Optical fiber strain sensor with double S-tapers

Abstract. With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion.

In the present study, real-time measurement of internal and residual strains in GFRP laminates with an optical fiber strain sensor was performed in an autoclave molding. An Extrinsic Fabry-Perot Interferometric (EFPI) type optical fiber strain sensor was embedded into prepreg sheets before the molding. The ion viscosity and the temperature of the epoxy resin matrix were also measured simultaneously with the internal strain measurement. The results of the experiments showed that the internal and residual strain of GFRP laminates during the molding could be monitored by the optical fiber sensor embedded in the parallel direction to the reinforcing fibers. It was also found that the circumstances around the embedded optical fiber strain sensor should be taken into account for the interpretation of the measured internal and residual strain.

Wearable sensors have been developed for various healthcare applications with human motion detection. For strain sensors to be adopted in smart wearable devices for human joint motion detection, it is necessary that they have excellent mechanical properties such as stretchability, flexibility, softness, and they should be non-toxic to the human body. In this paper, we report on a possible candidate for such a sensor: the polymer fiber composited with polydimethylsiloxane (PDMS) and Zinc Oxide (ZnO) nanoparticles. This polymer fiber exhibits softness, flexibility, stretchability, and reproducibility. Moreover, it has a tensile property of over 100% and excellent resilience. The strain evaluation of the ZnO-PDMS polymer fiber with silica fibers on the ends was carried out with a light emitting diode (LED) and photodetector. The strain sensor with polymer fiber has good properties of strain measurement; stable signal recovery, as well as rapid, immediate, and repeatable response in real-time. The polymer strain sensor was evaluated on the human finger and wrist joints for detection of the bending motion. Moreover, because this polymer strain sensor is waterproof, it can be used for measuring finger bending and releasing even in wet conditions. The results demonstrate that the ZnO-PDMS optical fiber strain sensor can be employed in wearable devices for healthcare applications such as sports and rehabilitation.

Intensity-modulated optical fibre strain sensor for continuous measurements of below-the-surface food deformation during drying

High sensitivity D-shaped optical fiber strain sensor based on surface plasmon resonance

Introduction Need for Integrated Structural Monitoring Introduction to Lightwaves Light Sources and Detectors Fiber Optic Technology Fiber Optic Structural Sensors and Their Merits Fiber Optic Strain and Temperature Sensitivity Sensor Installation and Material Integration Issues Short Gauge Sensor and Applications Long Gauge-Length Fiber Optic Sensing Multiplexed Fiber Optic Structural Sensing Distributed Strain and Temperature Sensing Future Prospects and Summary References Index

In this study, we developed a health monitoring system using a fiber optic distributed strain sensor for International America's Cup Class (IACC) yachts. Most structural components of an IACC yacht consist of an aluminum honeycomb core sandwiched between carbon fiber reinforced plastic (CFRP) laminates. In such structures, delamination, skin/core debonding and debonding between adhered members will be result in serious fracture of the structure. We equipped two IACC yachts with fiber optic strain sensors designed to measured the distributed strain using a Brillouin optical time domain reflectometer (BOTDR) and to detect any deterioration or damage to the yacht's structures caused by such failures. And based on laboratory test results, we proposed a structural health monitoring technique for IACC yachts that involves analyzing their strain distribution. Some important information about structural conditions of the IACC yachts could be obtained from this system through the periodical strain measurements in the field.

Wet deck slamming experiments with a FRP sandwich panel using a network of 16 fibre optic Bragg grating strain sensors

Wood drying, such as boxed-heart square timber, often involves surface checks, which spoils the appearance of wood products and reduces their market value. Pretreatments are performed to avoid the surface checks, but detailed information about surface strain during timber drying remains unclear. In this study, surface strain detections were performed using two types of strain sensors (strain gauge and optical fiber strain sensor) to understand the surface dimensional changes during a series of pretreatments, consisting of steaming and high temperature and low humidity treatment. Simultaneous strain measurements based on the optical fiber sensor grasped the surface strain distributions in each lateral surface during timber drying; contraction behaviors were observed in the middle part of most surfaces in the early steaming stage, while one surface showed expansion. A remarkable expansion was detected in one surface during the high temperature and low humidity treatment, although most of the other surfaces showed gradual contraction behaviors. It was also discovered that the above detected behaviors were gradually reduced with the progress of each pretreatment.

In this study, the method to estimate the bridge deflection is developed using the fiber optic Bragg-grating strain sensors. Most of the evaluation of structural integrity, it is very important to measure the geometric profile, which is the major factor standing for the global behavior of civil structures, especially bridges. In the past, for the lack of the appropriate method to measure the deflection curve of bridge on site, the measurement was restricted to just a few of discrete points along the span length, which were also limited to the locations installed with the displacement transducer in advance. Hereby, with an application of classical beam theory, a formula is established estimating the continuously deflected profile from the measurement of strains at several points. In addition, strains could be measured by the use of fiber optic strain sensors, which are electro-magnetic noisy-free, and into which several points of sensing could be installed. With the strain data acquired, the proper strain curve is fitted, and finally, deflection curve would be estimated. The experimental test was carried out to verify the developed algorithm.

Optical fiber sensors have been extensively adapted as structural health monitoring devices. Due to the existence of the adhesive layer, a portion of the strain is absorbed by the adhesive. As a result, the structural strain sensed by the optical fiber is underestimated and required to be corrected. An analytical solution is presented through which it is possible to establish the relationship between the strains in the host structure and the surface bonded optical fiber sensor. Experimental measurements based on the Mach–Zehnder interferometric technique were performed to validate the theoretical prediction and reveal the differential strains between the optical fiber strain sensor and test specimen. Parametric studies show that the percentage of the strain in the test specimen actually transferred to the optical fiber is dependent on the bonding length of the optical fiber and the adhesive. The strain transfer is increasing from 56% to 82% as the bonding length increases from 5 cm to 12 cm with the epoxy adhesive. The general trend of the strain transfer obtained from both experimental tests and theoretical predictions shows that the longer the bonding length and the stiffer the adhesive, the more strain is transferred to the optical fiber.

The presented research addresses the interfacial behavior of polyimide-coated optical fiber sensors embedded in neat resin matrices and their use in gaining information for strain sensing applications. The interfacial shear strengths of glass fiber/coating/resin systems were examined via a single fiber pull-out test. The optical fiber strain sensor was developed in-house employing polyimide-coated, polarization-maintaining optical fiber and was characterized for sensitivity to strain and other strain related phenomena. Optical fiber strain experiments are compared to an established data base of single fiber pull-out test results. Initial experimental comparisons are presented.

Polymer modified concrete repair materials (PMCRMs) offer significant protection to concrete reinforcement, if the PMCRM is compatible with the concrete substrate, and the instructions given by the manufacturer are followed. However, if the PMCRM is not compatible with the concrete substrate, or the manufacturer’s instructions are not followed, volume changes within concrete repair materials (CRMs) caused by shrinkage may result in the repair system failing by means of delamination or cracking. This paper demonstrates the use of an optical fibre strain and temperature sensor to provide valuable information regarding the movement and internal temperature changes which occur within concrete repairs, which may help extend the service life and provide a better understanding of PMCRMs. A strain measuring (fibre Bragg grating) FBG sensor was implemented to measure strain within a polymer modified concrete repair patch (PMCRP). The optical fibre monitoring system consisted of an internal temperature measuring FBG sensor and a strain measuring FBG sensor, which monitored the temperature and movement changes within a PMCRP. The device allowed for easy installation of the strain measuring FBG, and allowed for the optical fibre sensor to be reused, thus it was possible to measure both the internal temperature and the movement of the PMCRP soon after placement of the repair material.

A review of our recent work on ultrahigh resolution optical fiber sensors in the quasi-static region is presented, and their applications in crustal deformation measurement are introduced. Geophysical research such as studies on earthquake and volcano requires monitoring the earth’s crustal deformation continuously with a strain resolution on the order of nano-strains (nɛ) in static to low frequency region. Optical fiber sensors are very attractive due to their unique advantages such as low cost, small size, and easy deployment. However, the resolution of conventional optical fiber strain sensors is far from satisfactory in the quasi-static domain. In this paper, several types of recently developed fiber-optic sensors with ultrahigh resolution in the quasi-static domain are introduced, including a fiber Bragg grating (FBG) sensor interrogated with a narrow linewidth tunable laser, an FBG based fiber Fabry-Perot interferometer (FFPI) sensor by using a phase modulation technique, and an FFPI sensor with a sideband interrogation technique. Quantificational analyses and field experimental results demonstrated that the FBG sensor can provide nano-order strain resolution. The sub-nano strain resolution was also achieved by the FFPI sensors in laboratory. Above achievements provide the basis to develop powerful fiber-optic tools for geophysical research on crustal deformation monitoring.

This paper presents the design, development, and testing of an active textile-based wearable compression device that is capable of delivering a controlled pulsatile compression. The device uses low spring index nickel titanium (NiTi) coil actuators to produce an applied dynamic pressure of up to 5.5 kPa. The selected NiTi coil actuators produce pressure when thermally stimulated with Joule heating via an applied current (0.3 A), and generate recoverable strains up to 75% in extension. An optical fiber strain sensor was developed to monitor the textile strain and enable the indirect estimation of the applied pressure. A new approach using a passive NiBR spring (in parallel with the NiTi actuators) was also developed to assist the NiTi coils in recovering the detwinned martensite form after cooling. The pressure distribution around a rigid cylindrical shape was also evaluated, showing higher applied pressures (5.5 kPa) where the NiTi coil actuators were located. The strain sensor exhibits high accuracy compared to a reference commercial sensor (as indicated by the high correlation indexes of up to 0.97 between compression cycle measurements with both solutions).