Inhomogeneous Strain Distribution Research Articles

Visualization of strain distribution around fatigue crack developed by low cycle fatigue test in high-strength steels using digital image correlation method for replica films

Strain distributions around the fatigue crack during the low cycle fatigue of high-strength ferrite + martensite dual-phase (DP) and tempered martensite (Mt) steels were visualized during cycles from the initial state to the crack initiation and crack propagation using a digital image correlation analysis applied to replica films. The replica film successfully transcribed the microstructure on the specimen surface. Although both the DP and Mt. steels exhibited a high tensile strength, the work-hardening rate of the DP steel was higher than that of Mt. steel. Although the numbers of cycles to failure (Nf) in the DP and Mt. steels were almost the same at the total strain amplitudes (εt) of 0.005–0.008 and 0.02–0.03, the Nf in the DP steel was higher than that in the Mt. steel at the εt of 0.008–0.02. The cracks were generated and propagated along the high-tensile-strain regions in both steels and were initiated when the accumulated strain along the loading direction exceeded a critical value regardless of the value of εt or the steel applied. The increment of the accumulated strain in the Mt. steel was higher than that of the DP steel at the εt of 0.01, whereas it was almost the same between those at the εt of 0.03. The inhomogeneity of the strain distribution, which was quantitatively evaluated from the histogram of the strain distribution, increased as the number of fatigue cycles increased. The strain was rather inhomogeneously introduced in the Mt. steel in comparison to the DP steel at the εt of 0.01, whereas almost no difference in the inhomogeneity of the strain distribution was detected between the steels at the εt of 0.03. Thus, the inhomogeneous strain distribution may promote crack initiation and propagation, resulting in a low Nf at the εt of 0.008–0.02 in Mt. steel.

Formation and dissociation of shear-induced high-energy dislocations: insight from molecular dynamics simulations

Solid-phase processing (SPP) allows one to create complex microstructures, not achievable via thermal processing alone. The resulting structures exhibit a rich palette of defects, both thermal and non-thermal, including defect substructures, such as dislocation networks. It is essential to understand the mechanisms of deformation and defect structure formation to guide SPP towards achieving desired microstructures and material properties. In this study, large-scale molecular dynamics simulations are used to investigate the effects of inhomogeneous strain distribution, that mimics deformation conditions of tribological tests, on the evolution of defects under severe shear deformation in polycrystalline Al. Analysis of defect nucleation and reaction pathways reveals that strong geometric constraints suppress the nucleation and slide of low energy dislocation 1/2⟨110⟩{111} but promote the nucleation and slide of high energy dislocations, such as (001) and 1/2(11). A rough contact surface, characteristic to tribological tests, imposes an inhomogeneous stress field leading to inhomogeneous defect substructures due to location-dependent activation of slip systems. The results suggest that high-energy dislocations can dominate the evolution of grain structures in highly constrained environments, which should be considered in modeling plastic deformation and grain refinement during SPP.

Highly Stretchable and Skin Adhesive Soft Bioelectronic Patch for Long‐Term Ambulatory Electrocardiography Monitoring

AbstractWearable devices offer a revolutionary approach to ambulatory electrocardiography (ECG) for cardiovascular disease diagnosis. Herein, an adhesive, highly stretchable and conformal (ASC) patch for long‐term ambulatory ECG monitoring is presented. The ASC patch is designed with a “three‐bridge” structure to provide inhomogeneous strain distribution during stretching. Meanwhile, the electrical stability is achieved by stretchable liquid metal interconnects at the domain experiencing larger strain. Moreover, the long‐term usability can be attained by an adhesive layer made of polydopamine/polyacrylamide glycerol–water hydrogel, maintaining good adhesiveness and stretchability for more than two weeks. Altogether, the patch can successfully measure stable high‐quality ECG signals in relaxed, stretched, or underwater conditions. Enhanced mechanical and electrical properties exhibited by our ASC patch confer superior skin‐device interface on either dry or wet conditions than that of current electrodes. The hydrogel‐based patch displays conformal deformation along with the stretched skin. This work provides a scalable prototype for multilead wearable ECG devices and promises new opportunities for medical applications in soft electronics.

Coal strain and permeability evolution in a 2D fractured model based on computerized tomography image reconstruction

Coal heterogeneity, especially fracture distribution, affects the mechanical response and fluid seepage in the coal reservoir under complex stress conditions, which is important for exploiting coalbed methane and preventing gas outburst. The internal pores and fractures of coal samples were scanned using an X-ray industrial CT detection system in the State Key Laboratory of Coal Resources and Safe Mining of China University of Mining and Technology (Beijing). Based on CT images, 3D models of coal samples were reconstructed using MIMICS reconstruction and digital terrain model threshold segmentation. The number, size, and fractal dimension of pores and fractures were calculated basing from the vertical section of the 3D models to evaluate the heterogeneity of coal samples, and 2D numerical models of fractured coal samples were established using the Image Import module of COMSOL Multiphysics. Coal strain and permeability evolution during the reduction of gas pressure under constant confining stress were calculated in the numerical models. Results show that the distribution of pores and fractures is dominant to coal permeability. With gas pressure decreasing, the distribution of strain and gas flow rate in the coal samples is inhomogeneous, the maximum of which is observed at the fractures. Furthermore, the shrinkage and gas flow rate of the samples reduce with decreasing gas pressure, thereby alleviating the inhomogeneous distribution of coal strain and gas flow rate. As the gas pressure is decreased, the porosity of the coal samples decreases, the initial width of the fractures increases, and the ratio of current flow to initial flow decreases. When the gas pressure is changed from 2.0 MPa to 1.5 MPa, the ratio of current permeability to initial permeability of the coal samples initially decreases and then increases. Therefore, a dominant switch of effective stress and gas desorption on the permeability and mechanical response occurs. When the gas pressure is decreased, the effect of coal matrix shrinkage due to gas desorption on the permeability of the coal samples exceeds the effect of effective stress.

The tensile deformation behavior of AZ31B magnesium alloy sheet under intermittent pulse current

In this paper, to investigate the effect of loading method of pulse current on the tensile deformation behavior of AZ31B magnesium alloy sheet, the intermittent pulse current with short-time and high-frequency was introduced in uniaxial tensile tests and the influence of duty ratio and loading time of pulse current on the deformation behavior of AZ31B magnesium alloy sheet was discussed. The strain and temperature field distributions on the specimens were measured during the intermittent pulse electrically-assisted tension (IPEAT), and the microstructure and fracture morphology under different pulse current conditions were observed. Results shows appropriate pulse current parameters can effectively improve the elongation of AZ31B magnesium alloy sheet. The strain of the sample is closely related to temperature distribution. With the deformation of the sample, the temperature on the sample increases gradually and the temperature distribution is non-uniform along the tensile direction, resulting in an inhomogeneous strain distribution of the sample. In addition, grain growth and dynamic recrystallization were observed on the AZ31B magnesium alloy sheet in different degrees under intermittent pulse current. Fracture morphology analysis shows that the number of dimples and tearing edges increased on the fracture obtained under IPEAT. The microhardness analysis shows that when intermittent pulse current is applied in the tensile test, the hardness of the sheet may change. This research provides an effective idea for the forming process of magnesium alloy sheets, which can be used to form large size thin-walled sheet components, and can significantly improve the forming quality of the sheets.

Effect of PWHT on the Microstructure and Mechanical Properties of Friction Stir Welded DP780 Steel

In the present study, the effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of friction stir welded (FSW) DP780 steel sheets was investigated. FSW was carried out at a constant tool rotation speed of 400 rpm and different welding speeds (200 mm/min and 400 min/min). A defect free weld was witnessed for both of the welding conditions. The mutual effect of severe plastic deformation and frictional heat generation by pin rotation during the FSW process resulted in grain refinement due to dynamic recrystallization in the stir zone (SZ) and thermo-mechanically affected zone (TMAZ). Lower tensile elongation and higher yield and ultimate tensile strengths were recorded for welded-samples as compared to the base material (BM) DP780 steel. The joints were subsequently annealed at various temperatures at 450–650 °C for 1 h. At higher annealing temperature, the work hardening rate of joints gradually decreased and subsequently failed in the softened heat-affected zone (HAZ) during the uniaxial tensile test. Reduction in yield strength and tensile strength was found in all PWHT conditions, though improvement in elongation was achieved by annealing at 550 °C. The digital image correlation analysis showed that an inhomogeneous strain distribution occurred in the FSWed samples, and the strain was particularly highly localized in the advancing side of interface zone. The nanoindentation measurements covering the FSWed joint were consistent with an increase of the annealing temperature. The various grains size in the BM, TMAZ, and SZ is the main factor monitoring the hardness distribution in these zones and the observed discrepancies in mechanical properties.

Mode-sensitive magnetoelastic coupling in phononic-crystal magnomechanics

The acoustically driven spin-wave resonance in a phononic-crystal cavity is numerically investigated. The designed cavity enables confinement of gigahertz vibrations in a wavelength-scale point-defect structure and sustains a variety of resonance modes. Inhomogeneous strain distributions in the modes modify the magnetostrictive coupling and the spin-wave excitation susceptible to an external-field orientation. In particular, a monopole-like mode in the cavity having a near-symmetrical pattern shows a subwavelength-scale mode volume and can provide a versatile acoustic excitation scheme independent of the field-angle variation. Thus, the phononic-crystal platform offers an alternative approach to acoustically control the spin-wave dynamics with ultrasmall and inhomogeneous mode structures, which will be a key technology to integrate and operate large-scale magnomechanical circuits.

Finite element analysis on shape deformation of the macro‐structure region during hot‐rolling process for gear steel FAS3420H and its modification

AbstractThe rectangular macro‐segregation region in the core of hot‐rolled steel bar has a great influence on the heat treatment distortion of an automotive gear steel FAS3420H. In order to understand the distortion process of the segregation region during hot‐rolling process, a finite element (FE) analysis of the whole hot‐rolling process was carried out by software DEFORM‐3D, and a constructed constitutive model was embedded into DEFORM‐3D to describe the deformation process. The results show there is an inhomogeneous strain distribution at the cross‐section of hot‐rolled round bar, which results in uneven distortion of the segregation region in different directions. A square segregation region, which can reduce inhomogeneous distortion of gear in the subsequent heat treatment, is obtained after being rolled by simulation calculation verified through experiment.

Influential factors on the deformability of colonies in pearlitic steel

The inhomogeneous strain distribution introduced by the tensile deformation of pearlitic steel was visualized using digital image correlation method, and the morphology of cementite was quantitatively evaluated. The high strain was introduced near the block boundaries. Within the blocks, the strain was inhomogeneously distributed. The ratio (Lcem/Lcol) of cementite length to colony size, which is related to the degree of spheroidization, was well correlated with the average strain within the colonies; spheroidized colonies exhibited a relatively high strain. However, although the Lcem/Lcol values were nearly identical, the values of the introduced strain were largely dispersed in the colonies. Based on the Lcem/Lcol values, the colonies were classified as spheroidized, partially-spheroidized, and lamellar colonies. In majority of the lamellar colonies, the introduced strain was lower than the average strain and tended to be higher in the colonies located near the block boundaries. On the other hand, for the partially-spheroidized colonies, the values of the introduced strain varied considerably and were sufficiently correlated with the Schmid factor (SFLF_min) for slip system, where dislocations can glide long distances without interrupting the cementite plate. Additionally, the difficulty of slip propagation across colony boundaries was found to affect the deformability of the partially-spheroidized colonies. The spheroidized colonies exhibited the highest strain among the three types of colonies and were not correlated with the distance from the block boundary or SFLF_min. Therefore, it was concluded that the degree of spheroidization was the main influence on the deformability of the colonies in the pearlitic steel. Meanwhile, the SFLF_min, distance from the block boundary, and difficulty of slip propagation across the colony boundary sufficiently affected the deformability of the colonies, causing variation in the strain value for colonies with equal Lcem/Lcol values.

In-situ investigations of the inhomogeneous strain on the steel case of 18650 silicon/graphite lithium-ion cells

The silicon-graphite composite electrode is the main focus of the high energy and high power density 18650 Li-ion battery cells, while the large volume expansion of the composite electrode may induce the steel case of the cell to deform largely, even rupture. The mechanical reliability of the steel case is very important to the safety and durability of the battery cell. In the paper, the hoop strain of the steel case is in-situ monitored during 18650 Li-ion battery cell operation. At the early cycle stage, the hoop strain of the steel case is induced by the thermal expansion. After a long-term cycle (cycled number > 300 times), inhomogeneous strain distribution, even negative strain, is found on the steel case. Based on the CT images and non-concentric numerical model, it is found that the gap between the jelly roll and steel case is the root of the different hoop strain on the steel case during the cycle. It is suggested that the gap (the diameter ratio of the jelly roll to steel case) need be focused in the engineering design for the cylindrical Li-ion battery cell with next-generation silicon composite electrode.

Strain-dependent luminescence and piezoelectricity in monolayer transition metal dichalcogenides

The modification of optical and electronic properties of transition metal dichalcogenides via mechanical deformation has been widely studied. Their ability to withstand large deformations before rupture has enabled large tunability of the bandgap, and further, the spatially varying strain has been shown to control the spatial distribution of the bandgap and lead to effects such as carrier funneling. Monolayer transition metal dichalcogenides exhibit a significant piezoelectric effect that could couple to a spatially inhomogeneous strain distribution to influence electronic and optical behavior. We investigate both experimentally and theoretically an example case of photoluminescence in structures with a strain distribution similar to that employed in single-photon emitters but generated here via nanoindentation. Using a mechanical model for strain induced by nanoindentation, we show that piezoelectricity can result in charge densities reaching 1012 e/cm2 and can generate electrostatic potential variations on the order of ±0.1 V across the suspended monolayer. We analyze the implications of these results for luminescence and exciton transport in monolayer transition metal dichalcogenides with spatially varying strain.

A prototype for light-electric harvester based on light sensitive liquid crystal elastomer cantilever

In this paper, a prototype of a light-electric mini-generator based on light sensitive liquid crystal elastomer cantilever is proposed. Firstly, the mechanism of this light-electric mini-generator is introduced. Then the governing equation of motion of the cantilever is established considering the light induced inhomogeneous strain distribution. By means of superposition method and Duhamel integral, the closed-form expressions for the vibration deflection, induced electromotive force and energy conversion efficiency are obtained. Numerical results show that the induced electromotive force and energy conversion ratio can be tuned by adjusting the light parameters and cantilever dimensions. The maximum voltage and conversion efficiency are in proportion to the light intensity, the contraction coefficient, the second power of thermal relaxation time and light decay distance ratio, but inversely proportional to the third power of damping factor. Based on the present material parameters, a maximum voltage up to 75.8 mV with a preferable energy conversion efficiency of 9.50 × 10−3% per magnetic induction intensity and circuit resistance can be achieved, which obviously could be greatly improved with the subtler design of cantilever and circuit materials.

Microstructural Changes During Static Recrystallization of Austenitic Stainless Steel 304L: Cellular Automata Simulation

Static recrystallization and microstructural changes in austenitic stainless steel 304L were studied. The rolling experiments at 200 °C were carried out, and then, annealing treatment was made at temperatures ranging between 500 and 830 °C. A model was also developed to simulate the kinetics of non-isothermal recrystallization within the rolled steel. The distribution of plastic strains during rolling was predicted utilizing an elastic–plastic finite element formulation performed in ABAQUS/Explicit, while the predicted results were used to generate the as-rolled microstructure and to estimate the stored energy. Finally, microstructural–thermal model based on cellular automata was developed to evaluate the rate of static recrystallization within the rolled steel. The comparison between experimental and simulations showed a good consistency. The predictions illustrated that inhomogeneous distribution of plastic strain was produced during multi-pass rolling leading to different rates of recrystallization in the center and the surface regions of the rolled plate. The onset temperature of recrystallization was found about 700 °C, and the activation energies for nucleation and growth for recrystallization were determined as 180 kJ/mol and 240 kJ/mol, respectively. It was found that homogenous nucleation mechanism can be operative in recrystallization of multi-pass rolled steel, i.e., for reduction of 40% or higher.

Deformation Mechanisms of 316L Austenitic Stainless Steel Tubes under Equal Channel Angular Pressing

This study investigates the deformation mechanisms of 316L austenitic stainless steel tubes processed by equal channel angular pressing (ECAP) at room temperature. Nanoindentation tests were used to study the influence of the 1-pass ECAP process on mechanical properties. The microstructure evolution of specimens subjected to 1-pass ECAP process was systematically analyzed using a variety of characterization methods. The results from microstructures observed through cross sections and longitudinal sections of deformed samples at different areas showed that the grains were refined significantly by the 1-pass ECAP process, and numerous deformation twins were generated. The microstructures also showed that the 1-pass ECAP process can cause uneven refinement of grains due to inhomogeneous strain distribution. Significant changes in grain orientation and micro-texture were found during the 1-pass ECAP process. The deformation mechanisms of samples subjected to the 1-pass ECAP process consisted of two stages: dislocation slip and twinning. The observed plastic deformation by dislocation slip occurred prior to activate twinning, and severe plastic deformation mainly occurred near grain boundaries or twin boundaries.

Raman Activity of Multilayer Phosphorene under Strain.

Using computational tools, we study the behavior of activities of lattice vibrational Raman modes in few-layered phosphorene of up to four layers subjected to a uniaxial strain of −2 to +6% applied in the armchair and zigzag directions. We study both high- and low-frequency modes and find very appreciable frequency shifts in response to the applied strain of up to ≈20 cm–1. The Raman activities are characterized by Ag2/Ag1 activity ratios, which provide very meaningful characteristics of functionalization via layer- and strain-engineering. The ratios exhibit a pronounced vibrational anisotropy, namely a linear increase with the applied armchair strain and a highly nonlinear behavior with a strong drop of the ratio with the strain applied along the zigzag direction. For the low-frequency modes, which are Raman active exclusively in few-layered systems, we find the breathing interlayer modes of primary importance due to their strong activities. For few-layered structures with a thickness ≥4, a splitting of the breathing modes into a pair of modes with complementary activities is found, with the lower frequency mode being strain activated. Our calculated database of results contains full angular information on activities of both low- and high-frequency Raman modes. These results, free of experimental complexities, such as dielectric embedding, defects, and size and orientation of the flakes, provide a convenient benchmark for experiments. Combined with high-spatial-resolution Raman scattering experiments, our calculated results will aid in the understanding of the complicated inhomogeneous strain distributions in few-layered phosphorene or the manufacture of materials with desired electronic properties via strain- or layer-engineering.

Magnetic ripple domain structure in FeGa/MgO thin films

The magnetic domain structure is studied in epitaxial Fe100-xGax/MgO(001) films with 0 < x < 30 and thicknesses below 60 nm by magnetic force microscopy. For low gallium content, domains with the magnetization lying in the film plane and domain walls separating micrometric areas are observed. Above x ≈ 20, the magnetic contrast shows a fine corrugation, ranging from 300 to 900 nm, suggesting a ripple substructure with a periodic oscillation of the magnetization. We discuss the presence of a random magnetic anisotropy contribution, that superimposed to the cubic coherent anisotropy, is able to break the uniform orientation of the magnetization. The origin of that random anisotropy is attributed to several factors: coexistence of crystal phases in the films, inhomogeneous distribution of both internal strain and Ga–Ga next nearest neighbor pairs and interface magnetic anisotropy due to the Fe–O bond.

The Fracture Behavior and Thermal Stability of Commercially Pure Nickel Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP) is one of the most promising severe plastic deformation (SPD) techniques suitable for producing ultra-fine grained (UFG) sheet or plane metallic materials with unique physical, chemical, and mechanical properties. However, the heavily deformed sheets are usually with high strength but low ductility due to work hardening and micro-cracking, and the UFG microstructure and high strength are unstable especially when exposed to high temperatures. Thus, in this work, CGP was conducted on commercially pure nickel sheets and, firstly, the fracture mechanism of the processed sheets was examined. Then, the annealing behavior of CGP nickel sheets was investigated in detail to explore their thermal stability and improve their ductility. The results showed that significant grain refinement and strength improvement of the nickel sheets were achieved with great loss of ductility. The fracture surface morphology of the sheet sample exhibited typical characteristics of fatigue fracture, and inhomogeneous strain distribution and work hardening determined the micro-crack initiation position and propagation direction. The CGP sheets by one and two passes showed high thermal stability up to 650 and 600 °C, respectively, owing to different stored internal stresses and accumulated energy. In both cases, obvious recovery of elongation to failure from 12.7% and 10.6% to 29.3% and 27.3% were achieved by CGP with post-deformation annealing treatment, respectively, with acceptable drop of strength.

The effect of size on the mechanical properties of rolled specimen: predictions and experimental evaluation

Size effect is very important in microforming. Depending on the surface layer model, the effect of size on the mechanical properties is attributable to the difference between the mechanical properties of the surface and the interior of a specimen. The mechanical properties of the surface of rolled specimens are different from that of its interior due to the inhomogeneous strain distribution of the specimen during the rolling process. Therefore, the size effect on the mechanical properties of the specimen can be adjusted via the rolling process. The strain distribution of the specimen associated with rolling is affected by the material and process parameters. Therefore, the size effect on the mechanical properties of the rolled specimen is affected by these parameters. To investigate this idea, the effects of material and process parameters, such as the strain hardening exponent (n), tensile yield strength (σ0), strength coefficient (K), thickness of billet (t), and reduction ratio (RR), on strain distribution during rolling was examined using the finite element method. Moreover, the influence of the aforementioned parameters on the size effect characteristics of the mechanical properties of the rolled specimens was predicted according to the strain distribution. It was determined that the strain distribution of the specimens was more homogeneous with increasing n and t, or decreasing RR. The σ0 and K had slight effects on the distribution. The size effect increased with increasing n and RR without surface cracks. Overall, there was good agreement between the experimental data and the predicted results.

Finite element simulation of X20CrMoV121 steel billet forging process using the Deform 3D software

In this article, a three-dimensional finite element analysis using Deform 3D software has been employed to investigate the plastic deformation behavior during forging of X20CrMoV121 steel. The focus was on the influence of forging temperature on the strain, stress and particle flow velocity distribution during the forging process. From the results, it has been shown that the forging process results in inhomogeneous plastic deformation, hence inhomogeneous strain, stress, and particle flow velocity distribution. As the temperature increased from 800 to 1100 °C, the forging loads and the deformation resistance were observed to decrease with an increase. For each temperature, the load increased rapidly with the stroke and then the increase slowed. This behavior was related to the coefficient of friction at the tool sample interface. It was also shown that the maximum effective stress/strain decreased as the forging temperature increased to 1100 °C. It is suggested that 3D finite element simulation by Deform® 3D software is an efficient method for predicting metal flow behavior during the forging process.

Interfacial stress transfer in strain engineered wrinkled and folded graphene

Because of its large surface area and flexibility, graphene has been explored for various applications such as flexible electronics and energy storage. Topographical undulations are commonly generated in graphene artificially by pre-strain and annealing to allow more tunable properties in hierarchical patterns. It is also possible, however, that strain engineering may induce negative effects, such as inhomogeneous strain distribution and fracturing of graphene. However, the correlation between those negative defects and the interfacial stress transfer in graphene upon strain engineering is still not clear. This is practically crucial as the stress/strain state in graphene is one of the dominant factors in governing other properties such as conductivity and wettability etc. in flexible electronics. In this work, wrinkles/folds (i.e. without/with delamination) have been introduced into graphene by annealing and pre-straining. An interfacial stress transfer efficiency factor ηr, is proposed to quantify the effect of wrinkles/folds by comparing the area under the measured strain distribution curves with the ones calculated theoretically using the ‘shear-lag’ theory. It is found that the wrinkles, with small amplitudes, do not affect the value of ηr significantly due to good stress transfer being maintained by the graphene still conforming to the substrate, whereas the folds, with high amplitudes, reduce ηr remarkably as a result of the absence of stress transfer due to delamination. The effect of wrinkling can be predicted by a geometrical model that takes the amplitude and wavelength of the graphene wrinkling into account. The monitoring and evaluation of the interfacial stress transfer and the strain distribution of wrinkled and folded graphene provide insights into its interfacial instability, and will have wide impact and applications in the strain engineering of graphene and other 2D materials for flexible electronics and devices.