The oil leakage of off-shore pipelines will cause ocean contamination and economic losses. These accidents may happen
by the failures of offshore pipelines due to corrosion, impulse and free-spanning. So, it is very urgent on pipeline health
monitoring. Fiber optic distributed sensors should be used to know when and where failures may occur. In this study, a
feasibility of BOTDA (Brillouin Optical Time Domain Analysis) system is studied on off-shore pipeline distributed
strain monitoring influenced by free spanning. Strain distribution of an off-shore pipeline is calculated by numerical
analysis and strain measurement experiments are carried on a beam bending test using BOTDA system. BOTDA could
be an excellent tool to monitor the long-distance pipeline.
A fiber optic Bragg grating based acoustic emission sensor system is developed to provide on-line monitoring of cracks
or leaks in reactor vessel head penetration of nuclear power plants. Various type of fiber Bragg grating sensor including
the variable length of sensing part was fabricated and prototype sensor system was tested by using PZT pulser and pencil
lead break sources. In this study, we developed a cantilever type fiber sensor to enhance the sensitivity and to resonant
frequency control. Two types of sensor attachment were used. First, the fiber Bragg grating sensor was fully bonded to
the surface using bonding agent. Second one is that one part of fiber was partially bonded to surface and the other part of
fiber will be remained freely. The resonant frequency of the fiber Bragg grating sensor will depend on the length of
sensing part. Various kinds of resonant type fiber Bragg grating acoustic emission sensors were developed. Also several
efforts were done to enhance the sensitivity of FBG AE sensor, which include FBG spectrum optimization and electrical
and optical noise reduction. Finally, based on the self-developed acquisition system, a series of tests demonstrate the
ability of the developed fiber sensor system to detect a pencil lead break event and continuous leak signal.
There are to be some cracks on the material degradation part or the stress concentration parts of the main members,
which carry on over-loads, of structures. Because these cracks can be used to evaluate the structural health status, it is
important to monitor the crack growth for maintaining the structural safety. In this study, the fiber Bragg grating sensor
with a drop ball was developed as a sensor for crack growth detection of an existing crack. The crack growth detection
sensor was constructed with three parts: a probe part, a wavelength control light source and receiver part, and an impact
part. The probe part was just formed with a fiber Bragg grating optical fiber. The wavelength control light source part
was composed of a current supplying circuit, a DFB laser diode, and a TEC control circuit for wavelength control. Also,
the impact part was just implemented by dropping a steel ball. The performance of this sensor was confirmed by the
experiments of the crack detection with an aluminum plate having one existing crack. According to these experiments,
the difference of the sensor signal outputs was correlated with the crack length. So, it was confirmed that this sensor
could be applied to monitor the crack growth.
Acoustic emission (AE) has emerged as a powerful nondestructive tool to detect preexisting defects or to characterize
failure mechanisms. Recently, this technique or this kind of principle, that is an in-situ monitoring of inside damages of
materials or structures, becomes increasingly popular for monitoring the integrity of large structures. Concrete is one of
the most widely used materials for constructing civil structures. In the nondestructive evaluation point of view, a lot of
AE signals are generated in concrete structures under loading whether the crack development is active or not. Also, it
was required to find a symptom of damage propagation before catastrophic failure through a continuous monitoring.
Therefore we have done a practical study in this work to fabricate compact wireless AE sensor and to develop diagnosis
system. First, this study aims to identify the differences of AE event patterns caused by both real damage sources and
the other normal sources. Secondly, it was focused to develop acoustic emission diagnosis system for assessing the
deterioration of concrete structures such as a bridge, dame, building slab, tunnel etc. Thirdly, the wireless acoustic
emission system was developed for the application of monitoring concrete structures. From the previous laboratory study
such as AE event patterns analysis under various loading conditions, we confirmed that AE analysis provided a
promising approach for estimating the condition of damage and distress in concrete structures. In this work, the
algorithm for determining the damage status of concrete structures was developed and typical criteria for decision
making was also suggested. For the future application of wireless monitoring, a low energy consumable, compact, and
robust wireless acoustic emission sensor module was developed and applied to the concrete beam for performance test.
Finally, based on the self-developed diagnosis algorithm and compact wireless AE sensor, new AE system for practical
AE diagnosis was demonstrated for assessing the conditions of damage and distress in concrete structures.
Optical fiber mesh can be applied on some fences to secure some protective facilities. It is necessary to give the fiber
macro bending signal in order to detect an intruder at the initial intrusion stage with the off signal from fiber cutting.
Therefore, in this work, we made two step signal processing algorithm of the optical fiber mesh for detecting the intruder
effectively. First step is composed of the discrimination processing whether the fiber has some macro bending or not. If
the signal tells the macro bending, then it means that some intruder tries to invade. Second step is the signal destruction
by cutting the optical fiber. This step means that the intruder invaded by cutting the optical fiber mesh.
Continuous monitoring of soil movements is important to maintain civil structures. Fiber optic sensors can give the sensing solution for continuous displacement monitoring because they can be multiplexed with various sensors on one line of optical fiber, have the durability to harsh environments, and also are not affected by electro-magnetic effects. In this study, we work on the development of a multiplexed optical loss based fiber optic sensor for measuring displacements using the signal difference between the two reflected signals from a pair of optical connectors with various bending losses between them through an optical time domain reflectometer. We fabricate a multiplexed optical loss based fiber-optic sensor detecting linear displacements of 5 measuring positions of an object by setting these new 5 fiber-optic sensor probes on a single mode fiber simultaneously. We find that these sensors have a good capability to measure the displacements.