Foil Strain Gauges Research Articles

Railroad bridge monitoring using Dragonfly® piezoelectric strain gauge

Structural Health Monitoring (SHM) applied to civil structures such as bridges, high-rise towers, and electricity pylons can help plan their maintenance, detect damages or un-expected events, and extend their remaining operating lifetime. Metallic foil strain gauge and vibrating wire are the historic sensors used to measure strain on civil structures. However, civil structures are designed to undergo little deformation in service (0.1-50 µdef). Both SG and vibrating wire are limited by their resolution around 1 µdef. Moreover, they are known for their DC drift over time, which complexifies long term monitoring. There is therefore a need for more sensitive and less prone to drift sensors in the time domain. Dragonfly new piezoelectric strain gauges, with a resolution three order of magnitude over classic sensors, solve these issues and enable new applications. Dragonfly sensors consist in an extremely thin piezoelectric ceramic packaged in a flexible PCB and pre-wired with a SMA connector. The flexible sensor is glued with cyanoacrylate glue to the object to be studied. This paper presents the implementation of Dragonfly sensors on a railroad bridge. With a single sensor, it has been possible to identify the bridge girder first two resonance using ambient noise, train passage loading for fatigue-life analysis, train passage reproducibility, and pedestrian passage on the walkway. A complete comparison with classic metallic foil strain gauge is detailed.

Saw‐cut method verification for determining the most important factors influencing its applicability

AbstractThe current situation with prestressed structures raises questions about the ways how to determine residual prestressing which is the crucial factor for the evaluation of the prestressed structure. Therefore, this study aims to enhance existing knowledge of the so‐called Saw‐cut method which is considered a non‐destructive indirect technique. The presented study focuses on the determination of various factors which could influence the applicability and accuracy of this method such as both compressive and tensile normal stress release, the type of measuring instrument (linear foil strain gauge or newly developed FBG sensor), and the length of the normal stress release recording (up to almost 9 h). The obtained results suggest that tensile normal stress release could be more problematic for the evaluation than compressive normal stress release. Moreover, the application of FBG sensors seems to be more precise but also more complicated regarding the installation, recording and costs. Finally, the time span of the measurement can influence the residual normal stress release. For practical reasons, 60 min after the performance of the final sawing should be sufficient. Finally, when used at locations with adequate compressive normal stress, the Saw‐cut method seems to be a promising tool for prestressing evaluation.

Structural concrete measurements: New distributed approach for standard specimens

Concrete structures are widely used in construction because of their many advantages, including high corrosion resistance, durability, low operating costs, and the ability to design any geometry. However, they also have some disadvantages, such as their high dead weight and susceptibility to cracking. Although this material is widely used globally, new types of concrete, such as high-strength or lightweight aggregate-based, are still being developed. This requires constant control and testing in both laboratory and field conditions. Concrete is a heterogeneous material due to the influence of aggregate grains and local discontinuities, voids or pores. Given these characteristics, it is clear that most conventional diagnostic solutions, such as spot foil strain gauges or extensometers, are unable to accurately analyse these localised effects as they average local strains over the measurement base. In contrast, distributed fibre optic sensing (DFOS) enables strain readings to be taken with millimetre geometric resolution, significantly extending the structural assessment capability. The article describes research on standard concrete cylinders equipped with surface-mounted optical fibres for simultaneous measurements of distributed strains in both the longitudinal direction (compression zone) and circumference direction (tension zone). The novelty of the proposed approach, apart from the way of arranging the sensing fibres on the specimens, lies mainly in the new diagnostic approach that allow (1) the evaluation of the homogeneity of the concrete using statistical parameters, (2) the identification of microcracking and (3) the identification of near-to-failure state based on Poisson’s ratio analysis. Twenty cylindrical specimens were laboratory tested using reference techniques to validate the performance of the DFOS-based system and to provide data for statistical evaluation. The results indicate the possibility of detecting the approaching damage even without knowledge of the material properties of the specimen or the initial stress level at the moment of starting the measurements. The discussion also covered future research directions and potential improvements.

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.

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.

Validation of predicted stress in polypropylene fiber-reinforced self-consolidating concrete under restrained shrinkage conditions

Self-Consolidating Concrete’s high cement paste volume raises concerns of increased shrinkage and cracking. Evaluating concrete shrinkage potential is critical to designing mixes that limit cracking. AASHTO T334 tests restrained shrinkage, providing a relative method for comparatively assessing the performances of various mixes. In this study, a series of self-consolidating mixes incorporating fibers was prepared, and their cracking resistances were evaluated using the modified AASHTO T334 test. The modified restrained ring test includes two types of strain gages, vibrating wire strain gage, and foil strain gage. Accordingly, the concrete and steel strains were directly measured by two types of strain gages that provided the crack initiation and propagation ages. The measured concrete strain (and stress) and the predicted concrete stress based on the strain in the steel ring were compared and validated. Crack ages, lengths, and widths were measured using a digital microscope with 10x-50x magnification to quantify the cracking resistance of FR-SCC mixes. The results show that the use of polypropylene fibers could reduce the cracking area by 33%, and the use of the strain in the steel ring without modification could underestimate the predicted strain in concrete.

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.

Fatigue Performance of Type I and Type II Fibre Bragg Gratings Fabricated by Femtosecond Laser Inscription through the Coating.

Strain sensing technology using fibre Bragg grating (FBG) sensors is an attractive capability for aerospace structural health monitoring (SHM) and assessment because they offer resistance to harsh environments, low maintenance, and potential for high density and high strain sensing. The development of FBG inscription techniques through the fibre polymer coating using infrared (IR) lasers has overcome the mechanical weaknesses introduced by removal of the fibre coating, which is typically required for conventional UV laser inscription of FBGs. Type I and Type II femtosecond gratings are fabricated using through-coating inscription techniques, but the higher laser energy used for Type II gratings damages the glass fibre core, impacting mechanical performance. This paper investigates the fatigue performance of Type I and Type II through-coating FBG sensors with different fibre geometries and photosensitisation approaches to evaluate their overall reliability and durability, with a view to assess their performance for potential use in civil and defence SHM applications. The fatigue performance of FBG sensors was assessed under high-strain and high-frequency mechanical loading conditions by using a custom-designed electro-dynamically actuated loading assembly. In addition, pre- and post-fatigue microscopic analyses and high-resolution reflection spectrum characterisation were conducted to investigate the failure regions of the fibres and the effect of fatigue loading on reflection spectrum features. As expected, Type I gratings had a significantly higher fatigue life compared to Type II gratings. However, Type II gratings performed significantly better than conventional UV laser-inscribed FBGs and electrical foil strain gauges. Type II gratings withstand higher temperatures, and are therefore more suitable for application in harsh environments.

Geogrid stabilisation application – review of strain measurement methods, research and implications on design approaches

Geogrids have been used in the construction industry for stabilisation applications for decades, however, there is still a lot of conflicting information relating to the mechanical properties required to achieve this stabilisation function. Many designers adopt traditional tensile strength against tensile strain approaches that reflect membrane effect and ignore the actual mechanism of interlock and confinement. Recent ISO 10318-1:2015 defines geogrid stabilisation function by means of aggregate interlock and confinement, whereas tensile strength is seen as a function of reinforcement. Researchers across the globe have attempted to quantify the strain developed in geogrid used in a stabilising function exposed to heavy loads. Instruments used for monitoring typically comprise of standard wire strains gauges, fibre optics or foil strain gauges. Considering the harsh construction environment where the geogrids, and hence the strain gauges are exposed to damage or penetration by sharp aggregate edges, installation of sophisticated and sensitive monitoring systems can be challenging. There are several issues that can affect monitoring results such as the size of the monitoring instrument or the ability to achieve an adequate bond to the grid. Monitoring results have a major influence on geotechnical design and how practitioners perceive stabilisation mechanism in geogrids. Whilst the mechanisms of interlock and increased confinement have been studied, there still exists few design tools to provide designers with a straightforward analysis. This paper reviews research on the methods for in-situ strain measurement of geogrid installed in a stabilisation function, summarises results obtained by researchers and provides a summary of how these are reflected in geotechnical design approaches for geogrid stabilised soil.

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.

Gearbox Mechanical Efficiency Determination by Strain Gauges Direct Application

This article describes a unique method of measuring the efficiency of gearboxes using foil strain gauges, which allows maintaining the current configuration of the gearbox within the overall assembly of the machine and its functional condition. The presented method is applicable to gearboxes located in the original equipment assembly without the need to use a test rig. Using foil strain gauges, the torque at the input and output of the gearbox is detected. Therefore, the accuracy of torque measurement is key. The crucial step is the calibration of the instrumentation to the given application conditions, which, in this case, is ensured by a virtual calibration using a very accurate FEM analysis. The accuracy of the position of strain gauges and virtual calibration of measurements generate inaccuracies affecting the resulting uncertainty of the determined efficiency. The present article shows, on the example of several measurements, that when using 24-bit converters, after processing the obtained data, mechanical stress with a sensitivity better than hundredths of an MPa can be reliably detected even without signal amplification from strain gauges. It follows that the efficiency is determined with an accuracy of better than low units of tenths.

An experimental evaluation of geocomposite-reinforced soil sections

The present study aims to investigate the performance of soil-aggregate sections reinforced by three types of geosynthetics. The main focus of this research was on a geocomposite material comprised of a geogrid and a nonwoven geotextile. Along with investigating this reinforcement, its components (i.e., geogrid and nonwoven geotextile) were also studied individually to make a comparison between their functionalities in soil-aggregate systems. Two sandy soils, one of which was a case study, were chosen as the subgrade layer in this research. The California Bearing Ratio (CBR) was the index to examine the strength of specimens. The scale effect of the conventional CBR mold, as well as geosynthetics anchorage, were investigated through fabricating a modified CBR mold. Strain analysis was also carried out by utilizing some foil strain gauges to assess the strain mobilized in the geosynthetics in various reinforced sections. In the presence of both geocomposite and geogrid, load-penetration responses of the specimens enhanced significantly, while the geotextile inclusion effect was in a negative way. Improved functionalities of reinforcing materials were seen in the modified mold.

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

Distributed fibre optic sensors in FRP composite bridge monitoring: Validation through proof load tests

The distributed fibre optic sensors (DFOS) technique based on Rayleigh scattering has been chosen as the basic structural health monitoring (SHM) system of the first Polish fibre reinforced polymer (FRP) composite bridge. The validation of the DFOS technique is critical to address the development of a monitoring system with improved accuracy and spatial resolution tailored for the FRP bridge application. The validation through proof load tests and finite element analysis (FEA) has been performed to ensure accurate and reliable DFOS readings. Two additional strain measurements techniques were implemented to validate DFOS, i.e. common foil strain gauges and vibrating wire strain gauges. The DFOS strain profiles were first compared with discrete strain measurements and then converted into deflection profiles and validated against discrete deflection measurements performed with common linear variable differential transducers. The additional validation based on the comparison of the measurement profiles with the results of FEA has been also performed. Based on the experimental and numerical results it has been revealed the DFOS technique is a reliable and accurate method to be implemented for FRP bridge monitoring.

Using Fiber Optic Sensors for Bridge Monitoring

This paper presents the data obtained from monitoring a steel Struss bridge using Fiber Bragg Grating (FBG) sensors before and after a proposed repair for a crack propagation in the end plates. This paper details the operating mechanism behind the FBG sensors and advantages of using FBG sensors over resistive foil strain gauges for bridge structural health monitoring and also details how cracks on the outer web’s end plate originated and then provides a step-by-step guide to the completed repair. This technology can be use in other practical applications where structural health monitoring is needed.