A recently developed technique is presented for thermographic detection of delaminations in composites by performing
temperature measurements with fiber optic Bragg gratings. A single optical fiber with multiple Bragg gratings employed
as surface temperature sensors was bonded to the surface of a composite with subsurface defects. The investigated
structure was a 10-ply composite specimen with prefabricated delaminations of various sizes and depths. Both during and
following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the
temporal and spatial temperature variations. The data obtained from grating sensors were analyzed with thermal
modeling techniques of conventional thermography to reveal particular characteristics of the interested areas. Results
were compared and found to be consistent with the calculations using numerical simulation techniques. Also discussed
are methods including various heating sources and patterns, and their limitations for performing in-situ structural health
monitoring.
A recently developed technique is presented for thermographic detection of flaws in composite materials by performing
temperature measurements with fiber optic Bragg gratings. Individual optical fibers with multiple Bragg gratings
employed as surface temperature sensors were bonded to the surfaces of composites with subsurface defects. The
investigated structures included a 10-ply composite specimen with subsurface delaminations of various sizes and depths.
Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors
measured the temporal and spatial temperature variations. The data obtained from grating sensors were analyzed with
thermal modeling techniques of conventional thermography to reveal particular characteristics of the interested areas.
Results were compared with the calculations using numerical simulation techniques. Methods and limitations for
performing in-situ structural health monitoring are discussed.
Cryogenic temperature sensing was demonstrated using pressurized fiber Bragg gratings (PFBGs) with polymer
coating of various thicknesses. The PFBG was obtained by applying a small diametric load to a regular fiber Bragg
grating (FBG). The Bragg wavelengths of FBGs and PFBG were measured at temperatures from 295 K to 4.2 K. The
temperature sensitivities of the FBGs were increased by the polymer coating. A physical model was developed to
relate the Bragg wavelength shifts to the thermal expansion coefficients, Young's moduli, and thicknesses of the
coating polymers. When a diametric load of no more than 15 N was applied to a FBG, a pressure-induced transition
occurred at 200 K during the cooling cycle. The pressure induced transition yielded PFBG temperature sensitivities
three times greater than conventional FBGs for temperatures ranging from 80 to 200 K, and ten times greater than
conventional fibers for temperatures below 80 K. PFBGs were found to produce an increased Bragg wavelength shift
of 2.2 nm compared to conventional FBGs over the temperature range of 4.2 to 300 K. This effect was independent of
coating thickness and attributed to the change of the fiber
thermo-optic coefficient.
Cryogenic temperature sensing was studied using a pressurized fiber Bragg grating (PFBG). The PFBG was obtained
by simply applying a small diametric load to a regular fiber Bragg grating (FBG), which was coated with polyimide of a
thickness of 11 micrometers. The Bragg wavelength of the PFBG was measured at temperatures from 295 K to 4.2 K. A
pressure-induced transition occurred at 200 K during the cooling cycle. As a result, the temperature sensitivity of the
PFBG was found to be nonlinear and reach 24 pm/K below 200 K, more than three times the regular FBG. For the
temperature change from 80 K to 10 K, the PFBG has a total Bragg wavelength shift of about 470 pm, 10 times more
than the regular FBG. From room temperature to liquid helium temperature the PFBG gives a total wavelength shift of
3.78 nm, compared to the FBG of 1.51 nm. The effect of the coating thickness on the temperature sensitivity of the
gratings is also discussed.
A new technique has been developed for sensing both temperature and strain simultaneously by using dual-wavelength fiber-optic Bragg gratings. Two Bragg gratings with different wavelengths were inscribed at the same location in an optical fiber to form a sensor. By measuring the wavelength shifts that resulted from the fiber being subjected to different temperatures and strains, the wavelength-dependent thermo-optic coefficients and photoelastic coefficients of the fiber were determined. This enables the simultaneous measurement of temperature and strain. In this study, measurements were made over the temperature range from room temperature down to about 10 K, addressing much of the low temperature range of cryogenic tanks. A structural transition of the optical fiber was found when the temperature decreased. This transition caused splitting of the waveforms characterizing the Bragg gratings, and the determination of wavelength shifts was consequently complicated. The effectiveness and sensitivities of these measurements in different temperature ranges are also discussed.
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