Strain Sensitive Materials Research Articles

Stretch-tolerant interconnects derived from silanization-assisted capping layer lamination for smart skin-attachable electronics

Flexible strain sensor arrays hold great promise in on-skin monitoring of human signals and activities. Despite the development of strain-sensitive materials and patterning technologies for improved performance and device integration, the metal film serving as interconnects is always vulnerable upon stretch, which hinders the operation under large strains. Herein, a novel strategy is developed for achieving stretch-tolerant interconnects within a sensor array. Through introducing a high-modulus capping layer for the deposition of Ag interconnects, followed by silanization-assisted lamination onto the stretchable substrate where strain-sensitive graphene patches are inkjet-printed, the deformation of Ag interconnects is largely suppressed upon the global strain of the device, and a high working range of 40 % strain is achieved. Moreover, the chemical bonding between the capping layer and the stretchable substrate ensures a stable contact between the electrode and the sensitive layer under vigorous bending. The as-prepared sensor array demonstrates high sensitivity (gauge factor (GF) > 100) within a wide range (18 %), and could reliably monitor various physiological signals and human activities. A machine learning-assisted wearable gesture recognition system is developed based on the sensor array and a convolutional neural network (CNN), which could distinguish from 10 defined gestures with 100 % accuracy after 14 training processes. The facile and effective strategy could be universally applied for metal interconnects protection under stretch, and dramatically facilitate the design of smart flexible electronics.

Highly Inductive Coil Spring Strain/Compress Sensors Integrated with Shape Memory Alloy and Shape Memory Polymers-CNTs.

Future wearable electronic gadgets offer greatpotential for using stretchable, strain-sensitive materials to instantly detect human motion and record physiological information. This paper presens a strain/compress sensor made from a Shape memory alloy (SMA) coil spring covered with silver pastes and the composite of carbon nanotubes and Shape memory polymer (SMP). The combination of the shape memory materials that expand or contract automatically by temperature improved the mechanics of the sensor. First, the proposed sensors showed an excellent ability to broad-range strain of 250% and compress of 50% with a relative inductance (∆L/L0 ) range from -35% to 50%, respectively. Durability during 1000 loading and unloading cycles at 200% strain is included. Secondly, by monitoring changes in resistance, inductance, and time, it is determined how many silver layers appropriate for transformation should be in order to improve the recovery time of the SMA coil spring. Moreover, the presence of CNTs in the composite-covered outer of sensors helps to reduce the influence of the relation between resistance and temperature in the range from 30 °C to 110 °C. Finally, a device is suggested for monitoring arm and triceps brachii muscle movements based on the stretchable area as a key parameter.

Hierarchical Wrinkles for Tunable Strain Sensing Based on Programmable, Anisotropic, and Patterned Graphene Hybrids.

Flexible, stretchable, wearable, and stable electronic materials are widely studied, owing to their applications in wearable devices and the Internet of Things. Because of the demands for both strain-insensitive resistors and high gauge factor (GF) strain-sensitive materials, anisotropic strain sensitivity has been an important aspect of electronic materials. In addition, the materials should have adjustable strain sensitivities. In this work, such properties are demonstrated in reduced graphene oxide (RGO) with hierarchical oriented wrinkle microstructures, generated using the two-step shrinkage of a rubber substrate. The GF values range from 0.15 to 28.32 at 100% strain. For device demonstrations, macrostructure patterns are designed to prepare patterned wrinkling graphene at rubber substrate (PWG@R). Serpentiform curves can be used for the constant-value resistor, combined with the first-grade wrinkles. Strip lines can increase the strain-sensing property, along with the second-grade wrinkles. The patterned sensor exhibits improved GF values range from 0.05 to 49.5. The assembled sensor shows an excellent stability (>99% retention after 600 cycles) with a high GF (49.5). It can monitor the vital signs of the throat and wrist and sense large motions of fingers. Thus, PWG@R-based strain sensors have great potential in various health or motion monitoring fields.

Hydrostatic pressure mapping of barium titanate phase transitions with quenched FeRh

We report a pressure study of the metamagnetic/ferroelectric hybrid heterostructure of a quenched FeRh thin film (25 nm) grown on single crystal barium titanate (BTO). It has been previously reported that when the BTO undergoes a crystal transition a massive magnetization and coercivity change is triggered in the highly strain sensitive quenched FeRh thin film. Therefore quenched FeRh makes for an ideal probe for mapping a materials structural phase transitions. In this work we demonstrate this effect as a function of both temperature and hydrostatic pressure. As a result, we present the pressure dependence of the hybrid material which aligns identically with the BTO substrates pressure dependence reported in literature. The concept of combining a structural phase transitional (SPT) material with a magnetostrictive magnetic metal has been shown with vanadium oxides and our findings here prove that this methodology can be extended to strain sensitive metamagnetic materials systems in thin film, and possibly in bulk, heterostructures.

Sugar-templated conductive polyurethane-polypyrrole sponges for wide-range force sensing

Porous conductive polyurethane (PU) sponges were prepared by combining a sugar-templated fabrication of porous polyurethane elastomer and the in-situ polymerization of pyrrole inside the sponges. The conductive sponges had a uniform porous structure similar to the sugar cube templates and a unique micro-wrinkled structure of polypyrrole (PPy). Moreover, the conductive sponges exhibited good mechanical properties and electrical conductivity by the combination of elastomeric PU and conductive PPy, which make the conductive PPy-PU sponges as good candidates for pressure and strain sensitive materials. It was shown that a single PPy-PU conductive sponge was not only sensitive to large forces, but also sensitive to the tiny deformation by small forces such as 0.1 N, thus realizing a wide-range detection of pressure forces from 0.1 N to 200 N. Furthermore, the detection range could be further enlarged to more than 800 N by assembling a sponge array. A PPy-PU sponge matrix was also designed and showed potential application in multi-site position detection. These sugar-templated conductive sponges may find application for sensors in human physiological movement and intelligent monitoring.

All-organic microelectromechanical systems integrating electrostrictive nanocomposite for mechanical energy harvesting

Recent advances in the field of microelectromechanical systems (MEMS) have generated great interest in the substitution of inorganic microcantilevers by organic ones, due to their low cost, high flexibility and a simplified fabrication by means of printing methods. Here, we present the integration of electrostrictive nanocomposites into organic microcantilever resonators specifically designed for mechanical energy harvesting from ambient vibrations. Strain sensitive nanocomposite materials composed of reduced graphene oxide (rGO) dispersed in polydimethylsiloxane (PDMS) are integrated into all-organic MEMS by means of an innovative low-cost and environment friendly process by combining printing techniques and xurography. Static tests of the electrostrictive nanocomposite with 3.7wt% rGO show good performances with variations of capacitance that exceeds 4% for strain values lower than 0.55% as the microcantilever is bent. The results in dynamic mode suggest that the organic MEMS meet the requirements for vibration energy harvesting. With an applied sinusoidal acceleration (amplitude 0.5g, frequency 15Hz) a power density of 6μW/cm3 is achieved using a primitive circuit.

Flexible strain sensors with high performance based on metallic glass thin film

Searching strain sensitive materials for electronic skin is of crucial significance because of the restrictions of current materials such as poor electrical conductivity, large energy consumption, complex manufacturing process, and high cost. Here, we report a flexible strain sensor based on the Zr55Cu30Ni5Al10 metallic glass thin film which we name metallic glass skin. The metallic glass skin, synthesized by ion beam deposition, exhibits piezoresistance effects with a gauge factor of around 2.86, a large detectable strain range (∼1% or 180° bending angle), and good conductivity. Compared to other e-skin materials, the temperature coefficient of resistance of the metallic glass skin is extremely low (9.04 × 10−6 K−1), which is essential for the reduction in thermal drift. In addition, the metallic glass skin exhibits distinct antibacterial behavior desired for medical applications, also excellent reproducibility and repeatability (over 1000 times), nearly perfect linearity, low manufacturing cost, and negligible energy consumption, all of which are required for electronic skin for practical applications.

Organic microcantilevers based on reduced graphene oxide composite for electrostrictive energy harvesting

Electrostrictive materials are promising for mechanical energy harvesting applications because of their high power density, low cost and scalability. In this paper, strain sensitive nanocomposite materials based on reduced graphene (rGO) and PDMS are used for energy harvesting; they are characterized by a high electrostrictive coefficient (2.4x10-15 m2/V2) and a giant dielectric constant (ranging from 100 to 1000 at 100 Hz, depending of rGO concentration). Using these nanocomposite materials, electrostrictive MEMS microgenerators are fabricated with an innovative low-cost and environment friendly process in an all-organic approach. The fabricated microcantilevers exhibit excellent mechanoelectrical performances in dynamic mode. With an acceleration of 1 g of the microcantilever base using a shaker, experiment at the first resonant mode (≈ 16 Hz) generates an electrical power density of 8.15 μW/cm3.

Determination of 2A16 Constitutive Model

The stress-strain curves of 2A16 under different strain rate range (10-3s-1-103s-1) and different temperature range (293K-673K) were obtained through the quasi-static compression test and SHPB test by experimental method. The parameters were determined based on Johnson-Cook model and the strain rate hardening term in them was modified. The results show that 2A16 is a kind of strain rate and sensitive temperature materials. The flow stress increases with strain rate increasing, while that decreases with temperature increasing. The deviation is large between the unamended Johnson-Cook constitutive model and test data, while the modified constitutive model is a good agreement with experimental results. And the study is a preparation for the numerical simulation of 2A16 rivet electromagnetic riveting.

Modeling of the electro-mechanical behavior of ITER Nb3Sn cable in conduit conductors

The coupling of electrical and mechanical modeling of superconducting cable in conduit conductors (CICCs) can be useful for the understanding of the complex phenomena occurring in cables based on strain-sensitive materials like Nb3Sn. The MULTIFIL model is a detailed mechanical model aimed at computing the strain distribution at the strand level in multistrand superconducting cables. MULTIFIL is a finite element code dealing with the contacts between beam assemblies that includes plasticity and it allows modeling both transverse and longitudinal loads applied to the cable. The electromagnetic part of the THELMA code is based on a distributed parameter electrical circuit model of CICCs and is aimed at computing the current distribution and electrical losses at an arbitrary cabling stage. In this work, the maps of strain have been computed with MULTIFIL for a petal of a relevant conductor for the central solenoid of the ITER magnet system. These strain maps were computed in three main cases, namely applying only a thermal compressive strain due to cable contraction in cool-down, considering thermal strain plus the strain due to Lorentz force in a virgin state, and thermal strain plus Lorentz force after a given number of electromagnetic cycles. These strain maps have been implemented in the THELMA code, in order to compute the E–T curves corresponding to the different cases, such deriving important parameters to be compared with experimental results (Tcs, n value, effective strain). A methodology to account in the electrical model for the inhomogeneous strain on the strand cross section due to bending is described and the corresponding results are compared to the case of a homogeneous strain on the wire cross section. The model is able to explain the experimental difference between the physical strain that can be determined with Tc measurements and the effective strain that can be derived from the Tcs measurements.

Preparation and properties of ethylene propylene diene rubber/multi walled carbon nanotube composites for strain sensitive materials

Conducting rubber composites of ethylene propylene diene M-class rubber (EPDM) were prepared with multi walled carbon nanotubes (MWCNTs) and organo-clay, Cloisite® 15A to develop flexible strain sensitive materials. The composites were prepared with a constant amount of organo-clay and various amounts of MWCNTs from 5 to 50 wt.%. Organo-clay led to improved dispersion of MWCNTs in the rubber matrix. Upon increasing the MWCNT contents, the tensile strength, stiffness and electrical conductivity of the composites increased significantly but the elongation at break decreased in the EPDM/MWCNT composites. The organo-clay was able to improve tensile strength, stiffness, elongation at break and electrical conductivity of the composites. The non-symmetric linear resistance change was observed by the composites under deformation.

Textile-Based Electrogoniometers for Wearable Posture and Gesture Capture Systems

This paper introduces a method for detecting joint angles by using piezoresistive strain sensitive materials, as carbon loaded rubbers are. Materials used can be screen-printed onto fabrics to provide garments with a sensing apparatus able to reconstruct human postures and gestures. The main differences between this approach and the previous ones, and the core of this work, is the rigorous proof that for small local curvatures of the layers the constituting electrogoniometers, the resistance depends only on the total curvature of the layers and not on the particular form that the sensor keeps in adherence with the human body. In this paper, we show that the hypothesis of small local curvature does not severely restrict the set of angles which can be detected.

Theoretical and Experimental Investigation of the Spatial Filtering Properties of a Number of Concentric Annular Strain Sensors

As an extension to the work done on shaped sensors as spatial filters in one dimension, spatial filtering in two dimensions is considered. The spatial windowing function of disc shaped and annular strain sensors are derived. Experimental results are shown to validate the model. A numerical method for combining annular sensors is then presented. It is shown that low and band pass sensors can be created with relatively few annular elements if the evanescent field is not significant at the sensor. Theoretical and experimental results are presented as well as design criteria. Limitations due to spatial sampling, anisotropic properties of strain sensitive materials and the effect of evanescent and in-plane structural waves are discussed.

Ni–Ag thin films as strain-sensitive materials for piezoresistive sensors

Some results concerning the electrical, electromechanical and structural properties of Ni x –Ag 1− x (for x values between 0.35 and 0.50) thin films in view of their utilization for manufacturing pressure and force sensors are presented. As compared to the well known Ni–Cu (constantan) alloys, which are widely used for these type of sensors, Ni x –Ag 1− x thin films exhibit gauge factor values of the same order of magnitude, but they are much more corrosion resistant and adherent to the substrate. The influence of composition and post-deposition annealing on the electrical resistance, temperature coefficient of resistance and gauge factor of Ni x –Ag 1− x thin films is discussed.

Control of strain and temperature distribution in the ring rolling process

When rolling rings on radial-axial ring rolling mills using conventional rolling strategies, strain distribution is greatly non-homogeneous. Due to the constant deformation of the spread bulges, strain peaks are found in the cross section corners. This causes severe problems when rolling strain or temperature sensitive materials. In the case of titanium alloys the strain concentration in the corners leads to overheating in these areas and subsequent microstructure damage and form defects. A new rolling strategy was developed to even the strain distribution by decreasing the cross section areas in just one roll gap at a time, thereby reducing the deformation of the corner material. The new strategy has been tested in industry while rolling TiA16V4-rings conventionally and according to the new strategy. It showed, that severe microstructure damage and form defects that occurred when rolling conventionally are completely avoided and that the ring temperature is homogenised when applying the modified rolling strategy.

Limit strains in the sheet forming of strain and strain-rate sensitive materials

Forming-limit diagrams are a convenient way of displaying the limits of performance of a sheet material in pressing and stamping: they represent the limiting combination of strains after which unacceptable necks are formed in the material. Although originally empirical, there is clearly much to be gained from the prediction of the limit curves using more readily measured material and process parameters; for example, the material prestrain and strain and strain-rate hardening indices, its degree of anisotropy, as well as the effects of die design, lubrication, etc. In this paper the growth of the neck through the development of strain gradients is explored. The neck is assumed to originate at pre-existing thickness imperfections or material inhomogeneities and has a continous, but accelerating, development resulting in a very localised defect which can be interpreted as failure: it is assumed that in the sheet-stretching region the neck lies perpendicular to the major in-plane strain whilst in the drawing region the neck lies along the direction of zero extension in the plane of the sheet. The strain-rate sensitivity of the material produces a delay in the growth of the neck, and the analysis can be extended to cover materials which show considerable rate effects resulting in superplastic behaviour. A comparison between forming-limit curves predicted on the basis of limiting strain gradients and published data is made for a number of sheet materials.

Servocontrols for Automatic Tensile Testing

Greater speed and reliability are seemingly contradictory goals, but both are evident in all facets of metal production and manufacturing today. Nowhere is this truer than in mechanical testing for quality control purposes. On the one hand, there is the “time is money” syndrome which rewards speed, sometimes at the expense of accuracy. But if testing is to play the role it should in quality control, it must permit adjustments in the metal producing process; to do this, greater speed in obtaining meaningful testing results is essential. But speed can invite carelessness in the name of expediency, and, no matter how carefully done, variables in the way different technicans perform routine tests can have a very important effect on the outcome, especially with strain-sensitive materials such as titanium, high-alloy steels, and superalloys.