K.S.C. Kuang

Plastic Optical Fibre Sensors for Structural Health Monitoring: A Review of Recent Progress

While a number of literature reviews have been published in recent times on the applications of optical fibre sensors in smart structures research, these have mainly focused on the use of conventional glass-based fibres. The availability of inexpensive, rugged, and large-core plastic-based optical fibres has resulted in growing interest amongst researchers in their use as low-cost sensors in a variety of areas including chemical sensing, biomedicine, and the measurement of a range of physical parameters. The sensing principles used in plastic optical fibres are often similar to those developed in glass-based fibres, but the advantages associated with plastic fibres render them attractive as an alternative to conventional glass fibres, and their ability to detect and measure physical parameters such as strain, stress, load, temperature, displacement, and pressure makes them suitable for structural health monitoring (SHM) applications. Increasingly their applications as sensors in the field of structural engineering are being studied and reported in literature. This article will provide a concise review of the applications of plastic optical fibre sensors for monitoring the integrity of engineering structures in the context of SHM.

Assessment of an extrinsic polymer-based optical fibre sensor for structural health monitoring

Plastic optical fibre sensors offer remarkable ease of handling, and recent research has shown their potential as a low-cost sensor for damage detection and structural health monitoring applications. This paper presents details of a novel extrinsic polymer-based optical fibre sensor and the results of a series of mechanical tests conducted to assess its potential for structural health monitoring. The intensity-based optical fibre sensor proposed in this study relies on the modulation of light intensity as a function of a physical parameter (typically strain) as a means to monitor the response of the host structure to an applied load. Initially, the paper will reveal the design of the sensor and provide an outline of the sensor fabrication procedure followed by a brief description of its basic measurement principle. Two types of sensor design (fluid type and air type) will be evaluated in terms of their strain sensitivity, linearity and signal repeatability. Results from a series of quasi-static tensile tests conducted on an aluminium specimen with four surface-attached optical fibre sensors showed that these sensors offer excellent linear strain response over the range of the applied load. A comparison of the strain response of these sensors highlights the significant improvement in strain sensitivity of the liquid-filled-type sensor over the air-filled-type sensor. The specimens were also loaded repeatedly over a number of cycles and the findings exhibited a high degree of repeatability in all the sensors. Free vibration tests based on a cantilever beam configuration (where the optical fibre sensor was surface bonded) were also conducted to assess the dynamic response of the sensor. The results demonstrate excellent agreement with electrical strain gauge readings. An impulse-type loading test was also performed to assess the ability of the POF sensor to detect the various modes of vibration. The results of the sensor were compared and validated by a collocated piezofilm sensor highlighting the potential of the POF sensor in detecting the various eigen-frequencies of the vibration. Finally, preliminary results of a loading–unloading test of the same sensor design encased within a metal tube will be presented. The results obtained were encouraging offering the possibilities of employing the proposed device as an embedded sensor for damage detection in concrete beams.

The use of plastic optical fibre sensors for monitoring the dynamic response of fibre composite beams

This paper reports the findings of an evaluation of a low-cost, intensity-based plastic optical fibre (POF) sensor for monitoring the dynamic response of both cantilever and simply supported beams subjected to dynamic loading conditions. The sensors were either surface bonded to a plastic beam or embedded in a carbon-fibre composite and subjected to a series of forced and free vibration tests to assess the ability of the sensors to monitor these dynamic events. In the free vibration tests, the natural frequencies of the beams based on the results of the POF sensor were found to compare well with the predicted theoretical values as well as the readings obtained from an electrical strain gauge. Impact tests on simply supported carbon-fibre reinforced beams demonstrated the sensors ability to monitor out-of-plane deflections during the impact event. A laser Doppler velocimeter was also used in conjunction with a piezoelectric load cell as a means to validate the results of the POF sensor.

Fiber Optic Sensing for Monitoring Corrosion-Induced Damage:

This paper reports the feasibility of using embedded Fabry–Pé rot fiber optic sensors to detect and monitor the propagation of cracks and delamination within concrete beams induced by corrosion of the reinforcing bars. In this research, four series of reinforced concrete beams were subjected to varying degrees of corrosion-induced damage by modifying the composition of the concrete mix and subjecting all specimens to the same accelerated corrosion environment. The concept employed in this study involves embedding the Fabry–Pé rot sensor between two reinforcing bars to measure the transverse tensile strains associated with the longitudinal crack along the reinforcing bars (and in severe cases, delamination of the concrete beam) resulting from the radial expansion of the corroding rebars. Excellent correlation was obtained between the Fabry–Pé rot strain data and the amount of steel loss resulting from accelerated corrosion. In addition, the optical sensor strain readings and the reductions in the load-carrying and deflection capacities were also observed to exhibit strong positive correlation highlighting the potential of the optical sensor to monitor the progression of the rebar damage and the loss of structural integrity of the beams resulting from the extensive corrosion. The technique used in this study demonstrates the possibility of detecting corrosion-induced damage in reinforced concrete structures, particularly those where visual inspection is not possible.

An Application of a Plastic Optical Fiber Sensor and Genetic Algorithm for Structural Health Monitoring

This article demonstrates the potential of a new extrinsic plastic optical fiber (POF) sensor for vibration-based structural health monitoring (SHM) applications. A simple genetic algorithm (GA) is used in conjunction with the POF sensor. The construction and principle of operation of the intensity-based POF sensor used in the study is described, highlighting the advantages of the present sensor over previous designs based on glass fiber. A series of mechanical tests are conducted to evaluate the performance of the POF sensor and the results demonstrate the potential of the sensor for SHM. To assess the potential of the POF sensor for vibration-based damage identification, the sensor is attached to a beam to measure the global dynamic response of a carbon fiber composite beam. The POF sensor is surface-bonded to the cantilever beam and collocated with a polyvinyl idine fluoride-based piezofilm sensor for comparison. The time-history responses of the sensors following an impulse-type excitation of the beam are obtained and analyzed using classical Fourier transform techniques to identify the modal frequencies of the beam. Based on the basic principle that changes in structural properties will lead to changes in its vibration signatures, the sensor is used to monitor and acquire the vibration signatures. To simulate the vibration signature shifts, weights are used which are located sequentially at specific distances along the beam. The global dynamic responses of the beam are recorded using an oscilloscope following the excitation of the beam. The results show that the POF sensor is capable of detecting the relevant modal frequencies. Using a GA approach, the mass and location of the weights are identified based on the optical signal acquired through the POF sensor. This article demonstrates the potential of the POF–GA system for damage identification in a simple cantilever beam.

A breathing rate sensor with plastic optical fiber

A breathing rate sensor has been developed using plastic optical fiber and the test results are presented in this letter. The principle of coupling loss was used in designing this sensor to take advantage of the large core size of plastic optic fiber. The sensor was placed near the nostril to determine the rate of breathing as air was exhaled. The results demonstrated the ability to quantify the breathing rate and monitor different breathing patterns up to a resolution of 1 breath/s (1 Hz).

Detection of Impact Damage in Thermoplastic-Based Glass Fiber Composites Using Embedded Optical Fiber Sensors

Step-index multi-mode optical fibers have been embedded in a thermoplastic glass fiber polypropylene (GF/PP). Three types of commercially available optical fibers have been investigated to evaluate their potential for detecting impact-induced damage in thermoplastic-based composite structures. Preliminary findings have confirmed the feasibility of using these inexpensive multi-mode optical fibers as damage sensors in high-performance composite materials. When embedded between the uppermost ply of a cross-ply unidirectional [0/90] s laminate, impact energies as low as 0.8 J have been detected using these optical fibers. The optical fibers have been embedded at different locations within the composite structure to investigate the effect of embedment locations on their sensitivity for detecting impact loading and damage.

In situ process monitoring of a thermoplastic-based fibre composite using optical fibre sensors

This paper reports the use of optical fibres to obtain real-time information during the processing of thermoplastic-based glass fibre-reinforced polypropylene. The technique used to monitor the melting and solidification process in this study is based on monitoring the modulation of the refractive index of the polymer matrix. Before embedding the sensor in the composite laminate, the cladding layer of the optical fibre was removed to expose the optical fibre core thus allowing intimate contact with the host polymer matrix. As the matrix undergoes melting and solidification cycles, the refractive index of the polymer at the sensor–host interface changes which in turn varies the amount of light transmitted through the optical core. This signal intensity modulation was monitored during the composite processing cycle. Differential scanning calorimetry was carried out to provide a reference for evaluating the validity of the optical fibre data.

Hybrid optical fiber sensor system based on fiber Bragg gratings and plastic optical fibers for health monitoring of engineering structures

In this paper, packaged fibre Bragg grating (PFBG) sensors were fabricated by embedding them in 70mm x 10mm x 0.3mm carbon-fibre composites which were then surface-bonded to an aluminium beam and a steel I-beam to investigate their strain monitoring capability. Initially, the response of these packaged sensors under tensile loading was compared to bare FBGs and electrical strain gauges located in the vicinity. The effective calibration constant/ coefficient of the PFBG sensor was also compared with the non-packaged version. These PFBG sensors were then attached to an I-section steel beam to monitor their response under flexural loading conditions. These realistic structures provide a platform to assess the potential and reliability of the PFBG sensors when used in harsh environment. The results obtained in this study gave clear experimental evidence of the difference in performance between the coated and uncoated PFBG fabricated for the study. In another experimental set-up, bare FBG and POF vibration sensors were surface-bonded to the side-surface of a CFRPwrapped reinforced concrete beam which was then subjected to cyclic loading to assess their long-term survivability. Plain plastic optical fibre (POF) sensors were also attached to the side of the 2-meter concrete beam to monitor the progression of cracks developed during the cyclic loading. The results showed excellent long-term survivability by the FBG and POF vibration sensors and provided evidence of the potential of the plain POF sensor to detect and monitor the propagation of the crack developed during the test.

Polymer-based optical fiber sensors for health monitoring of engineering structures

This paper describes the design of an extrinsic optical fibre sensors based on poly(methamethycrylate) for structural health monitoring applications. This polymer-based optical fiber sensor relies on the modulation of light intensity and is capable of monitoring the response of the host structure subjected to either static or dynamic load types. A series of mechanical tests have been conducted to assess the response of the plastic optical fiber (POF) sensor. The readings of the sensors attached to an aluminium bar were found to compare well to electrical strain gauge response. The POF sensors were also attached to rebar concrete beams and exhibited encouraging response under flexural loading. Static and cyclic loading tests were also performed and the sensor was shown to exhibit excellent strain linearity and repeatability. Free vibration tests on a cantilever beam set-up in which the POF sensor was surface-bonded to a composite beam were also conducted. The results obtained highlight the capability of the sensor to accurately monitor the dynamic response of the beam. Impulse-type dynamic response of the sensor was also conducted and the POF sensor demonstrated potential for detecting the various modal frequencies of the host structure. POF sensors were also attached to a series of impacted composite beams with varying degree of damage to assess their potential to detect and quantify the damage in the host structure. The results demonstrated the feasibility of using the sensor for structural health monitoring applications.