Large Strain Region Research Articles

PtS2/GeC van der Waals heterostructure: a promising direct Z-scheme photocatalyst with high solar-to-hydrogen energy conversion efficiency for overall water splitting under acidic, alkaline, and neutral conditions and in large-strain regions

In this study, first-principles calculations were used to investigate in detail the structural, electronic, optical, and photocatalytic properties of PtS2/GeC heterostructures.

Cyclic true stress true strain relation in the large strain region of common steel for welded structures

Cyclic true stress - true strain curves were obtained using general-purpose shipbuilding steel in the large strain region. Test methods are incremental step test and static tensile test after cyclic loading. The stress–strain curves for static and cyclic loading were compared to investigate the cause of the difference between each other. A simple method for estimating the cyclic stress-strain curves was proposed. The diameter and curvature of the smallest cross-section of the specimen were measured using telecentric measurement device, and the true stress - true strain curve was obtained using the Bridgman correction method. Using the measurement method, cyclic stress-strain diagrams were obtained in the region where the true strain exceeded 1% by performing cyclic tensile tests using the incremental step method and static tensile tests after cyclic loading. The influence of the test method and maximum displacement conditions on the cyclic stress-strain curve was small within the scope of this study. There was little difference in the elongation between the static tensile test and the static tensile test after cyclic loading. The difference in yield stress between static and cyclic loading was discussed in terms of macro-yield mechanisms at intergranular and transgranular. A simple method for estimating cyclic stress-strain curves from a static stress-strain curve was proposed. The specimens used in this study are general-purpose shipbuilding steels, and the results should be applicable to similar steels for welding and structures.

Deformation mechanism diagram and deformation instability of a Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy during the hot compression

Controlling the hot deformation mechanism of the titanium alloy and avoiding deformation instability are the key points in the hot forging of the titanium alloy. A high throughput experimental method was used to establish a deformation mechanism map efficiently. A continuously varying strain distribution from 0.3 to 1.4 was obtained by the deformation of the double-cone specimens. At the temperature of 910 °C and the strain rate of 1.37 s−1, Combining the analysis of metallography and strain-stress curves, the deformation mechanisms in different regions of the double-cone specimens were divided. Dynamic recovery (DRV) and dynamic recrystallization (DRX) work together in the low strain region (＜0.62), and DRX gradually becomes the dominant deformation mechanism in the medium strain region (0.62–1.02). In the large strain region (1.02–1.4), amounts of deformation bands (DBs) formed. Electron back scatter diffraction (EBSD) technique was used for the microstructure observation and the analysis of deformation mechanism. The results demonstrate that with an increase in strain, the recrystallization fraction increased from 83.8% to 96.3%. Combining the several results of the double-cone specimens, the hot deformation mechanism diagram with a temperature range of 850–930 °C, and a strain range of 0.3–1.4 was established. When the strain reaches a certain degree, it is difficult to further form new recrystallized grains at the grain boundaries. The formation of recrystallized grains within the deformation band has been characterized using transmission electron microscopy (TEM). Under the action of shear force, fine recrystallized grains form within the elongated grains. The concentrated fine recrystallized grains aggravate the microstructural nonuniformity of the Ti-55511 titanium alloy in this area, which finally leads to the deformation instability.

Characterization of metal-core piezoelectric fiber/aluminum composite in a large-strain region

To evaluate the suitability of metal-core piezoelectric fiber/aluminum composites for structural health monitoring, their output characteristics and mechanical properties were evaluated in large strain regions exceeding 10%. Tensile vibration tests were adapted to the composites to evaluate the output charges in large strains; simultaneously, the mechanical properties were evaluated. Consequently, it was possible to obtain outputs even at approximately 20%, and the output was divided into three stages. The fracture behavior of the fiber was related to the change in output. The first stage involves the region where fiber fragmentation occurs due to mechanical load on the fiber and the fragmentation reduces the stress, resulting in a decrease in output charges. The second stage is a region where the fragment length reaches a critical length and the stress transmitted from the matrix becomes constant, resulting in a constant output. The third stage involves a sudden decrease in output due to core rupture. Further, the effect of residual stress was confirmed. The Weibull distribution of fiber strength, decreased with relaxation of the residual stress in the scale parameter from 9.31 MPa to 6.36 MPa, indicating that residual stress has a significant effect on the mechanical properties of the fiber.

Deformation mechanism diagram of a Ti–2.5Zr–2Al titanium alloy forged in the α+β region and grain refinement

The α+β forging is a key pass for the hot forging process of a Ti-2.5Zr–2Al titanium alloy. In this paper, the hot deformation mechanism of a Ti-2.5Zr–2Al titanium alloy was obtained by the double-cone compression test, which provides a theoretical basis for designing reasonable α+β forging process to obtain ideal microstructure. A wide range of gradient strains and corresponding microstructure were obtained by using the double-cone compression test. The double-cone compression process was simulated by the finite element software. Electron backscatter diffraction technique (EBSD) and transmission electron microscope (TEM) were used to characterize the deformed microstructure of the double-cone specimen. The hot deformation mechanism diagram in the strain range of 0.2–1.2 and the temperature range of 700 °C–850 °C was constructed. At 800 °C, in the low strain region (<0.476), the main deformation mechanism is dynamic recovery (DRV), and continuous dynamic recrystallization (CDRX) begins to occur. In the medium strain region (0.476–1.006), the deformation mechanism is mainly CDRX, accompanied by a small amount of the deformation bands (DBs). Nevertheless, in the large strain region (>1.006), a large number of the DBs occur. The ultra-fine grains with an average size of about 0.46 μm were obtained according to the hot compression under the strain of about 0.717 at 800 °C. The ultra grain refinement mechanism during hot deformation was summarized.

Correlation of stacking fault energy with deformation mechanism in Cu–(2, 20)Zn alloys

Copper–zinc alloys with different zinc contents contain different stacking fault energies (SFE). The influence of SFE on the deformation mechanism during surface mechanical attrition treatment was studied in this work. Research results indicate that the deformation mechanism directly correlated with the SFE in Cu–Zn alloys. Deformation twinning plays a paramount role during original deformation in Cu–20Zn (19 mJ/m2). However, for Cu–2Zn alloy with low-medium SFE (38 mJ/m2), the deformation mechanism is dominated by the dislocation slipping. Microbands are the predominant microstructural features in large strain and high strain rate regions for both Cu–Zn alloys in the present study. They are likely to be formed by the splitting of the high-density dislocation walls.

Simulation Study of the Effects of Interfacial Bonds on Adhesion and Fracture Behavior of Epoxy Resin Layers.

The adhesion and fracture behavior of tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM)/4,4'-diaminodiphenyl sulfone (44DDS)-bisphenol A diglycidyl ether (DGEBA)/44DDS layer interfaces were investigated by molecular dynamics (MD) simulation, mainly focusing on the role of covalent and noncovalent interactions. To accurately investigate the bond dissociation processes, the force field parameters of several bond potentials of the epoxy resin polymers were optimized by density functional theory calculations. In the MD simulations under a tensile load, small voids gradually developed without covalent bond dissociation in the plateau region. In the final large strain region, the stress rapidly increased with bond breaking, leading to failure. When the chemical bonds across the interface between the two layers were removed, the stress-strain curve in the initial elastic region was almost the same as that with interfacial bonds. This showed that the nonbonded interactions governed adhesion strength in the initial elastic region. In contrast, the bonded interactions at interfaces played important roles in the hardening regions because the bonded interactions made the major contribution to the fracture energies. We also investigated the effect of the etherification reaction in cross-linking. It was found that the etherification reaction mainly contributed to the behavior in the late region with large strain. These simulation results revealed that the nonbonded interactions, especially, van der Waals interactions, are important factors for adhesion of the different polymer layers in the small strain region up to the yield point.

Proposing a phase diagram for BNT-BKT-KNN and finding a new MPB and large strain region in the system

Detailed investigation on “1-x-yBNT-xBKT-yKNN” was done for better understanding of its phase diagram which is crucial for estimating a useful composition range for actuation applications. A phase diagram is proposed along with a MPB between “rhombohedral ferroelectric and tetragonal ferroelectric” phases in the range x = 0.05 (0.04 ≤ y < 0.08) and x = 0.10 (0.02 ≤ y < 0.06). However addition of KNN above the critical value y ≥ 0.12 is observed to drive the system toward cubic structure. A region having large “electric field induced strain (EFIS)” value (∼0.45%) is also identified in the phase diagram.

A method of reverse and revision of material mechanics performance parameters for vehicle collision simulation

In the process of vehicle collision, the steel material will be deformed by the impact speed. The stress under the same strain will be affected by the change of strain rate. The accuracy of the strain rate curve directly determines the accuracy of the model simulation. This article studies the optimization of material mechanical performance parameters and the correction reprocessing methods of the stress that cannot be obtained by the test in the strain interval. This paper proposes the principle of the modified constitutive equations and the corresponding formula. Based on the approximate model of thin-walled beam under the dynamic impact, the correction coefficient is obtained reversely. While ensuring the accuracy between fitting curve based on the material constitutive equation and the experimental data, the modified constitutive model solves the effect of the non-convergence of stress in a large strain region on the accuracy of the collision simulation model. The dynamic drop weight test results of thin-walled beams show that the modified constitutive equation has great significance for improving the accuracy of simulation models under high-speed impact conditions.

Relationship between mesoscale structure and ductility of drawn high carbon steel wire

This study describes the evolution of a mesoscale structure that was characterized using fiber textures and mechanical properties versus drawing strain up to the drawing limit: the wire was drawn without causing rupturing, for the drawn high carbon steel wires (initial diameters: 0.276, 0.444 and 0.936 mm). Crystal orientation analysis using an electron backscatter diffraction pattern showed that the evolution of the mesoscale structure followed four steps with increasing drawing strain, regardless of the initial wire diameter. First, the wire consisted of only a primary fiber texture {100}<110>−{111}<110>. Second, the wire had primary and secondary {110}<110>−{111}<110> fiber textures in the outer and inner sides, respectively. Third, the wire had subprimary {100}<110>−{111}<110> and secondary fiber textures in its outer and inner sides, respectively. Fourth, the wire only consisted of a subprimary fiber texture. Results obtained through tensile testing showed that uniform elongation increased but the reduction of area decreased as the initial diameter increased over the entire drawing strain range, when there were no differences in lamellar spacing and tensile strength for the patented wires. Furthermore, uniform elongation decreased but the reduction of area increased when the ratio of thickness of secondary fiber texture to the wire radius increased. This study suggested that maintaining the thickness of secondary fiber texture in a large drawing strain region contributes to the improvement of drawability.

Flexible Ti3C2Tx MXene/ink human wearable strain sensors with high sensitivity and a wide sensing range

Flexible strain sensors still face great challenges in combining high sensitivity, wide detection range, good mechanical stability and simple manufacturing processes into sensors. Herein, this paper reports a simple and low cost strain sensor fabricating method based on two-dimensional (2D) material (MXene) and ink, which can be large-scale fabricated into multiduty sensors with high sensitivity and excellent durability by spray-coated process and wrapped by nylon fabric. Owing to the fact that MXene has excellent dispersion in printing ink material, the sensor obtains low percolation threshold of 3.6 wt% and a percolation network based on Ti3C2Tx MXene/ink composites was precisely designed. The migration of the MXene lamination leads to a large change in resistance, while the ink coating restricts the large displacement of the nanometer lamella to maintain the connectivity of the conductive path in the large strain region. The strain sensing test shows that the MXene/ink encapsulated by nylon fabric strain sensor exhibits a high sensitivity (gauge factor of 27.2 and 170.9 under 0 %–25 % and 25 %–29.7 % strain, respectively), good linearity (R-Square >0.967), and excellent reliability and stability (>2950 cycles). The as-developed MXene/ink strain sensor was demonstrated to monitor real-time both tiny and large human activities, showing that it has great potential in the field of wearable strain sensor.

Design and Biomechanical Analysis of a Novel Retrievable Peripheral Vascular Stent

Abstract Structurally retrievable drug-eluting stents may have valuable clinical applications because they do not leave any foreign materials inside the patient's body. This article presents a novel design of retrievable peripheral vascular stent and the results from biomechanical analysis of its performance. Using the finite element analysis method, principal parameters of the stent were studied. Moreover, to ensure the practicability of the retrieval process, simulation, and in vitro experiments were performed. The retrieval force reached the maximum value when the whole retrievable part had been retrieved. Furthermore, the force was gradually increased during the retrieval process and remained constant after the main part had been retrieved. When the stent was being compressed, the maximum strain of the stent occurred at the connection between the stent's retrieval part and the main body part, at a value of 4%. The index of nonuniformity of the stent was too small to be counted both at the end of the compression and self-expansion processes. With the increase of moment, the bending stiffness (EI) of the stent decreased gradually. After bending moment was applied, the large strain region was mainly located in the stent's main body part rather than the retrieval part. The results of preliminary stent retrieval experiments demonstrated that the stent could be retrieved successfully. This novel retrievable stent displays promising biomechanical performance. The preliminary experiments demonstrated that the stent could be retrieved smoothly from the blood vessels.

Evaluation of strength of stainless steel bolt without heat treatment considering Bauschinger effect during manufacturing process

Abstract Isotropic hardening models for elastic-plastic materials are often used to simulate cold forging processes, although it cannot represent the Bauschinger effect. However, the Bauschinger effect must be considered for precise simulation of the strength in service of cold-forged components without heat treatment, as well as their geometry. A cold-forged bolt made of stainless steel is a typical example. In this report, finite element analysis of the forming process and the strength was performed for two multi-stage cold forging processes of non-heat-treated stainless steel bolts. The Yoshida-Uemori model was used as a kinematic hardening model that takes into consideration the Bauschinger effect of a material. The Yoshida-Uemori model was modified so that they can represent the strain hardening behavior in a large strain region appropriately for applying the model to cold forging process. As a result, it was possible to account for the Bauschinger effect during the forming process. Moreover, it was possible to estimate the difference in strength between the isotropic and kinematic hardening models by analysis of the tensile test results. Furthermore, the impact of the forming process on the strength of the cold-forged bolt was also evaluated.

Mechanical considerations for design and implementation of peripheral intraneural devices

Objective. Neural interfaces designed to stimulate or record electrical activity from peripheral nerves have applications ranging from the electrical modulation of nerve activity as a therapeutic option (e.g. epilepsy and depression) to the design of prosthetics. Currently, most peripheral nerve interfaces are either cuff-style devices that wrap around the target nerve or intraneural devices that are implanted within the nerve. While the latter option offers higher specificity and signal-to-noise ratio, penetrating devices can cause significant damage to the nerve due to the high degree of mechanical mismatch. Because of this, there is interest in developing penetrating devices fabricated from soft or softening materials (materials having a low elastic modulus). However, there is currently a lack of understanding regarding implantation forces required for successful insertion, which is a constraint for soft device design. Softer devices require robust designs to achieve a critical buckling force that is larger than forces experienced during device insertion. Approach. This study comprehensively assesses insertion force under different implantation conditions, with three variations for implantation speed, angle, and device tip angle, during insertion of silicon shanks in rat sciatic nerve. Additionally, we report compression moduli for rat sciatic nerve at different compression rates to inform computational modeling. Main results. We found that insertion speed and angle had significant effects on peak insertion force. We observed lower insertion forces (10–60 mN) when the device was implanted at higher angles relative to perpendicular insertion (80–125 mN). We also demonstrate the use of a nerve-stabilizing device to keep the nerve immobile during implantation. Additionally, we found that compression moduli were significantly different in small and large strain regions of the stress–strain curve with values between 1500–4500 Pa depending on compression rate. Significance. This study provides information imperative to the design and successful implementation of soft penetrating peripheral nerve interfaces.

Experimental characterization and finite element modeling the deformation behavior of rubbers with geometry defects

Rubbers have broad applications in various areas. The engineering application requires accurately capturing the response of these materials under large deformation. In this work, we investigate the deformation behavior of two engineering rubbers (EPDM and Silicone rubbers) until failure occurs. The digital image correlation technique is used to capture the strain field of rubber specimens with different types of geometry defects. The results show that EPDM rubbers are more ductile than Silicone rubbers. For both rubbers a homogenous deformation behavior can be observed for specimens without defects, while strain localization behaviors occur for specimens with geometry defects. Finite element analysis (FEA) is also performed. The Arruda-Boyce eight chain model is used to represent the constitutive relationship of both rubbers with the parameters determined from the stress-strain response of specimens without geometry defects. The strain field obtained from FEA shows good agreement with the experimental data even in the large strain region close to failure. However, the simulated force-displacement is consistent with the experimental results only for specimens without geometry defects. For specimens with geometry defects, discrepancies exist in all the cases, which indiates that more mechanisms need to be incorporated into the constitutive relationship.

Effect of Anisotropic Plasticity on the Prediction of Formability of E-Form Magnesium Alloy Sheet

In this paper, the formability of E-FORM magnesium alloy sheet (as one of recent alloys for automotive magnesium sheets) is analyzed based on the comparison between the results of actual magnesium (Mg) roof forming and those of finite element (FE) simulations. The FE model considers anisotropic mechanical properties and forming limit diagram (FLD) of the investigated magnesium alloy sheet. Through the coupled experimental and numerical procedure, the dominating factors for improving the accuracy of the numerical simulations are further studied. A commercial finite element analysis program, AUTOFORM®, is used for the simulations, in which identified mechanical properties and forming limit criteria are implemented as functions of temperature and strain rate. The improvement in the prediction of formability during the warm forming of the E-FORM magnesium alloy sheet can be obtained by considering a modified hardening law at large strain region, sheet anisotropy, and the properly measured forming limit curve.

3-Dimensional Homogenization Finite Element Analysis of Foamed Rubber

In this study, 3-dimensional homogeneous finite element analysis of a truncated octahedral unit cell which consisted of hyperelastic beams was conducted to predict macroscopic mechanical characteristics of the foam rubbers considering its microscopic structure. The homogenization theory was applied to finite element analysis code and the unit cells for simulation were assumed to have the periodic boundary condition. An original finite-strain homogenization FEM code for hyperelastic material and truncated octahedral unit cell were developed. The relative density of the homogenization analysis was adjusted by width of the beams in unit cell model. The rubber matrix was assumed to be represented by the nearly-incompressible hyperelasticity. The Mooney-Rivlin model and nearly-incompressible condition were applied to the hyperelasticity. The material parameters of the Mooney-Rivlin model for foamed rubber matrix were identified by the tensile loading test results. The developed code showed enough stability to predict the mechanical property of foamed rubber in the large strain region.

Heterogeneous Strain Distribution of Elastomer Substrates To Enhance the Sensitivity of Stretchable Strain Sensors.

Stretchable strain sensors, which convert mechanical stimuli into electrical signals, largely fuel the growth of wearable bioelectronics due to the ubiquitous, health-related strain in biological systems. In contrast to rigid conventional strain sensors, stretchable strain sensors present advantages of conformality and stretchability, solving the mechanical mismatch between electronics and the human body. However, the great challenge of stretchable strain sensors lies in achieving high sensitivity, which is required for both signal fidelity and cost considerations. Recent advances to solve this sensitivity challenge have focused on material optimization, in search of the optimum combination of conductive active materials and elastomer substrates among a myriad of artificial or natural materials. However, high sensitivity with a gauge factor larger than 50 remains a grand challenge, especially within large-strain regions. Here we present heterogeneous strain distribution of elastomer substrates as a powerful strategy to significantly enhance the sensitivity of stretchable strain sensors. The theoretical foundation of this strategy is mathematically proven on the basis of Ohm's law in electrics and mechanics of materials. First, the extent of the sensitivity enhancement is proved to be determined by the local strain in resistance-testing segments of heterogeneous strain sensors. Next, the local strain is proved to be quantitatively decided by material properties such as section area and Young's modulus. Thus, the necessary and sufficient condition to achieve high sensitivity in heterogeneous strain sensors is that the Young's modulus reciprocal or section area reciprocal in the resistance-testing segment is larger than the mean value. This provides a theoretical design guideline to achieve high sensitivity via heterogeneous strain distribution. On the basis of this guideline, we systematically summarize concrete instances of heterogeneity-induced sensitivity improvement in stretchable strain sensors, in sequence of increasing dimensionality. A typical example of a one-dimensional heterogeneous strain sensor is a structured fiber with microbeads, where the varied section area along the fiber axis results in heterogeneous strain and sensitivity improvement. Two-dimensional heterogeneous sensors in the form of thin films contain thickness gradient sensors and auxetic mechanical metamaterial sensors. The former exhibit heterogeneous section area via the self-pinning method, while the latter show heterogeneity in both the strain direction and amplitude, leading to a 24-fold improvement in sensitivity. Three-dimensional strain sensors include rationally structured sensors for out-of-plane force detection and asymmetric active materials in electronic whiskers. The resultant enhanced sensitivity in these heterogeneous strain sensors is beneficial for applications such as continuous health monitoring, biomedical diagnostics, and replacement prosthetics, taking advantage of augmented detection accuracy and declined device cost. Finally, we discuss possible future work in exploiting heterogeneous strain distributions, involving extended methodology to achieve heterogeneity, employing suppressed strain for stretchable electrodes, cyclic durability for long-term applications, and multifunctional system-level integration. We believe that this strategy of using heterogeneous strain distribution to enhance sensitivity can strongly promote the development of stretchable strain sensors for both practical and theoretical requirements.

Work-hardening behaviour of sheet steels in large strain regions and its simple approximation

The equivalent plastic strains in stretch flanging and bending frequently exceed the strain equivalent to the uniform elongation in uniaxial tension. However, the uniaxial tensile test does not directly provide the true-stress and true-strain relationship in this strain region. In this study, the simple shear tests and the hydraulic bulge tests of an interstitial free steel, and 590 and 980 MPa grade dual-phase high-strength steels were carried out to investigate the work-hardening behaviour, i.e., the relationship between equivalent stress and equivalent plastic strain as well as the evolution of instantaneous n-value, in such a large strain region. A parameter k that correlates stress and strain for a simple shear (or equi-biaxial stretching) to an uniaxial tension leads to a good agreement of stress-strain curves for these testing modes. It was also found that k is practically independent of strain region in the present study. The work-hardening behaviour obtained by a simple shear or bulge test well agrees with that by a uniaxial tensile test. On the other hands, work-hardening equations proposed by previous studies do not reproduce the evolution of instantaneous n-value, i.e., a strain dependency of the n-value, for the present steel grades. Thus, the authors propose a new work-hardening equation that can reproduce the strain dependency of the n-value for any steel grade in this study.

Modelling of silk-reinforced PDMS properties for soft tissue engineering applications.

Polydimethylsiloxane (PDMS) is widely used in biomedical research and technology, but its mechanical properties should be tuned according to the desired product specifications. Mixing ratio of base polymer to curing agent or additives enables its mechanical properties to be manipulated and fit to mechanical properties of biological tissues. In this paper, we analysed the effect of mechanical load on silk-reinforced PDMS depending on silk concentration. We prepared cylinder-type PDMS samples with different silk concentrations and performed cyclic uniaxial compression tests with a fixed magnitude of applied strain. Next, we analysed the mechanical charascteristics of PDMS using computational modelling. The stress-strain data within the large-strain region of different PDMS cylinders without silk and with 1%, 5% and 10% silk concentrations was fitted to non-linear second order Mooney-Rivlin, and third-order Ogden models. The results show the equivalence of both models for investigated strain region of PDMS. On the other hand, PDMS cylinders with 10% silk concentration allowed the successful fitting of experimental data just for the second-order Mooney-Rivlin model, while all numerical probes to find an appropriate fitting parameters for third-order Ogden models were unsuccessful. The second-order Mooney-Rivlin model is preferable for analysing the properties of silk-reinforced PDMS over the entire measurement range.