damage models has been proven in terms of ULCF failure of structural steel. Based on GTN theories, a new micromechanical cyclic void growth model (GCVGM) is proposed, considering the detailed processes of void nucleation, growth, and coalescence under monotonic and cyclic loadings. Moreover, the joint effects of stress triaxiality and strain range can be explicitly described within a unified framework, without additional empirical assumptions. Based on experimental and simulation results, the proposed GCVGM shows significant advantages over conventional models in accurately predicting ULCF failure under various loading protocols, with low average errors of 2.73 % and −0.91 % for two types of notched bar specimens, respectively.

Effect of heterogeneous microstructures on mechanical properties of thin gradient nanotwinned copper foils

Strengthening of metals generally weakens ductility, but gradient strengthening is an exception. Heterogeneous microstructures, characterized by variations in grain size and twin spacing (structural gradients), were incorporated into 20-μm thin nanotwinned copper (NT-Cu) foils to enhance their mechanical properties. Four distinct heterogeneous NT-Cu samples, including two-layer (2L) structures and three-layer (3L) structures, were produced through rotary electroplating. The mechanical properties, work hardening rates, and microstructural features of these samples were systematically analyzed and compared. The results showed that an increase of 9.5 %, 6.4 %, and 20.1 % in ultimate tensile strength, yield strength, and work hardening rate, respectively, could be achieved in 3L structures, and the elongation was maintained at 5.5 %. The significant structural gradients and two interfaces of 3L structures were confirmed to stimulate the production of geometrically necessary dislocations (GNDs), which increased strength and work hardening rate. Two interfaces of 3L structures also contributed to the efficient moderation of plastic strains and thus improved the ductility. This study significantly enhances the strength and ductility of thin electroplated NT-Cu foils by introducing heterogeneous microstructures, promoting the application of thin NT-Cu films in electric vehicles, batteries, and the semiconductor industry.

The Influence of the Parameters of the Skew Rolling Process for Bimetallic Elements on the Mechanical Properties and Structure of Materials.

This paper presents selected results of theoretical and experimental research into the manufacture of axisymmetric bimetallic components using three-tool skew rolling technology. In the tests, it was assumed that the outer layer would be a material intended for heat treatment. However, low-carbon steel was used for the core. Experimental investigations were carried out in an innovative CNC skew rolling mill. Tests were carried out at different technological parameters of the process. In addition, the geometric parameters of the billet and the way it was heated were analyzed in relation to the quality of the resulting weld between the two materials. The quality of the weld was assessed based on metallographic observation and on strength tests (shear method). On the other hand, theoretical studies were based on numerical modeling (FEM). The numerical analysis made it possible to determine the distribution of temperature, deformation and stress in the rolling bimetallic component. The results obtained indicated that it is possible to produce bimetallic materials from the proposed steel grades. In addition, a significant effect of the method of heating the billet in the chamber furnace on the microstructure in the joining zone and the shear strength was found. There was an increase in Rc strength of about 35% when using oxidation protection. The results indicated better strength when the billet is rolling with a smaller outer layer thickness (about 50 MPa). This was confirmed by the results obtained from the FEM analysis, which indicated higher values of plastic strain and the occurrence of higher compressive stresses in the near-surface zones of the rolled bimetallic forging, both of which facilitate the welding process. From the temperature distribution (in the range of (600-1200) °C) obtained during the rolling of the bimetal forging, it can be seen that contact with cold tools does not affect the temperature in the welding zone.

Fluorescent alginate fiber with super-strong and super-tough mechanical performances for biomedical applications

Emerging research attentions are focused on the development of fluorescent biomaterials for various biomedical applications, including fluorescence-guided surgery. However, it is still challenging to prepare biomolecules-based fluorescent fibers with both satisfactory biocompatibility and optimal mechanical properties. Here, we develop a fluorescent robust biofiber through using a tetraphenylethene-containing surfactant as the contact points between polysaccharide chains of alginate. This newly developed contact points not only strengthen the cross-linking network of polysaccharide chains, but also afford enough energy-dissipating slippage for polysaccharide chains. Consequently, the generated fluorescent fiber is endowed with highly improved mechanical performances from plastic strain stage. The experimental results indicate that the fluorescent fiber shows good mechanical properties of breaking strength of 1.10 GPa (12.09 cN/dtex), Young's modulus of 39.81 GPa and toughness of 137.26 MJ/m3, which are comparable to those of dragline silk and outperforming spider silk proteins and other artificial materials. More importantly, its satisfactory biosafety and wound healing-promoting ability as a fluorescent suture are solidly proved by both in vitro and in vivo assays, which opens an opportunity for its biological and biomedical applications. This study provides a novel strategy for the development of robust fluorescent biomaterials.

Research on the Gap Effect of Circular Concrete-Filled Steel Tubes Using the Improved Cohesive Zone Model

Understanding the influence of gap distribution characteristics on the mechanical properties of circular concrete-filled steel tubes (CCFSTs) under bending load is important for stability and support design in engineering projects. In this study, the improved cohesive zone model considering friction was used to describe the mechanical behavior of mortar interfaces. Meanwhile, the concrete damage plastic model and isotropic elastoplastic model were applied for core concrete and steel tubes. The improved cohesive zone model has a unified potential function that governs the Mode I and Mode II failure processes of mortar interfaces to realize the mechanical interaction between concrete and steel. A smooth frictional function was utilized in the elastic stage to calculate the accurate frictional effect. Furthermore, the capability of the model in addressing unloading and reloading was verified, and the fracture energy varied accordingly during the cyclic loading. Then, the mechanical response of CCFSTs was investigated under bending loads by setting different gap sizes and angles between the gap and loading direction. The results show that under three-point bending, the equivalent plastic strains at the middle part of CCFSTs are much larger and the peak bearing forces are much lower than the other degrees when the angles between the coronal gap axis and loading direction equal 0° and 180°. In addition, the order of the peak bearing forces, from highest to lowest, is when the height of the coronal-cap gap increases from 0.0 mm to 2.5 mm, 5.0 mm, and 7.5 mm. The significant effect makes it inappropriate to ignore the weakening of the structural performance caused by coronal gaps in structural design.

Analysis of high temperature and strain amplitude effects on low cycle fatigue behavior of pitting corroded killed E350 BR structural steel

High-tension structural steels are prone to accelerated fatigue damage from pitting corrosion and high-temperature. Despite adverse effects, research on their low cycle fatigue (LCF) behavior is limited. Specifically, studies analyzing temperature-dependent pit sensitivity effects, considering pit-related material’s susceptibility to surface topographic features variation and stress concentration are lacking. This study conducts LCF tests on pitting corroded killed E350 BR structural steel at multiple strain amplitudes and high temperatures. It develops temperature-dependent parameters, such as the cyclic softening pit sensitivity factor, suitable for integration into existing approaches like total cyclic plastic strain energy density (CPSED), power-law, average strain energy density (SED), Coffin-Manson, and pit stress intensity factor (pit-SIF). Further, it proposes multiple linear regression-based prediction models relating total CPSED and average SED with strain amplitude and temperature. Corroded specimens show higher plastic deformation and reduced peak stress, fatigue life, and total CPSED compared to uncorroded ones. The developed parameters, integrated with average SED approach, predicts LCF life within an error band of ±1.5, while power-law relationship reduces it to ±1.2. Moreover, pit-SIF approach estimates fatigue life within an error band of ±1.5. The findings provide critical knowledge for enhanced component design, leading to structural safety, performance, and fire resilience.

Incremental Growth Analysis of a Cantilever Beam under Cyclic Thermal and Axial Loads.

Ratcheting analysis for cantilever beams subjected to the thermomechanical loads is presented using the finite element method. The cantilever beam is constrained along the vertical direction, and plane stress conditions are assumed according to the bilinear isotropic hardening model. Two points are considered to obtain areas of ratcheting by using linear extrapolation. The results and output diagrams for ratcheting with elastic-perfect plastic behavior are illustrated. It was revealed that the beam behaves elastically after the first considerable plastic strain, which is seen in two shakedown regimes. The numerical results are verified with known and analytical results in the literature. The results indicate a strong correlation between the outcomes from the cyclic ANSYS Parametric Design Language (APDL) model and Bree's analytical predictions. This consistency between the finite element analysis and the analytical solutions underscores the potential of finite element analysis as a powerful tool for addressing complex engineering challenges, offering a reliable and robust alternative to traditional analytical methods.

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.