Fiber optic acousto-ultrasonic transducers offer numerous applications as embedded sensors for
impact and damage detection in industrial and aerospace applications as well as non-destructive
evaluation. Superficial contact transducers with a sheet of fiber optic Bragg gratings has been
demonstrated for guided wave ultrasound based measurements. It is reported here that this method
of measurement provides highly reproducible guided ultrasound data of the test composite
component, despite the optical fiber transducers not being permanently embedded in it.
Fiber optic hydrophones are useful for a variety of underwater monitoring applications, as they offer high sensitivity,
signal-to-noise ratio and multiplexing ability for marine acoustics and offer longer term reliability than conventional
electronic hydrophones. The frequency response of packaged fibers depends strongly on the material and mechanical
parameters of the sensor design. Two fiber bragg-grating (FBG)-based hydrophones are described and their frequency
response is measured. One consists of a diaphragm-linked FBG, and another is a polymer coated FBG. While the
diaphragm-linked FBG has predictable resonances, resonance features on the simpler coated FBG hydrophone are
observed as well.
Guided wave ultrasound is well suited for inspection of laminate composite structures. Compared to nearly flat or gently
curved composites, performing accurate NDT on sharply curved structures is more complex with standard ultrasound
test methodologies, such as pulse-echo methods. Ultrasound propagation in curved composite structures is investigated
for sharply curved geometries. Responses are predicted based on dispersion models. Experimental results are presented
on 0.25 inch thick curved carbon fiber reinforced plastic (CFRP) composite geometry of an aircraft structural component
and compared with predicted values.
Passive acoustic monitoring hydrophones with low power consumption for autonomous underwater vehicles (AUVs) are
desirable for long term unmanned monitoring of ocean acoustics including marine mammal acoustics as well as those
due to human activity. Fiber-optic hydrophones offer wider bandwidth and high sensitivity alternatives to conventional
piezoelectric transducer (PZT) devices. Deployment on board AUVs requires operation under a wide range of
temperature and pressure conditions that change with depth and location, hence, maintaining the sensitivity and
reliability over the operating range is crucial.
A read-out mechanism for a resonant hydrophone using a fiber Bragg grating (FBG) transducer is described. The read-out
uses a temperature tuned DFB laser diode to compensate for FBG changes with temperature and depth, enabling
operation over a wide temperature range. Its compact footprint and battery-powered readout system operation enables
portability on AUVs.
Passive hydrophones with a minimal footprint are useful for a variety of underwater monitoring applications; a fiber
optic hydrophone is being investigated. Fiber optic hydrophones have been used in offshore moored applications, and
have the capability to cover large areas. A resonant hydrophone using a fiber Bragg grating (FBG) transducer for limited
bandwidth operation is described and compared with predicted diaphragm resonances. A battery-power readout system
using a laser diode source that can typically operate for a full day, and can be used in off-mooring applications such as in
autonomous underwater vehicles, is outlined.
Non-destructive testing of critical structural components is time consuming, while necessary for maintaining safe
operation. Large aerospace structures, such as the vertical stabilizers of aircraft undergo inspection at regular intervals
for damage diagnostics. However, conventional techniques for damage detection and identification before repair can be
scheduled are conducted off-line and therefore can take weeks. The use of guided ultrasound waves is being investigated
to expedite damage detection in composites. We measure the frequency dependent loss of ultrasonic guided waves for a
structure comprising a boron-nitride composite skin sandwiching an aluminum honeycomb. A wide range of ultrasound
frequencies propagate as measured using PZTs, with the lowest attenuation observed about 200-250 kHz. These
measurements are confirmed using optical fiber Bragg grating arrays used as ultrasound transducers.