Thin Film Strain Research Articles

Concentric Array of Printed Strain Sensors for Structural Health Monitoring.

Civil infrastructure is expanding around the world. The ever-growing trend toward urbanization drives the demand for new investments. However, the new constructions and gradual deterioration of those already existing, especially bridges, give rise to concerns about their proper maintenance. To improve safety and drive down maintenance costs of civil structures, there is a need for inexpensive sensing systems capable of reliable and automated monitoring. In this study, we present a new concept of thin-film strain sensors arranged in an array with a concentric layout that is incorporated into a flexible substrate sheet. The designed sensor array is intended to analyze strains in the proximity of round holes made at the crack tips, found in the investigated construction elements of civil structures. In this study, the performance of the sensor array was demonstrated using measurements taken on a highway bridge in one of the largest cities in Japan. We show that it can measure local strain distribution and indicate a region with risk for crack formation. The demonstrated results show new area of potential applications for the printed strain sensors in monitoring civil structures.

Large-Area Resistive Strain Sensing Sheet for Structural Health Monitoring.

Damage significantly influences response of a strain sensor only if it occurs in the proximity of the sensor. Thus, two-dimensional (2D) sensing sheets covering large areas offer reliable early-stage damage detection for structural health monitoring (SHM) applications. This paper presents a scalable sensing sheet design consisting of a dense array of thin-film resistive strain sensors. The sensing sheet is fabricated using flexible printed circuit board (Flex-PCB) manufacturing process which enables low-cost and high-volume sensors that can cover large areas. The lab tests on an aluminum beam showed the sheet has a gauge factor of 2.1 and has a low drift of 1.5 . The field test on a pedestrian bridge showed the sheet is sensitive enough to track strain induced by the bridge’s temperature variations. The strain measured by the sheet had a root-mean-square (RMS) error of 7 compared to a reference strain on the surface, extrapolated from fiber-optic sensors embedded within the bridge structure. The field tests on an existing crack showed that the sensing sheet can track the early-stage damage growth, where it sensed 600 peak strain, whereas the nearby sensors on a damage-free surface did not observe significant strain change.

Measurement method for deformation and contact force of the fuel assembly for China fast reactor under thermal gradient

Deformation and contact force of the fuel assembly due to thermal gradient are of great concerns to the integrity design and safe operation of the core in the China Fast Reactor (CFR). Up to now, only the 2D deflection measurement of the sub-assemblies has been investigated, and almost no attention is paid to the contact force measurement between the sub-assemblies with a small clearance. In this paper, the measurement method and system are developed for measuring the deformation and contact force of the fuel assembly under thermal gradient with the test fuel assembly simplified based on the stiffness equivalent strategy. In addition, the 3D deformation and contact force are measured through the non-contact industrial photogrammetry system and the thin film pressure sensor or strain gauge, respectively. Free thermal bowing test for a single assembly is performed at first for verifying the reliability of the 3D deformation measurement by comparing with the results of finite element (FE) simulation. Afterwards, the 3D deformation and the contact force are measured for the single assembly restrained thermal bowing test. The accuracy of the proposed contact force measurement is investigated through comparison with the method of spoke-type force sensors. The developed measurement method and system can provide experimental basis for safety design of the CFR due to its potential for comprehensive deformation and contact force measurements.

Ultrasensitive micro/nanocrack-based graphene nanowall strain sensors derived from the substrate's Poisson's ratio effect

Thin film strain sensors composed of GNWs grown by MPCVD, showing ultrahigh sensitivity which can be applied for acoustic signature recognition, as well as electronic skin devices to detect both subtle and large motions of the human body.

Influence of the AlN/Pt-ratio on the electro-mechanical properties of multilayered AlN/Pt thin film strain gauges at high temperatures

In this work, the impact of the AlN/Pt ratio on the strain-sensitive properties of multilayered AlN/Pt thin films is investigated. These thin film systems consisting of 10 bi-layers of AlN and Pt each with a thickness of 3 nm AlN and 7 nm Pt or 5 nm AlN and 5 nm Pt are fabricated, evaluated and compared to those realized with a sequence of 7 nm AlN and 3 nm Pt thin bi-layers. The thin film systems are sputter-deposited on oxidized silicon wafers or sapphire substrates. The influence of different annealing steps at 900 °C up to 24 h in Argon (Ar) atmosphere on the electrical film resistivity and the temperature coefficient of the electrical resistance (TCR) is investigated for these different multilayers using Van-der-Pauw measurements up to 330 °C in air. Furthermore, the impact of the AlN/Pt-ratio on the gauge factor of the thermally stabilized multilayers is determined using a purpose-built measurement setup up to 500 °C in air. Transmission electron microscopy and X-ray diffraction analyses are utilized to examine the microstructure and the crystallographic phase composition of the multilayers before and after thermal loading. Annealing the different multilayers at 900 °C leads to diffusion effects between the AlN and Pt thin films and recrystallizations processes in the Pt sublayers depending on the individual sublayer thickness values. After thermal pre-conditioning at 900 °C for 1 h in Ar the samples were stable even in air up to 500 °C. Finally, it is shown that the TCR as well as the gauge factor of an AlN/Pt multilayer can be tuned through the AlN/Pt-ratio. The highest gauge factor with a value of 4.7 (±0.3) at room temperature is achieved with a multilayer consisting of 5 nm AlN and 5 nm Pt (pure Pt thin film on Cr adhesion promotor: 3.2), while the lowest linear TCR with a value of α = 7.4 ∙ 10−4 K-1 is measured for multilayers with 7 nm AlN and 3 nm Pt sublayers (pure Pt thin film on Cr adhesion promotor: 3.66 ∙ 10-3 K-1).

Temperature Self-Compensated Strain Sensors based on MWCNT-Graphene Hybrid Nanocomposite

Sensors based on carbon nanomaterials are gaining importance due to their tunable properties and their potentially outstanding sensing performance. Despite their advantages, carbon-based nanomaterial sensors are prone to cross-sensitivities with environmental factors like temperature. Thus, to reduce the temperature influence on the sensing material, compensation and correction procedures are usually considered. These methods may require the use of additional sensors which can themselves be subject to residual errors. Hence, a more promising approach consists of synthesizing a material that is capable of self-compensating for the influence of temperature. In this study, a hybrid nanocomposite based on multi-walled carbon nanotubes (MWCNT) and graphene is proposed, which can compensate, by itself, for the influence of temperature on the material conductivity. The hybrid nanocomposite material uses the different temperature behavior of MWCNTs, which have a negative temperature coefficient, and graphene, which has a positive temperature coefficient. The influence of the material ratio and dispersion quality are investigated in this work. Material composition and dispersion quality are analyzed using Raman spectroscopy and scanning electron microscopy (SEM). A composition of 70% graphene and 30% MWCNT exhibits a nearly temperature-independent hybrid nanocomposite with a sensitivity of 0.022 Ω/°C, corresponding to a resistance change of ~1.2 Ω for a temperature range of 25 to 80 °C. Additionally, a simple investigation of the strain sensing behavior of the hybrid material is also presented. The hybrid nanocomposite-based, thin-film strain sensor exhibits good stability over 100 cycles and a significantly high gauge factor, i.e., 16.21.

Presenting a New Wireless Strain Method for Structural Monitoring: Experimental Validation

The structural health monitoring (SHM) of large and complex infrastructures as well as laboratory tests of new structures and materials resorts to strain gauge measurements to check mechanical stress. A wireless measurement of the strain gauge response is desirable in many practical applications to avoid the cost and the difficulty of wiring, particularly in large structures requiring several sensors and in complex objects where the measurement points are difficult to access. In this paper, a wireless strain gauge which is a hybrid between an RFID tag and a usual thin-film resistive strain gauge is experimented. Installation and maintenance problems of the wireless sensor networks are overcome allowing a high level of measurement accuracy, comparable to that of wired strain sensors, together with a long measurement distance. A large set of measurements has been performed using reference specimens and readings in order to validate the sensor and to develop a calibration procedure that makes the sensor suitable for a large number of different applications in civil engineering.

Carbon based polyimide nanocomposites thin film strain sensors fabricated by ink-jet printing method

Carbon-nanotube (CNT) and graphene-nanoplate (GNP) are two popular fillers to fabricate carbon-based nanocomposite devices. In this study, CNT-polyimide (PI) and GNP-PI nanocomposite thin film piezoresistive strain sensors were fabricated on flexible polyimide substrates and their electromechanical properties were characterized. The percolation thresholds were first determined for the two nanocomposites by investigating the volumetric conductivity as a function of filler concentration. Inkjet printing technique was employed to produce the nanocomposite devices with filler concentrations around the percolation threshold. The interdigital electrodes pre-deposited on the substrate were used for the electrical measurement of the devices. The piezoresistive sensitivity of the nanocomposite strain sensors was characterized by uniaxial tensile tests. The optimal compositions of 1.8 wt% CNT-PI and 1.4wt% GNP-PI nanocomposites were suggested for maximum gauge factors of 3.5 and 26, respectively. The temperature coefficients of resistance (TCR) for the fabricated nanocomposite thin film strain sensors were also characterized. The results demonstrate that GNP-PI nanocomposite strain sensors exhibit higher strain sensitivity than the CNT-PI counterpart. Temperature compensation is therefore necessary in practical applications of these nanocomposite strain sensors.

High temperature static and dynamic strain response of PdCr thin film strain gauge prepared on Ni-based superalloy

Thin film resistance strain gauges are being developed to monitor the in-situ static and dynamic strain of engine blades in aerospace propulsion and power generation system. In this work, the palladium chromium thin film strain gauge, which consists of four layers: graded buffer layer, insulating layer, strain sensitive element and protective layer, is prepared on Ni-based superalloy by vacuum sputtering deposition technique. The static and dynamic strain responses are investigated from room temperature up to 800 °C. The results of static testing demonstrate that the resistance of the gauge exhibits a linear change dependent on strain and displays excellent repeatability. The gauge factor shows temperature dependence, which is calibrated to be 1.78, 1.94, 2.03 and 2.13 at 25, 400, 600 and 800 °C, respectively. For dynamic strain response, the resistance of palladium chromium thin film strain gauge varies periodically, which keeps the same frequency with the K465 test bar vibrates driven by the shaker system at the resonance frequency. The results indicate that PdCr thin film strain gauge has a fast response enough to monitor the dynamic strain change. Then, the dynamic strain can be derived from the dynamic variation of resistance of the thin film strain gauge up to 800 °C. Therefore, palladium chromium thin film strain gauge provides an effective approach for the measurement of in-situ static and dynamic strain of engine blades in aerospace propulsion.

High Thermal Stability of Cr-Al and Cr-Al-B Thin Film Strain Sensor

High Thermal Stability of Cr-Al and Cr-Al-B Thin Film Strain Sensor

Influence of Sputtering Temperature and Layer Thickness on the Electrical Performance of Thin Film Strain Sensors Consisting of Nickel-Carbon Composite

Hybrid laminates consisting of fibre-reinforced thermoplastic films and metallic thin sheets are successively replacing thermoset based systems due to their obvious advantages of higher formability and aptitude for mass production. In order to monitor the material under operating conditions, hybrid laminates need to be equipped with smart sensor units. Artifact-free integration of commercial strain gauges into hybrid laminates is almost impossible. Therefore, a new thin film strain sensor based on a PVD sputtering process was developed.The aim of this work was to evaluate the influence of the layer thickness as well as the elevated temperature during the sputtering process on the electrical performance of Ni-C strain sensors. The Ni-C films with different layer thickness and different sputtering temperatures manufactured by means of a magnetron sputtering process were investigated for the sheet resistance and the change of temperature coefficients of resistance. In addition, Raman spectroscopy was utilized to investigate the phase development with regard to different sputtering temperatures. It can be seen that the gauge factor gets doubled while optimizing the layer thickness. When the sputtering temperature was increased, the graphitic phase formation was preferred and the impurities were reduced. These results are discussed in this paper and appropriate solution concepts are provided.

Influence of a heterolayered Al2O3–ZrO2/Al2O3 ceramic protective overcoat on the high temperature performance of PdCr thin film strain gauges

The application of PdCr thin film strain gauges on gas turbine engines requires protective overcoats to prevent the oxidation of the PdCr sensitive element. In this work, a heterolayered Al2O3–ZrO2/Al2O3 ceramic film, which was fabricated by electron beam evaporation, is utilized as a protective overcoat over the PdCr sensitive thin film. The morphology and microstructure of the prepared films indicate that the composite Al2O3–ZrO2 film is smooth and compact, and the interface between the composite Al2O3–ZrO2 film and Al2O3 film is crack-free, which contributes to improving the protective performance of the heterolayered Al2O3–ZrO2/Al2O3 ceramic protective overcoat at high temperature. Moreover, the PdCr thin film strain gauge with heterolayered Al2O3–ZrO2/Al2O3 ceramic film as protective overcoat displays a minimum drift rate of less than 0.09%/h at 800 °C, and excellent cyclic repeatability such that the eight cyclic curves of the relative grid resistance are almost overlapped from room temperature to 800 °C, indicating the excellent protecting performance of the heterolayered Al2O3–ZrO2/Al2O3 ceramic overcoat. It also reveals the smallest temperature coefficient of resistance of 155.3 ppm/°C compared to the monolayer Al2O3 overcoat and composite Al2O3–ZrO2 overcoat.

Effect of thickness on the electrical properties of PdCr strain sensitive thin film

The electrical properties of palladium chromium strain sensitive films with various thicknesses were fabricated onto sapphire substrates by direct current magnetron sputtering are investigated. The results show that the electrical properties of PdCr strain films are strongly dependent on their thickness. The square resistance and the temperature coefficient of resistance of PdCr strain sensitive films are dramatically decreased when the thickness of PdCr films that thinner than 232 nm, then the downward trend of electrical properties tends to be stabilized until 933 nm. The PdCr films with 933 nm thickness show a relative minimum temperature coefficient of resistance and excellent repeatability from room temperature to 800 °C. Furthermore, the stability of PdCr thin film strain gauges are measured at 600 and 800 °C by drift test, which reveals that the average resistance drift rate of PdCr strain film with 48 nm is almost ten times larger than that of the films with thickness of 933 nm. The above results further verify that the PdCr film with thickness of 933 nm possesses relatively superior electrical properties. Moreover, the strain response of PdCr thin film strain gauge with 933 nm on DZ22 Ni-based superalloys is studied, which indicates that the values of resistance are synchronous response to the strain implying the PdCr thin film strain gauge has a fast response to strain, and the gauge factor is 1.80, 1.88, 1.90 and 1.93 at 25, 400, 600 and 800 °C, respectively.

Combining High Sensitivity and Dynamic Range: Wearable Thin-Film Composite Strain Sensors of Graphene, Ultrathin Palladium, and PEDOT:PSS

Wearable mechanical sensors have the potential to transform healthcare by enabling patient monitoring outside of the clinic. A critical challenge in the development of mechanical-e.g., strain-sensors is the combination of sensitivity, dynamic range, and robustness. This work describes a highly sensitive and robust wearable strain sensor composed of three layered materials: graphene, an ultrathin film of palladium, and highly plasticized PEDOT:PSS. The role of the graphene is to provide a conductive, manipulable substrate for the deposition of palladium. When deposited at low nominal thicknesses (~8 nm) palladium forms a rough, granular film which is highly piezoresistive (i.e., the resistance increases with strain with high sensitivity). The dynamic range of these graphene/palladium films, however, is poor, and can only be extended to ~10% before failure. This fragility renders the films incompatible with wearable applications on stretchable substrates. To improve the working range of graphene/palladium strain sensors, a layer of highly plasticized PEDOT:PSS is used as a stretchable conductive binder. That is, the conductive polymer provides an alternative pathway for electrical conduction upon cracking of the palladium film and the graphene. The result was a strain sensor that possessed good sensitivity at low strains (0.001% engineering strain) but with a working range up to 86%. The piezoresistive performance can be optimized in a wearable device by sandwiching the conductive composite between a soft PDMS layer in contact with the skin and a harder layer at the air interface. When attached to the skin of the torso, the patch-like strain sensors were capable of detecting heartbeat (small strain) and respiration (large strain) simultaneously. This demonstration highlights the ability of the sensor to measure low and high strains in a single interpolated signal, which could be useful in monitoring, for example, obstructive sleep apnea with an unobtrusive device.

Bio-compatible wireless inductive thin-film strain sensor for monitoring the growth and strain response of bone in osseointegrated prostheses

Many benefits can be derived from in situ monitoring of the growth, load response, and condition of human bone. In particular, bone monitoring offers opportunity to advance understanding and designing of osseointegrated mechanical components fixated into bones such as artificial joints and more recently osseointegrated prosthetic limbs. In this study, a bio-compatible wireless inductive strain-sensing system is proposed, which is designed to monitor the growth and strain response of bone-hosting implants. Thin-film circuit fabrication methods based on lithography are adopted to develop a conformable wireless strain sensor designed as a passive resistive–inductive–capacitive circuit. Two forms of strain sensing are designed into the thin-film sensor. First, parallel-plate capacitors fabricated from metal electrodes and a polyimide dielectric layer are introduced to modulate bone strain onto a resonant frequency of the thin-film sensor. A second resonant frequency is introduced in the sensor design to measure circumferential bone growth using a highly nonlinear titanium-resistive element, whose resistance exponentially increases well after 1000 µε under monotonic increasing hoop strain. To ensure the possibility for implantation in animal subjects in future study, the thin-film sensing system is fabricated using mainly bio-compatible polymers (e.g. polyimide) and metals (e.g. titanium and gold). Fabricated prototypes inductively coupled using an impedance analyzer are experimentally tested. Results reveal linear response of the first resonant frequency to low levels of strain with a sensitivity of 4.555 Hz per unit microstrain. The second resonant frequency is sensitive to the resistive fuse with nonlinear fuse behavior initiated above 1000 µε and impedance phase increasing exponentially thereafter.

Printed Strain Sensors Using Graphene Nanosheets Prepared by Water‐Assisted Liquid Phase Exfoliation

AbstractA novel approach to leverage the extraordinary properties of graphene for designing thin film strain sensors is demonstrated. Graphene nanosheets (GNS) are produced from graphite by a liquid phase exfoliation (LPE) method using water along with N‐methylpyrrolidinone (NMP) as cosolvents. It is found that the water‐NMP solvent system enhances the exfoliation yield and the stability of the GNS dispersion, thereby lowering the number of defects in the GNS basal plane. Both LPE‐based GNS and reduced graphene oxide (RGO) are synthesized for fabricating thin film strain sensors and for comparison purposes. Detailed micro‐Raman, X‐ray photoelectron spectroscopy, and transmission electron microscopy studies indicate that the as‐produced GNS exhibits lower defects than RGO nanosheets. The strain sensing study reveals that strain sensors fabricated using low‐defect GNS exhibit enhanced electrical and electromechanical properties, including higher electrical conductivity, lower noise floor, and more stable electromechanical response as compared to RGO‐based sensors.

Self‐Connected Ag Nanoporous Sponge Embedded in Sputtered Polytetrafluoroethylene for Highly Stretchable and Semi‐Transparent Electrodes

AbstractHigh‐performance wearable electronics require highly stretchable and conductive materials that are prepared by simple, scalable processes to be implemented in the essential roles of electrodes and interconnects. Here, room temperature fabrication of highly stretchable electrodes consisting of self‐connected Ag nanoporous sponge embedded in polytetrafluoroethylene (PTFE) matrix on stretchable polyurethane substrates using a metal‐polymer co‐sputtering process is demonstrated. The metallic Ag nanoporous sponge networks (NSNs) embedded in PTFE matrix show minimal changes to resistance even after stretching by 50% and after 10 000 cycles under 20% strain due to the well‐connected Ag NSNs and the stretchable PTFE matrix. The feasibility of this hybrid material as stretchable interconnects and electrodes for stretchable thin film heaters and strain sensors is also demonstrated. Because the Ag NSNs embedded in PTFE matrix are simply fabricated by a typical sputtering process, which is compatible with current electronics fabrication processes, it is strongly expected that the Ag‐PTFE hybrid electrodes can be easily applied as universal stretchable electrodes for wearable thin film heaters, stretchable interconnectors, and stretchable strain sensors.

A plate model for compressive strength prediction of delaminated composites

Damage tolerance is of critical importance to laminated composite structures. In this paper, we present a new semi-analytical method for predicting the strain at which delamination propagation will initiate following sublaminate buckling. The method uses a numerical strip model to determine the thin-film buckling strain of an anisotropic sub-laminate created by delamination, before evaluating the strain energy release rate for delamination propagation. The formulation assumes that all energy is available for propagation in a peeling mode (Mode I); avoiding an approximate mixed-mode criterion. Results are compared with twelve experimentally obtained propagations strains, covering a variety of laminates each containing a circular PTFE delamination. Comparison shows agreement to within 12% for balanced sublaminate tests in which delamination propagation occurred before intra-ply cracking. The method can be used to significantly improve the damage tolerance of laminates, opening up new opportunities for structural efficiency using elastic tailoring, non-standard ply angles and material optimisation.

Corrugated Photoactive Thin Films for Flexible Strain Sensor

In this study, a flexible strain sensor is devised using corrugated bilayer thin films consisting of poly(3-hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene)-polystyrene(sulfonate) (PEDOT:PSS). In previous studies, the P3HT-based photoactive non-corrugated thin film was shown to generate direct current (DC) under broadband light, and the generated DC voltage varied with applied tensile strain. Yet, the mechanical resiliency and strain sensing range of the P3HT-based thin film strain sensor were limited due to brittle non-corrugated thin film constituents. To address this issue, it is aimed to design a mechanically resilient strain sensor using corrugated thin film constituents. Buckling is induced to form corrugation in the thin films by applying pre-strain to the substrate, where the thin films are deposited, and releasing the pre-strain afterwards. It is known that corrugated thin film constituents exhibit different optical and electronic properties from non-corrugated ones. Therefore, to design the flexible strain sensor, it was studied to understand how the applied pre-strain and thickness of the PEDOT:PSS conductive thin film affects the optical and electrical properties. In addition, strain effect was investigated on the optical and electrical properties of the corrugated thin film constituents. Finally, flexible strain sensors are fabricated by following the design guideline, which is suggested from the studies on the corrugated thin film constituents, and the DC voltage strain sensing capability of the flexible strain sensors was validated. As a result, the flexible strain sensor exhibited a tensile strain sensing range up to 5% at a frequency up to 15 Hz with a maximum gauge factor ~7.

Annealing effects of high sensitive thin strain gauges consisting of nickel carbon nanocomposites

Hybrid laminates consisting of fiber-reinforced thermoplastic films and metallic thin sheets are successively replacing thermoset-based systems due to their obvious advantages of higher formability and the aptitude for mass production. In order to monitor the material under operating condition, hybrid laminates need to be equipped with smart sensor units. Therefore, the aim of the present work is to evaluate the temperature stability of the Ni-C thin film strain gauge sensors. The analyses of the deposited Ni-C films manufactured by means of a magnetron sputtering process were carried out by means of Raman spectroscopy and X-ray diffraction to investigate the phase development after a heat treatment at different temperatures and holding times. In addition, the four-terminal sensing was utilized to study the change of temperature coefficients of resistance after a single annealing treatment.