Broad Strain Range Research Articles

Fabrication of Stretchable Ag Nanowire Electrode and its Electrochromic Application

We report on stretchable electrochromic films of poly(3-hexylthiophene) (P3HT) fabricated on silver nanowire (AgNW) electrodes. AgNWs electrodes are prepared on polydimethylsiloxane (PDMS) substrates using a spray coater for stretchable electrochromic applications. On top of the AgNW electrode, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is introduced to ensure a stable resistance over the electrode under broad strain range by effectively suppressing the protrusion of AgNWs from PDMS. This bilayer electrode exhibits a high performance as a stretchable substrate in terms of sheet resistance increment by a factor of 1.6, tensile strain change to 40%, and stretching cycles to 100 cycles. Furthermore, P3HT film spin-coated on the bilayer electrode shows a stable electrochromic coloration within an applied voltage, with a color contrast of 28.6%, response time of 4-5 sec, and a coloration efficiency of 91.0 cm2/C. These findings indicate that AgNWs/PEDOT:PSS bilayer on PDMS substrate electrode is highly suitable for transparent and stretchable electrochromic devices.

A low-cost, printable, and stretchable strain sensor based on highly conductive elastic composites with tunable sensitivity for human motion monitoring

Strain sensors with high stretchability, broad strain range, high sensitivity, and good reliability are desirable, owing to their promising applications in electronic skins and human motion monitoring systems. In this paper, we report a high-performance strain sensor based on printable and stretchable electrically conductive elastic composites. This strain sensor is fabricated by mixing silver-coated polystyrene spheres (PS@Ag) and liquid polydimethylsiloxane (PDMS) and screen-printed to a desirable geometry. The strain sensor exhibits fascinating comprehensive performances, including high electrical conductivity (1.65 × 104 S/m), large workable strain range (> 80%), high sensitivity (gauge factor of 17.5 in strain of 0%–10%, 6.0 in strain of 10%–60% and 78.6 in strain of 60%–80%), inconspicuous resistance overshoot (< 15%), good reproducibility and excellent long-term stability (1,750 h at 85 °C/85% relative humidity) for PS@Ag/PDMS-60, which only contains ∼ 36.7 wt.% of silver. Simultaneously, this strain sensor provides the advantages of low-cost, simple, and large-area scalable fabrication, as well as robust mechanical properties and versatility in applications. Based on these performance characteristics, its applications in flexible printed electrodes and monitoring vigorous human motions are demonstrated, revealing its tremendous potential for applications in flexible and wearable electronics.

Spectral TRIP enables ductile 1.1 GPa martensite

Introduction of interlath reverted austenite is an effective method to design ductile lath martensitic steels. The challenge in this concept is that all reverted austenite films have similar mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. Here we propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as ‘spectral TRIP effect’. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600 °C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range.

Real-time SAXS study of a strain gauge based on a self-assembled gold nanoparticle monolayer

A strain gauge based on a monolayer of colloidal gold nanoparticles deposited on a flexible Mylar foil by a modified Langmuir-Schaefer method was tested in situ under external uniaxial stress by the small-angle X-ray scattering (SAXS) technique. Simultaneously, the stress-strain curve of the foil was measured. A high-flux laboratory X-ray source allowed fast data collection with 10s temporal resolution. The monolayer exhibits a linear response in terms of the interparticle distances up to the 13% substrate strain while keeping its full integrity. This result indicates the dominant role of the pair potential function between the nanoparticle surfactant molecules and molecules of the substrate and compares well with previous quasi-static measurements of a similar strain gauge, suggesting none or negligible effect of the transient strain phenomena longer than the sampling interval on the gauge response. Keeping a constant strain on finishing the stretching, fast transient effects with characteristic times down to the limit imposed by the X-ray detector time resolution were not observed either during the SAXS pattern collection. A different stress behavior of the interparticle distance in the direction perpendicular to stretching comparing with a monolayer of colloidal iron-oxide nanoparticles studied previously reveals the surfactant effect on the gauge response controlled by the interparticle pair potential function. The results obtained suggest that the colloidal gold-nanoparticle monolayer on a flexible substrate is a prospective strain gauge with a very fast linear response in a broad strain range.

Microstructure and Properties of Pearlitic Steel during Cold Wire Drawing: A Residual Stress Perspective*

Abstract The residual stress evolution during cold drawing of pearlitic steel wire was followed over a broad strain range by diffraction techniques. The present work discusses possible links between microstructural changes induced by severe deformation and their “footprints” left in the residual stresses on different – macro, micro and nano – scale levels. Energy dispersive synchrotron diffraction revealed a significant divergence in the grain microstress evolution among differently oriented ferrite grains in the high deformation regime beyond an accumulated true strain of about ∊t ≥ 2.3. A possible physical reason for the observed divergence is discussed in terms of a distinct microstructure development in this stage of the cold wire drawing.

Cracked titanium film on an elastomeric substrate for highly flexible, transparent, and low-power strain sensors

Strain-dependent cracking behaviors in thin titanium (Ti) films on polydimethylsiloxane (PDMS) substrates were systematically investigated for their application to sensitive, flexible, transparent, and portable strain sensors. When uniaxially elongated, vertical cracks were developed in the low-strain range, and beyond a critical strain, tilted cracks appeared to intersect the vertical cracks. The cracking behaviors were also dependent on Ti film thickness. The varying strain-dependent crack patterns produced a significant resistance change in response to the applied strain, particularly, in the high- and broad-strain range. For a 180-nm-thick Ti film on PDMS substrate, a gauge factor of 2 was achieved in the range of 30% to 50% strain. The operation power was extremely low. All the Ti films on PDMS substrates were transparent, highly flexible, and very easy to fabricate. These results suggest that cracked Ti films on PDMS substrates could be a viable candidate for realizing a low-cost, flexible, transparent, and portable strain sensor.

Normal Ranges of Left Ventricular Strain: A Meta-Analysis

The definition of normal values of left ventricular global longitudinal strain (GLS), global circumferential strain, and global radial strain is of critical importance to the clinical application of this modality. The investigators performed a meta-analysis of normal ranges and sought to identify factors that contribute to reported variations. MEDLINE, Embase, and the Cochrane Library database were searched through August 2011 using the key terms "strain," "speckle tracking," "left ventricle," and "echocardiography" and related phrases. Studies were included if the articles reported left ventricular strain using two-dimensional speckle-tracking echocardiography in healthy normal subjects, either in the control group or as a primary objective of the study. Data were combined using a random-effects model, and effects of demographic, hemodynamic, and equipment variables were sought in a meta-regression. The search identified 2,597 subjects from 24 studies. Reported normal values of GLS varied from -15.9% to -22.1% (mean, -19.7%; 95% CI, -20.4% to -18.9%). Normal global circumferential strain varied from -20.9% to -27.8% (mean, -23.3%; 95% CI, -24.6% to -22.1%). Global radial strain ranged from 35.1% to 59.0% (mean, 47.3%; 95% CI, 43.6% to 51.0%). There was significant between-study heterogeneity and inconsistency. The source of variation was sought between studies using meta-regression. Blood pressure, but not age, gender, frame rate, or equipment, was associated with variation in normal GLS values. The narrowest confidence intervals from this meta-analysis were for GLS and global circumferential strain, but individual studies have shown a broad range of strain in apparently normal subjects. Variations between different normal ranges seem to be associated with differences in systolic blood pressure, emphasizing that this should be considered in the interpretation of strain.

Determination of the resistance of drill-pipe steels to deformation

Cylindrical specimens of steels 32G2A and 25KhGMA were upset on a cam-operated plastometer within a broad range of strain, strain rate, and temperature to determine the deformation resistance of steels used to make drill pipe. The empirical relations that were obtained made it possible to qualitatively and quantitatively describe the effect of the main parameters of the hot-working operation on the strength of these steels and thus refine the mechanical and thermal regimes for upsetting the ends of drill pipes. The study results will also be used to develop processes for rolling pipes of steels 32G2A and 25KhGMA in rolling-and-heat-treatment shop No. 2 at the TAGMET plant.

An internal state variable plasticity-based approach to determine dynamic loading history effects on material property in manufacturing processes

Worked materials in large deformation processes such as forming and machining experience a broad range of strain, strain rate, and temperatures, which in turn affect the flow stress. However, the flow stress also highly depends on many other factors such as strain path, strain rate and temperature history. Only a model that includes all of these pertinent factors is capable of predicting complex stress state in material deformation. In this paper, the commonly used phenomenological plasticity models (Johnson–Cook, Usui, etc.) to characterize material behavior in forming and machining were critically reviewed. Although these models are easy to apply and can describe the general response of material deformation, these models lack the mechanisms to reflect static and dynamic recovery and the effects of load path and strain rate history in large deformation processes. These effects are essential to understand process mechanisms, especially surface integrity (residual stress, microhardness, and microstructure) of the manufactured products. As such a dislocation-based internal state variable (ISV) plasticity model was used, in which the evolution equations enable the prediction of strain rate history and temperature history effects. These effects can be quite large and cannot be modeled by the equation-of-state models that assume that stress is a unique function of the total strain, strain rate, and temperature, independent of the loading path. The temperature dependence of the hardening and recovery functions results in the prediction of thermal softening during adiabatic temperatures rises, which are common in metal forming and machining. The dynamic mechanical behaviors of three different benchmark work materials, titanium Ti-6Al-4V, AISI 52100 steel (62 HRc), and aluminum 6061-T6, were modeled using the ISV approach. The material constants were obtained by using a nonlinear regression-fitting algorithm in which the stress–strain curves from the model were correlated to the experiments at different (extreme) temperatures. Then the capabilities of the determined material constants were examined by comparing the predicted material flow stress with the test data at different temperatures, strains, and strain rate history. The comparison demonstrates that the internal state variable plasticity model can successfully recover dynamic material behavior at various deformation states including the loading path effect. In addition, thermal softening due to adiabatic deformation was also captured by this approach.

Effect of particle size on thermal residual stress in WC–Co composites

Thermal residual stresses (TRS) were studied in a series of tungsten carbide (WC)–cobalt (Co) composites using neutron powder diffraction. Samples with 10, 20, and 40 wt.% Co and WC particle sizes of 0.5, 1, 3, and 5 μm were used. As expected, the mean WC TRS increased in magnitude as the Co content increased, i.e. as the WC content decreased. The corresponding stresses in the Co phase were computed from force balance equilibrium requirements. For fixed Co content, the mean (compressive) stresses in the WC increased in magnitude with decreasing WC particle size. The change was most dramatic for the 40 wt.% Co samples, where the mean TRS increased in magnitude from −440 to −1137 MPa as the WC particle size varied from 5 to 0.5 μm, respectively. The stress distribution in the WC phase was studied using the breadths of the WC diffraction peaks. The full-width at half-maximum (FWHM) values indicate a broad range of strain within WC particles that increases with increasing stress in the WC and is attributed primarily to point-to-point variation in the angular WC particles.

Mechanical Properties of Hardened AISI 52100 Steel in Hard Machining Processes

This paper provides an approach using tensile tests at elevated temperatures to estimate mechanical properties of the work material for both elastic and plastic deformations in a broad range of strain, strain rate, and temperature in machining. The proposed method has been applied to estimate mechanical properties of hardened AISI 52100 steel in hard machining. Tensile testing is shown capable of estimating the mechanical properties of both elastic and plastic regions with large strains at elevated temperatures. Flow stresses at high strain rates in machining can be obtained by extrapolating the data from tensile tests by using the velocity-modified temperature. Flow stress data from tensile and cutting tests is consistent with regard to the velocity-modified temperature. Temperature is the dominant factor of mechanical properties of this material, while the effect of strain rate is secondary. Cutting forces and chip geometry predicted by the 3D FEM simulation of hard turning using the material property data obtained from the developed method agree well with the experimental data.

Transverse strain responses in electrostrictive poly(vinylidene fluoride-trifluoroethylene) films and development of a dilatometer for the measurement

A dilatometer based on the cantilever beam concept has been developed. The dilatometer is easy to use and capable of measuring transverse strain response of soft polymer films in a broad strain range (from 10−7 to 10−1) without mechanical constraining of the sample. It is capable of detecting strain over a relatively wide frequency range from mHz to above 100 Hz under different load and temperature. Using the setup, the electric field induced transverse strains of the electrostrictive poly(vinylidene fluoride-trifluoroethylene) copolymer films were characterized which shows that a large transverse strain can be achieved in this class of polymer. In addition, the effect of mechanical tensile load on the transverse strain was also evaluated and the results show that the strain response will be affected by the load. However, depending on the load level, the strain response of the polymer film under a given electric field may increase or decrease with load. Based on the phenomenological theory, it is shown that for a ferroelectric based material, the mechanical load will shift the Curie temperature. Hence, to a large extent, the change of the strain response with load observed here can be understood by linking it to the strain change with temperature.

Residual stress and stress distribution in a WC-Ni composite

The differential thermal stress state in a WC-15.6wt.%Ni cemented carbide composite has been studied using neutron powder diffraction. Absolute stress values have been obtained from the difference in WC cell parameters in a stress-free, loose WC powder and the WC phase in the composite over the temperature range 20–700 K. In addition, the stress distribution in the WC phase was determined by analysis of WC peak breadths in the reference powder and the composite. The peak breadths were corrected for particle size effects using a procedure based on the integral peak breadth method of particle size-strain analysis. The resultant full width at half-maximum values of the elastic strain distribution indicate that a broad range of strain, and thus stress, is present in the carbide phase of the composite. The distribution is centered on an average compressive stress and ranges from very compressive to very tensile values. The stress distribution results are consistent with prior results in other WC-Co and NbC-Co cemented carbides.

In-plane displacement measurement of composite material by objective speckle

The objective speckle was used to measure the in-plane displacement around the hole in a coupon specimen of composite material under tensile loading. Mirror transplantation method was adopted to ensure the high reflectivity of specimen surface and to obtain high quality double exposure specklegram. The adjustable spatial frequency of the whole-field pattern of the displacement made it possible to measure a broad range of strain, from elastic to plastic. The results obtained by objective speckle agree well with those by Moire method.