The US Navy is becoming increasingly interested in the implementation of composite materials for their marine structures because of high specific strength and favorable stiffness to weight ratios. This interest, coupled with austere spending budgets, creates the need for methods that can monitor and classify damage in real-time in order to drive maintenance based on current condition and not time accumulation.

For this project, The Navy is working with UC San Diego and Hitest Laboratories to develop a damage detection system for plate like composites. The sensing technology for the system is built on fiber Bragg gratings as the strain sensing method. The benefits for using this type of system as it relates to the operational constraints of this problem are numerous.

Fiber Bragg gratings are:

  • Lightweight
  • Immune to radio and electromagnetic interference
  • Can be embedded in the structure
  • Water indifferent
  • Can be multiplexed
  • Corrosion resistant
  • Small Scale

Figure 1 describes the sensing technology. A broadband light source is shown through an optical fiber. The etched gratings suddenly change the refractive index of the fiber core, causing a very narrow band of light to reflect back towards the source while the rest if the light continues to propagate down the fiber. The peak wavelength of this reflected band is called the Bragg wavelength and as the fiber is strained, that wavelength will shift, and the differential Bragg wavelength can be converted to strain via simple arithmetic.

The project is broken into several phases and will span several years if all options are included. I will just discuss the phases that I was personally involved in.

Phase IIa

Phase I included a rudimentary proof-of-concept test that proved that (1.) optical fibers with Bragg gratings can be embedded in a composite plate, (2.) accurate strain measurements can be made, (3.) the embedding of the fiber does not significantly affect the strength of the laminate, and (4.) strain measurements can accurately be taken even if the specimen is submerged in water. Phase IIa aimed to repeat Phase I, but include other layup techniques besides the glass prepreg used in the first phase.

We conducted three point bend tests (Figure 2 shows the test setup) on several 18″x10″x.25″ specimens, some fabricated with a VARTM technique and others were carbon prepreg. The load vs. strain curves generated by processing raw data from the fiber optic interrogator during the test was then match up with the results from a finite element model to validate the model and the accuracy of the tested strain values.

Phase IIb

After the initial proof-of-concept in Phase I and Phase IIa, Phase IIb will begin the development of the SHM algorithm. The testbed will be a composite specimen connected to a steel combing with 5 bolts. Damage will be incrementally introduced into the connection and we will test that our SHM algorithm can accurately detect damage in the connection.

The results of this phase are summarized in a conference paper published for

The 5th International Forum on Opto-electronic Sensor based Monitoring in Geo-engineering, Nanjing, China. Click the button to read the paper.

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