Strains In Excess Research Articles

Extreme Drop Durability of Sintered Silver Traces Printed With Extrusion and Aerosol Jet Processes

Abstract Sintered silver is a popular material for printing conductive traces in printed hybrid electronics (PHE). However, due to the novel materials and printing techniques in PHEs, reliability still needs to be adequately characterized for all types of life-cycle application conditions. This paper focuses on characterizing the reliability of printed silver traces fabricated with extrusion printing and aerosol jet printing (AJP) processes, under severe shock conditions up to 40,000 g peak acceleration, resulting in very high strain magnitudes and strain rates. This study utilizes test specimens of cantilever form factor to study the reliability of three-dimensional printed traces and substrates. Traces printed using both techniques were found to withstand repetitive drops at up to 40,000 g peak acceleration. However, extrusion-printed silver traces were found to be more reliable than their AJP counterparts, because of the extruded traces' superior adhesion to the FR4 substrate and lack of sintering shrinkage cracks. Strain gauges revealed strains in excess of 10,000 με during a 40,000 g shock event. A calibrated finite element (FE) model revealed that the strains at the trace location exceeded 15,000 με during a 40,000 g shock event.

Evidence for and Against Temperate Ice in Antarctic Shear Margins From Radar‐Depth Sounding Data

AbstractThe majority of ice mass loss from Antarctica flows through narrow, fast sliding regions of ice. The lateral boundaries of these regions, termed shear margins, are characterized by lateral shear strains in excess of ∼10−3 yr−1. Shear heating within these margins could warm ice significantly–even to the melting point–but other processes such as lateral advection of cold ice and fabric development compete with this effect. Radar observations can help constrain where temperate ice exists because englacial temperature increases electric conductivity which increases radar attenuation. We utilize the temperature‐dependent attenuation of ice to develop a novel method for constraining englacial temperature in shear margins by combining existing thermal models with very high frequency radar depth‐sounding data. We find evidence supporting temperate shear margins in 18 locations and find evidence for non‐temperate margins in 37 locations, notably in the Amundsen Sea Embayment.

Mechanical properties and deformation mechanisms of single crystal Mg micropillars subjected to high-strain-rate C-axis compression

The mechanical properties and deformation mechanisms of single crystal magnesium under c-axis quasi-static and high-strain rate compressions are investigated through in situ scanning electron microscope (SEM) experiments and post-mortem transmission electron microscope (TEM) characterization. The findings revealed that ductility and high rates of hardening are preserved for pillars as large as 15 μm. Furthermore, rate effects result in a mild increase in flow stress with plastic deformations controlled primarily by the slip of <a+c> type dislocations. Importantly and in contrast to other literature reports, plastic deformation occurs in the absence of twining. As the strain increases and plastic deformation exceeds about 4%, crystal rotation activates basal slip, <a> type dislocations, resulting in a more rate independent flow stress. TEM observation on micropillars compressed at a strain rate of 250/s, revealed the activation of {112‾2‾}<1‾1‾23> slip systems and high mobility of screw dislocations as major contributors to plastic strains in excess of 10% without fracture. These findings are relevant to the design of lightweight materials used in transportation systems, e.g., selection of material grain size. Moreover, the experimental data here reported provides the materials science community with a unique opportunity to validate discrete dislocation dynamics (DDD) formulations employed in multiscale design of materials.

RETHINKING THE 10% RULE: PATIENT-SPECIFIC MODELLING OF STRAIN IN DISTAL FEMUR FRACTURES

The objective of this study was to use patient-specific finite element modeling to measure the 3D interfragmentary strain environment in clinically realistic fractures. The hypothesis was that in the early post-operative period, the tissues in and around the fracture gap can tolerate a state of strain in excess of 10%, the classical limit proposed in the Perren strain theory.Eight patients (6 males, 2 females; ages 22–95 years) with distal femur fractures (OTA/AO 33-A/B/C) treated in a Level I trauma center were retrospectively identified. All were treated with lateral bridge plating. Preoperative computed tomography scans and post-operative X-rays were used to create the reduced fracture models. Patient-specific materials properties and loading conditions (20%, 60%, and 100% body weight (BW)) were applied following our published method.[1]Elements with von Mises strains &gt;10% are shown in the 100% BW loading condition. For all three loading scenarios, as the bridge span increased, so did the maximum von Mises strain within the strain visualization region. The average gap closing (Perren) strain (mean ± SD) for all patient-specific models at each body weight (20%, 60%, and 100%) was 8.6% ± 3.9%, 25.8% ± 33.9%, and 39.3% ± 33.9%, while the corresponding max von Mises strains were 42.0% ± 29%, 110.7% ± 32.7%, and 168.4% ± 31.9%. Strains in and around the fracture gap stayed in the 2–10% range only for the lowest load application level (20% BW).Moderate loading of 60% BW and above caused gap strains that far exceeded the upper limit of the classical strain rule (&lt;10% strain for bone healing). Since all of the included patients achieved successful unions, these findings suggest that healing of distal femur fractures may be robust to localized strains greater than 10%.

Dislocation-based high-temperature plasticity of polycrystalline perovskite SrTiO3

Dislocation networks have been demonstrated to substantially enhance functional properties. As-sintered samples are virtually devoid of dislocations, new innovative techniques for introducing sufficiently high dislocation densities into polycrystalline ceramics are needed. While dislocation-based plasticity at high temperatures has been demonstrated for a large range of ceramic single crystals, plasticity in polycrystals is much less understood. Here, we demonstrate plastic strains in excess of several % based on dislocation motion in polycrystalline SrTiO3 at ≈ 1100 °C with 3.9 µm grain size. Ultra-high voltage electron microscopy reveals an associated increase in dislocation density by three orders of magnitude. Achievable strain rates are comparable to creep-based mechanisms and much less sensitive to applied stress than observed for metals. A specialized testing protocol allows quantification of the deformability via stress exponent, activation volume and activation enthalpy giving additional quantification. In conjunction with TEM images, the mechanical data gives insight into the underlying mechanisms.

Auxetic boosting of confinement in mortar by 3D reentrant truss lattices for next generation steel reinforced concrete members

A study is presented on a new method for confining concrete/mortar materials using steel auxetic truss lattice reinforcement. The study builds on major advances in architected materials and a new concept for the confinement of structural members. Numerical results are presented on a unit cell of the new composite including a mortar matrix and a steel reentrant auxetic lattice as reinforcement, followed by a small scale experimental phase where 3D printed steel auxetic lattices are encased in mortar and tested under axial compression. Several findings emerge: the auxetic lattice applies enhanced confinement to the mortar matrix, the new composite exhibits significant strength increase compared to plain mortar specimens and the composite has a remarkably ductile behavior, with a high residual strength that endures to strains in excess of 20%. The experimental results are further validated through computational modeling. The findings of this paper demonstrate the high potential of using auxetic lattices as reinforcement in a concrete/mortar matrix with advantageous properties for structures in terms of strength and ductility which are crucial in the response of structures under extreme loading conditions such as earthquakes.

Considerations for measuring residual stresses in driven piles with vibrating wire strain gauges

Vibrating wire strain gauges are often the preferred technology for measuring strain in driven piles. However, measuring the residual strain after pile driving is challenging to accomplish using vibrating wire gauges. The driving process can cause a shift in the no-load reading from a relaxation of locked-in manufacturing strains in the pile or relaxation of the gauge wire tension. Also, there are temperature effects from installing piles below ground. A test pile program was developed using driven steel H-piles instrumented with vibrating wire strain gauges. The piles were subjected to dynamic forces by striking against a steel plate in an attempt to relax the locked-in manufacturing strain prior to installation. The strain gauges and thermistors were connected to a data logger during pile driving to record strain and temperature changes following installation. It was observed that applying a dynamic impact to the piles prior to installation resulted in a shift of 0–5 με (microstrain). Temperature effects from installing the piles in cooler ground resulted in a shift of strain in excess of 60 με in some strain gauges. It is concluded that temperature-induced shifts to strain must be measured following pile driving to interpret residual stresses.

Ligament size dependency of strain hardening and ductility in nanoporous gold

Nanoporous (NP) metals with ligament sizes up to a few tens of nm show exceptional mechanical properties such as high strength and stiffness per weight. While their elasticity and yield strength have been the subject of numerous studies, less is known about the plastic deformability of these materials under large compressive and tensile strains. In this study, the effect of ligament size is investigated using large-scale atomistic simulations to probe the elastic response, plastic response, and deformation mechanisms of nanoporous gold under uniaxial compression and tension to strains in excess of 60 percent. This paper explores the full range of the material response under uniaxial loading, focusing on the modifications to strain hardening under compression and ductility and delayed failure under tension. It was found that the elastic modulus experiences a compression-tension asymmetry that decreases with ligament size increase. Under compression, strain hardening is found to be ligament size dependent. This size dependency can be explained by the coupling effect of the change in surface area to solid volume ratio evolution and defects accumulation. Under tension, the material shows higher ductility with ligament size increase causing a delay in failure. This is attributed to differences in dislocation density. The results reported in this work will help in evaluating the effect of ligament size on the material response, and eventually enhance the design of novel nanoporous foams with tailored mechanical response.

Characterization and modeling of temperature effect on the shear mode properties of magnetorheological elastomers

The temperature dependency of the viscoelastic properties of magnetorheological elastomers (MREs) operating in the shear mode was experimentally investigated under broad ranges of strain amplitude, excitation frequency, and applied magnetic field. Experiments were performed with isotropic MRE samples with volume fraction of carbonyl iron particles dispersed in the silicone rubber matrix under controlled temperature, ranging from to together with different levels of magnetic field density ( to ). The results revealed significant effect of temperature on the linear and nonlinear viscoelastic properties of MREs, in addition to the effects of strain amplitude, rate, and applied magnetic flux density. The temperature dependency of mechanical properties of an unfilled rubber matrix was also measured, which served as a reference. The shear properties of the MRE revealed reductions in both the storage and loss moduli with increasing temperature. The temperature dependency of the MRE, however, diminished under shear strain in excess of the critical strain. The results also revealed more pronounced temperature effect at higher temperatures and excitation frequencies, while it was relatively lesser with increasing magnetic field. Increase in temperature resulted in enhanced magnetorheological effect, while a clear trend with regard to the excitation frequency could not be established. Finally, a phenomenological model has been developed to predict the storage and loss moduli of the MRE as a function of excitation frequency, applied magnetic flux density, and temperature. The superb performance of the proposed model in predicting viscoelastic moduli under various operating and environmental conditions has been demonstrated through comparison of the experimental and simulation results.

High strain damping for sands from load-controlled cyclic tests: Correlation between stored strain energy and pore water pressure

Undrained stress-controlled cyclic triaxial and direct simple shear test data on five different clean sands was analyzed in terms of dissipated and stored strain energy. A new method was developed to calculate damping ratio from undrained stress-controlled tests and its plausibility was examined with the help of pore water pressure data. Results from different methods in the literature to calculate damping ratio were also evaluated. The data analysis shows that the damping ratio results calculated with the new method were in some agreement with the damping ratio values for sands summarized by Seed & Idriss (1970) [1] for shear strains up to 0.4%. For shear strains larger than 0.4%, the new method provides more accurate estimates of damping ratio that are also conceptually more reasonable than those obtained with linear equivalent approximations. For strains in excess of 1%, damping ratio trends with strain obtained with the new method are clearly superior, although reliable experimental data is limited. In general, equivalent linear calculation of damping ratio can lead to an overestimation of damping for shear strains smaller than 1% and an underestimation of damping for the shear strains larger than 1%.

Prediction of the height of caving and fracturing above an isolated longwall extraction panel

ABSTRACT The height of collapse above a longwall extraction panel is simulated assuming a transversely isotropic continuum and an isotropic strength criterion. The transversely isotropic assumption allows consideration of the role of overburden conditions, specifically the spacing of bedding discontinuities. The shape and height of the collapse zone are like those produced in physical models and are consistent with subsidence measurements. The interaction between the disruption and break-through to the surface is examined using voussoir beam analysis. Break-through to the surface, which implies a fracture connection to the extraction level, may be associated with surface subsidence in excess of about 300 mm or a compressive strain in excess of about 4 mm/m.

Effect of strain on the structural and electronic properties of graphene-like GaN: A DFT study

We present ab initio density functional theory (DFT) calculations on the effect of in-plane equi-biaxial strain on the structural and electronic properties of graphene-like GaN monolayer (ML-GaN). For compressive strain in excess of 7.2%, ML-GaN gets buckled; buckling parameter increases quadratically with compressive strain. The 2D bulk modulus of ML-GaN was found to be smaller than that of graphene and graphene-like ML-BN, which reflects weaker bond in ML-GaN. More importantly, the bandgap and effective masses of charge carriers in ML-GaN were found to be tunable by application of in-plane equi-biaxial strain. In particular, when compressive biaxial strain of about 3% was reached, a transition from indirect to direct bandgap-phase occurred with change in the value and nature of effective masses of charge carriers; buckling and tensile strain reduced the bandgap — the bandgap reduced to 50% of its unstrained value at 6.36% tensile strain and to 0 eV at an extrapolated tensile strain of 12.72%, which is well within its predicted ultimate tensile strain limit of 16%. These predictions of strain-engineered electronic properties of highly strain sensitive ML-GaN may be exploited in future for potential applications in strain sensors and other nanodevices such as the nano-electromechanical systems (NEMS).

A comparative investigation into the influence of the constitutive model on the prediction of in-plane formability for Nakazima and Marciniak tests

The present study investigates the use of both Marciniak and Nakazima tests to generate the forming limit curves (FLCs) for a power-law hardening dual phase steel, DP980, and an aluminum-magnesium alloy, AA5182, that exhibits dynamic hardening and saturation-type behavior. The recently proposed methodology of Min et al. (2016) to compensate for the so-called process effects of non-linear strain paths (NLSP) and contact pressure was evaluated and applied to the Nakazima FLCs to enable comparison with the Marciniak FLCs and the formability predictions of the Modified Maximum Force Criterion (MMFC) of Hora et al. (1994, 2013) for in-plane stretching. An emphasis was placed upon the experimental determination of the constitutive model to plastic strains in excess of 0.5 using tensile and shear tests. A flexible variant of the Hockett-Sherby (1975) hardening model for large strain levels along with constraints upon the hardening model calibration are proposed. It is demonstrated that the choice of hardening model, test data, and the calibration procedure can have a marked influence on the Nakazima process corrections and the predicted FLC using the MMFC model. An extension to the Linear Best Fit (LBF) limit strain detection methodology by Volk and Hora (2011) was developed to account for bend severity and the material hardening response and is compared with the ISO 12004-2 limit strains. If the hardening model is accurately calibrated to large strain levels, the analytical MMFC predictions were in excellent agreement with the process-corrected Nakazima and Marciniak FLCs for DP980. The results for the AA5182 were found to be strongly dependent upon the choice of hardening model that influences both the MMFC and the contact pressure correction.

Signatures of physical aging and thixotropy in aqueous dispersion of Carbopol

In this work, we investigate signatures of physical aging in an aqueous dispersion of Carbopol that shows yield stress and weak enhancement in elastic modulus as a function of time. We observe that the creep curves, as well as strain recovery, show a significant dependence on waiting time elapsed since shear melting. The corrected strain, which is the strain in excess of the recovered strain, has been observed to show time–waiting time superposition in the effective time domain, wherein time is normalized by time dependent relaxation time that shows a power-law dependence. The corresponding power law exponent, which is close to unity in a limit of small stresses, decreases with stress and tends to zero as stress approaches the yield stress. For a range of stresses, the material shows time–stress superposition suggesting the shape of the evolving relaxation time spectrum to be independent of the time as well as the stress. This work, therefore, suggests the presence of physical aging in an aqueous dispersion of Carbopol even though the elastic modulus shows only a weak enhancement. We also discuss the Andrade type of creep behavior in aqueous Carbopol dispersion.

Liquid electromigration in gallium-based biphasic thin films

Liquid metals have recently gained interest as a material of choice for soft and stretchable electronic circuits, thanks to their virtually infinite mechanical failure strain and high electrical conductivity. Gallium-based thin films are obtained by depositing gallium in the vapor phase to form a class of liquid metal conductors. The films, with an average thickness below 1 µm, withstand mechanical strain in excess of 400%. However, modes of failure other than mechanical ones have not yet been thoroughly investigated. In particular, electromigration, a well-known cause of failure in solid thin film traces for integrated circuits, also occurs in bulk liquid metals. In this work, microscopic observation of the thin conductive traces reveals that gallium is displaced from the anode terminal toward the cathode terminal after direct current stressing. This results in a catastrophic increase in the trace resistance and electrical failure. The mean time to failure decreases with increasing current density, following Black’s equation, an empirical mathematical model originally developed to describe failure in solid metal thin-film tracks due to electromigration. We show that using alternating current, e.g., symmetric square wave, rather than direct current can extend the lifetime of the thin liquid metal film conductor by several orders of magnitude. These results may help stretchable circuit designers who select liquid metal thin-film conductors as the stretchable interconnect technology to predict devices’ lifetime and implement mitigation strategies at the system level or at the material level.

Shear response of 3D non-woven carbon fibre reinforced composites

We experimentally and numerically investigated the shear response of a three-dimensional (3D) non-woven carbon fibre reinforced epoxy composite with three sets of orthogonal tows and approximately equal fibre volume fractions in the orthogonal directions. Shear tests on two orientations of dogbone specimens showed significant strain hardening and an increasing unloading stiffnesses with increasing applied strain. Unloading was also accompanied by considerable strain recovery, with X-ray tomographic scans revealing minimal damage accumulation in specimens until near final failure at shear strains in excess of 50%. To understand the origins of this unusual mechanical response of the 3D carbon fibre composites, we developed a micro-mechanical model wherein all tows and matrix pockets in the composite are explicitly considered. The tows were modelled using a pressure-dependent crystal plasticity approach to capture texture evolution under large deformations and the model replicated many of the experimental observations with a high degree of fidelity. Importantly, the model illustrated the role of the 3D architecture in not only suppressing delamination but also enhancing the strain hardening response due to a 3D confinement effect of the tow architecture. On the other hand, a model wherein the tows were modelled using an anisotropic Hill plasticity framework (absent plastic spin) failed to replicate the observed strain hardening response or capture the associated strain recovery upon unloading. This highlights the importance of accounting for the evolution of the material substructure within the tows of these high ductility 3D composites. The results of this work illustrate the unique mechanical behaviour of 3D non-woven fibre composites and provide insight into how 3D fibre architecture can be used to enhance the mechanical performance of fibre composites.

Crystallization‐Directed Anisotropic Electroactuation in Selectively Solvated Olefinic Thermoplastic Elastomers: A Thermal and (Electro)Mechanical Property Study

AbstractDielectric elastomers (DEs), a class of soft electroactive polymers that change size upon exposure to an external electric field, constitute an increasingly important class of stimuli‐responsive polymers due primarily to their large actuation strains, facile and low‐cost fabrication, scalability, and mechanical robustness. Unless purposefully constrained, most DEs exhibit isotropic actuation wherein size changes are the same in all actuation directions. Previous studies of DEs containing oriented, stiff fibers have demonstrated, however, that anisotropic actuation along a designated direction is more electromechanically efficient since this design eliminates energy expended in nonessential directions. To identify an alternative, supramolecular‐level route to anisotropic electroactuation, we investigate the thermal and mechanical properties of novel thermoplastic elastomer gels composed of a selectively solvated olefinic block copolymer that not only microphase‐separates but also crystallizes upon cooling from the solution state. While these materials possess remarkable mechanical attributes (e.g., giant strains in excess of 4000%), their ability to be strain‐conditioned enables huge anisotropic actuation levels, measured to be greater than 30 from the ratio of orthogonal actuation strains. This work establishes that crystallization‐induced anisotropic actuation can be achieved with these DEs.

Response of hybrid isolation system during a shake table experiment of a full‐scale isolated building

SummaryA full‐scale 5‐story steel moment frame building was subjected to a series of earthquake excitations using the E‐Defense shake table in August, 2011. For one of the test configurations, the building was seismically isolated by a hybrid system of lead‐rubber bearings and low friction roller bearings known as cross‐linear bearings, and was designed for a very rare 100 000‐year return period earthquake at a Central and Eastern US soil site. The building was subject to 15 trials including sinusoidal input, recorded motions and simulated earthquakes, 2D and 3D input, and a range of intensities including some beyond the design basis level. The experimental program was one of the first system‐level full‐scale validations of seismic isolation and the first known full‐scale experiment of a hybrid isolation system incorporating lead‐rubber and low friction bearings. Stable response of the hybrid isolation system was demonstrated at displacement demands up to 550 mm and shear strain in excess of 200%. Torsional amplifications were within the new factor stipulated by the code provisions. Axial force was observed to transfer from the lead‐rubber bearings to the cross‐linear bearings at large displacements, and the force transfer at large displacements exceeded that predicted by basic calculations. The force transfer occurred primarily because of the flexural rigidity of the base diaphragm and the larger vertical stiffness of the cross‐linear bearings relative to the lead‐rubber bearings.

Using architectured materials to control localized shear fracture

A medium carbon steel was partially decarburized to create a compositionally graded material consisting of a surface layer with 0.1 wt% C and a core with 0.4 wt% C. The graded material was heat-treated to produce a martensitic microstructure which was subsequently cold-rolled in the as-quenched state. Localized shear bands developed within the core region during the rolling process. The shear bands did not, however, penetrate the outer decarburized layer. In this way, it was possible to roll 0.4 wt%C martensite, in the as-quenched state, to equivalent strains in excess of 2. The microstructure evolution of the cold-rolled martensite was followed using optical microscopy, Electron Back Scatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and nano-indentation. The development of shear bands is interpreted in terms of the collapse of martensite blocks during deformation, leading to geometric softening and shear localization within the high carbon core.

Structural health monitoring system for bridges based on skin-like sensor

Structural health monitoring activities are of primal importance for managing transport infrastructure, however most SHM methodologies are based on point-based sensors that have limitations in terms of their spatial positioning requirements, cost of development and measurement range. This paper describes the progress on the SENSKIN EC project whose objective is to develop a dielectric-elastomer and micro-electronics-based sensor, formed from a large highly extensible capacitance sensing membrane supported by advanced microelectronic circuitry, for monitoring transport infrastructure bridges. Such a sensor could provide spatial measurements of strain in excess of 10%. The actual sensor along with the data acquisition module, the communication module and power electronics are all integrated into a compact unit, the SENSKIN device, which is energy-efficient, requires simple signal processing and it is easy to install over various surface types. In terms of communication, SENSKIN devices interact with each other to form the SENSKIN system; a fully distributed and autonomous wireless sensor network that is able to self-monitor. SENSKIN system utilizes Delay-/Disruption-Tolerant Networking technologies to ensure that the strain measurements will be received by the base station even under extreme conditions where normal communications are disrupted. This paper describes the architecture of the SENSKIN system and the development and testing of the first SENSKIN prototype sensor, the data acquisition system, and the communication system.