Large Strain Limit Research Articles

The transport-structural correspondence across the nematic phase transition probed by elasto X-ray diffraction.

Electronic nematicity in iron pnictide materials is coupled to both the lattice and the conducting electrons, which allows both structural and transport observables to probe nematic fluctuations and the order parameter. Here we combine simultaneous transport and X-ray diffraction measurements with in-situ tunable strain (elasto X-ray diffraction) to measure the temperature dependence of the shear modulus and elastoresistivity above the nematic transition and the spontaneous orthorhombicity and resistivity anisotropy below the nematic transition, all within a single sample of Ba(Fe0.96Co0.04)2As2. The ratio of transport to structural quantities is nearly temperature independent over a 74 K range and agrees between the ordered and disordered phases. These results show that elasto X-ray diffraction is a powerful technique to probe the nemato-elastic and nemato-transport couplings, which have important implications to the nearby superconductivity. It also enables the measurement in the large strain limit, where the breakdown of the mean-field description reveals the intertwined nature of nematicity.

Multifunctional Two-Dimensional PtSe2-Layer Kirigami Conductors with 2000% Stretchability and Metallic-to-Semiconducting Tunability.

Two-dimensional transition-metal dichalcogenide (2D TMD) layers are highly attractive for emerging stretchable and foldable electronics owing to their extremely small thickness coupled with extraordinary electrical and optical properties. Although intrinsically large strain limits are projected in them (i.e., several times greater than silicon), integrating 2D TMDs in their pristine forms does not realize superior mechanical tolerance greatly demanded in high-end stretchable and foldable devices of unconventional form factors. In this article, we report a versatile and rational strategy to convert 2D TMDs of limited mechanical tolerance to tailored 3D structures with extremely large mechanical stretchability accompanying well-preserved electrical integrity and modulated transport properties. We employed a concept of strain engineering inspired by an ancient paper-cutting art, known as kirigami patterning, and developed 2D TMD-based kirigami electrical conductors. Specifically, we directly integrated 2D platinum diselenide (2D PtSe2) layers of controlled carrier transport characteristics on mechanically flexible polyimide (PI) substrates by taking advantage of their low synthesis temperature. The metallic 2D PtSe2/PI kirigami patterns of optimized dimensions exhibit an extremely large stretchability of ∼2000% without compromising their intrinsic electrical conductance. They also present strain-tunable and reversible photoresponsiveness when interfaced with semiconducting carbon nanotubes (CNTs), benefiting from the formation of 2D PtSe2/CNT Schottky junctions. Moreover, kirigami field-effect transistors (FETs) employing semiconducting 2D PtSe2 layers exhibit tunable gate responses coupled with mechanical stretching upon electrolyte gating. The exclusive role of the kirigami pattern parameters in the resulting mechanoelectrical responses was also verified by a finite-element modeling (FEM) simulation. These multifunctional 2D materials in unconventional yet tailored 3D forms are believed to offer vast opportunities for emerging electronics and optoelectronics.

Friction of Shear-Fracture Zones.

A shear fracture of brittle solids under compression undergoes a substantial evolution from the initial microcracking to a fully formed powder-filled shear zone. Experiments covering the entire process are relatively easy to conduct, but they are very difficult to investigate in detail. Numerically, the large strain limit has remained a challenge. An efficient simulation model and a custom-made experimental device are employed to test to what extent a shear fracture alone is sufficient to drive material to spontaneous self-lubrication. A "weak shear zone" is an important concept in geology, and a large number of explanations, specific for tectonic conditions, have been proposed. We demonstrate here that weak shear zones are far more general, and that their emergence only demands that a microscopic, i.e., fragment-scale, stress relaxation mechanism develops during the fracture process.

Stretchable, High-k Dielectric Elastomers through Liquid-Metal Inclusions.

An all-soft-matter composite with exceptional electro-elasto properties is demonstrated by embedding liquid-metal inclusions in an elastomer matrix. This material exhibits a unique combination of high dielectric constant, low stiffness, and large strain limit (ca. 600% strain). The elasticity, electrostatics, and electromechanical coupling of the composite are investigated, and strong agreement with predictions from effective medium theory is found.

Bounded elastic potentials for rubberlike materials with strain‐stiffening effects

Well‐known models for rubberlike elasticity with strain‐stiffening effects provide good predictions for unlimitedly, rapidly growing stress in a process of approaching a very large strain limit. According to such models, however, unbounded elastic strain energy would be generated as the strain limit is approached. To resolve this issue, a new, explicit approach is proposed to obtain multi‐axial elastic potentials based on uniaxial data and shear data. Then, a strain‐stiffening elastic potential is given to always yield bounded strain energy. Good agreement is achieved with a number of test data.

Optimum Specimen Geometry for Accurate Tensile Testing of Superplastic Metallic Materials

The high temperatures and large strain limits associated with superplastic materials amplify the possibility of the tensile test outcomes being sensitive to the shape and size of the specimen geometry. In spite of that, the disparities in the specimen geometries used throughout the numerous efforts on characterising this unique class of materials are rather astonishing. There is an urge to evaluate the dependency of a superplastic tensile test on specimen geometry, before a much-needed universally-adopted standard specimen can be designed; which is the main objective of this comprehensive experimental investigation. More than 20 geometries, covering multiple variations in gauge length, gauge width, grip length and grip width values, are tested at identical conditions, and the corresponding material behaviour is compared in terms of deformation uniformity, material flow and the extracted stress/strain curves. The results reveal the influences of each geometrical parameter, as well as their combined effects, and guide the selection of an optimum specimen geometry for accurate and unified tensile testing of superplastic metallic materials.

Polybutadiene model networks —synthesis, mechanical characterization and comparison with rubber elasticity models

Model networks of defined crosslink density are prepared via nonradical statistical crosslinking of polybutadiene in bulk and concentrated solution using a masked bistriazolinedione as crosslinker. The kinetics of crosslinking is monitored by FT-IR-spectroscopy. The reaction follows pseudo-1st-order reaction kinetics. The activation parameter of the crosslinking reaction is estimated from crosslinking at various temperatures. Networks of deuterated polybutadiene are prepared by this reaction in a wide range of crosslink densities. The stress strain behavior is analyzed according to the Junction Constraint-Theory of rubber elasticity (JCT) and to the approach introduced by Graessley accounting for trapped topological constraints. The analysis clearly demonstrates that trapped entanglements contribute to the mechanically effective cycle rank, i.e., to the modulus in this system in the small and large strain limit.

Impurity Conduction in Compensated P-Germanium at Large Strain Limit

The strain dependence of impurity conductivity has been measured in compensated p-type germanium containing 5×10 15 to 1.2×10 16 impurity atoms/c.c. using large uniaxial compression. For both and compression the ground state orbital of acceptor was found to have an ellipsoidal envelope along the stress direction and was enlarged. Impurity resistivity decreases linearly in a log-log plot of the resistivity versus the inverse strain when the stress is large enough to split off the degenerated valence bands. The decrease of resistivity shows a large difference between the cases of and stress, whereas the difference by the impurity concentration is small. Some clues for the mechanism of impurity conduction in the intermediate impurity concentration region may be found in this experiment.

Microdynamics of Plastic Flow at Constant Stress

Arguments are presented which conclude that the mean density of mobile dislocations in a crystalline solid increases linearly with plastic strain for small strains and decays exponentially for large strains, all at constant stress and low temperatures. The resulting analytic expression combined with the strain-rate equation can be integrated explicitly in terms of tabulated exponential integral functions. This creep equation has a general form that is consistent with many experiments and detailed comparison with a particularly well-documented experimental curve yields quantitative agreement. The large strain limit of the proposed creep equation is the familiar logarithmic creep law. A simple transformation of the creep equation yields the shape of a slowly propagating plastic front, and it is shown that the predicted shape corresponds to experimental observations for mild steel. The lengths of the incubation periods that precede fast flow, as predicted by the equation, have a stress dependence that agrees with experiment. Finally, the present arguments together with previous ones yield a strain-rate equation which generates both stress-strain and strain-time curves that are good reproductions of experimental observations.