Fiber Optic Strain Gauge Research Articles

Contact acoustic nonlinearity and damping properties of completely free vibrating curved and delaminated CFRP plates

This study reports an experimental investigation on the effect of plate curvature and circular inter-ply delamination on the completely free vibration properties of carbon fiber reinforced polymer (CFRP) plates. Impact tests on wire suspended plates were performed and variations in measured natural frequencies and modal damping are reported. Delamination clapping type contact acoustic nonlinearity (CAN) was observed in the presence of a series of higher harmonics in the vibration spectrum. These higher harmonics consisted of five to seven equally distanced peaks grouped in a frequency range of around the double of odd mode M (5,0). Modal damping was highly sensitive to delamination which caused a 33% average increase in damping loss factors. Odd mode M (3,0) was the most affected with an average increase of 51% in damping loss factor, explained by the delamination location relative to the wavelength of that mode shape. Natural frequencies were less sensitive to delamination, on average decreasing 1.0% in flat plates and increasing 3.1% in curved plates. The modal frequencies calculated from embedded fiber optic sensor (FOS) or electrical resistance strain gauge signals agreed with finite element modeling (FEM) to within a 3.1% difference for all modes considered. The damping loss factors deducted from suspended plate tests were consistent with complementary results from dynamic mechanical analysis (DMA). The measured delamination clapping CAN and its related influence on damping can serve to enhance structural health monitoring (SHM) methods.

Experimental evaluation of fatigue in wind turbine blades with wake effects

Large investments in the implementation of wind turbines were made in the last decades, which increased the importance of this source in the national electricity systems. The management of this asset can strongly benefit from the development of cost effective monitoring strategies. These may contribute to the extension of the lifespan of existing wind turbines, to the reduction of insurance costs by reducing the operation risk and to the optimization of the design, leading to an increase in the competitiveness of this energy source. This paper is dedicated to the implementation and validation of tools to assess the fatigue condition of wind turbine blades from strains measurements with a set of fiber optic strain gages installed at the blade roots. It describes the data processing needed to obtain accurate experimental estimations of the bending moments applied to the blade roots and the procedure to obtain the accumulated fatigue damage. The instrumentation of two wind turbine rotors aligned with the most frequent wind direction allowed quantifying the interaction between neighboring wind turbines and its effect on fatigue life consumption. The paper presents unique experimental data simultaneously collected in two 1.8 MW wind turbines of an onshore wind farm during 14 months that permitted a very clear illustration of wake effects on fatigue consumption.

Experimental Investigation on Fatigue Improvement of Orthotropic Steel Bridge Deck Using Steel Fiber Reinforced Concrete

AbstractThe high-performance topping concrete overlay would significantly reduce the fatigue stresses at susceptible details of orthotropic steel deck (OSD) and effectively improve the fatigue performance. Full-scale fatigue tests and numerical simulations were carried out to quantify the effects of steel fiber reinforced concrete (SFRC) topping pavement in reducing the OSD fatigue stresses based on a practical bridge design. Fatigue stress of the OSD critical details was measured from tests and cracking was observed in the SFRC overlay. Both an innovative pre-pump-pulse Brillouin optical time domain analysis (PPP-BOTDA) distributed optical fiber sensor and strain gauges were also used to measure the stress at OSD fatigue-prone details, including the crossbeam cut-out, rib-to-deck weld, rib-to-crossbeam weld and splice of U-rib. The corresponding three-dimensional (3-D) finite element model was then established and verified by the test results, and used to conduct a parametric study to obtain a quantitative expression of fatigue stress reduction factor, in terms of relative elastic modulus and thickness of overlay and steel deck plate. The proposed quantitative model of the fatigue stress reduction effect can guide the cost-effective design of composite bridge deck.

Axial cyclic loading of piles in low-to-medium-density chalk

Comprehensive field investigations into the axial cyclic loading behaviour of open steel pipe piles driven and aged in low-to-medium-density chalk identify the conditions under which behaviour is stable, unstable or metastable. Post-cycling monotonic tests confirmed that stable cycling enhanced pile capacity marginally, while unstable cases suffered potentially large losses of shaft capacity. Metastable conditions led to intermediate outcomes. The patterns by which axial deflections grew under cyclic loading varied systematically with the normalised loading parameters and could be captured by simple fitting expressions. Cyclic stiffnesses also varied with loading conditions, with the highest operational shear stiffnesses falling far below the in situ seismic test values. The monotonic and cyclic axial responses of the test piles were controlled by the behaviour of, and conditions within, the reconsolidated, de-structured, chalk putty annuli formed around pile shafts during driving. Fibre-optic strain gauges identified progressive failure from the pile tip upwards. Large factors of safety were required for piles to survive repetitive loading under high-level, two-way conditions involving low mean loads, while low-amplitude one-way cycling had little impact. A simple ‘global’ prediction procedure employing interface shear and cyclic triaxial tests is shown to provide broadly representative predictions for field behaviour.

Energy losses during driving due to tapered section of monopile

Efficient pile driving and accurate pile driving predictions are essential for the continued use of monopiles as an optimised foundation for the offshore wind energy sector, especially as the offshore wind industry continuously pushes limits of wind turbine and foundation size. For monopile foundation a crucial part of the structure from an installation perspective is the transition tapered section of the monopile between the top and bottom parts of the monopile and its interaction with the water column during pile driving. To evaluate the impact of this interaction on the driving energy, two monopile foundations installed at an offshore wind farm (OWF) site in the North Sea were instrumented and their installation was monitored in 2019. For the monitoring, optical fibre strain gauges and resistive accelerometers were placed at different levels above, below, and on to the tapered section of the monopiles. As an innovative part, all sensors were glued on the monopile shaft surface instead of the traditional welded/screwed approach. The driving energy loss between different sensor levels was calculated post installation. The measurement data were used to benchmark a series of finite element analyses (FEA) that were performed to identify and quantify the different factors affecting the energy loss.The study evaluates the effect of various parameters, such as wave reflections due impedance changes caused by diameter increase and wall thickness variation, bending of tapered section, and vertical acceleration of water due to interaction with the tapered section of the monopile. Measurements and the finite element analysis both reveal that a large part of the driving energy applied by the hammer is dissipated due to the above-mentioned factors with the water-pile interaction playing the most important role, resulting in a significant impact on the monopile installation.

Autonomous material composite morphing wing

Aeronautics research has continually sought to achieve the adaptability and morphing performance of avian wings, but in practice, wings of all scales continue to use the same hinged control-surface embodiment. Recent research into compliant and bio-inspired mechanisms for morphing wings and control surfaces has indicated promising results, though often these are mechanically complex, or limited in the number of degrees-of-freedom (DOF) they can control. Seeking to improve on these limitations, we apply a new paradigm denoted Autonomous Material Composites to the design of avian-scale morphing wings. With this methodology, we reduce the need for complex actuation and mechanisms, and allow for three-dimensional placement of stretchable fiber optic strain gauges (Optical Lace) throughout the metamaterial structure. This structure centers around elastomeric conformal lattices, and by applying functionally-graded warping and thickening to this lattice, we allow for local tailoring of the compliance properties to fit the desired morphing. As a result, the wing achieves high-deformation morphing in three DOF: twist, camber, and extension/compression, with these morphed shapes effectively modifying the aerodynamic performance of the wing, as demonstrated in low-Reynolds wind tunnel testing. Our sensors also successfully demonstrate differentiable trends across all degrees of morphing, enabling the future state estimation and control of this wing.

Measurement of Parachute Canopy Textile Deformation Using Mechanically Invisible Stretchable Lightguides

AbstractConventional strain gauges are not designed for accurate measurement over the large range of deformations possible in compliant textiles. The thin, lightweight, and flexible nature of textiles also makes it challenging to attach strain gauges in a way that does not affect the mechanical properties. In this manuscript, soft, highly extensible fibers that propagate light (i.e., stretchable lightguides) are stitched as a strain gauge to map the deformation of a nylon parachute textile under tension. When under load, these fiber optic strain gauges propagate less light, and this strain‐induced light modulation is used to accurately (absolute error≈2.93%; Std. Dev.: 3.02%) measure strain in the <30% range before these textiles fail. This system has directionality; strain in parallel to the sensor results in little light attenuation while perpendicular loading shows high sensitivity (Gauge factor⊥≈24.8 and Gauge factor||≈0.05 at the first 1% strain). Structural and optical simulations are coupled to demonstrate that load transfer on the fiber optic by the stitchwork is the dominating cause of signal modulation. To further validate the hypotheses, digital image correlation was used under dynamic loading conditions to show that these sensors do not significantly affect the mechanical properties.

Compliant Mechanism-Based Sensor for Large Strain Measurements Employing Fiber Optics.

We propose a sensor design for measurement of large strains where direct application of a fiber optic strain gauge is impossible due to the stiffness mismatch between the optical fiber and the structure under test. The sensor design is based on a rhombus type compliant mechanism, which functions to attenuate input strain and transfer it to the ends of the sensing beam with the mounted optical strain gauge. We developed an analytical model of the sensor, which allows us to relate actuation forces, input displacement/strain, and output strain. The analytical model was verified with the finite element analysis and validated against an experimental prototype. The prototype sensor was able to handle input strains exceeding ±2.5 × 105 µε. Potential application areas of the proposed sensor include compliant elastomeric structures, wearables, and soft robotics.

Design a Building Information Modeling (BIM) project concept in combination with foundation monitoring on the example of the Abu Dhabi Plaza construction project

With the advent of new technology in the monitoring of buildings and highrise structures it has become possible to monitor damage and defects in existing structures and infrastructures. The need for timely detection of critical points of building structure defects through the use of sensors, as well as the lack of description tools for understanding, visualizing and documenting sensor output data increasingly encourages designers, builders and facility managers to use the capabilities of such a powerful system as the Building Information Modeling System (BIM). The primary goal of any construction project is safety for people, which can only be ensured by proper monitoring of building health, where the presentation and management of the received information about changes in the system and structure can be ensured by using BIM. The purpose of this paper is to describe the application of a distributed fiber optic strain gauge sensor system, and the possibility of integrating the data obtained from the sensors into the BIM system.

New data analysis methods for instrumented medium-scale monopile field tests

The PISA Joint Industry Research Project was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England, and in dense, normally consolidated marine sand at Dunkirk, northern France. This paper describes the experimental set-up for pile testing, with unique features of load-application mechanisms and built-in fibre optic strain gauges. New procedures are described for the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.

Comparison of Experimental Investigation of Deflection of the Sandwich Composite Beam by Optic-fibre Gauge with Theoretical Models

The aim of the article is to compare the experimental investigation of deflection of the sandwich composite beam with theoretical models. The experimental test specimens were composed of three layers. Skins were made using epoxy-resin-impregnated glass laminates with plain weave. The light weight foam Divinycell H100 was used as sandwich core. The three-point bending test was carried out. Fibre-optic strain gauges, SOFO SMARTape Compact deformation sensors, were used for determining the deflection of the sandwich composite structure. Experimentally obtained data were used for comparison with theoretical models – sandwich theory with the transverse shear, sandwich theory without the transverse shear, laminate theory with the transverse shear and laminate theory without the transverse shear.

Flight Test Applications of an Improved Operational Load Monitoring Device

This paper introduces a load monitoring device and its flight test applications for military aircraft. The device can digitize and record signals from optical fiber sensors and strain gauges. Some key flight parameters can be simultaneously recorded in the device, which is connected to an aircraft data bus cable. The device was installed on a low-speed aircraft and multiple sensors including fiber optic sensors and strain gauges were surface bonded on certain major structures. A total of 40 flight tests were conducted over 2 years. The recorded signals and parameters were successfully analyzed and reviewed to measure strains and vertical landing speeds and to estimate structural loads for over 200 flight hours. Structural load spectra were also obtained using the developed load monitoring device and compared with military specification and design spectra.

Fiber-optic sensors for monitoring the stress-strain state of construction objects

For the timely detection of significant changes in the stress-strain state in the bearing elements of various building structures, which may lead to a deterioration of the technical condition of the object or transfer it to the emergency state, the use of fiber-optic sensors is proposed. The main advantageous characteristics of fiber-optic sensors, describing the promise of their application for practical applications, are disclosed. As a parameter recorded by the measuring transducer, the fiber-optic sensors use the intensity of the light wave. The mechanism of changing the optical intensity can be due to reflection, absorption, refraction of light rays passing through the measurement zone. The authors have developed a fiber optic attenuator type deformation sensor capable of controlling mechanical deformations of structural elements caused by disturbed internal geometric characteristics, destruction of structural integrity and negative influence of external weather conditions, seismic factors, temperature drops during operation. When developing the design of a fiber-optic sensor, the fundamentals of circuit design of measuring devices, methods of geometrical and fiber optics are used. As a basic measuring transducer, a fiber-optic microdisplacement transducer with a limiting attenuator is used, in which the intensity of the optical signal is modulated by changing the position of the attenuator placed in its path or increasing the optical power. A design-structural diagram of a fiber-optic micro-displacement measuring transducer and a block diagram and the design of a new sensor are presented. The principle of operation of an attenuator-type fiber-optic strain gauge is described.

Numerical Simulation of the Piezo-Optical Strain Sensor Gauge Factor

The dependence of the piezo-optical strain sensor gauge factor on the geometric parameters of the photoelastic element is obtained by accurate numerical simulations. It is shown that the piezo-optical gauge sensitivity to the applied force is more dependent on the photoelastic element shape than the sensitivity to deformation. A comparative analysis of the gauge factors and other parameters of strain-resistive, piezoelectric, fiber-optic, and piezo-optical strain gauges was carried out. Strain-resistive and fiber-optic sensor gauge factors are three orders of magnitude lower than those of piezo-optical sensors. Correctly calculated piezoelectric sensor gauge factors are also two-three orders of magnitude inferior to those of piezo-optical sensors. A comparison of the basic properties and parameters of modern commerical strain gauges based on different physical principles is presented.

Improving the design of piles driven in chalk through the ALPACA research project

Chalk is present under large areas of NW Europe as a low-density, porous, weak carbonate rock. Large numbers of offshore wind turbines, bridges and port facilities rely on piles driven in chalk. Current European practice assumes ultimate shaft resistances that appear low in comparison with the Chalk’s unconfined compression strength and CPT cone resistance ranges and can impact very significantly on project economics. Little guidance is available on pile driveability, set-up or lateral resistance in chalk, or on how piles driven in chalk can sustain axial or lateral cyclic loading. This paper describes the ALPACA (Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing) project funded by EPSRC and Industry that is developing new design guidance through comprehensive field testing at a well-characterised low-to-medium density test site, supported by analysis of other tests. Field experiments on 36 driven piles, sixteen of which employ high resolution fibre-optic strain gauges, is supported by advanced laboratory andin situtesting, as well as theoretical analysis. The field work commenced in October 2017 and was largely complete in May 2019.

Testing and Analysis of Dynamic Response of a High-Voltage Substation Structure to Short-Circuit Current Forces

In substations, buswork, insulators, and supporting structures are subjected to electromagnetic forces caused by short circuits. The current industry design standard of practice is to apply the maximum instantaneous short-circuit force to the structure and perform simple static structural analysis to calculate supporting structure member. However, it is well known that this may be overly conservative since the applied forces and the response of the structure are dynamic in nature. To quantify the dynamic effects, full-scale short-circuit testing was performed on a typical 138 kV disconnect switch stand. The switch stand was instrumented with fiber optic strain gages and subjected to a variety of fault conditions. Extensive dynamic finite element analyses were performed on the subject structure, which were validated with the test data. The results confirm the reduced dynamic response of the structure. With further analyses, it is anticipated that a generalized quasi-static design methodology that considers dynamic response can be developed to replace the present short-circuit design approach.

Polymer optical fiber strain gauge for human-robot interaction forces assessment on an active knee orthosis

Abstract This paper presents the development of a polymer optical fiber (POF) strain gauge based on the light coupling principle, which the power attenuation is created by the misalignment between two POFs. The misalignment, in this case, is proportional to the strain on the structure that the fibers are attached. This principle has the advantages of low cost, ease of implementation, temperature insensitiveness, electromagnetic fields immunity and simplicity on the sensor interrogation and signal processing. Such advantages make the proposed solution an interesting alternative to the electronic strain gauges. For this reason, an analytical model for the POF strain gauge is proposed and validated. Furthermore, the proposed POF sensor is applied on an active orthosis for knee rehabilitation exercises through flexion/extension cycles. The controller of the orthosis provides 10 different levels of robotic assistance on the flexion/extension movement. The POF strain gauge is tested at each one of these levels. Results show good correlation between the optical and electronic strain gauges with root mean squared deviation (RMSD) of 1.87 Nm when all cycles are analyzed, which represents a deviation of less than 8%. For the application, the proposed sensor presented higher stability than the electronic one, which can provide advantages on the rehabilitation exercises and on the inner controller of the device.

Development of a Shipboard Remote Control and Telemetry Experimental System for Large-Scale Model's Motions and Loads Measurement in Realistic Sea Waves.

Wave-induced motion and load responses are important criteria for ship performance evaluation. Physical experiments have long been an indispensable tool in the predictions of ship’s navigation state, speed, motions, accelerations, sectional loads and wave impact pressure. Currently, majority of the experiments are conducted in laboratory tank environment, where the wave environments are different from the realistic sea waves. In this paper, a laboratory tank testing system for ship motions and loads measurement is reviewed and reported first. Then, a novel large-scale model measurement technique is developed based on the laboratory testing foundations to obtain accurate motion and load responses of ships in realistic sea conditions. For this purpose, a suite of advanced remote control and telemetry experimental system was developed in-house to allow for the implementation of large-scale model seakeeping measurement at sea. The experimental system includes a series of technique sensors, e.g., the Global Position System/Inertial Navigation System (GPS/INS) module, course top, optical fiber sensors, strain gauges, pressure sensors and accelerometers. The developed measurement system was tested by field experiments in coastal seas, which indicates that the proposed large-scale model testing scheme is capable and feasible. Meaningful data including ocean environment parameters, ship navigation state, motions and loads were obtained through the sea trial campaign.

Laboratory investigations of the thermal strain of frozen soils, using fiber-optic strain gauges based on Bragg gratings

Measurements of the thermal deformations of frozen soil samples were performed in the cold laboratory in temperature range from 0 °C to −12 °C. Fiber Bragg Gratings strain and temperature sensors were used to measure the deformation and temperature inside the samples. A number of tests with the samples prepared from Kaolin and Cambrian clay saturated with fresh water, and prepared from fine and silt sand saturated with fresh or saline water, are performed. Thermal deformations of the samples are analyzed depending on the cyclic changes of their temperature.

Development of a sensitive element for fiber-optic strain gauge based on Bragg gratings

Development of a sensitive element for fiber-optic strain gauge based on Bragg gratings