Surface Mounted Strain Gauges Research Articles

An experimental investigation into the effect two-phase flow induced vibrations have on a J-shaped flexible pipe

Multiphase flow inside of pipes occurs in a wide variety of engineering applications, including offshore deep-water oil and gas transport. Vibrations induced by the flow inside of the pipe can lead to its mechanical failure and thus lead to uncontrolled release of the fluids being transported. In subsea applications, flexible J-risers are often employed to deliver the produced fluids from the seafloor to the host platform. Despite the potentially significant liabilities associated with subsea hydrocarbon leaks, there has been a distinct lack of investigations into how flow induced vibrations in large scale, pressurised flexible J-risers can lead to system integrity loss. Previous investigations have generally focused on the response of rigid pipes or small scale, unpressurised flexible risers. This study presents an investigation into the response of a 10 m long, 50.8 mm internal diameter composite riser containing a tensile armour helical structure to a variety of two-phase, water-nitrogen flows at 10.8 barg of pressure and ambient temperature. High speed cameras were used to investigate the structure of the flow at either end of the flexible riser, whilst synchronised surface mounted strain gauges and accelerometers were used to investigate the response of the pipe. Time-averaged data were acquired to assess the general response of the pipe, whilst a statistical analysis of the fluctuations highlighted the movement of the pipe. One-dimensional and computational fluid dynamics simulations were used to define the experimental test matrix and provide further insight into the structure of the flow inside the J-riser. Single-phase gas flow was found not to cause the J-riser to move significantly, whilst multiphase flow led to significant in-plane movement of the pipe. Increasing the liquid flow rate (or decreasing the gas flow rate) increased the mean strain experienced by the pipe. At low gas flow rates, the pipe oscillated smoothly about its mean position, but at higher gas flow rates a violent intermittent whipping motion was observed. The latter produced large in-plane and out-of-plane movement of the pipe which could pose a threat to system integrity. This work offers new insights into fluid-structure interactions in large scale engineering applications, contributing to improved system design and control.

Mould free laminated composites using eccentric fibre prestressing

Mould free fibre reinforced composites are an emerging manufacturing process that allows for complex shapes to be produced, independently from complex moulding. Previous examples demonstrated reconfiguration through thermal stresses induced by an asymmetrical stacking sequence within a laminate. However, asymmetry within the laminate can adversely influence the stiffness and geometric coupling which results in heavier structures. This paper presents eccentric prestressing as a novel method to produce more structurally efficient mould free laminated composites. Euler-Bernoulli beam theory and finite element methods were employed to predict the strain and deformation of laminated composites with selective fibre prestress. The internal and external strains were measured with chirped fibre bragg grating sensors, and surface mounted strain gauges respectively to validate the numerical and analytical predictions where a difference of 3.13% was reported. Finally, a comparative study was conducted to show the benefits of eccentric prestress over asymmetric 4D printed laminates.

Lithium-Ion Battery Strain Gauge Monitoring and Depth of Discharge Estimation

Abstract Lithium-ion battery diagnostics and prognostics rely on measurements of electrical impedance, capacity, and voltage to infer the internal state of the battery. Mechanical changes to the cell structure represent an additional measure of the battery’s state because these changes are related to the overall battery health. As lithium-ion batteries are charged and discharged, lithium ions are inserted or removed from the anode and cathode, a process called intercalation and deintercalation. As lithium ions intercalate and de-intercalate, they can cause changes to the lattice of the electrode particles, resulting in volumetric changes. These volumetric changes cause mechanical stresses and strains on the lithium-ion battery electrodes, and subsequently, the whole cell’s thickness varies as it is charged and discharged. This paper presents a study on the use of surface-mounted strain gauges for in-situ measurement of structural changes to lithium-ion batteries, along with a characterization of the unit-to-unit differences in strain response. A neural network modeling structure is then used to predict the battery’s depth of discharge under dynamic discharge conditions.

Strain sensing in GFRP via fully integrated carbon nanotube epoxy film sensors

Structural health monitoring of composite structures enables early damage detection to prevent critical component failure. Surface-mounted strain gauges are commonly applied to monitor the integrity of composite structures in highly loaded areas. However, strain gauges can only measure strain on the structure’s surface and are exposed to environmental influences. Within this paper, fully integrated carbon nanotube thin-film sensors for strain and damage sensing in glass fibre reinforced polymers (GFRP) via electrical resistance measurements are presented. Single wall carbon nanotube epoxy thin-films were manufactured using a manual film applicator, partially pre-cured, placed on dry glass fabrics and infused in a resin transfer moulding process. The mechanical properties of the composites and strain sensing capabilities of the integrated sensors were studied for various load cases and different laminate lay-ups. Results of quasi-static and step-wise three-point bending and tensile tests show that the integrated films allow for localised strain measurements in GFRP without significant loss of mechanical properties. Open hole tensile tests proof the ability to monitor local strain and damage in highly loaded areas enabling failure prediction via threshold resistance change values. The proposed thin-films enable a tailored strain and damage monitoring of GFRP offering the possibility for measurements at different material depths, over large sections or selectively in highly loaded areas. The manufacturing process is easily automatable and suitable for large scale manufacturing.

A smoothed iFEM approach for efficient shape-sensing applications: Numerical and experimental validation on composite structures

A smoothed inverse finite element method (iFEM(s)) is developed by coupling the inverse finite element method (iFEM) and the smoothing element analysis (SEA) for real-time reconstruction of displacement field utilizing a network of discrete strain-sensor measurements. This reconstruction is commonly referred to as “shape sensing”. The shape-sensing capabilities of iFEM(s) in multilayered composite and sandwich structures are validated using both numerical and experimental strain data. The iFEM(s) approach first recovers continuous (smoothed, full field) strains from discrete strain measurements and subsequently employs these strains in the least-squares variational principle to obtain the deformed structural shape. To model through-the-thickness displacement distributions accurately, the kinematic relations of the refined zigzag theory (RZT) are incorporated into the mathematical formulation of iFEM(s). The least-squares functional accommodates the membrane, bending, zigzag, and full transverse-shear section strains. Moreover, simplified forms of this functional are derived for both woven composite and sandwich structures. Subsequently, a four-node quadrilateral inverse-plate element, iRZT4, is implemented for discretization of the geometry and approximation of kinematic variables. The high accuracy of present computational framework is successfully demonstrated by performing shape- and stress-sensing analyses using numerical strain data. Then, the predictive capabilities of iFEM(s) are also explored on a twill-woven wing-shaped sandwich laminate using experimental strain measurements from surface mounted strain gauges and embedded fiber Bragg grating (FBG) sensors. Finally, the improved shape-sensing predictions of iFEM(s) for both numerical and experimental cases are compared to the conventional iFEM application.

Using a low-cost bluetooth torque sensor for vehicle jerk and transient torque measurement

This paper presents the use and development of a specific wireless torque measurement system that is used to obtain the transient torque performance of vehicle transmissions. The torque sensor is strain-based, using surface-mounted strain gauges on a prop shaft. The gauges are connected to a compact printed circuit board, which is clamped to the shaft next to the strain gauges using a three-dimensional printed housing. The printed circuit board contains an amplifier, low-pass filter, analog-to-digital converter, microcontroller and bluetooth transceiver. The printed housing is impact resistant carbon-reinforced nylon and securely retains the printed circuit board and the battery powering the device. The transmitted torque data are received by a transceiver, which is interfaced to a PC through an RS-232 connection. NI LabVIEW is used to process, display and save data. The wireless torque sensor was installed to the Unit Under Test at the output shaft of the five-speed manual transmission. The Unit Under Test was installed on a dynamometer for verification purposes and the transient torque was recorded under various operational conditions. The transient output torque of the manual transmission is measured and compared with results obtained from simulations performed under similar operating conditions. The two sets of transient responses show a good correlation with each other and hence demonstrate that the torque sensor meets the major design specifications. The data obtained will be used to enhance the fidelity of the software model.

Application study on fiber Bragg grating sensors in damage monitoring of sandwich composite joints

This article studies the application of fiber Bragg grating sensors in damage monitoring of sandwich composite L-joints, which aims to reveal the interface damage between the GFRP skin and the foam core. Fiber Bragg grating sensors have been arranged based on numerical results. The ultimate load and failure location via numerical modelling have been experimentally verified. Strains measured by fiber Bragg grating sensors have shown anomalies with load increased, indicating the onset and propagation of damage. Based on the damage-induced strain anomalies, four stages have been addressed to reveal the emergence, accumulation and propagation of damage during the loading process. Measurements by numerical simulation and surface mounted strain gauges have been found to be less sensitive to internal damage, proving the great significance of embedding fiber Bragg grating sensors to monitor interface damages of sandwich L-joint.

Early Damage Detection in Composites during Fabrication and Mechanical Testing.

Fully integrated monitoring systems have shown promise in improving confidence in composite materials while reducing lifecycle costs. A distributed optical fibre sensor is embedded in a fibre reinforced composite laminate, to give three sensing regions at different levels through-the-thickness of the plate. This study follows the resin infusion process during fabrication of the composite, monitoring the development of strain in-situ and in real time, and to gain better understanding of the resin rheology during curing. Piezoelectric wafer active sensors and electrical strain gauges are bonded to the plate after fabrication. This is followed by progressive loading/unloading cycles of mechanical four point bending. The strain values obtained from the optical fibre are in good agreement with strain data collected by surface mounted strain gauges, while the sensing regions clearly indicate the development of compressive, neutral, and tensile strain. Acoustic emission event detection suggests the formation of matrix (resin) cracks, with measured damage event amplitudes in agreement with values reported in published literature on the subject. The Felicity ratio for each subsequent loading cycle is calculated to track the progression of damage in the material. The methodology developed here can be used to follow the full life cycle of a composite structure, from manufacture to end-of-life.

Early Damage Detection in Composites by Distributed Strain and Acoustic Event Monitoring

Advances in the development of fibre reinforced polymer composites and their manufacturing techniques have led to their increased use as structural materials. They offer superior corrosion resistance and high specific strength and stiffness, compared to metals, while designing anisotropic structures can provide weight reductions. However, there is uncertainty associated with understanding the consequences of out-of-plane damage in composites. Non-destructive evaluation techniques are adopted in many cases, but represent significant down-time and labour costs. Attached and/or embedded structural health monitoring systems have shown promise in improving the reliability and safety of composites, while reducing lifecycle costs, and improving design and manufacture processes. In this work, a distributed optical fibre sensor is embedded in a six ply carbon fibre-epoxy composite laminate during fabrication, to give three strain sensing regions near the top, middle, and bottom surface of the laminate, which allow in-situ and real time monitoring of strain development in the panel during resin infusion and curing. Four point bending is then conducted on the plate. Acoustic emission events are collected using four bonded piezoelectric wafer active sensors, thus allowing the comparison of strain data with damage formation and growth during progressive loading cycles. Distributed strain data demonstrates the sensitivity of the optical fibre through-the-thickness of the panel. The top, middle, and bottom sensing regions clearly indicate the development of compressive, neutral, and tensile strain, respectively. The strain values obtained from the optical fibre are in good agreement with strain data collected by surface mounted strain gauges. Acoustic event detection suggests the formation of matrix (resin) cracks, with measured damage event amplitudes in agreement with values reported in published literature on the subject.

Estimation of transverse shear force during slamming impacts on a simply supported composite panel using a strain derivative method

Use of sandwich constructions in slamming regions of high speed marine craft has led to increased consideration of the applied transverse shear force. Low shear strength core materials can lead to transverse shear failure becoming a high risk failure mode. Direct measurement of transverse shear force is difficult without altering the structure of the hull panels. This work utilises a non-invasive strain derivative method to estimate the applied transverse shear force. The basis of this method is the correlation between applied bending moment, determined from surface mounted strain gauges, and transverse shear force. A simply supported 1000×500mm instrumented sandwich panel has been tested in the Servo-hydraulic Slam Testing System. Impacts have been undertaken at 10° with vertical velocities from 1.0 to 3.5m/s. The shear force to bending moment ratio has been compared with the ratio based on a uniformly distributed load, as frequently used in design. An increase of up to 68% for the slamming experiments is observed. This significant difference illustrates a greater applied transverse shear force in slamming regions than would be predicted through the application of a uniformly distributed load.

Measurement of bar strain during pull-out tests: use of electrical resistance gauge methods under large displacement

Two issues make measuring the local bond stress–slip relationship difficult: first, the bar is embedded within the concrete, making measurement of local strain values complex; second, yielding of the steel bar influences the bond stress–slip behaviour. Global bond stress–slip behaviour is generally obtained by measuring the slip that occurs at the free end of the bar during pull-out testing. Several distinct mounting methods have been used to install electrical resistance strain gauges on embedded bars in concrete to obtain local strain data. These methods are reviewed in relation to their ability to deliver reliable and robust data at both small and large values of slip. The use of such strain gauges is found to provide a viable method for assessing the variation in strain along embedded steel bars during pull-out testing. The use of surface-mounted strain gauges leads to high failure rates due to physical damage to either the gauges or their connections. The use of near-surface- and central-groove-mounted strain gauges enables measurement of local bar strain over large displacements. Using data from near-surface-mounted strain gauges, it is shown that in the post-peak region the bond stress increases linearly from the free end of the bar.

Doubly Reinforcing Cementitious Beams with Instrumented Hollow Carbon Fiber Tendons

Surface mounted strain gages are used to characterize the behavior of polymer-enhanced cementitious beams designed to withstand reverse loadings. These unique composite structures are doubly reinforced with hollow carbon fiber (graphite) tendons equipped with strain gages and the study includes section design, materials considerations, structural testing, and finite element analysis. The primary purpose of strain gage integration is to insure that the stress in the materials remains within the elastic range so that damage does not occur. A finite element model is developed to characterize the structural response in the elastic range and a hybrid approach is suggested in which displacement, strain, and stress can be obtained with a single strain gage. The ability to characterize structural performance beyond the elastic range is also demonstrated by analyzing data obtained from displacement-controlled tests.

Use of a surface-mounted chirped fibre Bragg grating sensor to monitor delamination growth in a double-cantilever beam test

A surface-mounted chirped fibre Bragg grating (CFBG) sensor has been used for the first time to monitor delamination growth within a composite material (a transparent, unidirectionally reinforced glass fibre/epoxy resin double-cantilever beam (DCB) specimen). The specimens were tested using a constant displacement rate, with the delamination length being measured using complementary techniques: (i) in situ photography, (ii) surface-mounted strain gauges, and (iii) the surface-mounted CFBG sensors. The unidirectionally reinforced DCB specimens showed characteristics typical of such material which complicate the curvature of the beams, i.e. the development of extensive fibre bridging and pronounced R-curve behaviour. To validate the interpretation of the CFBG reflected spectrum, the experimentally determined strains from the surface-mounted strain gauges have been used, together with in situ photographs of the position of the delamination front. Using the CFBG sensor technique, the delamination length was measured to within about 4mm over the 60mm sensor length.

Determination of dynamic strain profile and delamination detection of composite structures using embedded multiplexed fibre-optic sensors

This paper studies the dynamic strain measurement and delamination detection of clamped–clamped composite structures using embedded multiplexed Fibre-optic Bragg grating (FBG) sensors through experimental and theoretical approaches. A dynamic strain calibration of the FBG sensors and surface mounted strain gauges to find out the correlation between the strain and photovoltage is reported. The embedded FBG sensors were used to measure the natural frequency and dynamic strain of intact and delaminated composite structures. The strain profile of these structures subjected to external excitations was evaluated by using experimental measured data and a modified vibration theory. The results revealed that the use of the embedded FBG sensors is able to actually measure the dynamic strain and identify the existence of delamination of the structures. This allows the continuous estimation of fatigue life and minimises the need of in-site inspection of the structures.

A comparison of bone strain measurements at anatomically relevant sites using surface gauges versus strain gauged bone staples

Instrumented bone staples were first introduced as an alternative to surface-mounted strain gauges for use in human in vivo bone strain measurements because their fixation to bone is secure and requires not only minimally invasive surgery. Bench-top bone bending models have shown that the output from strain gauged bone staples compares favorably to that of traditional mounted gauges. However their within- and across-subject performance at sites typically instrumented in vivo has never been examined. This study used seven human cadaver lower extremities with an age range of 23–81 years old and a dynamic gait simulator to examine and compare axial strains in the mid tibial diaphysis and on the dorsal surface of the second metatarsal as measured simultaneously with strain gauged bone staples and with traditional surface-mounted gauges. Rosette configurations were used at the tibial site for deriving principal compression and tension, and shear strains. Axial outputs from the two gauge types demonstrated strong linear relationships for the tibia ( r 2=0.78–0.94) and the second metatarsal ( r 2=0.96–0.99), but coefficients (slopes) for the relationship were variable (range 7–20), across subjects and across sites. The apparent low reliability of strain gauged staples may be explained by the fact that both strain gauged staples and surface strain gauges are inexact to some degree, do not measure strains from exactly the same areas and strain gauged staples reflect surface strains as well as deformations within the cortex. There were no relationships for the principal tibia compression, tension or shear strain measurements derived from the two rosette gauge types, reflecting the very different anatomical areas measured by each of the constructs in this study. Strain gauged bone staples may be most useful in comparing relative axial intra-subject differences between activities, but inter-subject variability may require larger sample sizes to detect differences between populations.

Development and testing of a modular strain measurement clip

A novel, multi-use, low-stiffness and low-cost transducer for measuring in vitro strains has been developed and tested. Currently available strain measurement methods are either too expensive, too complicated or too inflexible for multi-use strain measurement. The stainless-steel modular strain measurement clip introduced here was instrumented with four 350 Ω axial strain gauges in a full Wheatstone bridge configuration to take advantage of commonly available strain gauge amplifier equipment. Adjustable extension arms were designed to allow greater application versatility. The clip was calibrated and produced a linear response ( R 2>0.99) over a minimum of 1.04 mm at high amplifier gain. With reduced amplifier settings, testing showed a linear response over a range of 30.5 mm ( R 2>0.99). Clip stiffness was 0.6 N/mm of extension arm tip displacement for minimal instrumentation artifact. A validation test was conducted through a comparison of strain clips, surface-mounted strain gauges and theoretical strain in an aluminium rod subjected to axial tensile loading. The two measurement techniques were used to determine the modulus of elasticity of the aluminium rod. Results were within 6% of the known modulus of elasticity for aluminium. A comparative biomechanical test was also performed on an equine third metacarpal specimen. The traditional bonded strain gauging method produced similar results as the new strain clip, but failed to measure ultimate strains since all strain gauges failed prior to specimen failure. Further investigations into the multiple uses of the clip are underway and recommendations for future versions of the clip are given.

An extensometer for global measurement of bone strain suitable for use in vivo in humans

An axial extensometer able to measure global bone strain magnitudes and rates encountered during physiological activity, and suitable for use in vivo in human subjects, is described. The extensometer uses paired capacitive sensors mounted to intraosseus pins and allows measurement of strain due to bending in the plane of the extensometer as well as uniaxial compression or tension. Data are presented for validation of the device against a surface-mounted strain gage in an acrylic specimen under dynamic four-point bending, with square wave and sinusoidal loading inputs up to 1500 μ ε and 20 Hz, representative of physiological strain magnitudes and frequencies. Pearson's correlation coefficient ( r) between extensometer and strain gage ranged from 0.960 to 0.999. Mean differences between extensometer and strain gage ranged up to 15.3 μ ε. Errors in the extensometer output were directly proportional to the degree of bending that occurs in the specimen, however, these errors were predictable and less than 1 μ ε for the loading regime studied. The device is capable of tracking strain rates in excess of 90,000 μ ε/s.

Strain monitoring in composite-strengthened concrete structures using optical fibre sensors

In this paper, the mechanical behaviour of the composite-strengthened concrete structures is addressed. Optical fibre sensor presents a great deal of potential in monitoring the structural health condition of civil infrastructure elements after strengthening by externally bonded composite materials. The use of embedded optical fibre sensor for strain and temperature monitoring enables to reveal the status of the composite-strengthened structure in real-time remotely. In this paper, an experimental investigation on the composite-strengthened concrete structures with the embedment of fibre-optic Bragg grating (FBG) sensors is presented. Single- and multiplexed-point strain measuring techniques were used to measure strains of the structures. Frequency modulated continuous wave (FMCW) method was used to measure strains in different points of the structure with using only one single optical fibre. All strains measured from the sensors were compared to conventional surface mounted strain gauges. Experimental results show that the use of the embedded FBG sensor can measure strain accurately and provide information to the operator that the structure is subjected to debond or micro-crack failure. Multiplexed FBG strain sensors enable to measure strain in different locations by occupying only one tiny optical fibre. Reduction of strength in composite laminate is resulted if the embedded optical fibre is aligned perpendicular to the load-bearing direction of the structure.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

In-situ process and condition monitoring of advanced fibre-reinforced composite materials using optical fibre sensors

This paper presents a general overview of a number of optical fibre sensor systems which have been developed and used in advanced fibre-reinforced composites for in-situ process and condition monitoring. The in-situ process monitoring techniques were optical-fibre-based evanescent wave spectroscopy, transmission near-infrared spectroscopy and refractive index monitoring. The optical fibre sensors were successful in tracking the cure reaction. The condition monitoring of advanced fibre-reinforced composites was carried out using two intensity-based optical fibre sensor systems: an extrinsic multi-mode Fabry-Pérot sensor and Bragg gratings. In addition to this, the feasibility of using the reinforcing fibre as a light guide was demonstrated. These sensor systems were evaluated under quasi-static, impact and fatigue loading. The test specimens consisted of prepreg-based carbon-fibre-reinforced epoxy and glass-fibre-reinforced epoxy filament-wound tubes. Excellent correlation was obtained between surface-mounted strain gauges and the embedded optical fibre sensors. The feasibility of using these sensor systems for the detection of impact damage and stiffness reduction in the composite due to fatigue damage was successfully demonstrated.