Increase In Tensile Strain Research Articles

Magnetic properties of monolayer WSe2 doped with nonmagnetic metal and nonmetal atoms

Electronic and magnetic properties of WSe2 monolayer with different nonmagnetic metal and nonmetal dopants have been studied by the DFT method. Nonmagnetic metal (Ag, Al, Li, Mg, and Na) and nonmetal (B, C, F, N, and O) dopants prefer to W and Se substitutions, respectively. Magnetism has been induced by Al doping. Then, we further have considered the effect of strain on the magnetism in Al-doped WSe2 monolayer. We have applied an isotropic strain on the system. On one hand, the magnetism disappears while the compressive strain is applied. On the other hand, the magnetism keeps stable under the tensile strain. Moreover, we also investigate the magnetic coupling in two-Al-doped WSe2 structures under the tensile strain. Interestingly, a tunable magnetic coupling has been observed. As the tensile strain increases, the magnetic coupling of two Al atoms switches from nonmagnetic (NM) to ferromagnetic (FM) due to the p-p hybridization between Al and Se atoms. These fascinating controllable magnetic properties are desired for spintronic applications.

Aligned-SWCNT film laminated nanocomposites: Role of the film on mechanical and electrical properties

In this study, well-aligned single-walled carbon nanotube (SWCNT) films were utilized as lamellae to prepare a multifunctional nanocomposite by a feasible ‘layer-by-layer’ preparation method. This structural design can not only dramatically reduce the agglomeration effect of SWCNTs, but also achieve high in-plane stiffness. It was found that the well-aligned SWCNT films contribute to the high stress-transfer between CNTs and present good wettability with epoxy matrix, thereby improving the mechanical enhancement. Interestingly, this laminated structure possesses distinct electrical behaviors under different loading conditions. The electrical resistance linearly increases with the increase of in-plane tensile strain but is insensitive to bending. This work could provide in-depth understanding on the mechanical and electrical properties of CNT-based laminated nanocomposites.

Rate effect on the stress-strain relations of synthetic carbon dioxide hydrate-bearing sand and dissociation tests by thermal stimulation

Methane hydrates (MHs) have been recognized as an important material for use as a new energy resource. Recently, not only MHs, but also carbon dioxide hydrates (CO2-hydrates), have been attracting attention from the viewpoint of CO2 storage in the form of CO2-hydrates. It is essential, therefore, to investigate the mechanical behaviour of gas hydrate-bearing sediments in order to achieve safe MH extraction and to ensure the long-term stability of CO2-hydrate-storaged submarine sediments. In order to gain further knowledge of CO2-hydrate-bearing sediments, we carried out three kinds of laboratory experiments on synthetic CO2-hydrate-bearing sand specimens: (1) undrained triaxial compression tests with a constant strain rate, (2) undrained triaxial compression tests with a step-changed strain rate, and (3) dissociation tests on CO2-hydrate-bearing sand specimens using the thermal stimulation method.The main findings obtained from these three experiments are as follows: First, it was found that CO2-hydrate-bearing sand shows larger strength and larger positive dilatancy than water-saturated sand without CO2-hydrates under undrained conditions. Second, CO2-hydrate-bearing sand clearly exhibits strain rate dependency with an increase in hydrate saturation. It is interesting to note that hydrate saturation’s dependency on strength does not appear in cases where the strain rate is quite slow. Third, a drastic increase in pore pressure and extensive tensile strain are observed simultaneously during the dissociation of CO2-hydrates. The pore gas pressure of CO2, produced by the hydrate dissociation, can exceed the liquefied boundary of CO2-gas, and the increase in pore gas pressure can be limited by the liquefaction of CO2-gas.

Effect of Crack Length on Mechanical Field of Stress Corrosion Crack Tip of Cold Working 316L Stainless Steel

The effect of crack length at different cold working level on stress and strain state and fracture parameters of stress corrosion cracking tip of 316L stainless steel in high temperature water environment of the nuclear pressure vessel was studied by sub-model technology of elastic plastic finite element method. It combined with the mechanical properties of 316L stainless steel under different cold working level, and the effect of crack length on the Mises stress, equivalent plastic strain, tensile stress, tensile strain and J integral of 316L stainless steel under different cold working level was compared. The results indicate that crack length of stress corrosion cracking has certain effect on stress and strain state at crack tip. With the increase of crack length, Mises stress and equivalent plastic strain, tensile stress and tensile strain increase at crack tip, at the same time, J integral increases with increase of crack length. Cold working level has changed the mechanical properties of 316L stainless steel. When the stress intensity factor K is constant, with the increase of cold working level, Mises stress increases and equivalent plastic strain decreases at crack tip, while tensile stress increases and tensile strain decreases at crack tip. The rate of change of stress strain curve from 0° to 90° is different because of cold working level. The higher the level of cold working, the faster the Mises stress and the tensile stress increase at 90° in front of the crack tip, and equivalent plastic strain and tensile strain decrease most slowly at 90° in front of the crack tip. As the degree of cold working increases, the J integral of crack tip also increases. The increase of crack length and the level of cold working aggravate the stress corrosion cracking of 316L stainless steel under high temperature and high pressure in a certain range.

Laboratory Investigation of Microsurfacing Asphalt Modified with Nanosilica and Nanoclay Combined with Polyethylene Fibers

Abstract Microsurfacing asphalt is a thin surface treatment comprising asphalt emulsion, filler, and crushed stone materials that can be defined as a protective asphalt that has low thickness and is adaptable to the environment. In this study, the mix design was specified in terms of the optimal percentage of additives to asphalt emulsion; the additives contained 4 wt % nanosilica and 3 wt % nanoclay in addition to 0.4 wt % polyethylene fibers that can be defined as a kind of flexible fiber-reinforced microsurfacing. The results of the abrasion test indicate that an increase in the resistance to long-term moisture sensitivity in fiber-reinforced microsurfacing occurs when resistance against abrasion in this sample is increased 30 % and the contents of nanoclay and fiber are in an optimal combination. The results of the sand adhesion test demonstrate that an increase in the amount of nanosilica is far more effective than an increase in nanoclay and fiber. Also, a comparison of the results of the cohesion and loaded wheel tests reveals that the specimen with the greatest possible cohesion between its components still has a low vertical displacement; thus, it is recommended as the best combination for producing and manufacturing highly flexible fiber-reinforced microsurfacing. This indicates that nanosilica and fiber effectively modify surfaces. Additionally, a 46 % improvement in rutting resistance and a 27 % increase in the flexural tensile strain was obtained in the fiber-reinforced microsurfacing.

Anisotropy and microstructural evolutions of X70 pipeline steel during tensile deformation

Abstract

Effect of temperature on the mechanical properties of adhesively bonded basalt FRP-aluminum alloy joints in the automotive industry

To study the effect of temperature on the mechanical properties of adhesively bonded basalt FRP-aluminum alloy joints (BFRP-Al joints), the two-component epoxy adhesive Araldite® 2015 was selected to fabricate bulk tensile specimens, butt joints (BJs), scarf joints (SJs) with angles of 45° and thick adherend shear joints (TASJs), which were tested at − 40, − 10, RT (25 °C), 50 and 80 °C using a universal testing machine. A differential scanning calorimetry (DSC) analysis of Araldite® 2015 and BFRP was performed, and the strain of bulk specimens at different temperatures was obtained by the 3D digital image correlation (DIC) method. The results show that as temperature increases, the Young's modulus and tensile strength of Araldite® 2015 decrease. Additionally, the tensile strain increases and the mechanical properties change significantly as the glass transition temperature (Tg) is approached or exceeded. The failure strength of BFRP-Al joints at different temperatures is influenced by the performance of the adhesive and the BFRP, and the strength decreases at elevated temperatures. The shear load capacity of BFRP-Al joints is the highest, and the joints are more likely to fail under the normal stress state and combined shear and normal stress state. BFRP damage tends to occur when the joint is subjected to normal stress or low temperatures. It is noted that the failure criteria of adhesively bonded BFRP-Al joints accord with stress criteria under test temperatures. The equation of stress failure criterion at any temperature (from − 40°C to 80 °C) is established to provide a more comprehensive reference for the application of adhesively bonded BFRP-Al joints in automobile industry.

Microtissue engineering root canal dentine with crosslinked biopolymeric nanoparticles for mechanical stabilization.

To evaluate the functional strain distribution pattern in root dentine following canal preparation and root canal surface engineering with crosslinked biopolymeric nanoparticles using digital moiré interferometry (DMI). Root dentine specimens were prepared, grating material replicated and tested for 10-50N, compressive loads in a customized high-resolution, whole-field moiré interferometry set-up. Digital moiré fringes were acquired to determine the strain distribution pattern at specific regions of interest before and after canal enlargement, and dentine surface engineering with a chitosan nanoparticle crosslinker solution. Fringe patterns were acquired, and strain distribution pattern in the direction perpendicular to dentinal tubules (U-field) and parallel to dentinal tubules (V-field) was analysed with custom digital image-processing software. Data were analysed with a statistical method on trend analysis at 0.05 significant level. Distinct deformation patterns perpendicular to the dentinal tubules were observed in root dentine. Root canal dentine removal following instrumentation resulted in an increase in strain distribution, which increased with an increase in applied loads (P<0.01). The root canal dentine engineered with crosslinked nanoparticles demonstrated a conspicuous decrease in previously increased strain distribution in both coronal and apical root dentine (P<0.01). A significant increase in tensile strain in root dentine was observed subsequent to instrumentation in the direction parallel to dentinal tubules (P<0.01). There was a significant reduction in the tensile strain formed at the apical region of the instrumented root dentine following crosslinked nanoparticle treatment (P<0.05). This study highlighted the potential of root canal dentine microtissue engineering with crosslinked chitosan nanoparticle to improve radicular strain distribution patterns in instrumented canals.

(Invited) CMOS-Compatible Germanium Light Sources

Integration of high efficiency light sources on a Si platform is desirable for realization of optoelectronic integrated circuits (OEICs). Ge is one of the most promising candidates toward realization of monolithic-integrated laser sources on a Si substrate. Ge has the narrower bandgap energy than Si, allowing us to extract light emissions from Ge with wavelength ranges corresponding to telecommunication band. Also the emitted light from Ge is transparent in Si, making the Ge the most proper light source for optical interconnection based on Si waveguides. In addition, since the Ge has been already involved in the commercial CMOS circuit, the Ge-based optical devices are considered to be fully compatible with CMOS processes, and thus integration of Ge devices on the Si platform is more cost effective than that of compound semiconductors. Whilst there are many advantages of Ge as light sources on the Si chip, one critical drawback is the fact that Ge is indirect band gap material like Si, which means that the light emission efficiency from Ge is naturally difficult. To overcome this problem, however, various types of Ge light emitting structures have been researched so far, from bulk to nano structures. For most cases, key mechanisms to enhance the light emission efficiency are based on band engineering by strain introduction and doping control. Here, we show recent progresses of our researches on strained GOI (Ge-on-Insulator), highly strained Ge microbridges and n-doped Ge QDs. By inducing tensile strain and n-type doping, energy band structure of Ge can be modified to a direct one, leading to significantly enhanced light emission. The tensile strain can be induced into a Ge directly grown on a Si (Ge-on-Si) due to thermal expansion mismatch between Si and Ge, making the Ge-on-Si a very attractive light emitter. Toward realization of low threshold laser on silicon chip, higher efficiency light emitting structures are highly required. To this end, we show Ge-on-Insulator structure can eliminate defective Ge, the source of non-radiative recombination, leading to enhancements of the light emission. In addition, as a resonator we fabricate Ge microdisks with circular Bragg gratings (CBGs) to enhance the reflectivity and obtain sharper resonant light emission. For Ge-on-Si, the amount of strain is limited up to ~0.2%, which is not sufficient for direct-gap transition. To further increase amounts of the tensile strain, fabrication of a micro-structure is effective as a relatively large elastic stress is locally induced into the micro-region. The strained GOI is suitable for fabrication of the micro-structure as the underlying buried oxide is easily selective-etched and a free-standing Ge can be obtained. We fabricated a GOI microbridge as a micro-structure containing uniaxial strain and obtained high efficiency room-temperature PL. With the tensile strain increase, the PL peaks are found to shift to lower energies, indicating the direct band gap decreases with the strain. At higher strain levels (> ~ 0.5%), two peaks appear in the spectra, implying the splitting the heavy-hole (HH) and light-hole (LH) valence bands. Furthermore, we have recently observed resonant light emission corresponding to the resonance in Fabry-Perot (FP) cavities formed transversely to the bridge direction, implying a possibility of combining the microbridge with optical resonators. We also numerically designed FP cavities by adding distributed Bragg reflectors (DBR) laterally to the microbridge and obtained improved Q-factors without modification of the microbridge structure and resultant strain loss. A multilayer of self-assembled Ge quantum dots (QDs) is another attractive light emitting structure. To date, strong electroluminescence has been obtained from micro cavity devices involving Ge QDs. For further improvements of the light emission efficiency, stronger confinements of both electrons and holes in QDs are essential, whereas the Ge QD can confine only holes due to its type II band diagram. To confine electrons near the Ge QDs, phosphorous delta-doping at the Ge dot interface was attempted and enhancement of the PL intensity was obtained with optimal doping densities. In conclusion, by controlling higher tensile strain and n-type doping, Ge-based light sources offer highly enhanced light emission, indicating Ge has high potential for monolithic integrated laser diodes on the Si platform. Further performance improvements can be made by optimization of resonant cavity structures, increase in the tensile strain and reduction of defects toward realization of low-threshold laser operations.

Mechanical property statistical analysis of Gd50Al30Co20 amorphous wires for providing reference to design requirements of cooling system

Gd50Al30Co20 wires show excellent magnetocaloric properties and high heat exchange rate due to the microsize. The Weibull and lognormal methods were used for systematically analyzing its mechanical properties for matching the design requirements in cooling system. The wire exhibits average fracture strength of ~ 969.5 MPa and typical fracture behavior of amorphous character. Moreover, the distributions of stresses for tensile strains at 10 values are estimated by probability plot and Chi-square goodness-of-fit test. The random stresses were best fitted by lognormal probability distribution for most studied cases; however, fracture strength was best fitted by Weibull probability distribution. It is interesting to note that the mean and standard deviation of the stresses (to reach specific tensile strain) increase as the tensile strain grows, accompanied by the coefficients of variation of stresses which decrease smoothly. It is concluded that the inhomogeneity of material does cause the scatter of stresses growth, and the scatter could be considerably large.

Phase transition studies of Na3Bi system under uniaxial strain

We investigated the electronic properties and phase transitions of Na3Bi in four structural phases (space groups P63/mmc, Pc1, Fmm and Cmcm) under constant-volume uniaxial strain using the first-principles method. For P63/mmc and Pc1-Na3Bi, an important phase transition from a topological Dirac semimetal (TDS) to a topological insulator appears under compression strain around 4.5%. The insulating gap increases with the increasing compressive strain and up to around 0.1 eV at a strain of 10%. However, both P63/mmc and Pc1-Na3Bi still keep the properties of a TDS within a tensile strain of 0–10%, although the Dirac points move away from the Γ point along Γ–A in reciprocal space as the tensile strain increases. The Na3Bi with space group Fmm is identified as a topological semimetal with the inverted bands between Na-3s and Bi-6p and a parabolic dispersion in the vicinity of Γ point. Interestingly, for Fmm-Na3Bi, both compression and tensile strain lead to a TDS which is identified by calculating surface Fermi arcs and topological invariants at time-reversal planes (kz = 0 and kz = π/c) in reciprocal space. Additionally, we confirmed the high pressure phase Cmcm-Na3Bi is an ordinary insulator with a gap of about 0.62 eV. It is noteworthy that its gap almost keeps constant around 0.60 eV within a compression strain of 0–10%. In contrast, a remarkable phase transition from an insulator to a metal phase appears under tensile strain. Moreover, this phase transition is highly sensitive to tensile strain and takes place only at a strain 1.0%. These strain-induced electronic structures and phase transitions of the Na3Bi system in various phases are important due to their possible applications under high pressure in future electronic devices.

Functional siloxanes with photo-activated, simultaneous chain extension and crosslinking for lithography-based 3D printing

A novel, poly(dimethyl siloxane)-based photopolymer that exhibits simultaneous linear chain extension and crosslinking was suitable for vat photopolymerization additive manufacturing. Photopolymer compositions consisted of dithiol and diacrylate functional poly(dimethyl siloxane) oligomers, where simultaneous thiol-ene coupling and free radical polymerization provided for linear chain extension and crosslinking, respectively. Compositions possessed low viscosity before printing and the modulus and tensile strain at break of a photocured, higher molecular weight precursor after printing. Photorheology and soxhlet extraction demonstrated highly efficient photocuring, revealing a calculated molecular weight between crosslinks of 12,600 g/mol and gel fractions in excess of 90% while employing significantly lower molecular weight precursors (i.e. < 5300 g/mol). These photocured objects demonstrated a 2× increase in tensile strain at break as compared to a photocured 5300 g/mol PDMS diacrylamide alone. These results are broadly applicable to the advanced manufacturing of objects requiring high elongation at break.

Impact of tensile strain on the thermal transport of zigzag hexagonal boron nitride nanoribbon: An equilibrium molecular dynamics study

The thermal conductivity of single layer strained hexagonal boron nitride nanoribbon (h-BNNR) has been computed using the Green—Kubo formulation of Equilibrium Molecular Dynamics (EMD) simulation. We have investigated the impact of strain on thermal transport of h-BNNR by varying the applied tensile strain from 1% upto 5% through uniaxial loading. The thermal conductivity of h-BNNR decreases monotonically with the increase of uniaxial tensile strain keeping the sample size and temperature constant. The thermal conductivity can be reduced upto 86% for an applied uniaxial tensile strain of 5%. The impact of temperature and width variation on the thermal conductivity of h-BNNR has also been studied under different uniaxial tensile strain conditions. With the increase in temperature, the thermal conductivity of strained h-BNNR exhibits a decaying characteristics whereas it shows an opposite pattern with the increasing width. Such study would provide a good insight on the strain tunable thermal transport for the potential device application of boron nitride nanostructures.

Scalable lattice-strain in preferentially oriented acceptor-doped cerium oxide film and its impact on oxygen ion transport kinetics

Fast ionic conduction through strain engineered heteroepitaxial film have been demonstrated well. However, this effect in the preferentially orientated film remains to be unexplored. Here, we report the effect of interatomic distance between cation-oxygen bonds on the oxygen ionic transport in preferetially (111) oriented Ce0.80Sm0.1M0.1O2-δ (M = Gd, Nd, Pr, La) film over an amorphous quartz substrate. With the increase in ionic radii of dopant cation, the films exhibited a substantial increase in tensile strain with a change in lattice constant (Δa) in the order of ∼1.14% (Δa: 1.14% (Sm3+ + La3+) > 1.04% (Sm3+ + Pr3+) > 0.69% (Sm3+ + Nd3+) > 0.14% (Sm3+) > 0.08% (Sm3+ + Gd3+)) has strong influence on defect association enthalpy (ΔHa) leading to the change in migration energy of oxygen ion during conduction process. Highest conductivity was observed from Ce0.85Sm0.1Nd0.1O2-δ, which is four times greater than that of Ce0.85Sm0.2O2-δ. As a consequence, it is clear that optimum co-dopant is needed to attain the critical ionic radii/tensile strain to achieve the maximum ionic conductivity in the preferentially oriented ceria-based thin film. This work demonstrates the importance of tuning the ionic radii of dopant cation, which can effectively facilitate the optimum expansion of lattice to attain the fast ionic conduction process in (111) oriented thin film based solid electrolyte.

Scale-Up Fabrication of Biodegradable Poly(butylene adipate-co-terephthalate)/Organophilic-Clay Nanocomposite Films for Potential Packaging Applications.

The development of biodegradable packing materials is a global priority due to the huge volumes of plastic refuse entering landfills and the environment. In this study, a series of biodegradable nanocomposite films based on poly(butylene adipate-co-terephthalate) (PBAT) and reinforced with an organophilic layered double hydroxide (OLDH) were scale-up fabricated. The OLDH nanosheets with a basal spacing of 4.07 nm were presynthesized on a large-scale by solvent-free high-energy ball milling. All of the PBAT/OLDH nanocomposite films (0.5–4 wt % OLDH) showed a uniform dispersion of OLDH nanosheets in the PBAT matrix. A PBAT/OLDH film containing 1 wt % OLDH (denoted herein as OLDH-1) demonstrated outstanding thermal, optical, mechanical, and water vapor barrier properties compared with a pure PBAT film (OLDH-0), including a 37% reduction in haze and a 41.9% increase in nominal tensile strain at break dramatically. Furthermore, the food packaging measurement revealed that the OLDH-1 film showed a better packaging effect than the pure PBAT film and commercial polyethylene packing materials. The feasibility of scale-up manufacture and the excellent processability, manufacturing scalability, mechanical performance, optical transparency, water vapor barrier properties, and food packaging performance of the PBAT/OLDH nanocomposite films encourage their future application as biodegradable packaging films.

Uniaxial strain modulation of the skyrmion phase transition in ferromagnetic thin films

Mechanical control of magnetic order through strain is an important and promising approach in the development of advanced spintronics devices. Here, using a real-space phase-field model based on Ginzburg-Landau theory, we explore the uniaxial strain modulation of topological phase transition in ferromagnetic thin films. In the phase-field model, a magnetoelastic coupling between magnetization and strain, rather than the strain-induced anisotropic Dzyaloshinskii-Moriya interaction, is introduced to describe the strain-modulated anisotropic deformation and topological phase transition of skyrmions. The phase-field simulations show that a uniaxial tensile strain in the ferromagnetic thin films can not only change the wavelength and propagation direction of the spin spiral in helical phase but also induce topological phase transitions of skyrmions. Under specific magnetic fields, the ferromagnetic thin film exhibits the ferromagnetic-to-skyrmion and skyrmion-to-helical phase transitions serially when the uniaxial tensile strain increases, which are different from those induced by uniaxial stress in bulk ferromagnets. The present work not only provides a magnetoelastic coupling mechanism on the uniaxial strain control of topological magnetic structures but also suggests an effective way to tune the electromagnetic properties of ferromagnetic thin films for a new generation of spintronic devices based on skyrmions.

First-principles study of polarization and piezoelectricity behavior in tetragonal PbTiO3-based superlattices**Project supported by the National Natural Science Foundation of China (Grant No. 11372085) and the Shenzhen Science and Technology Project (Grant No. JCYJ20150625142543461).

Using first-principles calculation, the contribution of A-site and B-site atoms to polarization and piezoelectricity d33 in the tetragonal PbTiO3/KNbO3 and PbTiO3/LaAlO3 superlattices is investigated in this paper. It is shown that PbTiO3/KNbO3 superlattice has larger polarization and d33 than PbTiO3/LaAlO3 superlattice, because there is stronger charge transfer between A(B)-site atoms and oxygen atom in PbTiO3/KNbO3 superlattice. In PbTiO3/KNbO3 superlattice, B-site atoms (Ti, Nb) make larger contribution to the total polarization and d33 than the A-site atoms (Pb, K) because of the strong covalent interactions between the transition metal (Ti, Nb) and the oxygen atoms, while piezoelectricity in PbTiO3/LaAlO3 superlattice mainly ascribes to piezoelectric contribution of Pb atom and Ti atom in PbTiO3 component. Furthermore, by calculating the proportion of the piezoelectric contribution from PbTiO3 component in superlattices, we find there is different response of strain to piezoelectric contribution from PbTiO3 component in two superlattices but still with a value larger than 50%. In PbTiO3/KNbO3 superlattice, the c-axis strain reduces the proportion, especially under tensile condition. Meanwhile in PbTiO3/LaAlO3 superlattice, PbTiO3 plays a leading role to the total d33, especially under compressive condition, and the proportion decreases as the tensile strain increases.

The Relationship between Tensile Strain and Residual Stress of High Strength Dual Phase Steel Sheet

The relation between residual stress and tensile strain is an important factor for evaluating plastic formation grade of steel sheet. The degree of plastic deformation (Δl) and elastic recovery (δ) were obtained by measuring the length of DP600 steel sheet sample under different tensile test conditions, i.e. five tensile strains (ε). Furthermore, the average residual stress value in the surface middle (the diameter of 10 mm) region of above tensile samples was analyzed by x-ray diffraction (XRD) in the crystal plane of (211). By processing the diffraction peak angle (2θ) with half width high method (FWHM), the relationship between sin2(ψ) and diffraction angle is attained by least squares method. On this basis, a mathematical model was established to correlate the tensile strain with the residual stress in the present study. The results show that the residual stress decreases and the elastic recovery increases with the increase of tensile strain (ε≤0.205). The relation between residual stress and tensile strain can be described with an exponential function . Finally, a function of tensile strain, elastic recovery and surface residual stress is established, by which a reasonable forming condition, viz. ε=0.205, δ=2.65 mm is determined for achieving the smallest σψ.

Investigating mechanical properties of polymethylmethacrylate/silver nanoparticle composites by molecular dynamics simulation

The molecular dynamics simulation (MD) was carried out to investigate the mechanical properties of pristine polymethylmethacrylate (PMMA) and the composites of PMMA mixed with the silver nanoparticles (PMMA/AgNPs) at two AgNP weight fractions at 0.60 and 1.77 wt%. From the stress–strain profiles by the tensile process, it can be seen that the improvement on Young’s modulus is insignificant at these lower AgNP fractions. The tensile strength of pristine PMMA can be slightly improved by the embedded AgNPs at 1.77 wt%, because the local density and strength of PMMA in the vicinity of AgNP surface within about 8.2 A are improved. For the temperature effect on the mechanical properties of pristine PMMA and PMMA/AgNP composite, the Young’s moduli and strength of pristine PMMA and PMMA/AgNP composite significantly decrease at temperatures of 450 and 550 K, which are close to the predicted melting temperature of pristine PMMA about 460 K. At these temperatures, the PMMA materials become more ductile and the AgNPs within the PMMA matrix display higher mobility than those at 300 K. When the tensile strain increases, the AgNPs tend to get closer and the fracture appears at the PMMA part, leading to the close values of Young’s modulus and ultimate strength for pristine PMMA and PMMA/AgNP composite at 450 and 550 K.

Mechanical behavior of ultrafine-grained eutectoid steel containing Nano-cementite particles

A eutectoid steel with an ultrafine-grained ferrite (α) + submicron/Nano-cementite particle (θ) structure was formed by combining warm deformation of martensite to a strain of 0.36 at 0.1s−1 at 500°C with subsequent annealing at 500°C for 6h. The characteristics of the microstructure were investigated by means of a scanning electronic microscope and transmission electron microscope, and the corresponding mechanical behavior was analyzed in comparison with that of the eutectoid steel with a typical ultrafine-grained α + θ structure. The results show that both ferrite matrix and cementite particles of the ultrafine-grained α + submicron/Nano-θ steel are finer than that of the ultrafine-grained α + θ steel, i.e., the average size of approximately 0.54µm vs. 1.0µm and 0.20µm vs. 0.56µm, accompanying with a continuous yielding and a discontinuous yielding, respectively. The yield strength of the ultrafine-grained α + submicron/Nano-θ steel is 264MPa higher that of the ultrafine-grained α + θ steel, i.e., 884MPa vs. 620MPa, resulting from the enhancement caused by refined ferrite grains and cementite particles. The intragranular cementite particles within the ultrafine-grained α + submicron/Nano-θ steel are in Nano-scale, i.e., an average size of approximately 60nm, resulting in plenty of geometrically necessary dislocations (GNDs) to bring its work-hardening rate higher than that of the ultrafine-grained α + θ steel during uniform deformation. As a result, the stress increments caused by work-hardening are 89MPa and 155MPa for the ultrafine-grained α + θ steel and the ultrafine-grained α + submicron/Nano-θ steel, respectively, and their interval length of uniform strain range are nearly equal, i.e., a true strain of approximately 0.08. The work-hardening rate of the ultrafine-grained α + θ steel decline continuously with the increase of tensile strain during uniform deformation. However, the ultrafine-grained α + submicron/Nano-θ steel shows that the work-hardening rate decrease rapidly at the initial stage of work-hardening and then raise within a small strain range, then following by a slowly continuous decline to necking, and, namely, there has a peak of work-hardening rate. Furthermore, the work-hardening rate curve were divided into three stages, and the work-hardening behavior of Stage I, II and III were discussed in view of the evolution of dislocation substructures and the analytical model based on the Kocks-Mecking model.