Structural Health Monitoring

plantcitybusinessUrban and Civil

Nov 26, 2013 (3 years and 4 months ago)

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Use of Fiber Optic Sensors in
Structural Health Monitoring

Joshua
H
i nnebusch

&
D
ani el
W
ri ght

Structural Health Monitoring (SHM)



Monitoring qualities of structures to assess overall structure health



Common qualities inspected:


Cracking


Deflection (bridges)


Vibrations/movement of a structure


Corrosion of metal elements



Classical methods of monitoring:


Visual inspection


Outdated technology


Engineers/civil workers

The Problem



‘Classical’ technology is becoming outdated



Visual inspection can be inaccurate


Only surface abnormalities can be found


Accuracy depends on human element


can vary greatly



Issues not found can lead to catastrophic failure of structures


Time and money spent to rebuild structures


Hinder efficiency of transportation/business


More prevalent in second/third
-
world countries


Fiber Optic Technology



First demonstrated during 1840s



Broad applications


Telecommunications


Illumination


Sensors



Immune to electrical & magnetic interference



Typically manufactured using silica


Fluorides and crystalline materials also used


Different elements can change heat
-
resistance


Applying Fiber Optic Sensors to SHM



Crack detection within bridges/structures



Monitoring bridge deflection



Vibration sensing



Monitoring steel rebar corrosion

Crack Discovery in Structures



Concrete can not flex, so
microcracks

can form


Caused by temperature changes, traffic, among others


Surface or internal



Current SHM methods are not
efficient or reliable


Visual inspection by workers


Inspection quality varies on the inspector


Very time
-
consuming



Visual inspections detect surface cracking only


Leaves internal cracks to increase in size


May lead to serious structural issues

Crack Discovery in Structures



Fiber optic sensing technology: SMARTape


Thin (.20mm) strip of thermoplastic tape


Embedded fiber optic cable


Stretches (strains) when a crack is present


Straining alters optic beam send through fiber


Used with the DITEST system


Signal pumped through length of cable


Output monitored to determine location of
cracking


Installation


Surface
-
mounted (concrete or steel)


Embedded within concrete during construction


Crack Discovery in Structures


Integration within structure allows for early detection of cracks


Allows for repair before problem becomes too serious


Solves problems of visual inspection


Able to be applied to existing structures


surface only


Can help increase the lifespan of a structure


Removes human element


More accurate & efficient


Readings can be given in as little as 5 minutes



Monitoring Bridge Deflection



Deflection
-

Relative vertical displacement by a span


Determines load capacity and overall well
-
being of the structure



Current methods of measuring deflection


Direct connection of bridge span and ground


Surveying crews measure spans over time


GPS to measure coordinates (Z+ and Z
-

for displacement)


Monitoring Bridge Deflection



Fiber optic sensing technology: Liquid
-
leveling transducers


Liquid
-
containing vessels (usually water)


Connected via pipe system


liquid flows freely between vessels


Archimedes’ principle of buoyancy applied


Force = Displacement


Uses fiber optic load sensors to measure height of float



Vessels mounted along bridge spans


Can be installed on existing structures



System output examined by inspectors


Can be accessed at any time, in real
-
time via fiber optic network


Deflections compared to reference measurement
-

located on ground


Monitoring Bridge Deflection





Transducers are self
-
sustaining


No outside anchoring (direct connection)


Minimal human interaction (surveying)


No reliance on satellites (GPS)



Simple design


Utilizes centuries
-
old principle of buoyancy


Easy integration into existing structures and new constructions


Sensing Vibrations Within Structures



Accelerometers made using network of fiber optic cables


A mass
-
spring system is used to allow a reading of acceleration


The mass
-
spring system is fitted with a discontinuous fiber optic cable system


The ends of the fiber optic cable have lines of transparent and opaque material (grating A and B) that are
used to transmit or block light transfer between the cables


The net intensity of light is measured and is used to calculate



the acceleration of the building



The correlation between acceleration and vibration


The acceleration determines the rate at which



the speed of an object changes


In this context, the speed is used to determine



the oscillation of the building




Sensing Vibrations Within Structures



Structural effects from vibrations


Blatantly speaking, building vibrations are to be mitigated


Every structure has a ‘normal frequency’ at which it vibrates most sporadically


Sensors that are used to measure this oscillation can be used to determine if a structure is close to
naturally vibrating near its normal frequency, that is very detrimental to its integrity.



Corrosion
in Structures


Corrosion, in particular steel, is one of the leading causes to structural disasters


Corrosion greatly lowers the strength and endurance of structures. Therefore, it is sought to be
eliminated by any means possible


This also raises the cost of maintaining structures immensely, if not detected early




Monitoring Steel Corrosion



Monitoring Structures’ Corrosion


Structures will be outfitted with sensors that include a series of fiber optic cables through which light is
transmitted


The change in intensity of light is used to interpret the impurities that are in the cable itself


This raw data is used to calculate the amount of corrosion INSIDE the structure, not just the exterior

Iron
-
coated film

Most light is reflected

Small fraction of light

i
s reflected

Light escapes core

of fiber

Monitoring Steel Corrosion



How to use this system


The fiber optic cables are implanted with small amounts of iron and other various materials


The iron is in an environment identical to the interior of the structure (
i.e.
what is being monitored)


The iron inevitably reacts with surrounding oxygen, forming iron oxide


what we consider to be
corrosion


This level of iron oxide is measured from the corresponding change in light intensity from the raw data



Benefits from contemporary and Existing Systems


Compared to competing sensor technology, this system can be measured to be on the order of
centimeters, thus severely diminishing the cost of implanting into structures, as well as maintaining the
integrity of the structures to a higher degree thanks to a smaller size factor


Availability of fiber optic technology due to its use in telecommunications and electronics.


Earlier detection of corrosion, saving an uncertainly vast amount of money for a myriad of projects

Ethical Impacts



Benefits over contemporary systems


As previously stated, fiber optic technology is implemented in other forms of technologies,
e.g.
telecommunications, electronics, and illumination


Concrete success is implied by widespread use of fiber optics.


The use of these cables are not affected by electromagnetic and magnetic fields due to the complete
absence of an electric current in fiber optic cables


Less interference is proceeded by more accurate results


The new technology can accurately detect structural defects more quickly by its easy retrofitting into
older buildings


This vastly reduces the labor needed to maintain structures if a problem is detected sooner and when it
is not as augmented



Sustainability Factors



Economic


F
iber optic sensors in SHM aids in detecting problems before they become serious


The sooner a problem is found, the sooner it can be repaired


Minor repairs cost less than replacing an entire structure


Sensors and associated technology are inexpensive



Efficiency


Immune to electrical and magnetic interference


Major bridge repairs/failures are detrimental to traffic flow


Durable sensors can outlive their host structures