Development of Thermoelastic Stress Analysis as a Practical Bridge Inspection Method
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Transcript of Development of Thermoelastic Stress Analysis as a Practical Bridge Inspection Method
Development of Thermoelastic Stress Analysis as a Practical Bridge
Inspection Method
Steven Chase1, Yaw Adu-Gyamfi2 and Paul Fuchs3
1&2Department of Civil Engineering, University of Virginia3Fuchs Consulting Inc.
Explain why this is an important advance in bridge inspection
Introduce thermoelasticity Provide overview of the project Answer any questions
Outline
Have you driven over a highway bridge recently?◦ Yes◦ No
Poll Question #1
Root cause of many collapses and failures
Fatigue and Fracture on Bridges
The current practice is to rely on hands on visual inspection◦ Tedious◦ Expensive◦ Dangerous◦ Unreliable
Damage already exists
Current Approach
Were you concerned about the safety of the bridge you drove over?◦ Did not even think about it.◦ I was concerned.◦ I was terrified.◦ I don’t drive.
Poll Question #2
Develop a device that automates detection and monitoring of fatigue cracks on steel bridge by imaging dynamic stress concentrations at fatigue-prone details.◦ Identify and quantify precursors to fatigue cracks◦ Improve detection of existing cracks◦ Assess effectiveness of any repair or retrofit actions
Project Goals
What is the Thermoelastic Effect?
The relationship between the temperature change and the strain in the object is expressed as:
Thermoelasticity
∆ 𝑇=α 𝑇𝜌𝐶 𝜀
∑ 𝛿𝜎𝛿𝑇
𝜀+𝑄𝜌𝐶 𝜀
α – coefficient of thermal expansion – strain change. – stress change. - specific heat at constant strain. – absolute temperature of the material.
Second term: Temperature change due to conduction
Thermoelasticity
∆ 𝑇= α 𝑇𝜌𝐶 𝜀
∑ 𝛿𝜎 𝑖𝑗
𝛿𝑇𝜀𝑖𝑗−
𝑄𝜌𝐶𝜀
0Neglect if stress change occurs fast enough
Thermoelastic Signal
∆𝑻𝒕𝑬=−𝐾𝑇 Δ(𝜎1+𝜎 2)
– Thermoelastic constant, – principal stresses. – absolute temperature of the material.
Research MethodologyComputer Simulations Laboratory Testing Field Deployment
Load Characterization
Data Capture
Signal Processing
Results
Key Components
Data Acquisition
Signal Processing
Results
Set up and Data Acquisition
Initial project funded by MAUTC
Proof of Concept Testing
Summary of Simulations
Simulated TSA response
50 100 150 200 250 300
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Gradient Image
50 100 150 200 250 300
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Stress Concentrations
TSA Concept
Successful Proof of Concept
• 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
Is this explanation of the thermoelastic effect understandable◦ Yes◦ You lost me with the first equation◦ I think I understand
Poll Question #3
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
Follow on VCTIR project
Steel Specimen: with ½ inch diameter Hole in plate.
Loading frame holds specimen.◦ Used to apply loads to specimen.◦ Actuator creates the forces.◦ Computer controller coordinates actuator
movement.
Laboratory Testing
MTS Hydraulic Grip.
Flat Plate with Hole Specimen
Uncooled micro-bolometer camera Frame rate: 60 Hz Resolution: 256 by 324 pixels
Thermal Camera
Embedded computer and touch screen interface
DAQ system: acquires data from load cell and IR camera simultaneously
High level signal from MTS or low-level signal from strain gage interface
IR-TSA System
Pre and Post Trigger Data Acuisition
Total samples per trigger (Strain Gauge and Infrared Camera )
Pre-trigger duration Post-trigger duration
Trigger Occurs (Begin Data Logging)
Time
Threshold
Signal Processing (Lock-In)
Correlation
Denoised output
Loading Event Temperature Variation
10 20 30 40
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Raw Data
20 40 60
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Thermoelastic Response
Processed Results
Fatigue Crack Detection
Crack
I-64 Delta Frame Bridge near Lexington, VA
Deploy system for extended period of time
Acquired data triggered by multiple truck events
Extract stress concentration of fatigue prone details due to random truck loading
Field Test Goals
Field Deployment and TestingCamera Viewing a DetailField Computer
Field Test Result
Large dynamic stresses due to heavy truck loads at connection plate web weld termination
Do you think the TSA system is a valuable addition to the methods available to detect and evaluate fatigue cracks?◦ Yes◦ No◦ Don’t know◦ Depends on cost, ease of use and interpretation
of data collected
Poll Question # 4
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.
The TSA system has been validated with computer simulations, laboratory and field tests.
Summary
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.
Concluding Remarks
Acknowledgements
FUCHS CONSULTING INC.
Questions?