Metal Foil Strain Gauges Research Articles

Graphene Nanoplatelets/Polydimethylsiloxane Flexible Strain Sensor with Improved Sandwich Structure.

In engineering measurements, metal foil strain gauges suffer from a limited range and low sensitivity, necessitating the development of flexible sensors to fill the gap. This paper presents a flexible, high-performance piezoresistive sensor using a composite consisting of graphene nanoplatelets (GNPs) and polydimethylsiloxane (PDMS). The proposed sensor demonstrated a significantly wider range (97%) and higher gauge factor (GF) (6.3), effectively addressing the shortcomings of traditional strain gauges. The microstructure of the GNPs/PDMS composite was observed using a scanning electron microscope, and the distribution of the conductive network was analyzed. The mechanical behavior of the sensor encapsulation was analyzed, leading to the determination of the mechanisms influencing encapsulation. Experiments based on a standard equal-strength beam were conducted to investigate the influence of the base and coating dimensions of the sensor. The results indicated that reducing the base thickness and increasing the coating length both contributed to the enhancement of the sensor's performance. These findings provide valuable guidance for future development and design of flexible sensors.

Wireless strain-field monitoring system for motion recognition via direct-ink-writing sensor-array

Large-area strain-monitoring is highly desired in various engineering applications. Despite the fact that the commercial strain gauges have been successfully used for strain measurement, there is still big challenge for this manually-pasting based solution especially for large-area monitoring. First, the wires of multiple commercial strain gauges for large-area monitoring will exhibit remarkably abundant and complex, and eventually reduce the reliability and convenience of the measurement system. Second, the commercial terminal instrument is usually function-redundant with complicated electronic configuration to match the low-sensitive metal-foil strain gauge. Meanwhile, the wire-based signal acquisition and processing system substantially limit the compatibility for moving objects. Here we take the advantages of direct-ink-writing (DIW) 3D printed sensor array, thereby propose a wireless-based solution for large-area strain field measurement, data transmission and processing. The sensitivity of the printed sensing unit is one order of magnitude higher than that of traditional strain gauges, while the sensor array layout is optimized for compatibility to the strain orientation and detecting domain. A high-frequency signal acquisitor is designed and assembled with a wireless transmitter for rapid multi-point strain detection and real-time data transmission. A remote data receiver and processing terminal are developed and integrated for twin-visualization of the detected strain field. In particular, the detected strain data are processed to construct a cascaded support-vector-machine (SVM) classifier, that successfully identify motion states of moving vehicles in high precision. This study provides insight into exploring the advantages of DIW printed sensor arrays combined with specifically-designed wireless system and strain-based dynamic recognition.

Online structural health monitoring of polymer composite structure using gold nanoparticles (AuNPs)

A mechanism for online structural health monitoring of composite structure was established by developing highly sensitive strain gauges based on colloidal Gold Nano Particles (AuNPs). They were synthesized using well known Turkevich-Frens method followed by phase transfer in chloroform using surfactant Cetyltrimethylammonium bromide (CTAB). Flexible Strain gauges / smart sensors were formed by mixing colloidal AuNPs-chloroform solution in polystyrene (PS) – chloroform solution and depositing their thick paste on hand-made flexible thermoplastic polyurethane (TPU) film. These sensors were used as strain gauges on composite substrates for tensile testing. The developed AuNP based strain gauges were found to have 10 times higher sensitivity than normal metal foil strain gauges.

Hypersonic aerodynamic force balance using temperature compensated semiconductor strain gauges

Metal foil strain gauges remain the state-of-the-art transducers for wind tunnel balances. While strain gauge technology is very mature, piezoresistive semiconductor sensors offer alternatives that are worth exploring to assess their unique benefits, such as better strain resolution and accuracy, which would enable balances to be designed with higher factors to safety and hence longer fatigue lifetimes. A new three-component balance, based on temperature compensated semiconductor strain gauges, is designed, calibrated and tested in a hypersonic low density wind tunnel. The static accuracy of the semiconductor balance is calibrated better than 0.3% FS, and the dynamic accuracy of the balance is established using a HB-2 standard model in a Mach 12 hypersonic flow. Good experimental repeatability is confirmed to be better than 2.5% FS, and the effectiveness of the balance is demonstrated by comparing the forces and moments of measured data with computational fluid dynamics simulations, as well as reference wind tunnel results under similar conditions.

Additive Manufacturing of Flexible Strain Sensors Based on Smart Composites for Structural Health Monitoring with High Accuracy and Fidelity

This research introduces a novel flexible spherical carbon nanoparticle‐based polyurethane conductive ink, which is employed to fabricate strain sensors by a lab‐developed direct ink writing/3D printing system. Rheological tests are performed, and sensors are pasted on glass fiber‐reinforced plastic specimens to study strain gauge behaviors under quasistatic loading. The gauge factor in tensile loading is found to be layer width dependent as decreasing the strain gauge's layer width increases the sensitivity of the strain sensor. A maximum gauge factor of 34 is achieved using a layer width of 0.2 mm, 17 times greater than commercially available metal foil strain gauges. The four‐point bend tests are performed under tension/compression to assess the sensor's strain‐sensing and damage‐monitoring ability. Fractographic analysis is coupled with strain monitoring using the developed sensor, which confirms that the failure progresses from intralaminar failure modes such as ply splitting in tension. At the same time, delamination leads to kink band formation under compression and the eventual failure of load‐bearing fibers. The developed sensor exhibits repeatable performance with low hysteresis and integrated nonlinearity errors for up to 1000 cycles. The developed sensors could be effectively employed for online in situ structural health monitoring of aerospace structures under static and dynamic loading.

Development of High Dynamic Range Six-Axis Force Sensor with Simple Structure

We propose a six-axis force sensor with a simple structure that can measure small forces while affording a high load capacity. For a robot to perform complex motions in an unknown environment, the force sensor used must exhibit a high resolution and high load rating. Various studies have been conducted to simultaneously satisfy these two requirements. However, the high processing cost due to the complicated sensor structure is problematic. Therefore, we develop a column-type high dynamic range (HDR) six-axis force sensor using two types of strain gauges. The sensor was composed of a drawn pipe to solve structural problems that arise during manufacturing. By attaching strain gauges to the surface of the drawn pipe, the forces and torques in the six axes can be measured. HDR measurement was realizable using a semiconductor strain gauge for small loads and a metallic foil strain gauge for large loads. Based on the simulation results, the rated loads of the sensor were 1400 N in the Fx and Fy directions, 9000 N in the Fz direction, and 120 Nm in the Mx, My, and Mz directions. The performance of the fabricated force sensor was evaluated. The maximum nonlinearity errors of the semiconductor strain gauge and the metallic foil strain gauges were 1.21% and 1.81%, respectively. In addition, when comparing the S/N ratios, the minimum measurable values were 0.035 N and 0.13 N for Fy and Fz, respectively.

Optimizing the Conductivity of a New Nanoparticle Silver Ink for Aerosol Jet Printing and Demonstrating Its Use as a Strain Gauge

Aerosol Jet Printing (AJP) shows promise for printable electronics and additive manufacturing as it is capable of printing conformally on nearly any substrate due to its direct write patterning, non-contact printing process, and compatibility with a wide range of printing materials. However, nearly all printed inks require some form of post-processing sintering to achieve acceptable properties, such as electrical conductivity. In this paper, a design of experiment (DOE) using Van der Pauw printed pads is presented which can be applied to characterizing any conductive ink. We focus on a new silver nano-particle ink from NovaCentrix (JS-A426) that was developed specifically for aerosol jet printing and study its electrical conductivity under a variety of sintering time and temperature conditions. From these results, a linear regression model is developed that accurately predicts the experimentally measured electrical conductivity of the printed ink. This DOE characterization process and resulting predictive model are especially useful for applications requiring limited thermal budgets, such as flexible electronics. SEM images are also used to analyze how the silver particles coalesce and densify due to thermal diffusion. Finally, the above results are used to design and fabricate an in-situ miniature strain gauge sensor using the NovaCentrix JS-A426 silver nano-particle ink on a thin flexible substrate. After printing and sintering, the miniature custom strain gauge had a nominal resistance of 355 Ω and a gauge factor of 1.74. These values are similar to commercial metal foil strain gauges and demonstrate that the aerosol jet printing is a viable manufacturing process for the production of custom miniature in-situ strain gauges. Advantages of this methodology include quick prototyping, custom digital design, conformal printing on essentially any surface, elimination of any adhesive layer, and manufacturing in less than a minute. This work is also the first to characterize the JS-A426 silver nano-particle ink thermal curing process and demonstrate its use as a viable sensing element using direct write aerosol jet printing.

CoFeB/MgO-based magnetic tunnel junctions for film-type strain gauge

We show that a CoFeB/MgO-based magnetic tunnel junction (MTJ) formed directly on a flexible substrate has considerable potential as a high-sensitivity strain gauge. A gauge factor of ∼1000, which represents the highest sensitivity reported, thus far, for a film-type strain gauge and is ∼500 times larger than that of the most prevalent metal-foil strain gauge, is realized in the flexible MTJ with a pseudo-spin valve structure under the condition of external magnetic-field assistance. Additionally, using a flexible MTJ with strain-insensitive exchange-biased pinned and strain-sensitive free layers, we demonstrate that a large resistance change due to the strain application can be achieved under magnetic field-free conditions. According to simulations based on the coherent magnetization rotation model, conditions are suggested for improving the gauge factor, as well as for using the flexible MTJ as a strain gauge in practical applications.

High Dynamic Range 6-Axis Force Sensor Employing a Semiconductor–Metallic Foil Strain Gauge Combination

This letter proposes a force sensor that combines semiconductor strain gauges and metallic foil strain gauges to present force information of a higher dynamic range to robots. The strain gauges have different sensitivities, with semiconductor strain gauge sensitivity being approximately 90-fold than that of the metallic foil strain gauges. Using this difference in sensitivity, large forces and small forces are both detected through the two types of strain gauges and high dynamic range force detection is achieved by combining the output signals of these two. It was confirmed from the SN ratio test that the proposed sensor has a measurement range from 0.005 N to 1000 N, the maximum load. The dynamic range is 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10^5$</tex-math></inline-formula> , extending the dynamic range of the 6-axis force sensor with the highest range in previous studies two-fold.

High Dynamic Range Uniaxial Force/Torque Sensor Using Metal Foil and Semiconductor Strain Gauge

High Dynamic Range Uniaxial Force/Torque Sensor Using Metal Foil and Semiconductor Strain Gauge

On the in-situ on-line structural health monitoring of composites using screen-printed sensors

This study aims at developing the screen-printed sensors as a viable means of depositing sensing tracks on composites for their on-line structural health monitoring. Conventional silk screen was employed in order to deposit a nano-composite solution comprising of a conductive nano-filler (carbon nano-particles) dispersed in a thermoplastic matrix (high density polystyrene) on laminated composite specimens. The solution was deposited using a squeegee and was allowed to dry. Commercially available metal foil strain gauges were also bonded alongside screen-printed sensor in order to compare the response of the screen-printed sensors with the commercially available strain gauges. The sensing ability of these screen-printed sensors was tested on a universal testing machine (MTS 810) in four-point bending configuration using a load cell of 100 kN. The sensor deposited using screen-printing technique underwent tensile loading at the lower side of the laminate. A data linearization and amplification module comprising of commercially available instrumentation amplifier (INA 118) was used in conjunction with data acquisition module (Keithley KUSB 3100). The results obtained show that the screen-printed sensors have higher gauge factors in tensile loading scenario with reasonably linear response as compared to traditional metal foil strain gauges. The ease of the deposition of a nano-composite solution via screen printing also makes the technique a viable alternative to the traditional resin bonded metal foil strain gauges which have to be bonded on the surface. Moreover, screen printing offers unlimited options for the development of smart composites in various configurations for a multitude of structural applications.

Creep adjustment of strain gauges based on granular NiCr-carbon thin films

Abstract. An important property of high-precision mechanical sensors such as force transducers or torque sensors is the so-called creep error. It is defined as the signal deviation over time at a constant load. Since this signal deviation results in a reduced accuracy of the sensor, it is beneficial to minimize the creep error. Many of these sensors consist of a metallic spring element and strain gauges. In order to realize a sensor with a creep error of almost zero, it is necessary to compensate for the creep behavior of the metallic spring element. This can be achieved by creep adjustment of the used strain gauges. Unlike standard metal foil strain gauges with a gauge factor of 2, a type of strain gauges based on sputter-deposited NiCr-carbon thin films on polymer substrates offers the advantage of an improved gauge factor of about 10. However, for this type of strain gauge, creep adjustment by customary methods is not possible. In order to remedy this disadvantage, a thorough creep analysis is carried out. Five major influences on the creep error of force transducers equipped with NiCr-carbon thin-film strain gauges are examined, namely, the material creep of the metallic spring element (1), the creep (relaxation) of the polymer substrate (2), the composition of the thin film (3), the strain transfer to the thin film (4), and the kind of strain field on the surface of the transducer (5). Consequently, we present two applicable methods for creep adjustment of NiCr-carbon thin- film strain gauges. The first method addresses the intrinsic creep behavior of the thin film by a modification of the film composition. With increasing Cr content (at the expense of Ni, the intrinsic negative creep error can be shifted towards zero. The second method is not based on the thin film itself but rather on a modification of the strain transfer from the polyimide carrier to the thin film. This is achieved by controlled cutting of well-defined deep trenches into the polymer substrate via a picosecond laser.

Methodology to quantify the real vertical load applied to the pavement

Highway networks attend society as routes for personal and commercial transports. Recurrent loadings, especially the vertical ones, imposed by the wheels of vehicles are assumed to deteriorate the pavement structure during its usage. The economics of pavement project consist of a compromise between the higher initial cost of high-strength pavement sections and the higher maintenance cost of low-strength alternatives. Regarding the importance of the road transportation and its correct maintenance nowadays, the evaluation of the vertical load applied to the pavement come to be a relevant theme, especially when few studies present results based on real measurements. Finally, the vertical load applied to the pavement can be simulated with some software available on the market, but the correlations with real measurements are not always presented. Within this scenario, the main objective of this paper is to present a proposal of a methodology to quantify experimentally this vertical load applied to the pavement in road transportation. Based on an instrumentation with metal-foil strain gauges, the measurements with this methodology were performed on a public highway in the state of Parana (Brazil), which had a known value of International Roughness Index (IRI - a standard scale for road roughness), as well as on a proving ground. By using the instrumented leaf springs as calibrated load cells, it was possible to observe the influence of some usual road events (such as speed bumps, braking, corners, lane changes and potholes) on the load applied to the pavement.

Foil Strain Gauges Using Piezoresistive Carbon Nanotube Yarn: Fabrication and Calibration.

Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoresistive characteristics that can be tapped to sense strain. This paper presents the details of novel foil strain gauge sensor configurations comprising carbon nanotube yarn as the piezoresistive sensing element. The foil strain gauge sensors are designed using the results of parametric studies that maximize the sensitivity of the sensors to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the foil strain gauge sensors are also provided and discussed in the context of their electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic mechanical loading. It was shown that these foil strain gauge sensors comprising carbon nanotube yarn are sensitive enough to capture strain and can replicate the loading and unloading cycles. It was also observed that the loading rate affects their piezoresistive response and that the gauge factors were all above one order of magnitude higher than those of typical metallic foil strain gauges. Based on these calibration results on the initial sensor configurations, new foil strain gauge configurations will be designed and fabricated, to increase the strain gauge factors even more.

Multi-axis dynamic displacement measurement based on a strain shunt structure

Transient gap or crack width monitoring is essential for structural health monitoring and failure analysis of large civil structures. In this paper, an innovative multi-axis displacement sensor, which utilises metal foil strain gauges on a strain shunt structure, has been proposed. This displacement sensor has the advantages inherited from metal foil strain sensing, such as low cost, high precision, fast dynamic response and low power consumption, and can also measure displacement in two axes independently. The working principles and sensitivity are derived theoretically from key geometrical parameters of the shunt structure, and the linear response of strain values to the given displacement of two translational axes has been demonstrated experimentally. Furthermore, modal response, stress concentration, optimal gauges installation positions and bending deformation due to moment of the third axis are studied numerically.

The effect of short range order on the thermal output and gage factor of Ni3FeCr strain gages

AbstractThe ability of alloy 70Ni‐27.5Fe‐2.5Cr to form an ordered crystalline structure upon application of elevated temperature and the resulting effects on a variety of physical properties such as magnetism, strength, electrical resistance, and specific heat are well known. This paper demonstrates that strain gages made with 70Ni‐27.5Fe‐2.5Cr foil with an ordered crystalline structure have both high gage factors and high thermal outputs. The thermal output for strain gages made of 70Ni‐27.5Fe‐2.5Cr foil is demonstrated to be about 1,374 μΩ/Ω/K. The gage factor is nonlinear ranging in magnitude from about 5.0 to 3.6 for applied strains of 300 and 1,300 μE, respectively. The magnitude of gage factor and thermal output correspond with transformation of the crystalline structure from a state of disorder to a state of order. Comparisons to strain gages made with 36Ni‐57Fe‐7Cr foil are provided because of its use in the manufacturing of metal foil strain gages and their demonstrated high gage factors and high thermal outputs. Practical application for 70Ni‐27.5Fe‐2.5Cr strain gages is demonstrated by dead‐weight loading of shear‐beam load cells at low applied strain levels to minimise the effect of nonlinear gage factor; and Wheatstone bridge cancellation of high thermal output.

The strain sensitivity of copper powder reinforced concrete

Earthquakes, material deteriorations and other environmental factors challenge the structural safety. In order to protect the lives, structural health monitoring is crucial. The metal foil strain gages have low durability, low sensitivity and can get point wise measurements which are disadvantages. In this study six different concrete mixtures were designed; one without any copper powder, the rest five having different copper powder volume fractions. Three cube samples from each mixture were cast and cured. Simultaneous measurement of electrical resistance and strain were conducted during the compression tests. A strong linear relationship between strain and electrical resistance change was obtained for copper powder reinforced concrete. The results are contribution to the development of “Smart Concrete” which can sense its strain and damage.

Passive wireless strain measurement based upon the Villari effect and giant magnetoresistance

A passive wireless radio frequency-identification (RFID) stress/strain sensor is presented. Stress is transformed into a change of magnetic field by utilizing an amorphous metal ribbon. This magnetic field change is measured by a giant magnetoresistance magnetic field sensor and converted into a digital value with a RFID chip for wireless access. Standard metal foil strain gauges have a gauge factor GF from around 2 to 5 and suffer from the disadvantage of a physically connected power supply and measurement equipment. For the presented sensor, a strain range of −10 μmm to 190 μmm results in a linear sensor response, a gauge factor of GF ≈ 245, and a detectivity of 4.10 nmm1Hz. The detectivity of the presented sensor is similar to the detectivity of a reference metal foil strain gauge. Due to low power consumption and easy signal analysis, this sensor is well suited for long term strain measurement inside closed spaces. RFID adds features like multiple tag detection, wireless passive operation and a user data storage.

Carbon based resistive strain gauge sensor fabricated on titanium using micro-dispensing direct write technology

Carbon based resistive strain gauge sensor suitable for medical implant technology has been directly fabricated on a titanium test-plate using Micro-Dispensing Direct Write (MDDW) technology. A 3.5μm biocompatible dielectric layer of parylene C was initially coated on the titanium test-plate. Commercially available screen-print carbon conductive paste was deposited on the parylene C and cured at 80°C for 3h; this was to ensure the physical properties and chemical integrity of the parylene C layer was maintained, whilst meeting the electrical conductivity curing requirements for the carbon tracks. The novel integrated strain sensor was experimentally tested and found to have a gauge factor of 10 making it approximately 5 times more sensitive than a commercially available metal foil strain gauge glued to the same titanium plate.

The effect of temperature and moisture on electrical resistance, strain sensitivity and crack sensitivity of steel fiber reinforced smart cement composite

Earthquakes, material degradations and other environmental factors necessitate structural health monitoring (SHM). Metal foil strain gages used for SHM have low durability and low sensitivity. These factors motivated researchers to work on cement based strain sensors. In this study, the effects of temperature and moisture on electrical resistance, compressive and tensile strain gage factors (strain sensitivity) and crack sensitivity were determined for steel fiber reinforced cement based composite. A rapid increase of electrical resistance at 200 °C was observed due to damage occurring between cement paste, aggregates and steel fibers. The moisture—electrical resistance relationship was investigated. The specimens taken out of the cure were saturated with water and had a moisture content of 9.49%. The minimum electrical resistance was obtained at 9% moisture at which fiber–fiber and fiber–matrix contact was maximum and the water in micro voids was acting as an electrolyte, conducting electrons. The variation of compressive and tensile strain gage factors (strain sensitivities) and crack sensitivity were investigated by conducting compression, split tensile and notched bending tests with different moisture contents. The highest gage factor for the compression test was obtained at optimal moisture content, at which electrical resistance was minimum. The tensile strain gage factor for split tensile test and crack sensitivity increased by decreasing moisture content. The mechanisms between moisture content, electrical resistance, gage factors and crack sensitivity were elucidated. The relations of moisture content with electrical resistance, gage factors and crack sensitivities have been presented for the first time in this study for steel fiber reinforced cement based composites. The results are important for the development of self sensing cement based smart materials.