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Description  |
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FIELD OF THE INVENTION
This invention pertains to an integral Fabry-Perot stress corrosion, strain
and material corrosion sensor and associated system that allows for
in-situ monitoring of stress-corrosion, strain and corrosion at a given
location of a specimen in real-time. The sensor uses an intrinsic
Fabry-Perot cavity with one mirror integrally mounted on the specimen that
provides a means for: i) transducing information of stress-corrosion of
the specimen; ii) measuring strain of the specimen and iii) measuring the
corrosive environment.
BACKGROUND OF THE INVENTION
Fiber optic sensors are popular for use in detecting changes in
temperature, pressure and strain. Fiber optic sensors in which the fiber
itself acts as the sensor, are of interest in the context of advanced
smart structures that use composite materials in combination with metallic
components. This is due to the fact that the fiber is generally compatible
with both thermoset and thermoplastic composites. This makes the the fiber
optic sensor particularly useful when embedded in the composite/metallic
material to function as an in-situ measurement device.
In particular, the invention herein can be used in the aircraft industry
where failure of aluminum components is a major concern. Current
inspection techniques for such failures include manual processes that may
involve disassembling parts, thereby removing the plane from service for
costly extended time periods. The proposed invention herein solves such a
problem by providing an on-line system for remotely monitoring the
structural components that operate in a corrosive environment.
Pertinent prior art that uses the Fabry-Perot fiber optical cavity to
perform physical measurements of a structural object include U.S. Pat. No.
5,202,939 by Belleville et al. entitled "Fabry-Perot Optical Sensing
Device for Measuring a Physical Parameter." This patent teaches of a
device that uses an optical fiber Fabry-Perot optical cavity with a
Fizeau-interferometer for multimode light transduction of physical
parameters such as temperature, pressure and mechanical strain. This
teaching requires the use of a halogen-quartz based lamp type light
source, not a single mode light source, i.e. a diode laser as in the
present invention which effectively allows for corrosion measurements.
Moreover, this patent teaches away from using a single mode light
transmission optical fiber, which is the only mode used in the present
invention. Moreover, this reference does not teach or suggest using the
interferometer to measure corrosion.
U.S. Pat. No. 5,237,630 by Hogg et al. entitled "Fiber Optic Device with
Reflector Located at Splice Joint" teaches of how to make fiber optic
Fabry-Perot strain gauges and system for monitoring along with various
ways of making the optical cavity, and how to make a Fabry-Perot
reflecting surface. However, this patent does not teach or suggest the
means for detecting stress-corrosion or corrosion of a structural specimen
as discussed below.
Another teaching of a typical prior art Fabry-Perot strain sensors appears
in the article entitled Fabry-Perot fiber optic sensors in full scale
testing of the F-15 aircraft by Murphy, Applied Optics, January 1992, Vol.
31, No. 4, pages 431-433 deals exclusively with extrinsic measurements of
strain and does not teach or suggest using the sensor for corrosion
detection.
SUMMARY OF THE INVENTION
The present invention provides an integral Fabry-Perot stress corrosion,
strain and material corrosion sensor and associated system that allows for
in-situ monitoring of stress-corrosion, strain and corrosion at a location
of a structural specimen in real-time. The sensor uses an intrinsic
Fabry-Perot cavity with one mirror integrally mounted on the specimen that
provides a means for: i) transducing information of stress-corrosion of
the specimen; ii) measuring strain of the specimen; and iii) measuring the
corrosive environment.
OBJECTS OF THE INVENTION
Accordingly, several objects and advantages of the present invention are:
(a) To provide a Fabry-Perot stress-corrosion sensor and system that can
monitor stress-corrosion, strain and corrosion at a given location of an
object under observation at the same time using a single sensor.
(b) To provide a Fabry-Perot stress-corrosion sensor and system that can
monitor the physical condition of metallic structural components designed
to operate in corrosive environments.
(c) To provide a Fabry-Perot stress-corrosion sensor and system that can
monitor the physical condition of metallic structural components, and in
particular the stress-corrosion of aircraft components for safety reasons.
Still further advantages will become apparent from a consideration of the
ensuing detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a Fabry-Perot stress-corrosion sensor.
FIG. 2 shows a phase modulated sine wave of a fiber sensor signal to
produce an intensity signal to derive structural strain of a specimen with
respect to time.
FIG. 3 shows a phase modulated sine wave of a fiber sensor signal to
produce an intensity signal to derive structural corrosion of a specimen
with respect to time.
FIG. 4 shows a block diagram of the corrosion sensor and associated system
for detecting stress-corrosion, stress and corrosion.
FIG. 5 shows an example of experimental data that relates reflectivity as a
function of time that establishes the corrosion environment data for a
look-up table for the stress-corrosion apparatus. This data can also be
used to develop the look-up table for this apparatus.
FIG. 6 shows the Fabry-Perot sensor response to damped vibration of a
cantilever beam over a short time to provide strain information for the
look-up table.
DETAILED DESCRIPTION
The stress-corrosion sensor as shown in FIG. 1 is capable of monitoring
strain and corrosion simultaneously in the system as described below.
Stress-corrosion is the reduction in structural strength due to the
combined effects of mechanical loading and chemical corrosion. These
mechanisms alone can reduce strength, but a combination of mechanical
loading and corrosion leads to an accelerated reduction of mechanical
strength in a structural entity. The strain and corrosion are measured
using mutually exclusive optical phenomenon in an intrinsic Fabry-Perot
fiber optic sensor which is known in the optical interferometer arts. The
strain is measured by monitoring the optical phase changes created in the
all-Fabry-Perot interferometer, and the corrosion is measured by
monitoring the reflected intensity from the thin film full mirror used to
create the Fabry-Perot cavity. This mirror is fabricated from a metal
similar to that of a metallic structural specimen being monitored so that
the corrosion rate of the full thin film mirror and the structural
specimen are the same.
The Fabry-Perot sensor as shown in FIG. 1 uses light from an external
single mode lead-in optical fiber that launches light into the Fabry-Perot
cavity of the sensor which is partially reflected, shown as (I.sub.2), and
transmitted through the cavity, shown as (I.sub.3), through a thin film
partial mirror that is formed between two cleaved fibers. I.sub.3 is then
reflected from the thin film full mirror which is part of the structural
specimen and is transmitted back through the partial mirror. I.sub.2 and
I.sub.3 optically interfere to produce an intensity function of the form
I.sub.out =A+B cos (4.pi.nL/.lambda.); (1)
where A and B are constants, n is the refractive index of the cavity, L is
the cavity length, and .lambda. is the wavelength of the light introduced
into the single mode of the optical fiber connected to the sensor. This
equation can be simplified to the form:
I.sub.out =A+B cos (.phi.) (2)
where .phi.=4.pi.nL/.lambda. is the optical phase detected. Since the
cavity length, L, and the refractive index, n, are functions of strain,
the optical phase can be used to measure the strain in the Fabry-Perot
cavity. The optical fiber Fabry-Perot cavity is also used to measure
structural strain by bonding or embedding it to the structural specimen
where a metallic element is part of the structure. Structures that the
sensor is compatible with also includes thermoset and thermoplastic
composites and can function as an in-situ measurement device.
Phase generated carrier based demodulation can be used to extract the phase
angle .phi., from the intensity I.sub.out. This technique preferably uses
a sine wave modulation of a wavelength .lambda., to produce an intensity
signal at any instant in time similar to that shown in FIG. 2. The
peak-to-peak value, shown as (2B), of this periodic function is governed
by the reflectivities of thin film full mirror and thin film partial
mirror as shown in FIG. 1.
When the thin film full mirror of the Fabry-Perot cavity is exposed to a
corrosive environment, the reflectivity of that mirror will be diminished
with time. This reduced reflectivity and resulting reduction in the
peak-to-peak value (2B) as shown in FIG. 3 do not change the actual phase
measurements obtained with time, but can be used to measure the corrosion
of the thin film full mirror that forms part of the observed structure
being monitored.
As corrosion attacks the coating material, its thickness and dielectric
properties are altered so as to reduce the reflectivity, i.e. defined as
I.sub.out /I.sub.in of the optical cavity, generally made of glass and the
metal interface under observation. Choosing the metallic coating to
possess a corrosion rate similar to the materials used to fabricate the
structural component enables remote monitoring of corrosion in the
structural system. Methods of making the thin film full mirror on the
structure is provided by information in U.S. Pat. No. 5,237,630 by Hogg et
al. entitled "Fiber Optic Device with Reflector Located at Splice Joint"
which is incorporated by reference e.g. the sputtering technique. The
Fabry-Perot sensor is surface mounted or adhered to the structural member
specimen being sensed.
The actual structural member being sensed with the Fabry-Perot corrosion
mirror can be aluminum, aluminum alloys, copper as well as other mettalic
materials that exhibit reflective properties. For example, aluminum is a
metal resistant to corrosion that justifies its' use in many aircraft
components. The higher the purity of aluminum, the better the corrosion
resistance becomes. Aluminum resistance to corrosion is due to a thin
oxide film that quickly grows to a limiting value on the surface once
exposed to oxygen or water contained in air or water. Under normal
atmospheric conditions, the oxide is about 5.0 nm thick, but can increase
if water vapor is present. Moreover, in a marine atmosphere, corrosion of
aluminum is produced by the amount of air-borne salt which when combined
with oxygen and moisture provides the electrolytic solution that initiates
corrosion. The dominant type of corrosion is pitting or plating out at
localized sites of the specimen. The invention can be used in such harsh
environments.
The corrosion sensor concept compliments the fatigue sensor in that the
transduction mechanism used for corrosion detection is independent of the
transduction mechanisms used to detect mechanical loading induced
degradation. In brief, the corrosion sensing mechanism is intensity based,
and therefore susceptible to noise due to fluctuations in optical source
intensity and optical fiber loss mechanisms such as microbending. Source
intensity can be independently monitored to eliminate source noise
effects. Other optical fiber loss mechanisms can be eliminated because
their time dependence is generally traceable to mechanical loading that
will therefore occupy a distinct and filterable region of the frequency
spectrum.
The instrumentation required to delineate the signals associated with each
mechanism can be integrated into a single unit using very reliable
detection techniques. FIG. 4 shows a preferred system configuration in
block form. The components include a temperature controlled diode laser
with optical isolator pair to provide a monochromatic light source capable
of wavelength modulation. The isolator prevents any reflected light from
entering the laser cavity. This light is launched through a polarization
controller into one port of a 2.times.2 coupler which splits the light
between the multitasking strain/corrosion sensor formed by the intrinsic
Fabry-Perot cavity whose corrosion mirror is integrally mounted on the
structural specimen and the photodetector D2. D2 is used to monitor
fluctuations in the source intensity for use in noise cancellation in the
corrosion sensing task of the sensor. The light entering the
lead-in/lead-out fiber reaches the sensor, is modulated by the strain
field that is taking place, and is subsequently reflected back to
photodetector D1. The sensor takes the form of any lead insensitive
intrinsic Fabry-Perot optical fiber sensor. The signal from D1 is
normalized by D2 and an envelope detector is used to extract the
normalized back-reflected intensity at any instant in time as depicted in
FIG. 3. The normalized reflected intensity is then processed through an
experimentally developed look-up table to convert intensity to corrosion
for outputting to a proper output means for indication or display of
stress-corrosion as shown in FIG. 4 of the specimens existing state. The
optical configuration shown in FIG. 4 can be fabricated with standard
optical fiber components or use high birefringent optical fibers for the
Fabry-Perot sensor and optical fiber means to the sensor.
The signal recorded by D1 in FIG. 4 is also tapped-off to the phase
demodulation electronic means such as serrodyne as taught by Jackson et
al. in Pseuodoheterodyne Detection Scheme for Optical Interferometry, in
Elect. Letters, 18, pp. 1274-78, 1982 or a Homodyne demodulation technique
as taught by Danbridge et al. in Homodyne demodulation Scheme for Fiber
Optic Sensors Using Phase Generated Carriers, in IEEE Journal Quantum
Electronics, QE-18(10), pp. 1647-53, 1982 which provides the phase history
of the structural specimen. The phase history exiting the demodulation
electronic means is directly dependent on the indigenous strain history of
the structural specimen. These signals can be processed with a
microprocessor based calibration look-up tables for example by providing
the state of fatigue of the structural specimen and reveal the remaining
fatigue life. This allows for monitoring the physical condition of
metallic structural components designed to operate in corrosive
environments, and in particular alert maintenance personnel of the
stress-corrosion of aircraft components as to the safety thereof.
The look-up table shown in FIG. 4 can be incorporated in a microprocessor
or be incorporated in a central processing unit of a monitored structural
entity. The look-up table is determined by subjecting the Fabry-Perot
sensor to a series of tests in which the load history and the corrosive
enviromment are changed. The resulting time to failure and the
intensity-corrosion calibration curves for each combination of load and
corrosive environment are recorded and used in the look-up tables for the
apparatus. This set of data is material-specific and is used for comparing
actual Fabry-Perot sensor data to estimate the remaining life of the
structural component.
FIG. 5 shows an example of experimental data that relates reflectivity as a
function of time that establishes required corrosion input data for the
stress-corrosion look-up table for the apparatus shown in FIG. 4. The
reflectivity data is a function of time under relatively constant
temperature, humidity and salinity. For example, this data is found by
placing an unstrained Fabry-Perot sensor in an environmental chamber with
a temperature of 45+/-5.degree. C. and a humidity of approximately 72%.
This data shows the relationship between the metal reflectivity of the
Fabry-Perot sensor and the length of exposure to a corrosive environments.
FIG. 6 shows the Fabry-Perot sensor response to damped vibration of an
experimental cantilever beam which provides information of the strain
induced properties of the material during a very short time period that is
used for data incorporated with the look-up table.
Although the description above contains many specificities, these should
not be construed as limiting the scope of this invention but as merely
providing illustrations of some of the presently preferred embodiments of
this invention.
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