Steven Chase1, Yaw Adu-Gyamfi2 and Paul Fuchs3
1&2Department
of Civil Engineering, University of Virginia
3Fuchs Consulting Inc.
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Explain why this is an important advance in
bridge inspection
Introduce thermoelasticity
Provide overview of the project
Answer any questions
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Have you driven over a highway bridge
recently?
◦ Yes
◦ No
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Root cause of many collapses and failures
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The current practice is to rely on hands on
visual inspection
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Tedious
Expensive
Dangerous
Unreliable
Damage already
exists
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Were you concerned about the safety of the
bridge you drove over?
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Did not even think about it.
I was concerned.
I was terrified.
I don’t drive.
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Develop a device that automates detection and
monitoring of fatigue cracks on steel bridge by
imaging dynamic stress concentrations at fatigueprone details.
◦ Identify and quantify
precursors to fatigue cracks
◦ Improve detection of
existing cracks
◦ Assess effectiveness
of any repair or
retrofit actions
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The relationship between the temperature change and
the strain in the object is expressed as:
α𝑇
∆𝑇 =
𝜌𝐶𝜀
𝛿𝜎
𝑄
𝜀+
𝛿𝑇
𝜌𝐶𝜀
α – coefficient of thermal expansion
𝜀 – strain change.
𝛿𝜎 – stress change.
𝐶𝜀 - specific heat at constant strain.
𝑇 – absolute temperature of the material.
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Second term: Temperature change due to
conduction
α𝑇
∆𝑇 =
𝜌𝐶𝜀
𝛿𝜎𝑖𝑗
𝑄
𝜀𝑖𝑗 −
𝛿𝑇
𝜌𝐶𝜀
0
Neglect if stress change occurs fast enough
∆𝑻𝒕𝑬 = −𝐾𝑇Δ(𝜎1 + 𝜎2 )
𝐾 – Thermoelastic constant,
α
𝜌𝐶𝜀
𝜎1 , 𝜎2 – principal stresses.
𝑇 – absolute temperature of the material.
Computer Simulations
Laboratory Testing
Load Characterization
Key Components
Data Capture
Data Acquisition
Field Deployment
Set up and Data
Acquisition
Signal Processing
Signal Processing
Results
Results
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Initial project funded by MAUTC
Stress Concentrations
Simulated TSA response
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First TSA image obtained with proof of concept project.
Demonstrated the use of a low cost camera was feasible
MS Thesis by Matt Kantner
Follow on project funded by VCTIR to develop field system
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Is this explanation of the thermoelastic effect
understandable
◦ Yes
◦ You lost me with the first equation
◦ I think I understand
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Develop ability to use random events
Design and develop a complete field system
Test capabilities and limits in laboratory
Conduct field tests on actual bridge with
cracks
MTS Hydraulic Grip.
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Steel Specimen: with ½ inch diameter
Hole in plate.
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Loading frame holds specimen.
◦ Used to apply loads to specimen.
◦ Actuator creates the forces.
◦ Computer controller coordinates actuator
movement.
Flat Plate with Hole Specimen
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Uncooled micro-bolometer camera
Frame rate: 60 Hz
Resolution: 256 by 324 pixels
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Embedded computer and touch
screen interface
DAQ system: acquires data from
load cell and IR camera
simultaneously
High level signal from MTS or lowlevel signal from strain gage
interface
Threshold
Time
Pre-trigger duration
Post-trigger duration
Trigger Occurs (Begin Data Logging)
Total samples per trigger (Strain Gauge and Infrared Camera )
Temperature Variation
Loading Event
Correlation
Denoised output
Raw Data
Thermoelastic Response
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3390
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Crack
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Deploy system for extended period of time
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Acquired data triggered by multiple truck events
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Extract stress concentration of fatigue prone details due
to random truck loading
Field Computer
Camera Viewing a Detail
Large dynamic stresses due to heavy truck loads at
connection plate web weld termination
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Do you think the TSA system is a valuable
addition to the methods available to detect
and evaluate fatigue cracks?
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Yes
No
Don’t know
Depends on cost, ease of use and interpretation of
data collected
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A TSA system has been developed based on a low-cost
microbolometer thermal imager, a dedicated field
computer (for triggering data acquisition) and signal
processing algorithms for extracting small changes in
stress associated with dynamic loading events.
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The TSA system has been validated with computer
simulations, laboratory and field tests.
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Laboratory and field testing show that at moderate to
high stress levels, the TSA system can be used to image
stress concentrations.
The system will be delivered to VCTIR and will be
deployed in high stress locations in the future.
Future implementation will focus on introducing this
new method to bridge owners in US and globally.
FUCHS CONSULTING
INC.