High-speed full-spectrum interrogation of fiber Bragg gratings for composite impact sensing
Spencer Chadderdon, Tyrie Vella
Why do we need High Speed Full Spectrum Interrogation?
Composite materials (materials made of at least two materials of significantly different physical or chemical properties) are used to make some of the most efficient aircraft in the world. Fiber Bragg gratings are used to measure strain and determine composite
material failure modes. However, strain on composites is non-uniform and impacts may occur at high velocities. High Speed Full Spectrum interrogation combines the advantages of interrogation methods such as peak tracking and full spectrum interrogation because it can detect non-uniform
strain as well as high velocity impacts.
A basic interrogator feeds a broadband source into the fiber bragg grating and the optical filter is tuned across the reflected FBG spectrum.
This process is sped up using the MEMS optical filter.
An ASE source is combined with an erbium-doped fiber amplifier to create the broadband spectrum. A sine wave is applied to the signal that has been reflected across the FBG and data can be recorded as a function of time.
The top two graphs show power and wavelength as functions of time. These are used to show wavelength and power in the third graph at specific points in time. The false color representations make reading the data in the third graph easier.
The experiments were conducted by using drop-tower facility shown in this picture. The drop tower has an impact head that falls from a specific height onto a woven composite laminate which is seen in this picture. The composite specimens have embedded optical fiber sensors.
These figures show the data collected from a single composite specimen that was impacted with an energy of 11.6 J. Data was recorded for each successive strike until the composite specimen was perforated by the impact head. In this test the driving voltage was set to have a frequency of 100 kHz. The figures show wavelength and intensity as a function of time and represent the spectral response of the FBG. This tight region is where the FBG is at rest but at these various times we see the impact occurring. Notice the changes in the spectrum with each successive strike. First we see multiple peaks at every strike, which indicates non-uniform strain. The peak moves and broadens and the relaxation time increases as the strike number increases. With this data we see the dynamics of this test.