Dynamic Strain Measurement Research Articles

Single-End-Access BOCDA Enabled by Multi-Core Fiber for Dynamic Strain Measurements

Single-End-Access BOCDA Enabled by Multi-Core Fiber for Dynamic Strain Measurements

Enhancing the performance of DAS using a dual-wavelength UWFBG array with alternating arrangement

We propose a novel distributed acoustic sensor (DAS) based on a dual-wavelength ultra-weak fiber Bragg grating (UWFBG) array. The UWFBGs in the array have two wavelengths that are arranged alternately. Two double-pulses with different wavelengths and staggered launching times are used for sensing in the UWFBG array. This approach successfully overcomes the trade-off between spatial resolution and sensing distance in the traditional UWFBG-based DAS and effectively doubles the maximum achievable sampling rate constrained by the sensing distance. The crosstalk of different wavelengths is avoided by time division multiplexing. Comprehensive analyses and simulations are conducted to validate the performance enhancements, compared to the traditional method. In our proof-of-concept experiments, employing a 2 km alternating arranged dual-wavelength UWFBG array, we achieve a spatial resolution enhancement from 10 m to 5 m, expand the dynamic strain measurement range from 34.2 rad to 68.2 rad, and increase the frequency response from 9 kHz to 19 kHz.

Analysis of PZT Piezoelectric Sensors in Buckling Mode for Dynamic Strain Measurement

Fatigue Life Monitoring is crucial to ensure engineering structures' safety and durability. This paper introduces an innovative approach for fatigue life monitoring using a soft PZT (Lead Zirconate Titanate) piezoelectric sensor operating in buckling mode to measure applied cyclic strains. A particular emphasis is on the utilization of a 3D-printed 'Extension' that serves as the platform upon which the PZT sensor is installed and operates in buckling mode while subjected to cyclic strains. An analytical framework is presented, which encompasses the development of the governing equations to establish a direct relationship between dynamic strain values and the sensor's output. Critical strain attributes, such as amplitude and occurrence strain rate also taken into account. The analytical solutions are validated by a series of experimental tests under various dynamic loading conditions. The PZT sensor, when integrated with the 3D-printed extension and operated in buckling mode, exhibits high sensitivity and functions as a passive dynamic strain measurement sensor, with high potential to be used as self-powered sensor nodes for fatigue life monitoring of engineering structures.

Bayesian structural model updating with multimodal variational autoencoder

A novel framework for Bayesian structural model updating is presented in this study. The proposed method utilizes the surrogate unimodal encoders of a multimodal variational autoencoder (VAE). The method facilitates an approximation of the likelihood when dealing with a small number of observations. It is particularly suitable for high-dimensional correlated simultaneous observations applicable to various dynamic analysis models. The proposed approach was benchmarked using a numerical model of a single-story frame building with acceleration and dynamic strain measurements. Additionally, an example involving a Bayesian update of nonlinear model parameters for a three-degree-of-freedom lumped mass model demonstrates computational efficiency when compared to using the original VAE, while maintaining adequate accuracy for practical applications.

Dynamic stress prediction for a Pump-Turbine in Low-Load Conditions: Experimental validation and phenomenological analysis

To meet the increasing demand for higher flexibility in hydropower, an operating range extension in an existing hydroelectric powerplant is considered. To this end, the dynamic flow and structural behavior of the low-head pump-turbine is investigated in turbine mode in partial load and deep partial load conditions, to evaluate the feasibility of this flexibilization due to the increased high cycle fatigue damage. In a first step, different numerical simulation approaches that combine fluid dynamics and structural dynamics are applied and compared to in-situ dynamic strain and pressure measurements on the impeller. As a result, the modelling requirements for a numerical workflow that leads to unmatched accuracy in predicting the broad-band dynamic stresses of deep partial load are identified. In a second step, the operating-point-dependent fatigue crack initiation spots are identified based on dynamic stresses. Herein, the emergence of an additional dominant critical spot near the blade leading edge is observed, which is unexpected for low-head pump turbines. Thanks to the detailed numerical flow and structural analyses, the root cause for the dynamic stress concentration is studied and the emergence of the new critical spot is understood. It turns out in deep partial load, that the main source of pressure oscillations on the impeller blade surfaces is the vortex-dominated flow detachment zone around the blade leading edge, which, in combination with the geometry of the investigated impeller, translates into the main stress oscillations. The results of this work were later used to extend the operating range of the hydroelectric powerplant from initially 25% to 100% power output to now 0% to 100%.

A Mode Sensitivity Enhancement Method for Beam Bridge Using High-Density Strain Feedback

Abstract Closed-loop damage diagnosis method has attracted some attention in recent years; however, for lack of sufficient measuring points, it is difficult to achieve robust control of large structures cause the output dimension is not enough to meet the demand of system controllability and observability. On this basis, a mode sensitivity enhancement method for beam bridge using high-density strain feedback is proposed, in which high density dynamic strain measurements of the girder structure can be collected by distributed fiber sensor. Utilizing strain–displacement transformation relationship of the girder structure, the dynamic displacements can be obtained and used as output with high dimension to achieve the feedback control for eigenvalue sensitivity enhancement. To verify the proposed method, a series of numerical case studies of a beam bridge structure are performed, and it is demonstrated that the eigenvalue sensitivity can be enhanced effectively.

Hybrid BOFDA/BOCDA system for distributed static and dynamic strain measurements.

We present a distributed optical fiber sensor based on a hybrid Brillouin optical frequency/correlation-domain analysis (BOFDA/BOCDA) configuration for both static and dynamic strain measurements. Distributed static strain (or temperature) measurements are realized using the conventional BOFDA method, i.e., acquiring the baseband transfer function of the fiber through a vector network analyzer (VNA). With little modifications, the same setup can perform dynamic, position-selective measurements synthesizing a correlation peak through a frequency modulation of the laser source while operating the VNA at a single modulation frequency. Experimental tests, carried out at a sampling frequency up to 40 Hz and a spatial resolution of ≈5 cm, demonstrate the validity of the proposed approach.

Ultrasensitive optomechanical strain sensor

We demonstrate an ultrasensitive optomechanical strain sensor based on a SiN membrane and a Fabry-Perot cavity, enabling the measurements of both static and dynamic strain by monitoring reflected light fluctuations using a single-frequency laser. The SiN membrane offers high-quality-factor mechanical resonances that are sensitive to minute strain fluctuations. The two-beam Fabry-Perot cavity is constructed to interrogate the motion state of the SiN membrane. A static strain resolution of 4.00 nɛ is achieved by measuring mechanical resonance frequency shifts of the SiN membrane. The best dynamic resolution is 4.47 pɛHz-1/2, which is close to that of the sensor using high-finesse cavity and optical frequency comb, overcoming the dependence of ultrasensitive strain sensors on narrow-linewidth laser and high-finesse cavity with frequency locking equipment. This work opens up a promising avenue for a new generation of ultrasensitive strain sensors.

Cyclical hydraulic pressure pulses reduce breakdown pressure and initiate staged fracture growth in PMMA

Using unique experimental equipment on large bench-scale samples of Polymethylmethacrylate, used in the literature as an analogue for shale, we investigate the potential benefits of applying cyclical hydraulic pressure pulses to enhance the near-well connectivity through hydraulic fracturing treatment. Under unconfined and confined stresses, equivalent to a depth of up to 530 m, we use dynamic high-resolution strain measurements from fibre optic cables, complemented by optical recordings of fracture development, and investigate the impact of cyclical hydraulic pressure pulses on the number of cycles to failure in Polymethylmethacrylate at different temperatures. Our results indicate that a significant reduction in breakdown pressure can be achieved. This suggests that cyclic pressure pulses could require lower power consumption, as well as reduced fluid injection volumes and injection rates during stimulation, which could minimise the occurrence of the largest induced seismic events. Our results show that fractures develop in stages under repeated pressure cycles. This suggests that Cyclic Fluid Pressurization Systems could be effective in managing damage build-up and increasing permeability. This is achieved by forming numerous small fractures and reducing the size and occurrence of large fracturing events that produce large seismic events. Our results offer new insight into cyclical hydraulic fracturing treatments and provide a unique data set for benchmarking numerical models of fracture initiation and propagation.

Full-field static and dynamic strain measurement by an inverse conjugate beam method with two-type sensor placement

This paper presents a novel inverse conjugate beam method (ICBM) to full-field static and dynamic strain measurement using long-gauge fiber Bragg grating (FBG) and vision sensors. By applying reverse analysis of conjugate beam theory, the ICBM establishes a displacement–strain transformation model that effectively explains the correlation between displacement, distributed strain and desired strain. The static and dynamic strain can be reconstructed directly by combining the monitored displacement and strain responses at the displacement monitoring locations. The vision sensor is employed to complement the installed long-gauge strain sensor for monitoring the displacement of the location without a long-gauge sensor. This method helps overcome the difficulty of monitoring the full-bridge strain due to insufficient sensors or inaccessible monitoring positions. At the same time, according to this method, it is necessary to use the displacement from the visual sensor to determine the residual stiffness of each unit as prior information for the ICBM. Both numerical studies and laboratory tests are carried out on a simply supported beam for conceptual verification. The results demonstrate that the proposed ICBM successfully achieves static and dynamic strain response reconstruction at displacement monitoring locations.

Embedded Fiber Bragg Grating (FBG) Sensors Fabricated by Ultrasonic Additive Manufacturing for High-Frequency Dynamic Strain Measurements

Embedded Fiber Bragg Grating (FBG) Sensors Fabricated by Ultrasonic Additive Manufacturing for High-Frequency Dynamic Strain Measurements

On the effectiveness of sparse regularization for structural damage diagnosis using dynamic strain measurements

Strain measurement is considered inherently suitable for structural health monitoring due to the ease of extracting damage-sensitive features. However, the need for pre- and post-damage feature extraction, especially in the absence of baseline data for aging infrastructures in service, has engendered skepticism concerning the practical efficacy of strain-based damage diagnosis. To address this, this study incorporates limited prior information on structures and sparse regularization to ascertain the locations and severities of structural damages based on dynamic strain response data. A model-assisted approach is presented, including the typical three-step process of damage feature extraction, finite element modeling, and analytical sensitivity-based model updating. In particular, sparse regularization technique aids in robustly identifying structural damage by tackling the hindrance encountered during model updating, involving ill-posedness and the impact of measurement and modeling uncertainties. The approach’s effectiveness is substantiated through a numerical investigation of a Bailey truss bridge model exposed to operating loads and an experiment with a steel beam equipped with an array of fiber Bragg grating sensors. The results demonstrate that using the proposed damage diagnosis approach enables accurately locating and quantifying element-level damages, even when significant modeling errors or a lack of baseline features exist.

Long range static and dynamic strain measurement by using phase-noise-compensated OFDR

Long range static and dynamic strain measurement by using phase-noise-compensated OFDR

High-Spatial-Resolution Dynamic Strain Measurement Based on Brillouin Optical Correlation-Domain Sensors

Brillouin-scattering-based sensors have been widely applied in distributed temperature or strain measurement in recent 20 years. Brillouin optical correlation-domain technology has extensive development and application prospects because of its millimeter-level spatial resolution, distribution measurement, and high accuracy. Traditional Brillouin-scattering-based sensors, requiring a time-consuming frequency-sweep process, struggle to achieve dynamic strain measurement. In this article, Brillouin optical correlation-domain analysis and reflectometry based on fast-sweep frequency and slope-assisted methods will be reviewed. The main merits, drawbacks, and performances of these schemes are compared, and the avenues for future research and development of these two technologies are also explored.

Influence of Adhesive Bonding on the Dynamic and Static Strain Transfers of Fibre Optic Sensors

The influence of the bonding procedure (the adhesive type, application procedure, etc.) on the static and dynamic strain transfers of bonded optical fibre sensors is studied theoretically and experimentally at room temperature. The achievable performances with four different types of adhesives (three urethane and one epoxy adhesive), and with different fibre types, are evaluated: acrylate-coated, polyimide-coated, and bare single-mode optical fibres. Static strain measurements, ranging from 20 to 200 µ strain, are performed using both fibre Bragg gratings (FBGs) and optical frequency domain reflectometry (OFDR), and are compared to reference strain-gauge measurements, and to the proposed analytical model, which is developed on the basis of stress equilibrium relations. This model is valid for bonding to all types of linear and elastic materials, as long as there is no sliding between the host material, the adhesive, and the optical fibre. The results agree between the analytical model and the experiments. Regarding the dynamic sinusoidal strain measurements, the studied dynamic range is from 10 to 100 Hz, and only the FBGs are tested. The results demonstrate that the sensitivities of strain sensors based on bonded uncoated fibres or bonded polyimide-coated fibres are comparable to those of strain gauges, and that it is possible to use bonded FBGs for precise dynamic strain measurements.

Dynamic Strain Measurement of Rotor Blades in Helicopter Flight Using Fiber Bragg Grating Sensor.

Fiber Bragg grating (FBG) sensor has a lot of advantages over the resistance strain gauge and has been used in many applications. However, there are few applications of rotor blade dynamic measurement in helicopter flight. In this paper, a method for blade dynamic strain measurement using an FBG sensor in a helicopter's real flight is presented. The corresponding measurement system is established and can eliminate the effects of the helicopter's electromagnetic environment on the electrical sensing components in the measurement system with the orthogonal frequency-division multiplexing modulation. The measured dynamic strains on the rotor blades of the helicopter in real flight contain six harmonic frequencies with the vibration characteristics of rotor blades, indicating that the established FBG measurement method and system have practical engineering applications.

Cumulative Fatigue Damage of Small-Bore Piping Subjected to Flow Induced Vibration

The structural fatigue of a vertically oriented small-bore connection due to flow induced turbulence emanating from an upstream piping manifold is experimentally investigated. Dynamic strain measurements are taken at two perpendicular locations on a small-bore connection and the method of rainflow counting is used to determine the cumulative damage incurred. A number of factors is investigated and their influences on the cumulative damage are explained. These include the effects of (1) single phase and multiphase flow, (2) the upstream flow path through the manifold, and (3) steady-state and transient conditions. From the experimental analysis, key observations that may be useful to piping designers and engineers are reported. For instance, for single-phase water, the largest bending stresses are due to the out-of-plane vibration of the small-bore piping, whereas the pulsating characteristics of the single-phase air and multiphase flow result in significantly larger in-plane bending stresses. It is also observed that under certain manifold outlet conditions, transient effects upon pump start-up can produce more than 300 times the cumulative fatigue damage compared to steady-state operation. As part of this study, the finite element method is also used to investigate the influence of piping support stiffness on the fatigue life of a small-bore connection. Through a series of parametric analysis, it is demonstrated that there exists a minimum support stiffness for which the fatigue life of a small-bore connection excited by flow-induced vibration, achieves its upper bound value.

Dynamic large strain measurement under high-temperature environment using a modified FBG sensor and plasma surface treatment.

Large strain measurement under high-temperature environment has been a hot but difficult research issue in the fields of measurement and metrology. However, conventional resistive strain gauges are susceptible to electromagnetic interference at high temperature, and typical fiber sensors will be invalid under high-temperature environment or fall off under large strain conditions. In this paper, aiming to achieve effective and precision measurement of large strain under high-temperature environment, a systematic scheme combining a well-designed encapsulation of a fiber Bragg grating (FBG) sensor and a special surface treatment method using plasma is presented. The encapsulation protects the sensor from damage while achieving partial thermal isolation and avoiding shear stress and creep, resulting in higher accuracy. And the plasma surface treatment provides a new bonding solution which can greatly improve the bonding strength and coupling efficiency without damaging the surface structure of the object under test. Suitable adhesive and temperature compensation method are also carefully analyzed. Consequently, large strain measurement up to 1500 µɛ under high-temperature (1000°C) environment is experimentally achieved in a cost-effective way.

Dynamic measurement of internal strain on composite solid propellant constituents during laser induced combustion

Binder melt flow and ammonium perchlorate (AP) cracking have been topics for research for a long time. Both impact the burning rate dependence on pressure and the binder flow. Rapid depressurization of the combustion vessel is the most common tool used to study binder melt flow by studying the ripples on the quenched surface. Laminate propellants were used to study fundamental combustion phenomena such as the melt flow over the oxidizer/binder interface AP/hydroxyl-terminated polybutadiene (HTPB). This study starts with thermally induced strain measurements in single crystals of AP with thicknesses 1- to 2.5-mm and neat HTBP slabs then finishes with laminates of AP/HTPB. A CO2 laser ignited and burned samples over a range of heat fluxes. First ever in- situ mapping of the two-dimensional thermally-induced internal strain generated at ignition to steady state combustion were imaged using a high-speed polarization camera. Heating of the surface produced strain fields in the AP, HTPB, and laminate provide information on the thermal diffusion during the ignition and fully developed combustion. This paper demonstrates how using polarized high-speed video, the binder melt flow was more accurately observed and AP cracking mechanisms can be better studied.

Distributed dynamic strain measurement with a direct detection scheme by using a three-step-phase-shifted double pulse in a UWFBG array.

We demonstrate a stable homodyne phase demodulation method with a double pulse based on an ultra-weak fiber Bragg grating (UWFBG) array. The technique divides one of the probe pulses into three sections and introduces successive 2π/3 phase differences into each section. By using a simple direct detection scheme, it can achieve distributed and quantitative vibration measurement along the UWFBG array. Compared to the traditional homodyne demodulation technique, the proposed technique is more stable and easier to accomplish. Moreover, the reflected light from the UWFBGs can provide a signal that is modulated uniformly by the dynamic strain and multiple results for averaging, resulting in a higher signal-to-noise ratio (SNR). We experimentally demonstrate the technique's effectiveness by monitoring different vibrations. The SNR for measuring a generic 100 Hz, 0.08 rad vibration in a 3 km UWFBG array with a reflectivity of -40 to -45 dB is estimated to be ∼44.92 dB.