Ultrahigh Strain Research Articles

Non-equilibrium MD modeling and simulation to extract mechanical properties of copper nanoparticles under ultra-high strain rate loading

Molecular dynamics (MD) simulations have been carried out to evaluate the mechanical properties of Cu nanoparticles under tensile and compressive loading at ultra-high strain rate regime. Simulations have been carried out in a way analogous to real experiments; but at strain rates of magnitude several orders greater than those used in the experiments for bulk Cu. The engineering stress-strain curves obtained from tensile loading have shown an initial linear elastic region, followed by a plastic region that involves deformation by local necking. However, under the ultra-high strain rates considered in the present simulation, no uniform plastic deformation region is observed. A significant rise in temperature of the nanoparticles has been observed during the tensile and compressive deformation, as a natural consequence of any deformation process. The yield strength and Young’s modulus of Cu nanoparticles have been calculated from the engineering stress-strain curves generated by a set of MD simulations and correlated with strain rate, equilibration temperature, particle size, etc. The tensile deformation at various ultra-high strain rates shows the ductile-to-brittle transition behavior of the Cu nanoparticles after exceeding a certain critical strain velocity which decreases with the increase in particle size. The extrapolated results have indicated that the Cu nanoparticles are several times stronger than bulk Cu and their mechanical properties are highly dependent on the particle size. The present study would provide a useful guideline for the design of nanoparticles under the condition of mechanical loading in advanced applications.

Fracture-resistant thin-film metallic glass: Ultra-high plasticity at room temperature

We report the first example of room-temperature rubber-like deformation in thin-film metallic glasses (TFMGs), 260-nm-thick Zr60Cu24Al11Ni5 layers, under ultra-high shear strain. The TFMGs were deposited, with no external heating, on Zr-based bulk metallic glass (BMG) and Si(001) substrates by rf magnetron sputtering in a 3 mTorr Ar plasma. Cross-sectional transmission electron microscopy (XTEM) analyses and nanoindentation results reveal that the TFMGs undergo an incredibly large shear strain, estimated to be ∼4000%, during fatigue tests, and thickness reductions of up to 61.5%, with no shear-banding or cracking, during extreme nanoindentation experiments extending through the film and into the substrate. TFMG/BMG samples also exhibit film/substrate diffusion bonding during deformation as shown by high-resolution XTEM.

Implementation of distributed feedback fiber laser sensor for acoustic measurements in hydraulic fracturing

A distributed feedback (DFB) fiber laser strain sensor was implemented to measure acoustic emission induced by the hydraulic fracturing process. A study of practical sensor mounting configurations and their characteristics was carried out to find a practical solution. Combining the suitable mounting configuration and ultrahigh strain sensitivity of the DFB fiber laser, the evolution of the hydraulic fracturing process was well monitored. This study shows that fiber lasers can be useful alternatives to piezoelectric sensors in the field of hydraulic fracturing for gas and oil extraction.

A high Zr-containing Ti-based alloy with ultralow Young's modulus and ultrahigh strength and elastic admissible strain

Ideal metallic biomaterials for use in implants are required to meet the mechanical compatibility, i.e., low Young's modulus combined with high mechanical strength, however, these constraints are generally contradictory. Here, an exceptional combination of ultralow Youngʹs modulus of E~45GPa, ultrahigh tensile strength of σb~1185MPa and ultrahigh elastic admissible strain of δ~2.2% is achieved in a new high Zr-containing Ti-based alloy (Ti-50Zr-5Al-4V (wt%)), which is subjected to solution treatment and cold rolling. The ultralow Youngʹs modulus is attributed to a mixture of bcc β and orthorhombic α″ phases while the ultrahigh strength results from the solution strengthening and work hardening. This study can be of significance for developing excellent removable implant materials.

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.

The size effect on deformation behavior in microscale laser shock flexible drawing

A microscale laser shock flexible drawing (µLSFD) is a novel ultrahigh strain rate manufacturing technology that provides an effective means for fabricating complicated microparts shapes in foil. However, the size effect phenomenon in ultrahigh strain rate microforming is still largely unknown. In this work, the micro-mold and process parameters were designed to investigate the size effects based on the similarity theory. The parts were formed using annealed copper foils with four different grain sizes to study the grain size effect. The parts were fabricated by use of different micro-molds and copper foils with varying thicknesses but with the same annealing temperature to investigate the effect of feature size. The experimental results indicated that the depth of the formed parts increased with an increase in the grain size; the forming depth decreased significantly when the feature dimension was smaller than a critical value. The surface roughness and the thickness thinning ratio of the formed parts increased when the grain size and feature dimension increased. The maximum thinning ratio appeared at the bottom of the formed parts. The regression analysis revealed that the material deformation was more homogeneous with a decrease in the grain size and an increase in the feature dimensions when the µLSFD process was employed. This study provides a theoretical basis for the investigation of size effects in laser shock flexible forming.

Recovery Kinetics in Commercial Purity Aluminum Deformed to Ultrahigh Strain: Model and Experiment

A new approach to analyze recovery kinetics is developed from a recent model, and microstructural observations are introduced to supplement hardness measurements. The approach involves two steps of data fitting, and the second step of fitting enables an estimation of the apparent activation energy for recovery. This approach is applied to commercial purity aluminum (AA1050) cold rolled to ultrahigh strain (99.6 pct reduction in thickness) and annealed at temperatures from 413 K to 493 K (140 °C to 220 °C). The annealing data fit the recovery model well, and the analysis shows that the apparent activation energy increases during recovery and approaches 190 kJ/mol at the end of recovery, suggesting that solute drag is an important rate-controlling mechanism. The recovery rate for the highly strained Al is found to be higher than that for Al deformed to a lower strain, an effect which is related to an increase in the stored energy (driving force). These findings form the basis for a discussion of recovery mechanisms and the increase in the apparent activation energy during annealing, suggesting an application of the model when optimizing the structure and strength through annealing of nanostructured materials produced by high strain deformation.

Nano-Impact Tests with Ultra-High Strain Rate Loading Using Graphene and Ion Impact

Strain rate is essential in study of the physics of fluids and solids undergoing deformation. State-of-art high strain rate tests have mainly been in macro-scale with an upper limit of [Formula: see text]. A graphene-based layered system is proposed to conduct nanometer-scale high strain rate testing. The process is investigated by molecular dynamics simulations. Accelerated single ion or group of ions are used to impact on the proposed system to generate ballistic or plate-like impact scenarios. The effects of impact energy, shape of impact absorber and the impaction location are investigated. The graphene layer is the key of the proposed system to spread the load and protect the sample. The results indicate that ion group and single ion impacts produce strain rates of [Formula: see text]–[Formula: see text] and [Formula: see text]–[Formula: see text], respectively. Ion group impact produces a more significant signal than single ion impact on the sensing nano-layer in the system. The ultimate strength of an Al-Cu alloy sample during ion impact is estimated to be 215[Formula: see text]MPa to 251[Formula: see text]MPa, significantly lower than predicted by the Johnson–Cook model because of the rapidly increased temperature and melting in the sample. The results demonstrate new possibilities for understanding high strain rate effects at nano-scale.

Nanocrystallization mechanism of beta phase in Ti-6Al-4V subjected to severe plastic deformation

Nanocrystallization mechanism of beta phase in the bulk coarse-grained Ti-6Al-4V by high energy shot peening was investigated via high-resolution transmission electron microscopy. The results suggested that dislocation gliding was first initiated in beta phase at and around the intersections of the phase boundary with the high-density dislocation structures in alpha phase followed by the formation of dislocation tangles and dislocation walls. As the short axis sizes of alpha phase approached that of beta phase, the dislocation tangles and dislocation walls gradually evolved into the high-angle grain boundaries and subdivided the original grains into the equiaxed ultrafine grains. Under the ultrahigh strain and strain rate, the equiaxed ultrafine grains were eventually refined to randomly oriented nanograins via dynamic recrystallization. In addition, the nanograins would be further refined via dislocation motion upon further straining.

Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues.

Phosphorene and phosphorene analogues such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calculation evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temperature nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelectric switching readily readable via electrical, thermal, optical, mechanical, or even spintronic detection upon the swapping of the zigzag and armchair direction. In addition, it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temperature, suggesting high potential for practical device application.

Molecular dynamics study of brittle fracture in epoxy-based thermoset polymer

A novel molecular dynamics (MD) simulation methodology to capture brittle fracture in epoxy-based thermoset polymer under mechanical loading is presented. The ductile behavior of amorphous polymers has been captured through traditional MD simulation methods by estimating the stress-strain response beyond the yield point; however, brittle fracture in highly crosslinked polymer materials such as epoxy thermoset has not been addressed appropriately and is the primary objective of this work. In this study, a numerically cured epoxy system comprising molecules of epoxy resin and hardener is generated. During the virtual deformation test, it is observed that the inherent molecular vibration due to temperature re-equilibrates the elongated covalent bonds and this molecular vibration impedes further stretching the covalent bond leading to scission. In order to overcome the influence of thermal vibration, an approach that employs deformation tests at absolute zero temperature condition – a concept borrowed from the quasi-continuum method, is developed. Bond dissociation energy is measured to quantify the extent of failure in the system by calculating the bond potentials during the deformation tests. Applying zero temperature condition to the deformation test, however, requires a large amount of computational time due to intermediate energy minimization processes. To improve the computational efficiency, an ultra-high strain rate (UHSR ≈ 1013 s−1) approach is developed by which the thermal vibration is decoupled from the deformation test using a strain rate higher than molecular vibration frequency. Note that the deformation test performed by traditional MD simulation methods used strain rates ranging 108–1010 s−1. Simulation results show that the UHSR approach successfully captures brittle fracture in epoxy polymer due to covalent bond dissociation with high computational efficiency.

Ultrahigh resolution optical fiber strain sensor using dual Pound-Drever-Hall feedback loops.

We present an ultrahigh resolution optical fiber strain sensor with a broad frequency range from quasi-static to several hundred hertz. The sensor consists of a π-phase shifted fiber Bragg grating for strain sensing and a fiber Fabry-Perot interferometer as reference. The laser carrier and sideband are locked to the reference and sensing elements, respectively, via two individual feedback loops, in which the Pound-Drever-Hall technique is employed to generate the error signals. The sampling rate is up to 500 samples/s in the demonstrational experiments, only limited by the updating rate of the frequency counter. The strain resolution exhibits a 1/f characteristic in the bandwidth of 0.01-250 Hz, and is better than 0.01 nϵ at 10 Hz with a dynamic range up to 149 dB. Compared with the traditional static strain sensors, the proposed sensor shows a great improvement in both resolution and sensing bandwidth, and can be a powerful tool for geophysical applications.

Macrodeformation Twins in Single-Crystal Aluminum

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum at scales beyond nanotwins. Here, we present the first experimental demonstration of macrodeformation twins in single-crystal aluminum formed under an ultrahigh strain rate (∼10^{6} s^{-1}) and large shear strain (200%) via dynamic equal channel angular pressing. Large-scale molecular dynamics simulations suggest that the frustration of subsonic dislocation motion leads to transonic deformation twinning. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.

Characterization of flow stress at ultra-high strain rates by proper extrapolation with Taylor impact tests

This paper is to provide a novel systematic procedure to obtain the dynamic flow stress of a material at ultra-high strain rates ranging from 104 s−1 to 106 s−1 where hardening behaviors are difficult to acquire from conventional experiments. Uniaxial material tests with AISI 4340 steel are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1 by using the INSTRON 5583, a high-speed material testing machine (HSMTM), and a tension split Hopkinson pressure bar (SHPB) testing machine. From the uniaxial tests above, stress–strain curves are obtained at the strain rates ranging from 10−3 s−1 to 103 s−1. However, stress–strain curves cannot be obtained at the strain rates higher than 104 s−1 due to the lack in experimental techniques. In order to characterize hardening behaviors at strain rates ranging from 104 s−1 to 106 s−1, Taylor impact tests are performed when the speed of a projectile is 200 m/s, 253 m/s, and 305 m/s, which entail ultra-high strain rates, high temperature, and large plastic deformation. Flow stresses at the ultra-high strain rates are characterized through an inverse optimization process by comparing the numerical simulation results with the experimental results of the sequentially deformed shapes of a projectile during the Taylor impact test. The thermal softening effect at different strain rates is also considered due to the elevated temperature caused by large plastic deformation. The flow stresses calibrated by the comparison are implemented to numerical simulation resulting in a good coincidence with the Taylor impact tests at different impact velocities. It is noted from the comparison that the yield stress and the comprehensive hardening curves proposed well describe the deformation behavior up to the strain rate of 106 s−1 beyond the strain rate range for conventional material testing.

Heat transfer modeling in asymmetrical sheet rolling of aluminium alloys with ultra high shear strain

Asymmetrical sheet rolling is a method of severe plastic deformation (SPD) for production of aluminium alloys with UFG structure. Prediction of sheet temperature during SPD is important. The temperature of sheet is changed due to the conversion of mechanical work into heat through sliding on contact surfaces and high shear strain. Paper presents the results of FEM simulation of the effect of contact friction, rolling speed and rolls speed ratio on the heating of aluminium sheets during asymmetrical rolling.

Experiment research on dynamic response of copper film at high strain rate by chirped pulse spectral interferometry

That the femtosecond laser pulses irradiate metallic materials thereby inducing ultrahigh strain rates, is an important experimental approach to studying the material behavior under extreme conditions. Femtosecond laser-generated shock waves in metal films have rise times of several picoseconds, the corresponding diagnostic technique is required to work with a higher time resolution, which makes the experimental measurements difficult. Chirped pulse spectral interferometry (CPSI) possesses capabilities of ultrafast time resolution and continuous measurement, thus it provides a diagnostic technique for studying the ultrashort shock wave. In this article, we carry out an experiment on femtosecond laser driven shock wave in copper film and the measurement by CPSI. Laser pulse of 25 fs duration at the central wavelength 800 nm is used, the tested samples are copper films of (5025) nm in thickness fabricated by electron beam sputtering deposition onto cover slip substrate of 180 m in thickness, pump beam focuses onto front surface of the copper film through the transparent substrate and this laser intensity is 2.31013 W/cm2. Chirped pulse spectral interferometry is used to detect the movements of the free rear surfaces of the copper films with temporal and spatial resolution. In the spectral interferometry, linearly chirped pulse is required and obtained by stretching the femtosecond laser pulse with a pair of gratings. The relation between frequency and time of the chirped pulse is accurately measured using asymmetric spectral interference method, which is required for explaining the experimental data. Since CPSI is a single shot diagnostic technique, we obtain the displacement and velocity history of the free rear surface with picosecond time resolution in a single measurement. From the results, the average shock velocity is calculated to be (5.60.2) km/s and the shock wave rise time is determined to be 6.9 ps. According to the shock wave relations, impact pressure and strain rate in the copper film are (57.18.8) GPa and 8109 s-1 respectively, the strain rate is so high that it is hard to achieve by long-pulse laser driven or other loading approaches. Additionally, experimental results also show that the free rear surface alternately experiences acceleration and deceleration, which indicates the spallation in the copper target. It is obvious that chirped pulse spectral interferometry is a reliable approach to studying ultrashort shock waves in metal films.

Residual Stress Distribution and Microstructure at a Laser Spot of AISI 304 Stainless Steel Subjected to Different Laser Shock Peening Impacts

The effects of different laser shock peening (LSP) impacts on the three-dimensional displayed distributions of surface and in-depth residual stress at a laser spot of AISI 304 stainless steel were investigated by X-ray diffraction technology with the sin2φ method and MATLAB 2010a software. Microstructural evolution in the top surface subjected to multiple LSP impacts was presented by means of cross-sectional optical microscopy (OM) and transmission electron microscopy (TEM) observations. Experimental results and analysis indicated that residual stress distribution and microstructure at a laser spot depended strongly on the number of multiple LSP impacts, and refined grain and ultra-high strain rate play an important role in the improvement of compressive residual stress of AISI 304 stainless steel. The analysis of treatment of the extended surface was presented to obtain uniform surface properties on the top surface of AISI 304 stainless steel.

Giant piezoelectricity and ultrahigh strain response in bismuth sodium titanate lead-free ceramics

The obvious conflicts between large piezoelectricity and high strain could be solved by adding new components in BNT. Moreover, this urgent problem has been solved and induced a large strain as well as a high piezoelectricity in BNT. Large normalized strain (d33*=526–597pm/V) and high strain (0.342%–0.388%) were achieved at the composition range of 0.0025≤x≤0.0050, suggesting that such a system is a promising lead-free candidate for electromechanical actuator applications. Furthermore, high d33 values of 141–204pC/N have also been attained at the above composition range due to adding new components.

Extremely stretchable and conductive water-repellent coatings for low-cost ultra-flexible electronics.

Rapid advances in modern electronics place ever-accelerating demands on innovation towards more robust and versatile functional components. In the flexible electronics domain, novel material solutions often involve creative uses of common materials to reduce cost, while maintaining uncompromised performance. Here we combine a commercially available paraffin wax–polyolefin thermoplastic blend (elastomer matrix binder) with bulk-produced carbon nanofibres (charge percolation network for electron transport, and for imparting nanoscale roughness) to fabricate adherent thin-film composite electrodes. The simple wet-based process produces composite films capable of sustained ultra-high strain (500%) with resilient electrical performance (resistances of the order of 101–102 Ω sq−1). The composites are also designed to be superhydrophobic for long-term corrosion protection, even maintaining extreme liquid repellency at severe strain. Comprised of inexpensive common materials applied in a single step, the present scalable approach eliminates manufacturing obstacles for commercially viable wearable electronics, flexible power storage devices and corrosion-resistant circuits.

Application of a nanosecond laser pulse to evaluate dynamic hardness under ultra-high strain rate

The paper presents results of experimental tests of plastic metals deformation generated by a shock wave induced by laser pulse. Tests were carried out on the Nd:YAG laser with a wavelength of 1064nm and the laser pulse of 10ns duration. The shock wave generate by the laser pulse was used to induced local plastic deformation of the material surface. The study examined the possibility of application the process to develop a new method of measuring the dynamic hardness of materials under ultra-high strain rate. It has been shown that the shock wave induced by the laser pulse with an energy of 0.35–1.22J causes a repeatable plastic deformation of surface of commercially available metals and alloys without thermal effects on the surfaces. Based on the knowledge of an imprint geometry, it is possible to evaluate the dynamic hardness of materials at strain rate in the range of 107s−1.