Imposition Of Strain Research Articles

Local structural models of complex oxygen- and hydroxyl-rich GaP/InP(001) surfaces

We perform density-functional theory calculations on model surfaces to investigate the interplay between the morphology, electronic structure, and chemistry of oxygen- and hydroxyl-rich surfaces of InP(001) and GaP(001). Four dominant local oxygen topologies are identified based on the coordination environment: M-O-M and M-O-P bridges for the oxygen-decorated surface; and M-[OH]-M bridges and atop M-OH structures for the hydroxyl-decorated surface (M = In, Ga). Unique signatures in the electronic structure are linked to each of the bond topologies, defining a map to structural models that can be used to aid the interpretation of experimental probes of native oxide morphology. The M-O-M bridge can create a trap for hole carriers upon imposition of strain or chemical modification of the bonding environment of the M atoms, which may contribute to the observed photocorrosion of GaP/InP-based electrodes in photoelectrochemical cells. Our results suggest that a simplified model incorporating the dominant local bond topologies within an oxygen adlayer should reproduce the essential chemistry of complex oxygen-rich InP(001) or GaP(001) surfaces, representing a significant advantage from a modeling standpoint.

Volume of polymer gels coupled to deformation

Polymer gels are soft solids with thermodynamically semi-open features which enables the solvents to flow in and out of the gels. The volume of gels is coupled to deformation due to this unique feature. The imposition of strain or stress on gels drives a finite change in volume, and the resultant volume change influences the mechanical properties of the gels. Significantly less attention has been paid to the strain-induced volume change in the conventional mechanical testing of gels, because the characteristic time of this phenomenon (τsw) governed by diffusion process is considerably long for millimeter- or centimeter-sized gels, as compared to the experimental time scale represented by −1 or ω−1 where and ω are the strain rate and angular frequency, respectively. The stress relaxation experiments in the long time scale (t ≈ τsw), and the tensile or compression experiments using extremely slow strain rates (−1 ≈ τsw) reveal that Poisson's ratio and stress of gels in solvents are pronouncedly dependent of time, varying from the values for isovolumetric deformation to the values for the equilibrium deformation where the induced volume change saturates. The strain rates satisfying the relation −1 < τsw are easily attainable for micrometer- or nanometer-sized gels with markedly short τsw. The strain-rate-dependent mechanical properties of gels need to be considered in the design of soft actuators based on micro-fabricated gels and fine gel fibers. The volume of the gels is also coupled to other types of mechanical stimulus such as solvent flow and centrifugal force. The analysis of these phenomena enables us to evaluate various properties of the gels such as the frictional coefficient, osmotic bulk modulus and shear modulus. In this review article, we summarize the experimental and theoretical studies on a rich variety of phenomena caused by the strain-induced volume change under various types of strain and force such as constant uniaxial strain, sinusoidal force, ultraslow stretching and compression, solvent flow, and ultracentrifugal force.

Ultrafine-grained steel fabricated using warm multiaxial forging: Microstructure and mechanical properties

Grain refinement of bulk metals using severe plastic deformation (SPD) is a popular approach to improve both strength and toughness. In this paper, grain refinement of steel processed by warm multiaxial forging (MAF) and its mechanical behavior has been investigated. Coarse-grained, plain low carbon steel was deformed using MAF at 500 °C. Microstructural evolution is characterized using electron backscattered diffraction and mechanical behavior has been studied. Fraction of low angle grain boundaries (LAB) is observed to increase with strain up to total engineering strain of 1.3 thereafter it starts decreasing whereas, high angle grain boundaries showed just the opposite trend. It appears that initially grain subdivision takes place with imposition of strain thereby increasing the fraction of LAB. After a critical strain these LAB transforms into the high angle boundaries (HAB). The initial coarser grains of average 30 μm size subdivided into grains of the size finer than 0.5 μm. This has been confirmed by TEM micrographs. Improved tensile strengths and hardness values are obtained after warm MAF.

Stress relaxation in fine fibrin films: Comparison of films prepared with thrombin and ancrod

AbstractMeasurements of stress relaxation in uniaxial extension have been made on fibrin film prepared from fine bovine fibrin clots (i.e., clots in which there is minimal lateral aggregation of protofibrils), both ligated and unligated, and polymerized with both thrombin and ancrod, plasticized with either aqueous buffer or glycerol. The stress 100 s after imposition of strain was approximately proportional to In λ, where λ is the stretch ratio. Ligated thrombin films showed comparatively little relaxation over a period of one day and almost complete recovery after release of stress. In unligated thrombin films, there was substantial relaxation in two stages, as previously observed for coarse films, and substantial irrecoverable deformation. The extent of relaxation and the proportion of strain that was irrecoverable increased with the magnitude of the strain. In ancrod films (unligated), there was much more relaxation (stress decaying by as much as a factor of 10) and much more irrecoverable deformation (about 70% of the initial deformation); these results did not depend on the magnitude of the strain. When an ancrod film was released after relaxation and submitted to a second stretch, the extent of the second relaxation was much less. These observations are discussed in relation to the structure of fine films and possible mechanisms for relaxation and irrecoverable deformation.

Rheology of Fibrin Clots. VI. Stress Relaxation, Creep, and Differential Dynamic Modulus of Fine Clots in Large Shearing Deformations

A fine, unligated clot of human or bovine fibrin prepared from purified fibrinogen is subjected to a large torsional deformation (maximum shear strain γ up to 1.37) with superposed small oscillating deformations (Δγ ca. 0.03) at frequencies from 0.2 to 1 Hz. The “secant modulus” is defined as Gi(γ)=σi/γ, where σ is stress and the subscript i refers to an initial measurement about 25 s after imposition of strain. The storage modulus G′(ω,γ) refers to a differential oscillating measurement at frequency ω superposed on a static strain γ. For γ up to about 0.1, Gi(γ) was independent of γ and equal to G′(ω,0) measured at about 1 Hz and zero static strain. At large static deformations, the differential storage modulus G′(ω,γ) could be used to monitor changes in structure. Since there is very little time dependence of the relaxation modulus in the range from 1 to 60 s, and the loss tangent is very small, Gi′(ω,γ) could be considered simply as the differential modulus Gi(γ)+γdGi/dγ, and agreed with the latter exp...

Studies of fibrin film. II. Small-angle x-ray scattering.

AbstractMeasurements of small‐angle x‐ray scattering have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 12–15% fibrin plasticized with either aqueous buffer or glycerol. Most of the experiments were made with water‐plasticized films because of the better x‐ray contrast. Both unligated and ligated (i.e., with α‐α and λ‐λ ligation by fibrinoligase, factor XIIIa) films were studied. Theoretical angular scattering profiles were calculated for a simplified model of the fibrin protofibril as a function of stretch ratio in uniaxial elongation based on an orientation distribution derived in the preceding paper. Measurements were made on unstretched films both with slit geometry and with pinhole geometry and two‐dimensional position‐sensitive detection. From the former, which had better resolution, the fundamental peak corresponded to a repeat spacing of 230 ± 5 Å, in agreement with early results of Stryer et al. [(1963) Nature 197, 793–794] and electron micrographs of stained fibrin, and with the half‐staggered overlapping model of the protofibril. Measurements were made with pinhole geometry on film stretched up to a stretch ratio λ = 1.59 at different times after imposition of strain and after release and recovery. The changes in meridional and equatorial scattering profiles, including the more prominent appearance of a harmonic in the former, show the orientation of the protofibrils as previously deduced from birefringence and related quantitatively to stretch ratio. In addition, the original repeat spacing is gradually replaced by one of about 290 Å. The “internal” stretch ratio λx, averaging about 1.28, is independent of the macroscopic stretch ratio λ from 1.14 to 1.59. The change, which is more nearly complete for unligated film and is reversible on release and retraction of the stretched strips, is attributed to an internal transition in the fibrin monomer units, probably involving extension of either the helical connectors or the terminal nodules. The results of stress and birefringence relaxation reported in the preceding paper of this series are interpreted on this basis.

The mechanical properties of the abdominal cuticle of Rhodnius larvae.

1. The mechanical properties of loops of cuticle cut from the abdomens of 5th instar Rhodnius have been investigated. The cuticle shows pronounced viscoelastic behaviour. 2. Stress-relaxation tests show a continuously falling modulus over a wide range of times after the imposition of a strain. 3. Plasticized samples of cuticle show stress-relaxation curves which are shifted along the time axis towards earlier times by up to times 10-3. The modulus at any particular time after the imposition of strain is about 10 times lower than that of the unplasticized cuticle. 4. It is concluded that the mechanical properties of this cuticle are determined, at least for maintained stresses, largely by the matrix material. Chitin microfibrils may act as a reinforcing filler for short-term, rapid stresses. The cuticular macromolecules are probably not extensively cross-linked by primary bonds, though secondary interactions between them are probably important in the viscoelastic properties of the cuticle. 5. Plasticization probably involves a change in either the number or the strength of secondary interactions between the cuticle macromolecules, or both.