Large Strain Research Articles

Hot Torsion Tests of AA6082 Alloy

Materials characterization and the knowledge of their elastic-plastic behavior are of fundamental importance for the design of industrial manufacturing processes. Nowadays, FEM simulation is the main tool used to optimize product quality and minimize scraps, and the numerical codes have evolved over the years to obtain accurate solutions with reduced computational times. Nevertheless, in order to perform reliable simulations, it is necessary to include accurate modeling of the material flow stress. Hot torsion is a powerful method for the characterization of the material flow stress because, tests can be carried out at constant speeds and temperatures, reaching large strain values, and thus getting over the limits of compression and tensile tests. In this paper the hot torsion characterization applied to AA6082 alloy is presented: tests were performed with equivalent strain rates of 0.01, 0.1, 1, and 10 s-1, in the temperature range from 440 to 550 °C (from 713.15 to 823.15 K). The results are presented in terms of equivalent stress vs equivalent strain. Finally, the material flow stress curve was predicted by the Hyperbolic sine model and Hensel-Spittel law, and the material parameters A, m1-9 are provided for the temperature expressed in °C and K.

Constitutive Modeling, Processing Map Optimization, and Recrystallization Kinetics of high-grade X80 pipeline steel

Hot compression tests were used to examine the hot deformation behavior and of high-grade pipeline X80 steel at strain rates of 0.001-1 s-1 and temperatures of 950-1100 ºC. Two approaches were used to model material flow curves. The first used the Zener-Hollomon parameter, Z, to show the impact of deformation temperature and strain rate on flow stress, using the Arrhenius constitutive equation. The second used a rule of mixture to represent the material's flow curve, considering work hardening and dynamic recrystallization flow stresses. The first method was accurate at low Z and low strains, while the second method accurately displayed peak stress, was suitable for large strains, and predicting flow curves at industrial strain rates. The microstructure of X80 steel exhibits strain rate dependence under hot deformation conditions, with finer and more equiaxed microstructures observed at higher strain rates. These findings align with processing map predictions for stable deformation behavior, suggesting potential for optimized hot working processes. The static recrystallization kinetics showed that the Avarmi exponent, n, ranges from 1 to 2 for strains of 0.2 and 0.35 at 950°C, but higher values were observed under hot deformation conditions.

Three-dimensional interconnected nanofiber network derived from bacterial cellulose for strain sensing and energy harvesting

Flexible strain sensors have garnered widespread attention due to their potential applications, including human health monitoring, human-machine interfaces and intelligent robotics. However, the tradeoff between sensitivity and detection range needs to be considered. Herein, a dual-functional film based on bacterial cellulose is fabricated with a cost-effective freeze-drying and carbonization process, which is used for strain sensing and energy harvesting. The effect of the carbonization temperature on the structure is explored. Taking advantage of three-dimensional interconnected nanofiber of carbonized bacterial cellulose (CBC) network and the stretchability of the Ecoflex substrate, the optimized sensor demonstrates impressive strain sensing capabilities, featuring exceptional sensitivity (gauge factor = 100), large tolerable strain (100 %), swift response time (70 ms), and robust durability (3000 cycles). These features endow the CBC strain sensor with the capability to effectively discern and monitor multiple complex human activities, such as pulse rate and various human movements. Moreover, the CBC structure can serve as triboelectric nanogenerator (TENG) for energy harvesting. This work provides a green, efficient and cost-effective manufacturing method for the development of multifaceted wearable devices for human health monitoring and portable energy sources.

Nonlinear NMM analysis for large deformation and contact problems: Using full strain-rotation decomposition algorithm and augmented Lagrangian method enhanced open-closed iteration

Nonlinear analysis deals with problems that involve large displacements, strains, rotations, and contact problems. Accurate results can be obtained by choosing a reasonable method for decomposing strain and rotation to avoid errors and ensure the correct contact force. A novel full strain-rotation (S-R) decomposition and an augmented Lagrangian enhanced contact model are established within the numerical manifold method (NMM) framework. Limitations in simplified S-R NMM are overcome using our redesigned resolution procedure. A new method has been applied to analyse beams with large deflections, block columns under compression, block sliding, rock falling, and semi-ring contact with block problems. Promising results from the analysis indicate that proposed method is more accurate and effective in theory and numerically than prior approaches when resolving contact problems and large deformations.

Improving the ablation performance of largely deformed silicone rubber-based composites under coupled mechanical-thermal-oxidative conditions by implementing deformable carbon fiber fabrics

Silicone rubber-based composites are used as thermal protection materials due to their large deformability and excellent thermal insulation properties. In this study, two types of commercially available silicone rubbers were selected as the matrices for preparing flexible thermal protection materials. The influence of applied strain rates on the microstructure, ablation and ceramifiable behavior of silicone rubbers was studied. The research showed that the ablation performance of silicone rubbers deteriorated greatly at large strain rates. The reinforcement using deformable carbon fiber fabrics was proposed to effectively counter the deterioration of the ablative properties incurred by applying external strain. The back-face temperature reached as low as 186 °C when the samples were ablated at above 1000 °C for 50s at a strain rate of 20 %. The proposed strategy was proved helpful in developing high performance flexible thermal protection systems that exhibit promising application in the fields of aerospace and fire protection among others.

Room temperature shape self-adjustable tough hydrogel based on multi-physical crosslinking

Room temperature shape adjustable hydrogels (rtSAH) can be (re)processed into different stable shapes at ambient conditions, making them appealing for various applications. However, elastomer-like and mechanically tough rtSAH is hard to obtain because of the elastic recovery force that prevents a deformed new shape from being fixed. Herein, we demonstrate a supramolecular elastic rtSAH having high tensile strength (2.6 MPa) and large strain at break (1770 %), whose network structure is designed to rely on a combination of strong and weak non-covalent interactions through three types of physical crosslinking: host–guest interaction, hydrogen bonding and coordination interaction. When deformed at room temperature and held in that state for sufficient time, the hydrogel adapts to a stable new shape while remaining elastic. The reconstruction of the dynamic Fe3+-carboxylate coordination interaction in the deformed hydrogel is the key to self-locking the new shape with stored elastic energy and enabling the shape memory function. Exposure the rtSAH to UV light or an acid solution disrupts the coordination crosslinking and allows the hydrogel to recover its initial shape. The demonstrated structural design represents an effective strategy to develop all-physically crosslinked shape memory hydrogels whose shapes can be reprocessed and self-locked at room-temperature.

Wearable fiber-based visual strain sensors with high sensitivity and excellent cyclic stability for health monitoring and thermal management

Visual strain sensors have attracted significant attention in the smart wearables field due to their ability to intuitively monitor human health and movement through color displays. However, their development is limited by restricted sensitivity over a large strain range and poor cyclic stability. The conductivity and electricity-induced heating performance can be enhanced by filling microcracks with conductive nanoparticles to increase the conductive paths. Inspired by this concept, nanocarbon powder/silver nanoparticles@carboxylic multi-walled carbon nanotubes/polyurethane (NAMP) fiber-based visual strain sensors were developed using wet spinning, in-situ polymerization, and ultrasonic impregnation techniques. The NAMP sensors exhibit high sensitivity (GF=3528), a wide strain range (0–107 %), and excellent cyclic stability (over 5000 cycles), demonstrating high reusability, stability, and durability. Meanwhile, a wide temperature range from 27.8 °C to 75.2 °C and corresponding color display changes triggered by applied voltages from 0 to 2.5 V were achieved, indicating excellent visualization performance. In addition, the integration of NAMP fibers into temperature-adjustable electrothermal fabric can be utilized for human thermal management therapy and deicing. This work provides valuable insights into the design and potential applications of intelligent fibrous sensor, paving the way for the development of wearable textiles from fibers to fabrics.

Ferromagnetism in High Entropy Cantor alloy triggered and tuned by severe plastic deformation

The process of severe plastic deformation (SPD) introduces unique conditions to materials, leading to the emergence of novel properties. This research specifically investigates the observation of deformation-induced ferromagnetism resulting from SPD processing. High-pressure torsion (HPT) is utilized to process both single-phase and nanocomposite high entropy alloys (HEAs). Vibrating sample magnetometry (VSM) confirms that HPT processing triggers the development of ferromagnetic properties. The distribution and alignment of magnetic domains post-SPD treatment are further examined using differential phase contrast scanning transmission electron microscopy (DPC STEM). Atom probe tomography (APT) analysis suggests that this phenomenon stems from the localized enrichment of ferromagnetic elements, particularly Ni, induced by deformation. This observation underscores the ‘cocktail effect’ in HEAs, whereby interactions between different elements has led to this unique magnetic behavior. Additionally, deliberate mechanical mixing of the CoCrFeMnNi alloy with a HfNbTaTiZr HEA, comprising non-ferromagnetic constituents, introduces a large rotational strain that alters the ferromagnetic properties. This mixing facilitates a transition from a random orientation of magnetic domains, as seen in the HPT-processed single CoCrFeMnNi alloy, to a coordinated alignment within the nanocomposite HEA.

Numerical modeling of site response at large strains with simplified nonlinear models: Application to Lotung seismic array

Abstract Site response analyses at large strains are routinely carried out neglecting the shear strength of soil and the stiffness degradation due to the increase in pore pressures, leading to unrealistic predictions of the seismic response of soil deposits. The study investigates the performance of a simplified nonlinear (NL) approach, implemented in the Deepsoil code, constituted by coupling a hyperbolic model incorporating shear strength with a strain-based semi-empirical pore pressure generation model. The first part of the study, based on a large one-dimensional parametric study, shows that above a shear strain of 0.1%, it is necessary to include shear strength in the site response modelling to get more realistic results. Then, the approach has been evaluated with reference to the well-known downhole Large-Scale Seismic Test array located in Lotung (Taiwan): numerical results have been compared with recordings in terms of acceleration response spectra and pore water pressure time histories at different depths along the soil profiles. The comparison shows that the NL simplified model is characterized by an accuracy comparable with more sophisticated advanced elasto-plastic NL analyses adopting essentially the same input data of the traditional equivalent linear approaches(shear modulus and damping curves) and simple physical-mechanical properties routinely determined during geotechnical surveys (i.e., shear strength, relative density, fine content). This approach is therefore recommended for site response analyses reaching large strains (i.e., soft soil deposits and moderate-to-high input motions).

Combined Electrostatic and Strain Engineering of BiFeO3 Thin Films at the Morphotropic Phase Boundary

AbstractMultiferroic BiFeO3 exhibits a morphotropic phase boundary at large compressive strain that merges metastable phases of tetragonal (T) and rhombohedral (R) character resulting in giant ferroelectric and electromechanical responses. To utilize this functionality in devices, it is essential to understand the response of these ferroelectric phases to the environment of a nanoscale heterostructure. Here, the emergence of ferroelectricity near the morphotropic phase boundary in BiFeO3 is explored directly during thin‐film growth, using optical second harmonic generation. It is found that the epitaxial films form at the growth temperature purely in the T phase with zero critical thickness for the spontaneous polarization. Signatures of monoclinic T‐like and R‐like phases only appear upon sample cooling. The robustness of a single‐domain configuration in the high‐temperature T phase is furthermore demonstrated during growth of capacitor‐like metal | ferroelectric | metal heterostructures. Here, a reduction in tetragonality, rather than multidomain formation, lowers the electrostatic energy in the few‐unit‐cell thickness regime. For this lower tetragonality, density‐functional calculations and scanning transmission electron microscopy point to the stabilization of a novel metastable monoclinic structure upon cooling toward room temperature. The synergistic combination of strain and electrostatic phase stabilization in BiFeO3 heterostructures hence provides a basis for designing new ferroelectric phases and ultrathin ferroelectric devices.

Nanoporous amorphous carbon nanopillars with lightweight, ultrahigh strength, large fracture strain, and high damping capability

Simultaneous achievement of lightweight, ultrahigh strength, large fracture strain, and high damping capability is challenging because some of these mechanical properties are mutually exclusive. Here, we utilize self-assembled polymeric carbon precursor materials in combination with scalable nano-imprinting lithography to produce nanoporous carbon nanopillars. Remarkably, nanoporosity induced via sacrificial template significantly reduces the mass density of amorphous carbon to 0.66 ~ 0.82 g cm−3 while the yield and fracture strengths of nanoporous carbon nanopillars are higher than those of most engineering materials with the similar mass density. Moreover, these nanopillars display both elastic and plastic behavior with large fracture strain. A reversible part of the sp2-to-sp3 transition produces large elastic strain and a high loss factor (up to 0.033) comparable to Ni-Ti shape memory alloys. The irreversible part of the sp2-to-sp3 transition enables plastic deformation, leading to a large fracture strain of up to 35%. These findings are substantiated using simulation studies. None of the existing structural materials exhibit a comparable combination of mass density, strength, deformability, and damping capability. Hence, the results of this study illustrate the potential of both dense and nanoporous amorphous carbon materials as superior structural nanomaterials.

Soft Thermoplastic Polyurethane/Silver Nanowire Membranes with Low Hysteresis for Large Strain Sensing and Joule Heating

Soft Thermoplastic Polyurethane/Silver Nanowire Membranes with Low Hysteresis for Large Strain Sensing and Joule Heating

Molecular dynamics simulation of the influence of nanocutting parameters on microstructure of AISI M2

Molecular dynamics simulation has become the main analysis method of ultra precision machining of various materials. In this research, molecular dynamics method was used to explore the influence of nanocutting parameters on amorphous atoms, atomic coordination numbers, atomic strain and dislocations of AISI M2 workpiece. LAMMPS was used to construct the molecular dynamics model for CBN tool cutting AISI M2. The polycrystalline model of the workpiece contained 12 grains and iron, chromium and tungsten atoms. The polycrystalline model of the tool contained 3 grains and boron and nitrogen atoms. Tersoff, EAM, Morse and L-J potential energy functions were used to characterize the interaction between atoms. The following results were obtained through 8 groups of simulation experiments: (1) Both cutting speed and cutting depth promoted the amorphous transformation of workpiece atoms. (2) Under the extrusion of the tool, atoms with high coordination numbers were produced in the contact area between the tool and the workpiece. (3) The cutting depth was positively correlated with the average strain of workpiece atoms. Large strain atoms mainly occurred in the cutting area and gradually extended into the workpiece. (4) 1/2 < 111> dislocation was the most common dislocation form in nanocutting. This research plays a rich role in molecular dynamics simulation of nanocutting polycrystalline alloys.

Comparison of AlN/GaN heterojunctions grown by molecular beam epitaxy with Al and Ga assistance

Although conventional III-nitride molecular-beam-epitaxy growth theories indicate that high-quality AlN should be prepared in Al-assisted regimes, AlN/GaN heterojunctions grown in Ga-assisted regimes actually demonstrate superior performances. In this letter, we compared Al-assisted and Ga-assisted grown AlN/GaN heterojunctions and explored the underlying mechanisms. Aberration-corrected transmission electron microscope, energy dispersive spectrometer, and atomic force microscopy measurements indicate that the Al-assisted growth results in partly-relaxed AlN of high-density defects, deteriorating AlN/GaN interfaces, and surface morphology consisting of irregular and truncated atomic steps. The Ga-assisted growth produces high-quality pseudomorphic AlN with excellent AlN/GaN interfaces, surface morphology, and electrical properties. It is suggested that the better crystalline quality and higher performances of the Ga-assisted grown samples are attributed to a lower N subsurface diffusion barrier resulting from a higher energy of the fcc adsorption positions in the adlayer-enhanced-lateral-diffusion model and intense thermal motions of Ga adatoms. It helps to understand the kinetics of metal-adlayer-enhanced growth of AlN and AlN/GaN heterojunctions at atomic scale and optimize the growth processes. Meanwhile, it is demonstrated that amorphization and Al diffusion near the AlN surfaces occur due to Pt bombardment on highly tensile-strained AlN during focused-ion-beam fabrications. It implies that large strain in AlN/GaN heterojunctions might be an important issue to be considered for device reliability.

Conformal Ordered Solid–Liquid Coupled Piezoelectric Units for Programmable Adaptive Optics

AbstractOrdered piezoelectric units are designed to achieve unnaturally large strains and artificial vibrational modes, owing to the synergistic effects of multiple units. However, when interfacing with another physical field, the challenge for ordered piezoelectric units is to achieve precise control of individual unit independently while maintaining a large manipulation range. In this study, conformal ordered solid–liquid coupled piezoelectric units are introduced, which effectively reduce stress transfer by as much as 84% between units via incorporating a liquid phase into each unit. This design allows for the generation of considerable strain levels (up to ɛZ = 1.13%) when used with a flexible layer and optimized structure. Hence, these conformal ordered piezoelectric units achieve simultaneous production of large strains and independent electromechanical control for each individual unit. As a practical illustration, a biomimetic piezoelectric adaptive lens and a lens array based on solid–liquid coupled piezoelectric units are designed and fabricated. They offer an extensive zoom range from 100.3 mm to infinity, ultra‐high diopter sensitivity of 70 D mV−1, ultra‐rapid response time of 50 µs, and programmable optical focusing through individual manipulation of each unit. This research paves the way for inspiring new designs for piezoelectric metamaterials and devices for intelligent electromechanical systems.

Multifunctional 1D/2D silver nanowires/MXene-based fabric strain sensors for emergency rescue

Abstract Monitoring the vital signs of the injured in accidents is crucial in emergency rescue process. Fabric-based sensing devices show a vast range of potential applications in wearable healthcare monitoring, human motion and thermal management due to their wearable flexibility and high sensitivity. Nevertheless, flexible electronic devices for both precise monitoring of health under low strain and motion under large strain are still a challenge in extremely harsh environment. Therefore, development of sensors with both high sensitivity and wide strain range remains a formidable challenge. Herein, a wearable flexible strain sensor with a one-dimensional/two-dimensional (1D/2D) composite conductive network was developed for healthcare and motion monitoring and thermal management by coating 1D silver nanowires (AgNWs) and 2D Ti3C2Tx MXene composite films on nylon/spandex blended knitted fabric (MANS). The MANS strain sensor can simultaneously achieve high sensitivity (gauge factor for up to 267), a wide range of detection (1–115%), excellent repeatability and cycling stability (1000 cycles). The sensor can be utilized for human health monitoring including heartbeat, pulse detection, breathing and various human motion. Moreover, the MANS sensor also has the electrical heating properties and voltage control temperature between 20-110 °C can achieved at low voltage. In addition, the MANS shows hydrophobicity with water contact angle of 137.1°. The MXene/AgNWs composite conductive layer with high sensitivity under low and large strains, electrical thermal conversion, and hydrophobicity has great potential for precisely monitoring health and motion of the injured in emergency rescue in harsh environment.

Dislocation accumulation-induced strength-ductility synergy in TRIP-aided duplex stainless steel

In this study, we investigate the intrinsic mechanism of intensive and progressive transformation-induced plasticity (TRIP) effects and their different strength-ductility synergies using a resource-efficient 15Cr-2Ni duplex stainless steel. The progressive TRIP material exhibits a ductility that is more than twice that of the intensive TRIP material, as well as, a larger product of the ultimate tensile strength and ductility. This is attributed to the dislocation accumulation caused by different grain sizes of strain-induced martensite depending on the stability of the γ phase, which determines the strength and work hardening of steel. When the stability is low, the γ phase is sensitive to loaded stress and transformed into dispersed fine martensite immediately after yielding at a high rate. It induces a sigmoid-shaped dislocation accumulation to an approximately 10-fold increase in the dislocation density at a limited strain, resulting in intensive work hardening and a large ultimate tensile strength. As the stability is adequate, the γ phase is transformed into coarse martensite laths with a high critical load stress, which is initiated from a delayed strain at an extremely low rate and steadily accelerated as the strain increases. This process induces a gradually increased dislocation accumulation to a 2–3-fold increase in the dislocation density at large strains, resulting in progressive work hardening and an excellent ductility.

Generalized grain boundary constitutive description implemented in a strain-gradient large-strain FFT-based formulation: Application to nano-metallic laminates

This paper presents a general treatment of grain boundary constitutive behavior in the context of strain-gradient (SG) plasticity, and its numerical implementation in a large-strain (LS) elasto-viscoplastic (EVP) fast Fourier transform (FFT)-based micromechanical model. Two novel grain boundary constitutive equations are proposed, allowing for more accurate description of the Burgers vector flow at the grain boundary. The capabilities of the generalized SG-LS-EVPFFT formulation are illustrated for the case of kink-band formation during layer-parallel compression of nano-metallic laminates (NMLs), requiring consideration of the interaction between dislocations and interfaces.

Role of grain-level chemo-mechanics in composite cathode degradation of solid-state lithium batteries

Solid-state Li-ion batteries, based on Ni-rich oxide cathodes and Li-metal anodes, can theoretically reach a high specific energy of 393 Wh kg−1 and hold promise for electrochemical storage. However, Li intercalation-induced dimensional changes can lead to crystal defect formation in these cathodes, and contact mechanics problems between cathode and solid electrolyte. Understanding the interplay between cathode microstructure, operating conditions, micromechanics of battery materials, and capacity decay remains a challenge. Here, we present a microstructure-sensitive chemo-mechanical model to study the impact of grain-level chemo-mechanics on the degradation of composite cathodes. We reveal that crystalline anisotropy, state-of-charge-dependent Li diffusion rates, and lattice dimension changes drive dislocation formation in cathodes and contact loss at the cathode/electrolyte interface. These dislocations induce large lattice strain and trigger oxygen loss and structural degradation preferentially near the surface area of cathode particles. Moreover, contact loss is caused by the micromechanics resulting from the crystalline anisotropy of cathodes and the mechanical properties of solid electrolytes, not just operating conditions. These findings highlight the significance of grain-level cathode microstructures in causing cracking, formation of crystal defects, and chemo-mechanical degradation of solid-state batteries.

Enantioselective Synthesis, Crystal Structures, and Stereoisomerism of Substituted o,m,o,p-Tetraphenylenes.

We have achieved the enantioselective synthesis of highly strained, substituted o,m,o,p-tetraphenylenes (≤98% ee) via the cationic Rh(I)/(R)-H8-BINAP complex-catalyzed chemo-, regio-, and enantioselective intermolecular cross-[2+2+2] cycloaddition of teraryl diynes with dimethyl acetylenedicarboxylate. X-ray crystallographic analyses demonstrate the highly bent structures of the para-substituted benzene moieties, and density functional theory calculations reveal the large local strain of the paraphenylene unit. 1H nuclear magnetic resonance analyses and theoretical calculations elucidate the stereoisomerism, indicating that the nonrotatable ortho-disubstituted biphenyl structure results in cis and trans isomers.