Unstrained State Research Articles

Unraveling the mechanics of directional coarsening in single crystal Ni-based superalloys via laser peening: Insights from experimental characterization and microstructural analysis

Surface treatment techniques, like laser peening, enhance the longevity of Ni-based superalloy components by reinforcing their microstructure against surface-initiated damage. Recent findings suggest that these methods may also influence precipitate coarsening behavior at high temperatures, leading to rafting phenomena. This study extensively examined γ’ rafting in single crystal Ni-based superalloy CMSX-4 post laser peening and heat treatment. X-ray diffraction revealed surface compressive stresses (-400 MPa to -800 MPa), transitioning to tensile stresses at greater depths before returning to an unstressed state. Correlation with electron microscopy indicated horizontal coarsening in compressive regions and vertical coarsening in tensile regions due to misfit and residual stresses aiding diffusion. Plastic strain near the LPed surface was measurably increased with lattice misorientation values around 5° before returning to an unstrained state after heat treatment due to rafting-aided recovery and defect reorganization. Secondary γ’ precipitates with a radius of approximately 10 nm occupied γ channels between rafted precipitates, indicating solute element diffusion and supersaturation. Energy dispersive x-ray spectroscopy showed a significant depletion of γ’-forming elements around rafted primary precipitates, highlighting preferential solute diffusion.

Deformation-induced band gap variation in phosphorene: tight-binding model vs. first-principles simulations

We focus on estimating the influences of uniaxial tensile and shear strains on a band gap in the electronic structure of monolayer black phosphorus. To study numerically the dependence of the band gap on the deformation type and strength, we apply two approaches: the tight-binding model (with the exponential and inversely quadratic strain-induced bond-length-dependent hoppings) and the density-functional-theory-based calculations. Both approaches corroborated that phosphorene as a direct semiconductor in the unstrained state can become a semimetal at certain types and strengths of deformations. The critical values of the semiconductor–semimetal transition are different depending on approximations and model parameters.

Mechanical and sensing performance under hydrothermal ageing of wearable sensors made of polydimethylsiloxane with graphitic nanofillers

The effect of hydrothermal ageing on flexible wearable sensors made of PDMS reinforced with graphitic nanofillers (carbon nanotubes, CNTs, and graphene nanoplatelets, GNPs) is explored. It is observed that the nanofiller addition, especially in case of GNPs, promotes a much higher water uptake at saturation due to the higher water diffusion as the generalized porosity is much more significant. However, it does not harm the mechanical properties in comparison with unaged samples, as these water molecules are not prevalently incorporated into the polymer network (around 0.3% water uptake in neat resin). Furthermore, the electrical properties at unstrained state are not severely affected but the electromechanical ones, which are much more sensitive to any change in the electrical network, drastically change with hydrothermal ageing. The gauge factor significantly increases in the case of GNP-doped samples, due to the higher separation between neighbouring nanofillers induced by the slight polymer swelling. The ultra-sensitivity of GNP-doped sensors (up to 100 at very low strains and above 106 at high strain levels) makes them very applicable for the detection of small human movements. For this purpose, a trial with a 2-month-aged GNP-doped sensor was successfully carried out in terms of neck and wrist pulse monitoring.

Metallo-Supramolecular Rod–Coil Block Copolymer Thin Films for Stretchable Organic Field Effect Transistor Application

Herein, to decrease the synthetic burden of the rod–coil diblock copolymers, the coordination-driven self-assembly approach is utilized to readily construct a series of polystyrene (PS)-block-poly(3-hexylthiophene) (P3HT) copolymers with dynamic heteroleptic coordination bonds between the two blocks. In detail, the chain length of the PS is varied for a fixed P3HT chain length to produce block copolymers, namely, PS85–Zn–P3HT187 (P1) and PS161–Zn–P3HT187 (P2), with comparable electrical performance to that of homopolymer P3HT (P0) in the unstrained state, along with enhanced crack onset strain and much less reduced mobility under 25–100% strain. The ability to maintain the charge transport characteristics is due to the longer coil chain length and amorphous PS regions for the dissipation of strain energy. The organic field effect transistors of P2 with a longer PS segment show only a slight change in mobility from 5.48 × 10–3 to 1.40 × 10–3 cm2 V–1 s–1 under 25–100% and maintain this mobility even after being subjected to 100 repeated stretching/releasing cycles at 50% strain. This proof-of-concept demonstration of the proposed molecular engineering approach based on the metallo-supramolecular method provides a suitable route to the molecular design of stretchable polymer semiconductors with balanced mechanical and electrical behaviors.

Volume-matched piezoelectric LaN/REN superlattices from first-principles

LaN/rare earth nitride (REN) superlattices, having magnetic REN as one of the parent components, are constructed and studied by first-principles calculations. In particular, they are found to be mechanically and dynamically stable with (anti-)ferromagnetic and ferroelectric orderings. We reveal that the volume matching condition is applicable to these superlattices, which results in the elastic constant C33 softening and, when combined with a small c/a value, induces a huge piezoelectric response near the unstrained state. We also show that in-plane biaxial strain can precisely control the nature (indirect or direct) and value of the electronic bandgap. Moreover, the unpaired magnetically active 4f-electrons reduce the c-direction off-centric distortion of the wurtzite structure, making possible the switching of the ferroelectric polarization. This work, therefore, reveals that the volume matching condition also applies to magnetic materials and provides guidance for the design of multiferroic rare-earth nitride superlattices in piezoelectric devices.

Elucidation of Strain-Dependent, Zinc Oxide Nanorod Response for Nanorod-Guided Fluorescence Intensity.

In this work, we examine how strain exerted on individual ZnO nanorods (NRs) can influence the fluorescence signals that are emitted from fluorophore molecules and subsequently coupled into and guided along the NR. We elucidate the relationships between the incremental levels of compressive and tensile strain on the NRs and measured fluorescence intensity of a model fluorophore, rhodamine 6G (R6G), as a function of the position on the NRs. We reveal that compressive strain on the NRs leads to a decrease in the guided fluorescence signal, while tensile strain leads to an increase in the fluorescence intensity. Compared to an unstrained state, approximately 35% decrease (increase) in R6G fluorescence intensity was observed from ZnO NRs when they were under compressive strain of −14% (tensile strain of +10%). Further, our systematic acquisition of the incremental addition of uniaxial strain result in a linear relationship of the coupled fluorescence signal and the amount of applied strain. The degree of fluorescence intensification on nanorod ends (DoF), which is a quantitative indicator for the amount of R6G signals coupled into and waveguided to the NR ends compared to those on the main body, also exhibits a linear relationship with strain. These outcomes, in turn, demonstrate that strain alters the waveguiding capabilities of ZnO NRs in a predictable manner, which can be exploited to modulate and optimize fluorescence and other light signals emitted by a nearby source. Considering the wide utility of ZnO NRs in photonics, optoelectronics, and sensors, insights from our study may be highly valuable to effectively controlling and enhancing optical signals from chemical and biological analytes through strain.

Anomalous behavior of strain modulated lattice thermal transport in piezoelectric crystals and the effect of polarization

Strain is known to influence the lattice dynamics of crystalline solids. Its effect is generally expressed in a decrease of the lattice thermal conductivity as the stress is varied from compression to tension. However, the results of our ab initio calculations suggest that under a uniaxial strain along the c-axis, the lattice thermal conductivity of PbTiO3, a typical piezoelectric material, actually increases from compression to tension going through a maximum at the unstrained state. We find that both tensile and compressive strains reduce considerably the phonon mean free path. Strain also modifies remarkably the phonon group velocity and lifetime of the low-frequency acoustic modes. This unusual strain dependence largely comes from a competing effect of the electrical polarization and the elastic stiffness on the phonon velocity due to their opposite strain dependence. Based on these results, we predict an unexpected thermal transport behavior due to the piezoelectric effect and reveal the significant role of the electrical polarization in lattice thermal transport. This work may thus inspire the development of polarization engineering as a new avenue towards manipulation of heat conduction bringing to the light the previously unrecognized potential of piezoelectric materials for thermal modulation applications.

Deformation of the colliding Luzon volcanic arc: Strain analysis using magnetic fabrics of syn-orogenic mudstones in the Coastal Range, eastern Taiwan

Exposed in the Coastal Range of eastern Taiwan is a portion of the Luzon volcanic arc that has collided with the South China continent since 6 Ma, as well as syn-orogenic sedimentary rocks deposited on the arc and subsequently intensely folded and thrusted. This study sampled syn-orogenic mudstones for the measurement of anisotropy of magnetic susceptibility (AMS) at four sites on the sides of the range. The AMS fabrics reveal a contribution from burial compaction and, less significantly, tectonic shortening which varies slightly across the range. This lateral variation is considered as, for simplicity, a result of two-phase deformation because these rocks were involved in an eastward propagating back thrust system on the colliding arc. A new strain analysis method is developed to estimate the overall tectonic strain by unstraining the pseudo-strains with respect to the AMS measurements at each site with the knowledge about the distribution of the susceptibility ellipsoids in the unstrained state. Strain results reveal that shortening phases occurred before and after the folding, with a shortening ratio of 0.177–0.195 and 0.181–0.248, respectively. The post-folding shortening on the western side is sub-parallel to the range, and the pre-folding shortening on both sides is layer-parallel and nearly perpendicular to the range. Therefore, the mudstones underwent layer-parallel shortening, fold-related tilting and eventually orogen-parallel shortening after deposition. This sheds light on the complexity in deformation of the colliding Luzon arc.

Nonlinear static response of an underwater elastic toroidal storage container

This paper investigates the behaviour of an underwater elastic toroidal shell storage container filled with compressible fluid under constraint volume condition. The fundamental form of the surface is used to define the geometry of a toroidal shell under an initial unstrained state (IUS), reference state (RS), and deformed state (DS). In the reference state, it is assumed that the toroidal shell storage container has a circular cross section. In the deformed state, the displacements of the shell are assumed to be large. The energy functional of the underwater toroidal shell storage container can be established based on the principle of virtual work, and it can be rearranged in an appropriate form that can conveniently be implemented in a finite element formulation. The nonlinear static analysis of the elastic toroidal storage container under external hydrostatic pressure can be performed by the nonlinear finite element method (FEM) via the fifth-order polynomials shape functions. The nonlinear responses under the variable water depth to cross-sectional radius, cross-sectional radius to wall thickness, and cross-sectional bend radii ratio are demonstrated and discussed. The results revealed that the change of internal pressure is indicated by the value of the Lagrange multiplier, which is calculated using the iterative method for finding the equilibrium configuration under the constraint volume requirement.

Material pre-straining effects on fatigue behaviour of S355 structural steel

A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.

Influence of substrate-induced thermal stress on the superconducting properties of V3Si thin films

Thin films of superconducting V3Si were prepared by means of RF sputtering from a compound V3Si target at room temperature onto sapphire and oxide-coated silicon wafers, followed by rapid thermal processing under secondary vacuum. The superconducting properties of the films thus produced are found to improve with annealing temperature, which is ascribed to a reduction in defects in the polycrystalline layer. Critical temperatures (Tc) up to 15.3 K were demonstrated after thermal processing, compared to less than 1 K after deposition. The Tc was always found to be lower on the silicon wafers, by on average 1.9±0.3K for the annealed samples. This difference, as well as a broadening of the superconducting transitions, is nearly independent of the annealing conditions. In situ XRD measurements reveal that the silicide layer becomes strained upon heating due to a mismatch between the thermal expansion of the substrate and that of V3Si. Taking into account the volume reduction due to crystallization, this mismatch is initially larger on sapphire, though stress relaxation allows the silicide layer to be in a relatively unstrained state after cooling. On oxidized silicon, however, no clear evidence of relaxation upon cooling is observed, and V3Si ends up with an out-of-plane strain of 0.3% at room temperature. This strain increases as the sample is cooled down to cryogenic temperatures, though the deformation of the polycrystalline layer is expected to be highly inhomogeneous. Taking into account also the reported occurrence of a Martensitic transition just above the critical temperature, this extrapolated strain distribution is found to closely match an existing model of the strain dependence of A-15 superconducting compounds.

The USGT Method for Suspender Tensioning of Self‐Anchored Suspension Bridges

Unlike earth‐anchored suspension bridges, self‐anchored suspension bridges (SASBs) involve a special construction stage, namely, suspender tensioning, in which the tensioning force and sequence are crucial and complicated. Against this background, an example bridge A, a SASB with a steel‐concrete composite beam, is introduced in detail. Using MIDAS finite element software, a suspender tensioning scheme is formulated based on a combination method of the unstrained state method and graded tension method (the USGT method), in which a suspender is tensioned according to its unstrained length. By analyzing the bending moment change of the beam and deflection of the main cable throughout the entire construction process, a “high‐to‐low” suspender tensioning sequence is proposed that also involves symmetrical tensioning from the main towers to the midspan or the anchor positions. In the optimized construction process, the deviation and stress of the main towers are controlled well, thereby ensuring the safety of the main beam and main towers in the construction process.

Depth-Sensing Hardness Measurements to Probe Hardening Behaviour and Dynamic Strain Ageing Effects of Iron during Tensile Pre-Deformation.

This work reports results from quasi-static nanoindentation measurements of iron, in the un-strained state and subjected to 15% tensile pre-straining at room temperature, 125 °C and 300 °C, in order to extract room temperature hardness and elastic modulus as a function of indentation depth. The material is found to exhibit increased disposition for pile-up formation due to the pre-straining, affecting the evaluation of the mechanical properties of the material. Nanoindentation data obtained with and without pre-straining are compared with bulk tensile properties derived from the tensile pre-straining tests at various temperatures. A significant mismatch between the hardness of the material and the tensile test results is observed and attributed to increased pile-up behaviour of the material after pre-straining, as evidenced by atomic force microscopy. The observations can be quantitatively reconciled by an elastic modulus correction applied to the nanoindentation data, and the remaining discrepancies explained by taking into account that strain hardening behaviour and nano-hardness results are closely affected by dynamic strain ageing caused by carbon interstitial impurities, which is clearly manifested at the intermediate temperature of 125 °C.

Oxidative Chemical Vapor Deposition of Conducting Polymer Films on Nanostructured Surfaces for Piezoresistive Sensor Applications

AbstractIn this study, a novel, fully polymeric setup for piezoresistive sensing is prepared and tested. Monolayers of polystyrene (PS) nanospheres are assembled on flexible polyethylene naphthalate substrates. Subsequently, thin layers (≈50–100 nm) of poly(3,4‐ethylenedioxythiophene) (PEDOT) are deposited conformally around the spheres by oxidative chemical vapor deposition (oCVD). Voltage−current characteristics and direct resistance measurements are performed to test the electrical properties of the samples in their unstrained state and their piezoresistive response during bending. Substrate deposition temperature (Tsub) and film thickness (tPEDOT) are used as parameters to alter properties of the PEDOT thin films; increased Tsub and tPEDOT lead to samples exhibiting lower intrinsic resistance. The electrical conductivity of the samples is estimated to range as high as tens of S cm−1. Dopant exchange of the oCVD‐PEDOT layer (intrinsically, chlorine‐doped) is performed by putting the samples in 0.5 m sulfuric acid, which decreases their resistance by ≈1/3. Regarding the piezoresistive properties of the devices, acid treatment, higher Tsub and tPEDOT (thus, lower intrinsic resistance) yield samples with increased response. As a result, gauge factors as high as 11.4 are achieved. Due to their flexibility and low‐cost, the proposed structures can be readily employed as skin‐inspired or wearable electronic devices.

Huge Piezoelectric Response of LaN-based Superlattices.

We construct LaN-based artificial superlattices to investigate the ferroelectricity and piezoelectricity using the volume matching conditions of the parent components that soften the elastic constant C33 and increase the piezoelectric modulus d33. The proposed superlattice consists of LaN and YN (or LaN and ScN) buckled monolayers alternately arranged along the crystallographic c-direction. The structure of polar wurtzite (w-LaYN/w-LaScN) is both mechanically and dynamically stable, and the computed energy barrier makes the ferroelectric polarization switching possible. We show that the epitaxial strain can modify the spontaneous ferroelectric polarization as well as d33. The LaN/YN superlattice exhibits a huge piezoelectric response in the unstrained state, due to their small c/a value and extremely soft C33. In addition, the epitaxial strain is revealed as effective control of the nature (indirect and direct) and value of the electronic band gap.

Gradient Photonic Materials Based on One-Dimensional Polymer Photonic Crystals.

In nature, animals such as chameleons are well-known for the complex color patterns of their skin and the ability to adapt and change the color by manipulating sophisticated photonic crystal systems. Artificial gradient photonic materials are inspired by these color patterns. A concept for the preparation of such materials and their function as tunable mechanochromic materials is presented in this work. The system consists of a 1D polymer photonic crystal on a centimeter scale on top of an elastic poly(dimethylsiloxane) substrate with a gradient in stiffness. In the unstrained state, this system reveals a uniform red reflectance over the entire sample. Upon deformation, a gradient in local strain of the substrate is formed and transferred to the photonic crystal. Depending on the magnitude of this local strain, the thickness of the photonic crystal decreases continuously, resulting in a position-dependent blue shift of the reflectance peak and hence the color in a rainbow-like fashion. Using more sophisticated hard-soft-hard-soft-hard gradient elastomers enables the realization of stripe-like reflectance patterns. Thus, this approach allows for the tunable formation of reflectance gradients and complex reflectance patterns. Envisioned applications are in the field of mechanochromic sensors, telemedicine, smart materials, and metamaterials.

Corrosion Resistance of Nitinol Wires After Deformation

The corrosion resistance of thin NiTi wires in unstrained state and at 6% of pseudoelastic strain was determined by cyclic potentiodynamic polarization, considering a surface area of 2.3 cm2, equivalent to that of an actual minimally invasive implant. Surface finishing was carried out by mechanical polishing, electropolishing, and subsequent heat treatment in a salt bath furnace. Topography, microstructure, and composition of the respective surfaces were characterized in detail using light and electron microscopy, as well as glow discharge optical emission spectroscopy. Whereas breakdown below 1000 mV was observed for wires in mechanically polished condition in unstrained and strained state, wires in electropolished and electropolished + heat-treated condition exhibit no breakdown up to 1000 mV. Multiple testing reproduced uniform potentiodynamic polarization curves in unstrained and strained state. Accordingly, the observed formation of cracks in the surface oxide layer of wires in electropolished + heat-treated condition is concluded to be negligible in unstrained and strained state. Cause for the observed high performance of both the electropolished and the electropolished + heat-treated wires is apparently the smoothing of the initial surface that results in a homogeneous oxide layer even after heat treatment.

A CONSTITUTIVE MODEL FOR BOTH LOW AND HIGH STRAIN NONLINEARITIES IN HIGHLY FILLED ELASTOMERS AND IMPLEMENTATION WITH USER-DEFINED MATERIAL SUBROUTINES IN ABAQUS

ABSTRACT Strain energy functions (SEFs) are used to model the hyperelastic behavior of rubberlike materials. In tension, the stress–strain response of these materials often exhibits three characteristics: (i) a decreasing modulus at low strains (<20%), (ii) a constant modulus at intermediate strains, and (iii) an increasing modulus at high strains (>200%). Fitting an SEF that works in each regime is challenging when multiple or nonhomogeneous deformation modes are considered. The difficulty increases with highly filled elastomers because the small strain nonlinearity increases and finite-extensibility occurs at lower strains. One can compromise by fitting an SEF to a limited range of strain, but this is not always appropriate. For example, rubber seals in oilfield packers can exhibit low global strains but high localized strains. The Davies–De–Thomas (DDT) SEF is a good candidate for modeling such materials. Additional improvements will be shown by combining concepts from the DDT and Yeoh SEFs to construct a more versatile SEF. The SEF is implemented with user-defined material subroutines in Abaqus/Standard (UHYPER) and Abaqus/Explicit (VUMAT) for a three-dimensional general strain problem, and an approach to overcome a mathematically indeterminate stress condition in the unstrained state is derived. The complete UHYPER and VUMAT subroutines are also presented.

Weakly-Emergent Strain-Dependent Properties of High Field Superconductors

All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets means we must understand and control the high sensitivity of critical current density Jc to strain ε. Here we present very detailed Jc(B, θ, T, ε) measurements on a high temperature superconductor (HTS), a (Rare−Earth)Ba2Cu3O7−δ (REBCO) coated conductor, and a low temperature superconductor (LTS), a Nb3Sn wire, that include the very widely observed inverted parabolic strain dependence for Jc(ε). The canonical explanation for the parabolic strain dependence of Jc in LTS wires attributes it to an angular average of an underlying intrinsic parabolic single crystal response. It assigns optimal superconducting critical parameters to the unstrained state which implies that Jc(ε) should reach its peak value at a single strain (ε = εpeak), independent of field B, and temperature T. However, consistent with a new analysis, the high field measurements reported here provide a clear signature for weakly-emergent behaviour, namely εpeak is markedly B, (field angle θ for the HTS) and T dependent in both materials. The strain dependence of Jc in these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence of Jc of any of their underlying component parts, but is amenable to calculation. We conclude that Jc(ε) is an emergent property in both REBCO and Nb3Sn conductors and that for the LTS Nb3Sn conductor, the emergent behaviour is not consistent with the long-standing canonical explanation for Jc(ε).

Surface viscoelasticity in model polymer multilayers: From planar interfaces to rising bubbles

In the present work, a polymeric transient viscoelastic network is used as a model system to investigate several fundamentals of interfacial viscoelasticity and nonlinear behavior, in simple shear, compression, and for simple mixed deformations. A supramolecular polymer bilayer, characterized by long but finite relaxation times, is created at the water-air interface using a layer-by-layer assembly method. The possibility of studying the individual layers starting from an unstrained reference state enabled the independent quantification of the equilibrium thermodynamic properties, and the viscoelastic response of the bilayer could be studied separately for shear and compressional deformations. Time- and frequency-dependent material functions of the layer were determined in simple shear and uniform compression. Moreover, a quasilinear neo-Hookean model for elastic interfaces was adapted to describe step strain experiments on a viscoelastic system by allowing the material properties to be time-dependent. The use of this model made it possible to calculate the response of the system to step deformations. Within the linear response regime, both stress-strain proportionality and the superposition principle were investigated. Furthermore, the onset of nonlinear behavior of the extra stresses was characterized in shear and for the first time in pure compression. We conclude by investigating the multilayer system in a rising bubble setup and show that the neo-Hookean model is able to predict the extra and deviatoric surface stresses well up to moderate deformations.