Multi-Functional Measurement Using a Single FBG Sensor

Abstract

This paper describes the measurement of average strain, strain distribution and vibration of a cantilever beam made of Carbon Fiber Reinforced Plastics (CFRP), using a single Fibre Bragg Grating (FBG) sensor mounted on the beam surface. Average strain is determined from the displacement of the peak wavelength of reflected spectrum from the FBG sensor. Two unstrained reference FBG sensors were used to compensate for temperature drift. Measured strains agree with those measured by a resistance foil strain gauge attached to the sample. Stress distributions are measured by monitoring the variation in the full width at half maximum (FWHM) values of the reflected spectrum, using a proposed optical analytical model, described in the paper. FWHM values were measured for both the cantilever test beam and for a reference beam, loaded using a four-point bending rig. The trend of the stress distribution for the test beam matches with our analytical model, however with a relatively large noise present in the experimentally determined data. The vibration of a cantilever beam was measured by temporal analysis of the peak reflection wavelength. This technique is very stable as measurements are not affected by variations in the signal amplitude. Finally an application of FBG sensors for damage detection of CFRP plates, by measuring the natural frequency, is demonstrated. With small defects of different sizes applied to the CFRP plate, the natural frequency decreased with damage size.

Lamb wave detection in prepreg composite materials with fibre Bragg grating sensors

“…Both systems are described below. Figure 4a shows the experimental setup used for the FBG measurements 12 . A super luminescent diode (Frankfurt Laser Company, 20 mW, 1520-1580 nm wavelength range at FWHM) was used as the light source.…”

Calculation of the Mean Strain of Smooth Non-Uniform Strain Fields Using Conventional FBG Sensors

“…To overcome this problem in practical scenarios, we introduce a slight modification to the above proposed algorithm. Instead of computing the cross-correlation between the reflected spectrum and the sinc function directly, we first align the two reflected spectra (recorded before and after applying the strain) based on their centre of mass [20] λ Bc = λ λR(λ)dλ λ R(λ)dλ (15) where λ is the wavelength region that covers the reflection spectrum. This shift, ∆λ Bc , can be used to find a rough estimate ofλ, sayλ = λ B + ∆λ Bc .…”

Strain and Displacement Controls by Fibre Bragg Grating and Digital Image Correlation

“…Equation (1). If no additional measurements from the I-MON interrogator are required (6), the programme is stopped (7). This is done by releasing the main PCB internal image data buffer, USB camera, and finally, the USB device and DLL.…”

“…When all the data are transferred, the image data buffer is overwritten with new image data while waiting for the next trigger signal by (3). When all the requested data are acquired and the LabVIEW application is terminated, the TCP/IP connection is closed by (7).…”