Distributed Strain Profile Research Articles

Innovations in Micropile Testing Using Distributed Optical Fibre Sensors

Micropiles are primarily used in situations where traditional foundation methods, such as large-diameter bored piles or driven piles, are not feasible due to various constraints, including limited access, low headroom, or the need to work in environmentally sensitive areas. They are often employed in urban areas with restricted space or for underpinning existing structures. Two case studies of instrumented micropiles are presented in this paper, with the aim to showcase innovations in pile testing using Distributed Optical Fibre Sensing (DOFS). In the first project, micropiles were installed as part of an underpinning work where several existing piles were removed to allow safe passage of Tunnel Boring Machine (TBM) underneath the building. An experiment pile was instrumented with conventional and DOFS sensors to measure pile responses due to tunnelling in proximity. In the second project, the micropile was tested to determine its load carrying capacity for buildings constructed in hilly area. In both cases, API pipes were applied and socketed into bedrocks. Optical cables were attached on four sides of the steel pipes and subsequently grouted. During the load tests, DOFS data were able to display precisely the shaft distribution of micropiles when subjected to vertical loads as well as bending behaviour due to the close proximity tunnelling. Interpretations of distributed strain profiles from DOFS have contributed to a better understanding of API-reinforced micropile performance under complex geological and loading conditions.

Novel Interrogation Method for Fully Distributed LCFBG Sensor Based on Microwave Photonic Filtering

A novel interrogation method for linearly chirped fiber Bragg grating (LCFBG) based fully distributed strain sensor which can simultaneously determine strain value and recognize position is proposed and experimentally demonstrated. Compared with the traditional distributed grating interrogation systems by analyzing the reflection spectrum of the LCFBG, the system is mainly based on frequency domain measurement from a microwave photonic filter (MPF) configuration, which has the features of fast responding and high resolution. By using this structure, when distributed strain is applied to the LCFBG, the reflective point on the LCFBG for a certain wavelength as well as the frequency response of the MPF will change. By detecting the resonance frequency shift of the MPF, the strain value can be achieved. Combined with the one-to-one correspondence between the wavelength and the spatial position of the LCFBG, the distributed strain profile along the LCFBG can be reconstructed. In a proof of concept experiment, interrogation of fully distributed grating sensors with ununiform strain distributions is experimentally demonstrated. A spatial resolution of ~0.2 mm over a gauge length of 27mm and a strain resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ &lt; 1\text{u}\varepsilon $ </tex-math></inline-formula> have been achieved.

Environmental Durability of an Optical Fiber Cable Intended for Distributed Strain Measurements in Concrete Structures.

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable.

Experimental and Numerical Investigation on the Strain Response of Distributed Optical Fiber Sensors Bonded to Concrete: Influence of the Adhesive Stiffness on Crack Monitoring Performance.

The present study investigated the strain response of a distributed optical fiber sensor (DOFS) sealed in a groove at the surface of a concrete structure using a polymer adhesive and aimed to identify optimal conditions for crack monitoring. A finite element model (FEM) was first proposed to describe the strain transfer process between the host structure and the DOFS core, highlighting the influence of the adhesive stiffness. In a second part, mechanical tests were conducted on concrete specimens instrumented with DOFS bonded/sealed using several adhesives exhibiting a broad stiffness range. Distributed strain profiles were then collected with an interrogation unit based on Rayleigh backscattering. These experiments showed that strain measurements provided by DOFS were consistent with those from conventional sensors and confirmed that bonding DOFS to the concrete structure using soft adhesives allowed to mitigate the amplitude of local strain peaks induced by crack openings, which may prevent the sensor from early breakage. Finally, the FEM was generalized to describe the strain response of bonded DOFS in the presence of crack and an analytical expression relating DOFS peak strain to the crack opening was proposed, which is valid in the domain of elastic behavior of materials and interfaces.

Reconstruction of Distributed Strain Profile Using a Weighted Spectrum Decomposition Algorithm for Brillouin Scattering Based Fiber Optic Sensor

Brillouin scattering based fiber optic strain sensor is used to measure distributed strain profile of a structure for structural health monitoring. A distributed strain profile is derived from the Brillouin spectrum peaks but error is induced when a local strain change occurs in a distance scale smaller than the spatial resolution. In this study, a new algorithm is proposed to reconstruct a Brillouin spectrum by eliminating low qualified data from the raw spectrum data. This is achieved by evaluating the strain gradient profile and statistically decomposing the power, width, and peak frequency of the raw spectrum data. This algorithm is versatile because it can be applied to various strain shapes and magnitudes. The reliability of the algorithm is verified experimentally by showing that the error of the strain profile obtained using the new method becomes 25% of that using the conventional peak detection method.

Distributed fiber optic sensing along driven ductile piles: Design, sensor installation and monitoring benefits

Efficient and economic foundations are essential to ensure the long-term integrity of structures. Driven ductile piles offer a safe and quick solution for foundations, which can be individually customized to changing soil conditions. Geotechnical load tests on a small subset of piles can be performed at large construction sites to examine the bearing capacity for optimization purposes. Arising deformations during these statical tests are usually measured using electrical sensors at the top, which, however, do not deliver information about the stress distribution along the pile. This paper presents a fiber optic monitoring approach, which provides distributed strain profiles with a spatial resolution of up to 10 mm along driven ductile piles. The high measurement resolution of about 1~{mu}m/m enables the detection of local effects in the load transfer from the pile to the surrounding grout and soil. The critical sensor installation on-site as well as results of various field applications with pile lengths of up to 25 m are presented. Verification measurements at the pile’s head and internal measurements of strain gauges prove the suitability of the developed monitoring approach and demonstrate the high potential of distributed fiber optic sensing for applications in soil mechanics.

Mode I fracture toughness of hybrid co-cured Al-CFRP and NiTi-CFRP interfaces: An experimental and computational study

In the spirit of advancing joining concepts as well as adoption of hybrid composites, the interface of a metallic foil and carbon fabric reinforced polymer matrix composite was explored experimentally and computationally. An out-of-autoclave manufacturing method, double-infusion VARTM, was successfully employed to create a hybrid composite laminate that was composed of a layer of metal foil (Al or NiTi) placed at the center of sixteen layer plain weave T300 carbon fabric with epoxy Epikote-Epikure 04908 matrix. The fracture behavior was obtained via the Double Cantilever Beam (DCB) tests at room and elevated temperatures and were monitored on-line using Rayleigh backscattering fiber optics technique. Distributed strain profiles were measured on both at the top and bottom surfaces of the specimen to validate/assist computational models. This new technique to measure distributed strains in-situ during testing has introduced a new dimension to the visualization of strain energy release upon crack propagation.The virtual crack closure method was employed to simulate crack propagation at the hybrid interface. The experimental results revealed the dominant mode of failure as cohesive. Load-displacement and strain profiles measured from the experiments were in good agreement with the numerical results. The foremost research highlight is the novel integration of computational simulation with fiber optics Distributed Strain System (DSS) in DCB experiments to elucidate crack growth at a hybrid interface.

Distributed fiber-optic strain sensing with millimeter spatial resolution for the structural health monitoring of multiaxial loaded GFRP tube specimens

Due to their high strength-to-weight ratio and excellent fatigue resistance, glass fiber reinforced polymers (GFRP) are used as a construction material in a variety of applications including composite high-pressure gas storage vessels. Thus, an early damage detection of the composite material is of great importance. Material degradation can be determined via measuring the distributed strain profile of the GFRP structures. In this article, swept wavelength interferometry based distributed strain sensing (DSS) was applied for structural health monitoring of internal pressure loaded GFRP tube specimens. Measured strain profiles were compared to theoretical calculation considering Classical Lamination Theory. Reliable strain measurements with millimeter resolution were executed even at elongations of up to 3% in the radial direction caused by high internal pressure load. Material fatigue was localized by damaged-induced strain changes during operation, and detected already at 40% of burst pressure.

Stringer debonding edge detection employing fiber optics by combined distributed strain profile and wave scattering approaches for non-model based SHM

The paper presents a comparison of two novel damage detection approaches for interrogating stiffened composites. A guided wave assessed methodology based on piezoelectric patches is integrated with a distributed fiber optic sensor providing a cost-effective and time-saving monitoring system. Being the two techniques characterized by different sample ranges, they are prone to be integrated without any interference or cross talk effect and can be applied to provide a redundant inspection system.The structural health monitoring by distributed fiber optic is a non-model based damage detection method. The relative aspect ratio between shape profile at different time stamps or at different locations along the fiber segments are taken into consideration and the first order derivative is applied, both in time and spatial domain, correlating sensors values characterized by high gradient events. The occurrences of a such dispersed pattern can be associated to a structural non-linearity induced by damage. In order to make this approach effective the PZT guided wave scattering methodology is used as reference. Both systems are tested to the same specific target, nominally a composite stiffened plate undergoing to impact. Good results are reported when comparing the resolutions of the two methodologies, indeed verified by ultrasonic nondestructive investigation analysis.

Characterization of Distributed Microfabricated Strain Gauges on Stretchable Sensor Networks for Structural Applications.

Smart structures mimic biological systems by using thousands of sensors serving as a nervous system analog. One approach to give structures this sensing ability is to develop a multifunctional sensor network. Previous work has demonstrated stretchable sensor networks consisting of temperature sensors and impact detectors for monitoring external environments and interacting with other objects. The objective of this work is to develop distributed, robust and reliable strain gauges for obtaining the strain distribution of a designated region on the target structure. Here, we report a stretchable network that has 27 rosette strain gauges, 6 resistive temperature devices and 8 piezoelectric transducers symmetrically distributed over an area of 150 × 150 mm to map and quantify multiple physical stimuli with a spatial resolution of 2.5 × 2.5 mm. We performed computational modeling of the network stretching process to improve measurement accuracy and conducted experimental characterizations of the microfabricated strain gauges to verify their gauge factor and temperature coefficient. Collectively, the results represent a robust and reliable sensing system that is able to generate a distributed strain profile of a common structure. The reported strain gauge network may find a wide range of applications in morphing wings, smart buildings, autonomous cars and intelligent robots.

Circular Patch Sensor Based on Distributed Fiber Optic Technology for Tensile and Bending Loads Identification

The design, manufacturing, and preliminary testing of a smart patch sensor named MonitoRing are herein presented. The sensor is conceived to identify amplitude and direction of structural loads by distributed strain profile detection along its circular geometry. The sensor is manufactured by using flexible glass/epoxy laminates hosting a single standard telecom fiber optic. The fiber optic is embedded according to three loops, different by radius and quote. The sensor is then externally bonded on a structural element and able to follow the deformations under tensile and bending loading condition. The optical Rayleigh backscattering technology provides an interrogation of strain with high spatial resolution all along the fiber path. The load and direction identification is hence, provided by comparing amplitude, phase and sign of deformation spectrum of each loop. Preliminary numerical and experimental result, are reported and analyzed for simple test cases.

A fully distributed fibre optic sensor for relative humidity measurements

In this paper we describe the characteristics of a 64-m-long distributed relative humidity sensor with cm spatial resolution. The sensing element is an optical fibre with a hygroscopic coating that undergoes a reversible expansion on water uptake, thereby transferring a strain on the fibre. The resulting distributed strain profile was interrogated using Rayleigh backscatter based optical frequency domain reflectometry. Investigations of the distributed sensor under controlled relative humidity and temperature were carried out. The distributed sensor is strongly humidity sensitive when polyimide coated fibre is used, and only weakly sensitive when acrylate coating is used. When using polyimide coated fibre the distributed sensor shows a near linear response across the entire range of relative humidity tested covering 15–92%, with a sensitivity ∼1.3μe/%RH and showing low hysteresis. The system is able to resolve relative humidity changes ∼0.1%. The response time of the sensor was found to be strongly dependent on temperature in the range tested, covering 30–50°C. Using reference temperature and humidity measurements, it was demonstrated that the distributed sensor accurately reported the combined effects of temperature and humidity variations over a cm spatial scale.

CHARACTERIZING ANOMALIES IN DISTRIBUTED STRAIN MEASUREMENTS OF CAST-IN-SITU BORED PILES

This paper describes the method of identifying typical defects of bored cast-in-situ piles when instrumenting using Distributed Optical Fiber Strain Sensing (DOFSS). The DOFSS technology is based on Brillouin Optical Time Domain Analyses (BOTDA), which has the advantage of recording continuous strain profile as opposed to the conventional discrete based sensors such as Vibrating Wire strain gauges. In pile instrumentation particularly, obtaining distributed strain profile is important when analysing the load-transfer and shaft friction of a pile, as well as detecting any anomalies in the strain regime. Features such as defective pile shaft necking, discontinuity of concrete, intrusion of foreign matter and improper toe formation due to contamination of concrete at base with soil particles, among others, may cause the pile to fail. In this study, a new technique of detecting such defects is proposed using DOFSS technology which can potentially supplement the existing non-destructive test (NDT) methods. Discussion on the performance of instrumented piles by means of maintained load test are also presented

Application of distributed optical fiber sensor for monitoring the mechanical performance of a driven pile

This paper proposes a special application of Brillouin Optical Time Domain Analysis (BOTDA) sensors for monitoring the mechanical performance of a steel H-pile during driving process in a field in Hong Kong. First, basic calibration and the related installation method of optical fiber sensors (OFS) on pile body were introduced. Second, distributed strain profiles along the H-pile during driving process were successfully obtained from optical fiber sensors. Finally, measured strain distributions of the H-pile subjected to two loading/unloading cycles monitored by OFS were used to interpret field performance of the driven pile. Monitoring results indicate that the vertical load applied on the pile head generated significant compressive deformation, which was however mostly released by the corresponding unloading process. The 1st unloading and 2nd unloading processes released around 66% and 72% of the total compressive pile deformation, respectively. The shear stress levels mobilized between H-pile and surrounding soils were limited after each loading/unloading cycle according to the monitoring results. In addition, loading locations on the driven pile can be clearly identified from strain/stress distribution profiles. It is found from the monitoring study that BOTDA based sensors are more useful than some traditional or other point OFS (such as strain gauges or fiber Bragg grating sensors) to evaluate the average strain distribution of large scale structures.

Performance Monitoring of a Secant-Piled Wall Using Distributed Fiber Optic Strain Sensing

An optical fiber strain-sensing technique, on the basis of Brillouin optical time domain reflectometry (BOTDR), was used to monitor the performance of a secant pile wall subjected to multiple props during construction of an adjacent basement in London. Details of the installation of sensors and data processing are described. Distributed strain profiles were obtained by deriving strain measurements from optical fibers installed on opposite sides of the pile to allow monitoring of both axial and lateral movements along the pile. Methods for analyzing the thermal strain and temperature compensation are also presented. Measurements obtained from the BOTDR were found to be in good agreement with inclinometer data from the adjacent piles. The relative merits of the two different techniques are discussed.

Detection of cracks in a reinforced concrete beam using distributed Brillouin fibresensors

This paper presents the results of using distributed Brillouin fibre sensors to detect crackformation in a simply supported reinforced concrete beam subjected to four-point loading.A Brillouin multiple-peak fitting method was used to enhance the spatial and strainresolutions of the measurements. By doing this, the distributed strain profile along thebeam was determined with a 5 cm read-out resolution in comparison with the15 cm spatial resolution of the fibres. The location of the cracks was identified bylocating the positions in the strain profile where the strain suddenly changes, bysearching for the maximum compressive or tensile peaks in the Brillouin frequencyspectrum, as opposed to conventional strain reading, which focuses solely on themaximum Brillouin peak. The amplitude of the Brillouin peak for the suddenlychanged strain (crack) was found to be smaller than half of the amplitude of themaximum Brillouin peak at the maximum strain location corresponding to the averagestrain of the material, which would have been neglected by standard peak or areafitting methods, especially for fine cracks or the initial crack build-up period.