In-line Long Period Grating and Brillouin Scattering Fiber Optic Sensors for Strain, Temperature, Chloride Concentration, and Steel Mass Loss Measurement in Bridge Applications (SN-3)

Universities: Missouri University of Science and Technology (Missouri S&T)
                           Georgia Institute of Technology

Principal Investigator: Dr. Genda Chen, Missouri S&T

PI Contact Information: Phone: (573) 341-4462  |  Email: gchen@mst.edu

Co-Principal Investigators: Dr. Yang Wang, Georgia Institute of Technology

Funding Sources and Amounts Provided:
Missouri S&T: $25,750.15
INSPIRE UTC: $114,084

Total Project Cost: $139,834.15

Match Agencies ID or Contract Number: Missouri S&T: Cash and In-kind   |   INSPIRE UTC: 00059239

INSPIRE Grant Award Number: 69A3551747126

Start Date: March 1, 2017
End Date:  December 31, 2018

Brief Description of Research Project:

Corrosion is the main reason for costly maintenance of aging transportation infrastructure in the U.S. Since 2008, the PI’s group has developed long period fiber grating (LPFG) sensors for point strain and steel mass loss measurements. When attached on a steel bar, a LPFG sensor doped with nano iron/silica particles and polyurethane can monitor the corrosion process of steel. However, the coating of particles with polyurethane was not robust. In addition, chloride concentration is important for the prediction of early corrosion in practice. Compared to grating sensors, Brillouin scattering based sensors are likely less accurate but offer a cost-effective solution to the monitoring of large-scale civil infrastructure. Therefore, integrating LPFG sensors into a distributed sensing system for multiple parameter measurements is important in bridge applications. Unlike fiber Bragg grating (FBG) sensors that have been recently applied to civil infrastructure, LPFG sensors and distributed sensing systems are mainly tested in laboratory. Their packaging is critical in field applications.

Approach and Methodology: A LPFG sensor is an in-line fiber device with its core refractive index changing periodically in the range of 0.1 mm to 1 mm for strain and corrosion monitoring, respectively. The gratings couple light energy between the core/guided mode and the cladding/lossy modes. This coupling condition is sensitive to the strain applied to the fiber, temperature and refractive index of surrounding medium (e.g. corrosion effect). Changes in these parameters can result in significant shifting of an attenuation band and resonant wavelength. Changes in strain and temperature also vary the density of the fiber and thus stimulate Brillouin scattering in pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA). With both LPFG and PPP-BOTDA measurements, strain and temperature at the location of gratings could be discriminated simultaneously.

Overall Objectives: This project aims to: (1) Develop a physically and optically protected LPFG strain sensor that is hermetically packaged in a fused silica capillary tube, (2) Develop a Fe-C coated LPFG sensor for life-cycle corrosion monitoring (chloride ion and mass loss) of nearby steel members, (3) Understand how many LPFG sensors of different types and wavelengths can be multiplexed to measure multiple parameters for the monitoring of large-scale bridges, and (4) Understand potential interference between the LPFG sensor interrogation and the PPP-BOTDA measurement.

Scope of Work in Year 1: (1) Characterize the sensitivity of wavelength change to the chloride concentration in corrosive environment and the mass loss of Fe-C coated LPFG sensors by simultaneously acquiring their transmission spectra and electrochemical impedance spectroscopy (EIS) diagrams, and (2) Establish a life-cycle deterioration model of reinforced concrete (RC) and steel members in 3.5wt.% NaCl solution using the proposed Fe-C coated LPFG sensors.

Scope of Work in Year 2: (1) Develop a multi-threshold corrosion sensor with several (6) Fe-C coated LPFG sensors integrated into a few (3) coaxial steel tubes for life-cycle corrosion monitoring, (2) Calibrate the wavelength change of a Fe-C coated LPFG sensor with corrosion rate at the threshold-level mass loss of each steel tube (controlled by wall thickness), and (3) Quantify and demonstrate the measurement specifications of the multi-threshold corrosion sensor through testing of RC beams.

Describe Implementation of Research Outcomes:
Research outcomes and implementation plan will be described towards the end of this project.

Impacts/Benefits of Implementation:
Impact/Benefits of Implementation will be summarized at the end of this project.

Project Website: http://inspire-utc.mst.edu/researchprojects/sn-3/
Progress Reports: