研究目的
To detect clamp looseness in hydraulic pipelines using modal strain estimated from FBG-based operational modal analysis.
研究成果
The proposed FBG-based OSMA method accurately identifies modal parameters, and the SMSD index effectively detects, localizes, and quantifies clamp looseness under different locations and degrees. Both analytical and experimental results validate the method's robustness, making it suitable for online structural health monitoring in industrial applications such as hydraulic pipelines.
研究不足
The method may be affected by the interaction between the pipe and excitation equipment (e.g., electric shaker). FBG sensors cannot be placed directly at clamp roots, limiting strain measurement accuracy. The study assumes linear behavior and does not account for non-linear effects like shocks or slips in loose clamps. Application to complex structures like pipe-in-pipe systems requires further investigation.
1：Experimental Design and Method Selection:
The study uses a novel Operational Strain Modal Analysis (OSMA) method with FBG sensing technology, integrating strain response transmissibility (SRT) to estimate strain mode shapes (SMSs) from a single output. A rational fraction composed of Forsythe Complex Orthogonal Polynomials (FCOP) is employed to fit the Positive Strain Power Spectral Density (PSPSD).
2：Sample Selection and Data Sources:
A three-span and four-span pipe model with fluid (hydraulic oil) is used. FBG sensors are glued on the pipe surface at intervals of 100mm. Data is acquired under random excitation (white noise or hammer excitation).
3：List of Experimental Equipment and Materials:
FBG sensors, dynamic FBG integrator with sampling frequency 2000Hz, industrial computer, optical fibers, signal generator, power amplifier, electric shaker, torque wrench, pipe with specified dimensions and material properties (e.g., outer diameter 12mm, wall thickness 0.75mm, length 1.2m, Young's modulus 200GPa), hydraulic oil (Kunlun brand, mass density 850kg/m3), glue (353ND).
4：75mm, length 2m, Young's modulus 200GPa), hydraulic oil (Kunlun brand, mass density 850kg/m3), glue (353ND). Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: Strain responses are measured using FBG sensors under healthy and damaged conditions (clamp looseness simulated by varying torque from 5Nm to 2Nm). Modal parameters (natural frequencies, damping ratios, SMSs) are identified from the OSMA method. SMS differences (SMSD) are calculated and compared to detect looseness.
5：Data Analysis Methods:
Least-squares fitting is used for polynomial fitting in OSMA. SMSD index is computed as the difference between normalized SMSs before and after damage. Analytical validation is done using Transfer Matrix Method (TMM) and Pseudo Excitation Method (PEM) for simulation.
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FBG sensor
Measure dynamic strain on the pipe surface
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Dynamic FBG integrator
Data acquisition for FBG sensors with sampling frequency 2000Hz
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Signal generator
Generate excitation signals (e.g., white noise)
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Power amplifier
Amplify signals to drive the electric shaker
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Electric shaker
Provide vibration excitation to the pipe
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Torque wrench
Tighten bolts to specific torques for simulating clamp looseness
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Industrial computer
Process and analyze data from FBG sensors
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Optical fibers
Transmit light for FBG sensing
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Glue
353ND
Attach FBG sensors to the pipe surface
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Hydraulic oil
Kunlun
Fill the pipe as the fluid medium
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