Conventional Foil Strain Gauges Research Articles

Experimental studies on carbon nanotube strain sensors

Conventional metal foil strain sensors only measure strain on the structural surface in predetermined directions and locations. These are made of Nichrome (Ni-Cr alloy) and Constantan (Cu-Ni alloy) and have gauge factor of 2. A carbon nanotube (CNT) as a strain sensing material is gaining research interest due to its outstanding electrical and mechanical properties. These sensors apart from multidirectional sensing are capable of making additional contributions such as structure strengthening or structure damping. Bucky paper (BP) alternatively called CNT film or CNT network made from single-walled carbon nanotube or multiwalled carbon nanotubes (SWCNT or MWCNT) has been used by some researchers in the past as strain sensor. However, they have observed nonlinear relationship between strain and change in resistance, and lower gauge factor. Literature review in the field indicates that there is scope for improvement in the performance of Bucky paper-based strain sensors. The objective of this experimental work is to investigate effect of CNT film constituents (MWCNT, SWCNT and mix of both), purification of CNTs and effect of preparation method on sensitivity. The work involves preparation of film from CNTs by vacuum filtration method and preparing the strain sensors from the same. These sensors are loaded along with conventional foil strain gauges on opposite side. The change in resistance of sensor due to applied load is measured to evaluate sensitivity or gauge factor of sensor for various cases. It was observed that the sensitivity of CNT strain sensor is dependent on the preparation and purification methods as well as on the aspect ratio.

Use of digital imaging correlation techniques for full-field strain distribution analysis of implantable devices and tissue in spinal biomechanics research

Few studies have used optical full-field surface strain mapping to study spinal biomechanics. We used a commercial digital imaging correlation (DIC) system to (1) compare posterior surface strains on spinal rods with those obtained from conventional foil strain gauges, (2) quantify bony vertebral body and intervertebral disc (IVD) surface strains on 3 L3-S cadaveric spines during gold-standard flexibility tests (7.5-Nm flexion–extension and 400-N compression), and (3) report our experience with the application and feasibility of DIC to comprehensively map strain in spinal biomechanics. Spinal rods were tested under zero load and using ASTM F1717 standard. For rod strain measures, the largest mean bias offset and baseline noise standard deviation under zero load for DIC were 7.6 με and 33.7 με, respectively. For tissue measures, the largest mean bias offset was 8 με for ε1 and −55 με for ε2 with baseline noise standard deviations of 19 με and 26 με, respectively. On average, DIC rod strain measurements were 5.3% less than strain gauge measurements throughout the load range. Principal IVD and bony surface strains were consistently measurable and showed marked regional differences in strain patterns under different load conditions. Strains measured on spinal rods using DIC techniques reasonably agreed with standard strain gauge measurements. Subregional strain analyses on soft and hard spinal tissues during standard flexibility tests were feasible. Optical strain mapping is a viable, accurate, and promising measurement technique for novel spinal biomechanical studies.

Distributed Optical Fiber Sensors for PCB-Strain Analysis

Printed circuit board (PCB)-strain tests are part of the quality control processes to detect and identify predamages on manufactured products. Such tests are commonly performed by instrumenting a PCB, and subjecting it to the manufacturing conditions. Among the various parameters to evaluate on the production equipment, the induced strain on the PCB is one of them, enabling the assessment of the manufacturing equipment performance. PCB instrumentation with foil strain gauges is currently highly complex due to small size and over populated boards, limiting the overall number of monitoring points. This paper aims at outlining an alternative approach based on distributed optical fiber technology sensors. To prove its feasibility, optical fiber sensors based on optical frequency domain reflectometry are considered and benchmarked against conventional foil strain gauges. A PCB was instrumented with both sensor technologies and subjected to an in-circuit test machine. The strain results showed a match between both technologies, with a maximum deviation of 3.5%, a high precision level for multiple tests, and a high sensitivity for both high and low induced strains. Overall, the fiber optic solution proved to be a suitable alternative technology for foil strain gauges in PCB-strain measurement application.

Development of a Spoke Type Torque Sensor Using Painting Carbon Nanotube Strain Sensors.

This study reports a hub-spoke type joint torque sensor involving strain gauges made of multiwalled carbon nanotubes (MWCNT). We developed the novel joint torque sensor for robots by means of MWCNT/epoxy strain sensors (0.8 wt%, gauge factor 2) to overcome the limits of conventional foil strain gauges. Solution mixing process was hired to fabricate a liquid strain sensor that can easily be installed on any complicated surfaces. We painted the MWCNT/epoxy mixing liquid on the hub-spoke type joint torque sensor to form the piezoresistive strain gauges. The painted sensor converted its strain into torque by mean of the installed hub-spoke structure after signal processing. We acquired sufficient torque voltage responses from the painted MWCNT/epoxy strain sensor.

Thin-film flexible sensor for omnidirectional strain measurements

Conventional strain sensors are both precise and inexpensive, but can only effectively measure strain in one specific direction. In this paper, we report an omnidirectional flexible strain sensor that operates regardless of the orientation of its installation with respect to the direction of the applied strain. The performance of the developed device was compared to that of conventional foil strain gauges and it was demonstrated that, in contrast to the conventional devices, the omnidirectional strain sensor developed here shows almost uniform sensitivity at various installation angles.

Polyaniline/multi-walled carbon nanotube composites for structural vibration damping and strain sensing

Polyaniline (PANI)/11% Multi-carbon nanotubes (MWCNT) nanocomposites sensors were synthesized through an in situ polymerization method. Frits compression method was adopted to make PANI/MWCNT. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) tests results showed that this technique produced a coating of PANI onto the MWCNT, which indicated that CNT were well dispersed in the polymer matrix. Several tests were run to evaluate the sensor’s capabilities. The free end vibration test results showed that the double sided attachment of the sensor had higher damping ratio values than single sided attachment. Also, damping ratios were higher when the sensor was placed at the clamped end. Further, the strain sensing properties of PANI/MWCNT sensors were compared with the conventional foil strain gage. The dynamic sensing test results showed that over the range of 10–1000 Hz, the PANI/MWCNT composite sensor was consistently superior to the traditional foil strain gage for sensing purposes.

Highly Stretchable Miniature Strain Sensor for Large Dynamic Strain Measurement

In this letter, a new type of highly stretchable strain sensor was developed to measure large deformation of a specimen subjected to dynamic loading. The sensor was based on the piezoresistive response of carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite thin films. The piezoresistive response of CNT composite gives fast response in strain measurement, whereas the ultrasoft PDMS matrix provides high flexibility and ductility for large strain measurement. Experimental results showed that the CNT/PDMS sensor is capable of measuring large strains (up to 26%) with an excellent linearity and ultrafast response (less than 30 μs). This stretchable strain sensor also exhibits much higher sensitivities, with a gauge factor of as high as 80, than the conventional foil strain gages. These strain sensing capabilities (large strain, ultafast response, high linearity, and high gauge factor) make the fabricated sensor suitable for high-rate dynamic loading test applications.

Strain Sensing Characteristics of Rubbery Carbon Nanotube Composite for Flexible Sensors.

In this study, the piezoresistive properties of CNT (Carbon Nanotube)/EPDM composite are characterized for the applications of a flexible sensor. The CNT/EPDM composites were prepared by using a Brabender mixer with MWCNT (Multi-walled Carbon Nanotube) and organoclay. The static and quasi-dynamic voltage output responses of the composite sensor were also experimentally studied and were compared with those of a conventional foil strain gage. The voltage output by using a signal processing system was fairly stable and it shows somehow linear responses at both of loading and unloading cases with hysteresis. The voltage output was distorted under a quasi-dynamic test due to its unsymmetrical piezoresistive characteristics. The CNT/EPDM sensor showed quite tardy response to its settling time test under static deflections and that would be a hurdle for its real time applications. Furthermore, since the CNT/EPDM sensor does not have directional voltage output to tension and compression, it only could be utilized as a mono-directional force sensor such as a compressive touch sensor.

Thin film sensors for measuring small forces

Abstract. Especially in the case of measuring small forces, the use of conventional foil strain gauges is limited. The measurement uncertainty rises by force shunts and is due to the polymer foils used, as they are susceptible to moisture. Strain gauges in thin film technology present a potential solution to overcome these effects because of their direct and atomic contact with the measuring body, omitting an adhesive layer and the polymer foil. For force measurements up to 1 N, a suitable deformation element was developed by finite element (FE) analysis. This element is designed for an approximate strain of 1000 μm m−1 at the designated nominal load. The thin film system was applied by magnetron sputtering. The strain gauge structure is fabricated by distinct photolithographic steps. The developed sensors were tested with different load increments. The functional capability of the single resistance strain gauges could be proven. Moreover, a developed sensor in a full bridge circuit showed a linear characteristic with low deviation and good stability.

Experimental accuracy of two dimensional strain measurements using Digital Image Correlation

The measurement of strain has always been central to developments in structural engineering. As structures around the world reach the end of their intended service life, there is a need for more robust field monitoring techniques to determine, in conjunction with numerical models, if these structures are still fit for purpose. Current strain gage technologies are only able to offer a limited number of discrete measurements, which often is not enough data to assess complex structures. This paper presents an alternative technique for measuring strain using digital images called Digital Image Correlation (DIC) that allows for the measurement of 2-D strain fields. The accuracy that can be achieved in physical experiments where sources of error such as lighting and camera quality are present is investigated. A series of tension tests on steel plates are conducted allowing measurements from the DIC technique to be compared to measurements from conventional foil strain gages. Out-of-plane motion is identified as one of the most significant potential sources of strain measurement error when using DIC. Five solutions for significantly reducing the effects of out-of-plane motion are presented and three of these solutions are applied to the steel plate experiment. Using these solutions it is possible to achieve mean strain errors of less than 5με when comparing DIC to foil strain gage measurements, which suggests that the DIC technique has the potential to replace conventional strain gages. Areas of future research are also introduced.

Variability in the Material Properties of Polymer Matrix Composites for Marine Structures

Abstract The test results from a composite material round robin manufacturing study are presented. The objective of the study was to investigate material property variability when different manufacturers were used to fabricate identical composite parts. The study was part of an ongoing Office of Naval Research project to determine the causes of material property variability of E-glass/vinyl-ester structural composites fabricated with a vacuum-assisted resin transfer molding process. The manufacturers that participated in the study consisted of five industrial composite fabricators that either had experience with U.S. naval fabrication projects or possessed commercial marine fabrication experience. The materials specified for the study were a 24 oz woven roving E-glass fabric and a rubber-toughened vinyl-ester resin system. The tests performed included constituent volume, tension, compression, in-plane shear, and flexure. The material coupon tests were performed on 5.1 mm (0.20 in.) thick cross-ply laminates in a warps parallel lay-up. A three-dimensional digital image correlation system was used to measure strain during testing to reduce the variability often experienced when using conventional foil strain gauges on heavy woven fabrics. The results of the testing indicated that compression had the most variability in both strength and modulus, which was attributed to the waviness of the fabric, while the flexural strength and tensile modulus had the least amount of variability.

フォトリソグラフィを用いたインテリジェントタイヤ用ゴムベース電気容量型ひずみセンサの作製

From a traffic safety point-of-view, there is an urgent need for intelligent tires equipped with strain sensors as a warning system for road conditions and for optimized braking control on poor road surfaces. However, since a conventional foil strain gage has high stiffness, it causes the analyzed region to behave unnaturally. The present study proposes a novel rubber-based strain sensor fabricated using photolithography. The rubber base has the same mechanical properties as the tire surface; thereby the sensor does not interfere with the tire deformation and can accurately monitor the behavior of the tire. This investigation details the design and manufacture of the rubber-based sensor. For verification, the sensor was attached to a beam specimen and found to have a fatigue life of over 10^6 and a measurable strain range up to 14%. The sensor was applied to a commercially available automobile tire and found to successfully measure the strain of the tire.

Rubber-based strain sensor fabricated using photolithography for intelligent tires

From a traffic safety point-of-view, there is an urgent need for intelligent tires equipped with strain sensors as a warning system for road conditions and for optimized braking control on poor road surfaces. However, since a conventional foil strain gage has high stiffness, it causes the analyzed region to behave unnaturally. The present study proposes a novel rubber-based strain sensor fabricated using photolithography. The rubber base has the same mechanical properties as the tire surface; thereby the sensor does not interfere with the tire deformation and can accurately monitor the behavior of the tire. This investigation details the design and manufacture of the rubber-based sensor. For verification, the sensor was attached to a beam specimen and found to have a fatigue life of over 10 6 and a measurable strain range up to 14%. The sensor was applied to a commercially available automobile tire and found to successfully measure the strain of the tire.

Multiwalled carbon nanotube film for strain sensing

We have studied the possibility of using multiwalled carbon nanotube (MWCNT) films asstrain sensors. The MWCNT films were prepared by a solution/filtration method and werebonded directly onto specimens by a nonconductive adhesive. For comparison, conventionalfoil strain gages were also bonded to the structure on the opposite side. The specimensthen underwent a uniaxial tensile load–unload cycle to evaluate them as strainsensors. To ensure good electrical contact between carbon nanotube film and thewires, a thin layer of copper was thermally deposited on both ends of the film aselectrodes, and the wires were connected to the electrodes by silver ink. Wheatstonebridges were used to convert the resistance changes of the MWCNTs to voltageoutput. Results indicated that the output voltages were proportional to the strainreadings from the stain indicator. The effect of temperature on the resistancewas measured and the MWCNT film resistance was found to be independent oftemperature over the range 273–363 K. The optimal film dimension for strainsensing was evaluated as well. Dynamic tests suggest that the MWCNTs were ableto extract the structural signature. Our results indicate that MWCNT film ispotentially useful for structural health monitoring and vibration control applications.

On-board Strain Measurement of a Cryogenic Composite Tank Mounted on a Reusable Rocket using FBG Sensors

This article presents the real-time strain measurement of a composite liquid hydrogen (LH2) tank using fiber Bragg grating (FBG) sensors. The tank was composed of carbon fiber reinforced plastic (CFRP), and an aluminum liner was fabricated by the filament winding method and mounted on a reusable rocket. This rocket (vertical takeoff and landing) is called a reusable rocket vehicle test (RVT) and was developed by the Institute of Space and Astronautical Science of the Japan Aerospace Exploration Agency (ISAS/JAXA). Considering the high operational pressure and the iterative use of the tank, its structural integrity must be guaranteed. Thus, the authors have attempted a real-time strain measurement of the composite LH2 tank using FBG sensors during rocket operations. First, the adhesive properties of the FBG sensors were investigated at cryogenic temperatures. As a result, UV-coated FBG sensors and polyurethane adhesives were adopted. An onboard FBG demodulator was then developed to be mounted on the rocket and its performance was assessed. Finally, the strain measurement was attempted during the flight experiments of the RVT using the onboard FBG demodulator. FBG sensors were glued on the surface of the composite LH2 tank and connected to the onboard FBG demodulator. During these rocket operations, the output of the onboard FBG demodulator was continuously monitored via a telemetry system. The results obtained by the demodulator agreed well with those of the conventional foil strain gage.

Structural integrity monitoring with fibre Bragg grating sensors

PurposeThis paper describes a recent collaborative project involving the development of a multiplexed fibre Bragg grating (FBG) sensor system for structural integrity monitoring.Design/methodology/approachThe system is described and field trials on both conventional and novel composite bridges are discussed. A FBG sensor‐based structural monitoring system was developed, based on a fluorescent fibre as the optical source. It used a tuneable, fibre‐coupled, Fabry‐Perot filter, actuated by piezoelectric transducers and operated over the bandwidth of the source at up to 250 scans/second. Light from the source was filtered and reflected back from the Bragg gratings, through optical couplers, to eight photodiode detectors. These detected the resulting time‐domain spectra of the sensors in each of the serially connected sensor arrays. The system was tested at City University and then subjected to trials on the Mjosund road bridge in Norway and on West Mill bridge in Oxfordshire, UK, which is the first bridge to be fabricated from a new type of composite material.FindingsDuring the Norwegian trials the system was arranged with four or five FBG sensors per channel giving a total of 32 measurement points with eight parallel channels. Twelve conventional foil strain gauges and a number of thermocouples were also installed. Different static and dynamic loads were applied over a period of 18 months and the results showed that the thermally compensated strain data obtained optically matched those from the resistive gauges to within <5 με. During the construction stage of the Oxfordshire bridge, sections of the decking and longitudinal composite support beams were instrumented with 40 FBG sensors with temperature compensation, placed at pre‐selected sites of maximum strain. These exhibited a resolution of ±5 με and an operating range of over ±2,000 με.Originality/valueThis research has shown that multiplexed, multi‐point FBG sensor systems can accurately and reliably monitor both static and dynamic strains in large structures over a range of temperatures and for extended periods of time.

Fundamental Understanding of Piezoelectric Strain Sensors

This paper investigates the behavior of piezoelectric elements as strain sensors. Strain is measured in terms of the charge generated by the element as a result of the direct piezoelectric effect. Strains from piezoceramic (PZT) and piezofilm (PVDF) sensors are compared with strains from a conventional foil strain gage and the advantages of each type of sensor are discussed, along with their limitations. The sensors are surface bonded and are calibrated by means of a dynamic beam bending setup over a frequency range of 5-500 Hz. Correction factors to account for transverse strain and shear lag effects due to the bond layer are analytically derived and validated experimentally. Additionally, design of signal conditioning electronics to collect the signals from the piezoelectric sensors is addressed. The superior performance of piezoelectric sensors compared to conventional strain gages in terms of sensitivity and signal to noise ratio is demonstrated.

Fundamental Understanding of Piezoelectric Strain Sensors

This paper investigates the behavior of piezoelectric elements as strain sensors. Strain is measured in terms of the charge generated by the element as a result of the direct piezoelectric effect. Strain measurements from piezoceramic (PZT) and piezofilm (PVDF) sensors are compared with strains from a conventional foil strain gage and the advantages of each type of sensor are discussed, along with their limitations. The sensors are surface bonded to a beam and are calibrated over a frequency range of 5-500 Hz. Correction factors to account for transverse strain and shear lag effects due to the bond layer are analytically derived and experimentally validated. The effect of temperature on the output of PZT strain sensors is investigated. Additionally, design of signal conditioning electronics to collect the signals from the piezoelectric sensors is addressed. The superior performance of piezoelectric sensors compared to conventional strain gages in terms of sensitivity and signal to noise ratio is demonstrated.

Shear Key Performance in Multibeam Box Girder Bridges

This report describes a series of field tests of multibeam prestressed box girder bridges. The objective of the test was to investigate the in-situ performance of the grouted shear keys, located at the longitudinal joints between adjacent girders. The longevity of these joints can be a problem with this type of bridge; failure of the joint will typically not only compromise the load-sharing mechanism between adjacent girders, but also lead to the failure of the deck waterproofing system, with attendant corrosion problems. The tests consisted of monitoring relative displacements occurring across the intergirder joints, as well as bending strains in the girders themselves during passes of a preweighted tandem-axle dump truck, with axle loads typically in the range of 84.7 kN (19 kips). Relative displacements were measured with an especially designed transducer, having a resolution on the order of .00254 mm (0.1 mil), which was bonded to the underside of the bridge across the intergirder joints. Bending strains in the girders were measured with conventional foil strain gauges, having a gauge length of 38 mm (1.5 in.). All bridges tested exhibited relative displacements across at least some of the joints, which indicated a fractured shear key.

Fiber optic sensors focus on smart systems

Materials with structurally integrated fiber-optic sensors, the first step toward smart structures, which continuously monitor their own vibration, internal strain and deformation, temperature, and structural integrity are described. It is shown that the Fabry-Perot fiber-optic strain sensor has capabilities that far exceed those of conventional foil strain gauges, which holds the promise of responding to acoustic emission signals while simultaneously tracking the high loads likely to be encountered in practical structures such as aircraft wings. The same sensing system may be able to monitor the degree of cure during fabrication of thermoset composite structures. The fabrication and characterization of such all-fiber, intrinsic Fabry-Perot strain sensors are discussed. Passive and active homodyne signal recovery techniques used in the fiber-optic sensor systems are also described. >