Pulse Pre-pump Brillouin Optical Time Research Articles

Experimental investigation on flexural behavior of steel-concrete composite floor slabs with distributed fiber optic sensors

This research investigates the flexural behavior of steel-concrete composite floor slabs with distributed fiber optic sensors based on pulse pre-pump Brillouin optical time domain analysis. The distributed sensors were embedded in trapezoidal concrete slabs and provided in-situ real-time monitoring of initiation and evolution of cracks in the profiled slabs of four composite floor slabs. This paper proposes a sensor deployment method for real construction of civil engineering structures and evaluates the effects of headed studs and concrete casting on the flexural behavior of composite floor slabs and the performance of distributed sensors. The composite floor slabs failed with concrete cracks, accompanied by yielding in the steel beams and slippage between concrete and steel. The distributed sensors captured complex crack patterns of the concrete slabs in the evolution of the damages in the composite floor slabs. The strengths and limitations of distributed sensors are discussed to promote applications and further development.

Experimental study on PPP-BOTDA-based monitoring approach of concrete structure crack

Cracks are the most common lesion in water-wading structures such as dams and sluices, which have an important impact on the durability of concrete materials and the safety of structures. The emergence of cracks is also regarded as a precursor to the destruction of buildings. However, the randomness and concealment of cracks are strong, which bring great difficulty to real-time and accurate capture. This paper focuses on Pulse Pre-Pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) and studies the principle and realization method of PPP-BOTDA optical fiber sensing for concrete structure strain. On this basis, several groups of monitoring experiment for crack initiation location and propagation process of concrete structures are carried out. In the paper, the relationship between force-light-opening is studied and the influence of angle between optical fiber and crack on monitoring results is analyzed. Finally, a monitoring model for concrete cracks based on optical fiber strain sensing is constructed.

Distributed fiber optic measurements of strain and temperature in long-span composite floor beams with simple shear connections subject to compartment fires

This study explores an instrumentation strategy using distributed fiber optic sensors to measure strain and temperature through the concrete volume in large-scale structures. Single-mode optical fibers were deployed in three 12.8 m long steel and concrete composite floor specimens tested under mechanical or combined mechanical and fire loading. The concrete slab in each specimen was instrumented with five strain and temperature fiber optic sensors along the centerline of the slab to determine the variation of the measurands through the depth of the concrete. Two additional fiber optic temperature sensors were arranged in a zigzag pattern at mid-depth in the concrete to map the horizontal spatial temperature distribution across each slab. Pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA) was used to determine strains and temperatures at thousands of locations at time intervals of a few minutes. Comparisons with co-located strain gauges and theoretical calculations indicate good agreement in overall spatial distribution along the length of the beam tested at ambient temperature, while the fiber optic sensors additionally capture strain fluctuations associated with local geometric variations in the specimen. Strain measurements with the distributed fiber optic sensors at elevated temperatures were unsuccessful. Comparisons with co-located thermocouples show that while the increased spatial resolution provides new insights about temperature phenomena, challenges for local temperature measurements were encountered during this first attempt at application to large-scale specimens.

A quasi-distributed monitoring method for ground settlement using pulse pre-pump Brillouin optical time domain analysis

In geotechnical engineering, it is critical to measure and control ground settlement. A new method is proposed in this work for quasi-distributed ground settlement monitoring. This method is based on the pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA). The optical fiber is arranged on the monitoring poles through special supports, forming a zigzag pattern. Based on geometric principles, the formulas converting strains in fibers to displacements of supports are derived. Afterwards, the feasibility of the method is verified through laboratory tests. The results show that the proposed method can achieve accuracy within 1 mm. Compared with traditional discrete-point measurement methods, the proposed method can monitor settlement up to 100 points simultaneously with satisfactory precision, facilitating remote and large scale ground settlement monitoring in geotechnical engineering applications.

Temperature measurement and damage detection in concrete beams exposed to fire using PPP-BOTDA based fiber optic sensors

In this study, Brillouin scattering-based distributed fiber optic sensor is implemented to measure temperature distributions and detect cracks in concrete structures subjected to fire for the first time. A telecommunication-grade optical fiber is characterized as a high temperature sensor with pulse pre-pump Brillouin optical time domain analysis (PPP-BODTA), and implemented to measure spatially-distributed temperatures in reinforced concrete beams in fire. Four beams were tested to failure in a natural gas fueled compartment fire, each instrumented with one fused silica, single-mode optical fiber as a distributed sensor and four thermocouples. Prior to concrete cracking, the distributed temperature was validated at locations of the thermocouples by a relative difference of less than 9%. The cracks in concrete can be identified as sharp peaks in the temperature distribution since the cracks are locally filled with hot air. Concrete cracking did not affect the sensitivity of the distributed sensor but concrete spalling broke the optical fiber loop required for PPP-BOTDA measurements.

Distributed fiber optic sensor-enhanced detection and prediction of shrinkage-induced delamination of ultra-high-performance concrete overlay

This study develops a delamination detection system for smart ultra-high-performance concrete (UHPC) overlays using a fully distributed fiber optic sensor. Three 450 mm (length) × 200 mm (width) × 25 mm (thickness) UHPC overlays were cast over an existing 200 mm thick concrete substrate. The initiation and propagation of delamination due to early-age shrinkage of the UHPC overlay were detected as sudden increases and their extension in spatial distribution of shrinkage-induced strains measured from the sensor based on pulse pre-pump Brillouin optical time domain analysis. The distributed sensor is demonstrated effective in detecting delamination openings from microns to hundreds of microns. A three-dimensional finite element model with experimental material properties is proposed to understand the complete delamination process measured from the distributed sensor. The model is validated using the distributed sensor data. The finite element model with cohesive elements for the overlay-substrate interface can predict the complete delamination process.

A study for optical fiber multi-direction strain monitoring technology

Distributed measurement is truly achieved by Pulse-Pre Pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) technique with its high spatial resolution and strain precision. A systematic distributed fiber-optic multi-direction strain monitoring (FOMDSM) technology is proposed according to the basic monitoring principle, testing parameters and layout scheme based on PPP-BOTDA. The reasonable layout scheme, temperature compensation formula, etc are also given. This article also determines the more reasonable specimen size, the position of optical fiber layout and the basic layout range. The article is an experimental feasibility inquiry into spatial multi-direction stain simultaneous monitoring, which is an urgent problem to be solved. The structure is not merely under drawing force in a practical project, and therefore this article also explores the studies common combination of bending and drawing in engineering, besides the tensile test. The technique utilizes the highest spatial resolution for the research field. From the monitoring efficacy, the high precision and good practical application have emerged. The article’s final purpose is to be proved reasonable and applicable in putting forward the spatial multi-direction stress and strain monitoring idea by the common 0°/45°/90° strain monitoring based on PPP-BOTDA technology. It illustrates important guiding significance for forming actually spatial FOMDSM ability.

Concrete pavement monitoring with PPP-BOTDA distributed strain and crack sensors

In this study, the feasibility of using telecommunication single-mode optical fiber (SMF) as a distributed fiber optic strain and crack sensor was evaluated in concrete pavement monitoring. Tensile tests on various sensors indicated that the SMF-28e+ fiber revealed linear elastic behavior to rupture at approximately 26 N load and 2.6% strain. Six full-scale concrete panels were prepared and tested under truck and three-point loads to quantify the performance of sensors with pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA). The sensors were protected by precast mortar from brutal action during concrete casting. Once air-cured for 2 hours after initial setting, half a mortar cylinder of 12 mm in diameter ensured that the protected sensors remained functional during and after concrete casting. The strains measured from PPP-BOTDA with a sensitivity coefficient of 5.43 10-5 GHz/ were validated locally by commercial fiber Bragg grating (FBG) sensors. Unlike the point FBG sensors, the distributed PPP-BOTDA sensors can be utilized to effectively locate multiple cracks. Depending on their layout, the distributed sensors can provide one- or two-dimensional strain fields in pavement panels. The width of both micro and major cracks can be linearly related to the peak strain directly measured with the distributed fiber optic sensor.

High-temperature measurement with Brillouin optical time domain analysis of an annealed fused-silica single-mode fiber.

The effect of annealing is experimentally studied for a fused silica, fully distributed fiber optic sensor based on the pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA). Within a heating rate of 4.3°C/min and 30.6°C/min, and a sustained peak temperature for 120 and 240 min, annealing extended the sensor's upper operation temperature from 800°C to 1000°C and reduced the sensor's measurement variability over a temperature range of 22°C to 1000°C with a maximum Brillouin frequency variation of 1%. The annealed sensor had a linearly decreasing Brillouin frequency sensitivity from 1.349×10-3 GHz/°C at 22°C to 0.419×10-3 GHz/°C at 1000°C. The time required to achieve a stable annealing effect decayed exponentially with annealing temperature.

Pulse-Pre Pump Brillouin Optical Time Domain Analysis-based method monitoring structural multi-direction strain

Pulse-Pre Pump Brillouin Optical Time Domain Analysis-based method monitoring structural multi-direction strain

Temperature-dependent strain and temperature sensitivities of fused silica single mode fiber sensors with pulse pre-pump Brillouin optical time domain analysis

This paper reports a distributed temperature and strain sensor based on pulse pre-pump Brillouin optical time domain analysis. An uncoated, telecom-grade fused silica single-mode fiber as a distributed sensor was calibrated for its sensitivity coefficients under various strains and temperatures up to 800 °C. The Brillouin frequency of fiber samples changed nonlinearly with temperature and linearly with strain. The temperature sensitivity decreased from 1.113 to 0.830 MHz /°C in the range of 22–800 °C. The strain sensitivity was reduced from 0.054 to 0.042 MHz /με as the temperature increased from 22 to 700 °C and became unstable at higher temperatures due to creep effect. The strain measurement range was reduced from 19 100 to 6000 με in the temperature range of 22–800 °C due to fused silica’s degradation. The calibrated fiber optic sensor demonstrated adequate accuracy and precision for strain and temperature measurements and stable performance in heating–cooling cycles. It was validated in an application setting.

Strain distribution and crack detection in thin unbonded concrete pavement overlays with fully distributed fiber optic sensors

This study aims at evaluating the feasibility of strain measurement and crack detection in thin unbonded concrete pavement overlays with pulse prepump Brillouin optical time domain analysis. Single-mode optical fibers with two-layer and three-layer coatings, respectively, were applied as fully distributed sensors, their performances were compared with analytical predictions. They were successfully protected from damage during concrete casting of three full-scale concrete panels when 5 to 10-cm-thick protective mortar covers had been set for 2 h. Experimental results from three-point loading tests of the panels indicated that the strain distributions measured from the two types of sensors were in good agreement, and cracks can be detected at sharp peaks of the measured strain distributions. The two-layer and three-layer coated fibers can be used to measure strains up to 2.33% and 2.42% with a corresponding sensitivity of 5.43×10−5 and 4.66×10−5 GHz/μϵ, respectively. Two cracks as close as 7 to 9 cm can be clearly detected. The measured strains in optical fiber were lower than the analytical prediction by 10% to 25%. Their difference likely resulted from strain transfer through various coatings, idealized point loading, varying optical fiber embedment, and concrete heterogeneity.

Measuring mortar shrinkage and cracking by pulse pre-pump Brillouin optical time domain analysis with a single optical fiber

In this study, a new method to measure strain distribution and detect cracking with pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA) is proposed. A mortar cylinder was cast in a corrugated polyethylene mold with two end plugs and tested according to ASTM C1698 for autogenous shrinkage. It was instrumented with two sensors made of a single optical fiber: one for combined strain and temperature measurement and the other for temperature compensation. The shrinkage-induced compressive strain was non-uniformly distributed over the sensor length. The average of the distributed strain was in excellent agreement with autogenous shrinkage that can be determined from the ASTM test. The location and magnitude of each peak of the distributed strain can be used to detect a corresponding invisible hidden crack.