Ultrahigh Strain Research Articles

Simultaneously enhanced strength and ductility in a metastable β-Ti alloy by stress-induced hierarchical twin structure

A novel metastable β-Ti alloy Ti-4Mo-3Cr-1Fe with high strength and high ductility was developed through controlling the alloyʼs stability and deformation mechanisms. The microstructure consists of randomly oriented β-grains containing an even distribution of athermal ω precipitates. Under tensile loading, the alloy exhibits unprecedented and comprehensive mechanical properties including a high yield strength of 870 MPa, excellent total elongation of 41% and an ultrahigh strain hardening rate of 2.5 GPa. Based on investigations of deformation microstructures, the superior mechanical properties are attributed to stress-induced formation of a complex nano-scale hierarchical twin structure which is promoted by reversion of ω precipitates.

Strain rate dependence of spall strength for solid and molten lead and tin

Dynamic tensile (spall) fracture of pure Pb and Sn in solid and molten states is investigated by MD simulations. The influence of unwettable inclusions on the spall strength is revealed. Mechanical model of fracture is fitted to MD data at the strain rate $$10^{\mathrm {9}}$$$$\hbox {s}^{\mathrm {-1}}$$ and used for calculation of the rate dependencies of spall strength in the range from $$10^{\mathrm {4}}$$$$\hbox {s}^{\mathrm {-1}}$$ to $$10^{\mathrm {9}}$$$$\hbox {s}^{\mathrm {-1}}$$ in comparison with the experimental data. The model takes into account homogeneous nucleation of pores, activation of pores on unwettable inclusions or other heterogeneities and change in pore size, which is viscous for melt and elastic-plastic for solid. In the case of pure uniform material, the homogeneous nucleation gives a slow decrease in spall strength with decreasing strain rate; the calculated values significantly exceed experimental results for moderate strain rates of $$10^{\mathrm {4}}$$–$$10^{\mathrm {5}}$$$$\hbox {s}^{\mathrm {-1}}$$. Accounting of unwettable inclusions removes this contradiction and provides correspondence to experimental data. A power-law size distribution of inclusions gives in the case of melt the power-law dependence of spall strength on strain rate that coincides with the experimental data for molten Sn. In the case of solid metal, the spall strength at moderate strain rates is determined by the yield strength. Therefore, the initial power law decrease in the spall strength is replaced by almost constant level at moderate strain rates. This behavior corresponds to the existing experimental data for solid Pb. Transfer to the homogeneous nucleation mode takes place for solid and molten metals at ultra-high strain rates, when the concentration of pores activated on the existing heterogeneities is not enough for the stress relaxation.

Dynamics of Entangled Networks in Ultrafast Perforation of Polystyrene Nanomembranes

The mechanical behavior of polymers at ultrahigh strain rates can be very different from their typical behavior because of the inertia effect, wave propagation, adiabatic processes, and rate-dependent polymer chain dynamics. Polymer chain entanglements play a significant role in the mechanical properties of polymers. By employing microscopic ballistic testing in vacuum, the high-strain-rate intrinsic behavior of polystyrene (PS) is demonstrated depending on its molecular weight and microprojectile’s perforation speed. Based on the experimentally quantified correlation between the specific penetration energy and the entanglement density, we predict the upper limits of the specific penetration energy of PS.

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy

Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.

Size Effect of Ultra Pure Ti Thin Strip under Asymmetrical Rolling

In order to study the size effect on properties of ultra pure Ti tensile tests were performed with ultra pure Ti foil (thickness range from 30μm to 280μm) by asymmetrical rolling. A pronounced size effect was observed at thickness of 34μm. The results were shown that the ultimate strength were increased as sample thickness down from 280μm to 34μm, however, were decreased as the thickness from 34μm to 30μm with ultrahigh strain. These features seemed to be link to the increasing of fraction of surface grain to volume, which led to the less districting on dislocation slip.

Single-Shot COTDR Using Sub-Chirped-Pulse Extraction Algorithm for Distributed Strain Sensing

Distributed optical fiber sensing, which uses telecom fiber as sensing medium, can realize high-precision temperature/strain monitoring along the fiber, making it an indispensable tool for structural health monitoring, security monitoring, seismic wave detection, etc. In this article, for the first time, a novel sub-chirped-pulse extraction algorithm (SPEA) is proposed to replace the time-consuming sweeping process in conventional COTDR, and a single-shot coherent optical time domain reflectometry (SS-COTDR) for dynamic strain sensing is demonstrated with very high performance. In order to quantitatively evaluate the maximum measurand change that can be measured per unit time, the measurement slew rate (SR) as an important parameter of COTDR has been analyzed in phase-demodultion scheme and in our scheme. As the proof-of-concept experiments, ultra-high strain sensitivities $\text{2.3 } p\varepsilon /\sqrt{\text{Hz}}$ is demonstrated in $\text{134} \text{ m}$ fiber with $\text{2} \text{ m}$ spatial resolution. Without distributed amplification, high-performance distributed strain measurements are demonstrated on $\text{75} \text{ km}$ fiber with strain sensitivity better than $\text{100} \,p\varepsilon /\sqrt{\text{Hz}}$ . In addition, this scheme is inherently immune to interference fading, which is disadvantageous for phase demodulation scheme.

Dislocation behavior in nickel and iron during laser shock-induced plastic deformation

Laser shock peening is one of the most effective surface strengthening techniques, which uses laser shock-induced plastic deformation to optimize surface stress state and microstructures of target material. In this paper, dislocation dynamics simulation was used to investigate laser shock induced ultra-high strain rate plastic deformation of face-centered cubic (FCC) nickel and body-centered cubic (BCC) iron. Molecular dynamics was employed to calculate dislocation mobility. Based on the obtained dislocation mobility coefficient, dislocation dynamics models of nickel and iron were established. Results show that the velocity of dislocation motion increases as temperature decreases. Under ultra-high strain rate deformation, dislocation density of nickel increases while dislocation density of iron decreases as temperature rises. Moreover, iron exhibits thermal softening while nickel exhibits thermal hardening under laser shock loading. Plastic deformation dominated by dislocations is sensitive to loading direction, depending on the Schmidt factor of the slip system. The ultra-high strain rate induced by laser shock can effectively increase dislocation density by promoting dislocation multiplication and suppressing dislocation annihilation.

Ultrahigh temperature and strain hybrid integrated sensor system based on Raman and femtosecond FBG inscription in a multimode gold-coated fiber.

In this paper, a novel approach for hybrid systems combining Raman distributed sensors with fiber Bragg grating (FBG) sensors to carry out distributed and quasi-distributed temperature/strain measurements was proposed and experimentally demonstrated. Three FBGs were inscribed by the point-by-point technique in a simple setup for type I femtosecond inscription in a pure silica core multimode gold-coated fiber. Employing a single fiber, temperatures up to 600 °C and strains up to 4144 μɛ approximately were measured simultaneously and without interferences between both distributed and point measurements. Moreover, a new calibration technique was implemented to calibrate the distributed temperature system using the FBG measurements as reference.

Amorphous structure in single-crystal magnesium under compression along the c axis with ultrahigh strain rate

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.

Ultrafast thermomechanical effects in aerosol deposition of hydroxyapatite nanoparticles on a titanium substrate

Hydroxyapatite [Ca10 (PO4)6 (OH)2] is the main mineral component of bone and teeth and is widely used for implant coatings due to its desirable osteoconductive and bioactive properties which are necessary for early bone formation. Aerosol deposition (AD), a novel coating method using a cold vacuum spray, offers the opportunity to produce robust, dense and pore-free coating by nanoceramic particles such as hydroxyapatite (HA). However, the high-velocity impact of HA particles with underlying substrates such as titanium is poorly understood. Here we use large-scale molecular dynamics simulations that reveal the mechanical deformation, stress and temperature during the fast impact of a 3 nm HA particle on a titanium surface. The effect of deposition velocity on deformation and the development of mechanical stresses and temperature of particle and substrate are discussed. We show that during an ultrahigh strain rate (>1010 s−1) deformation process, the temperatures of particle and substrate rise within a few picoseconds to ~1100–1253 K depending on the velocity. However, the particle and substrate temperatures remain lower than their melting temperatures. The simulations capture the initial elastic deformation and subsequent yield and plastic deformation of the particle. The dependence of particle penetration depth and structural transformation of the top layers of titanium on particle velocity is studied, revealing ultrafast mechanisms of particle–substrate bonding.

On the Constitutive Models for Ultra-High Strain Rate Deformation of Metals

Ultra-high strain rate deformation (> 104 s−1) is common in high speed manufacturing and impact engineering. However, a general constitutive model suitable for describing the material deformation at ultra-high strain rates is still unavailable. The purpose of this study is of two-folds. The first is to systematically evaluate the performances of four typical constitutive models, Johnson-Cook (J-C), Khan-Huang-Liang (KHL), Zerilli-Armstrong (Z-A), and Gao-Zhang (G-Z), in predicting the dynamic behaviors of materials. The second is to obtain an improved constitutive model to better describe the deformation of materials under ultra-high strain rates. To this end, high strain rate tests were carried out on different crystalline structures, i.e., BCC, FCC, and HCP over a wide range of strain rate from 102 s−1 to 1.5 × 104 s−1. It was found that before the critical strain rate, around 104 s−1, all of the previous models can predict the flow stresses. When the strain rate passes a critical point, however, these models fail to predict the sudden upsurge of the flow stresses. The improved model developed in this paper, by considering the dislocation drag mechanism, can successfully characterize the dynamic behaviours of materials over the whole range of strain rates.

A S-shaped long-period fiber grating with ultra-high strain sensitivity

A S-shaped long-period fiber grating (SLPFG) is presented in this paper as a new type of strain sensor with ultra-high strain sensitivity. The sensor is fabricated by using the CO2-laser-polished method. It is illustrated by experiments that the WLFPG has structural stability and repeatability. The strain sensitivity of the proposed sensor reaches 29.3 pm/με in the range of 0–1000 με, and the temperature sensitivity is 76.3 pm/°C in the range of 30–170 °C. Compared to other CO2-laser-induced long-period fiber gratings (CO2-LPFGs), the strain sensitivity of the SLPFG is nearly ten times higher.

Development of 3D printable engineered cementitious composites with ultra-high tensile ductility for digital construction

The current 3D concrete printing (3DCP) technology is limited by the reinforcing methods. Conventional steel reinforcement is hard to be incorporated in the 3DCP process. To overcome this limitation, this study aims to develop 3D-printable engineered cementitious composites (ECCs) exhibiting ultra-high tensile strain capacity of more than 8%, which can be used for digital construction of ‘self-reinforced’ concrete structures, reducing the reliance on the conventional steel reinforcement. Different volume fractions of polyethylene fibers (1%, 1.5% and 2%) were used to reinforce the ECC matrix. The fresh properties (including the workability, rheological properties and buildability) and the hardened properties (including the compressive and flexural strengths, and the uniaxial tensile performance), along with the microstructure of the developed 3D printable ECCs were experimentally investigated. Conventionally mold-cast ECCs were also prepared and tested for comparison purposes. The results showed that the developed 3D printable PE-ECCs exhibited strong strain-hardening behavior with the tensile strength and tensile strain capacity of up to 5.7 MPa and 11.4%, respectively. In addition, the results showed that the 3D printed PE-ECCs exhibited superior tensile performance to the conventionally mold-cast PE-ECC counterparts. This finding is in good agreement with the results of the microstructural analysis.

Microstructure evolution and mechanical properties of aging 6061 Al alloy via laser shock processing

Microstructural evolution and mechanical properties of an aging 6061 Al alloy subjected to laser shock processing treatment (post-aging LSP treatment) were investigated by transmission electron microscopy (TEM) and mechanical tests. Results showed that the precipitates formed during aging treatment still remained, and the grain was refined by the ultra-high plastic strain plastic deformation during subsequent LSP treatment. Moreover, the precipitates effectively interacted with the dislocations induced by LSP. These typical microstructures contributed to the improvement in mechanical properties of the Al alloy. Relatively higher microhardness and strength were observed for the post-aging LSP Al sample. Furthermore, the synergistic strengthening mechanism involving the interaction of precipitate with dislocation, dislocations, and refined grains were clearly revealed.

Microstructure characteristics of gradient nano-grained Ti-1300 titanium alloy induced by sliding friction treatment

A gradient nano-grained layer in Ti-1300 alloy is successfully fabricated by sliding friction treatment, and the microstructural characteristics and grain subdivision process are systemically investigated. The results show that uniform equiaxed β grains with an average size of around 70 nm in the topmost surface are generated due to the ultra-high plastic strain. The microstructure near substrate consists of high density dislocations and deformation slip bands, while the different microstructures featured of dislocation tangles, dislocation walls and lamella shaped subgrains are closer to the surface. The rotation dynamic recrystallization is responsible for grain refinement of Ti-1300 alloy. Moreover, the gradient nano-grained sample with the high strength properties possesses a high level of ductility in comparing to the initial coarse-grained one. The strain hardening behavior and grain refinement are considered as the main strengthening mechanism, and the high ductility is mainly attributed to the specific deformation mechanism for gradient nano-grained microstructure.

Estimates of Ta strength at ultrahigh pressures and strain rates using thin-film graded-density impactors

We present results from an experimental technique used to estimate the strength of Ta at extreme pressures (150 GPa) and strain rates (${10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). A graded-density impactor (GDI) was fabricated using sputter deposition to produce an approximately $40\ensuremath{-}\ensuremath{\mu}\mathrm{m}$-thick film containing alternating layers of Al and Cu. The thicknesses of the respective layers are adjusted to give an effective density gradient through the film. The GDIs were launched with a 2-stage light gas gun, and shock-ramp-release velocity profiles were measured over timescales of $\ensuremath{\sim}10$ ns. Results are presented for the direct impact of the film onto LiF windows, which allows for a dynamic characterization of the GDI, as well as from impact onto thin ($\ensuremath{\sim}40\ensuremath{\mu}\mathrm{m}$) sputtered Ta samples backed by a LiF window. The measurements were coupled with mesoscale numerical simulations to infer the strength of Ta, and the results agree well with other high-pressure platforms, particularly when strain-rate, microstructural, and thermodynamic-path differences are considered.

Ultrasensitive strain sensor based on Vernier- effect improved parallel structured fiber-optic Fabry-Perot interferometer.

A novel parallel structured fiber-optic Fabry-Perot interferometer (FPI) based on Vernier-effect is theoretically proposed and experimentally demonstrated for ultrasensitive strain measurement. This proposed sensor consists of open-cavity and closed-cavity fiber-optic FPI, both of which are connected in parallel via a 3 dB coupler. The open-cavity is implemented for sensing, while the closed-cavity for reference. Experimental results show that the proposed parallel structured fiber-optic FPI can provide an ultra-high strain sensitivity of -43.2 pm/με, which is 4.6 times higher than that of a single open-cavity FPI. Furthermore, the sensor is simple in fabrication, robust in structure, and stable in measurement. Finally, the parallel structured fiber-optic FPI scheme proposed in this paper can also be applied to other sensing field, and provide a new perspective idea for high sensitivity sensing.

Microstructural evolution of single-crystal magnesium under elevated temperature and ultra-high strain rate

Despite numerous studies of the deformation behavior of magnesium (Mg), its microstructural evolution at different temperatures and strain rates remains largely unexplored. In this paper, the evolution of dislocations and amorphous regions in single-crystal Mg under compressive loading along the c-axis is investigated using molecular dynamics simulations, and temperature and strain-rate dependence of the microstructural evolution is revealed. At a strain rate of 107 s−1, the dislocations are low in density, and they slip and evolve unevenly as the strain in the single crystal increases. Consequently, the stress in the single crystal varies in a zigzag manner with increasing strain. The dislocation density is higher at strain rates of 108 s−1 and 109 s−1, resulting in relatively smooth deformation and stress–strain curves. At a strain rate of 1010 s−1, the amorphous regions achieve a very high fraction during deformation, contributing to softening and smoother deformation of the single crystal. The fraction of amorphous regions also increases with increasing temperature, which is an important cause of the temperature softening effect. Furthermore, the initiation of dislocations and amorphous regions is also studied at different strain rates and temperatures.

Unravelling ultrafast deformation mechanisms in surface deposition of titanium nanoparticles

Abstract Deposition of metallic nanoparticles by Cold Spray (CS) method, has gained considerable attention from the research community. However, the process of deposition at the atomic scale is poorly understood. To gain insight in molecular mechanisms and to optimize practical coating processes, molecular dynamics simulations have been used to understand the influence of deposition velocity and size of the particles on deformation, and resulting stresses and temperatures during impact. We report molecular dynamics simulation of titanium nanoparticles deposition on a titanium substrate. Here local stress, temperature and particle deformation behaviour are studied in detail during the impact. Ultra high strain rates in the order of 1010 s−1, and very high temperatures lead to ultrafast deformation behaviour. The simulations are conducted for three different particle velocities, which are achievable in real experimental settings. Realistic potentials are used to allow for the deformation of the substrate and nano-particle upon impact. We show that there is a direct relationship between the deposition velocity and temperature. For a small particle of 2 nm size, the maximum temperature reaches to around 842 K at the highest impact velocity of 700 m/s. However for the 20 nm particle temperature locally and globally rises further up to 1369 K. We show at higher velocities (>500 m/s) deformation of the particle in the elastic regime is followed by strong plastic behaviour, and attribute this to very high temperatures. The calculated Young's modulus of titanium in the elastic regime is E = 95.7–194.3 GPa and are comparable to those reported in the literature for single crystal titanium. There is evidence of temperature and strain rate effects that leads to softening of the particle at higher velocities. We also show the local transformation of Ti into an amorphous state that could be due to high local temperatures.

Study on the Coupled Effects of Bentonite and High-Volume Fly Ash on Mechanical Properties and Microstructure of Engineered Cementitious Composites (ECC)

With the advantage of superior crack width control ability, stress transmission capacity and energy dissipation capacity, engineered cementitious composites (ECC) show ultra-high strain hardening characteristic, and its ultimate tensile strain can reach 3% to 7%, which is hundreds of times that of the normal concrete when cracking occurs. As a new kind of building material, ECC need to get more sustainable through adding vicinal materials. In the present research, we investigate the influence of ternary binder system of bentonite, fly ash (FA) and cement (C) on the mechanical behaviours and microstructures of ECC. The ratio of FA/ C were 1.2, 1.8, 2.4 and the amount of bentonite by mass of ternary binder system were 0%, 5%, 10%, 15%, and 20% in each FA/C ratio, respectively. The flexural strength, flexural deflection capacity and compressive strength were tested at 14, 28 and 90 curing time. The results show that ECC with combined supplementary cementitious materials have the behavior of strain hardening. Moreover, it was found that higher FA/C ratio and higher bentonite content can improve the flexural deflection capacity. In addition, there is a decrease in flexural strength and compressive strength with the increase in FA/C ratio and bentonite content. Furthermore, it could be seen that fly ash and bentonite are beneficial to the long-term strength development of ECC.