Development of Thermoelastic Stress Analysis as a Practical Bridge Inspection Method

1. 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 Virginia 3Fuchs Consulting Inc.

2. Outline • Explain why this is an important advance in bridge inspection • Introduce thermoelasticity • Provide overview of the project • Answer any questions

3. Poll Question #1 • Have you driven over a highway bridge recently? • Yes • No

4. Fatigue and Fracture on Bridges • Root cause of many collapses and failures

5. Current Approach • The current practice is to rely on hands on visual inspection • Tedious • Expensive • Dangerous • Unreliable • Damage already exists

6. Poll Question #2 • 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.

7. Project Goals • 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

8. What is the Thermoelastic Effect?

9. Thermoelasticity • The relationship between the temperature change and the strain in the object is expressed as: α – coefficient of thermal expansion – strain change. – stress change. - specific heat at constant strain. – absolute temperature of the material.

10. Thermoelasticity • Second term: Temperature change due to conduction 0 Neglect if stress change occurs fast enough

11. Thermoelastic Signal – Thermoelastic constant, – principal stresses. – absolute temperature of the material.

12. Research Methodology Computer Simulations Laboratory Testing Field Deployment Set up and Data Acquisition Load Characterization Key Components Data Capture Data Acquisition Signal Processing Signal Processing Results Results

13. Proof of Concept Testing • Initial project funded by MAUTC

14. Summary of Simulations Stress Concentrations

15. TSA Concept

16. 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

17. Poll Question #3 • Is this explanation of the thermoelastic effect understandable • Yes • You lost me with the first equation • I think I understand

18. Follow on VCTIR project • 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

19. Laboratory Testing MTS Hydraulic Grip. • 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. Flat Plate with Hole Specimen

20. Thermal Camera • Uncooled micro-bolometer camera • Frame rate: 60 Hz • Resolution: 256 by 324 pixels

21. IR-TSA System • 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

22. Pre and Post Trigger Data Acuisition Threshold Time Pre-trigger duration Post-trigger duration Trigger Occurs (Begin Data Logging) Total samples per trigger (Strain Gauge and Infrared Camera )

23. Signal Processing (Lock-In) Temperature Variation Loading Event Correlation Denoised output

24. Processed Results Raw Data Thermoelastic Response

25. Fatigue Crack Detection Crack

26. I-64 Delta Frame Bridge near Lexington, VA

27. Field Test Goals • 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

28. Field Deployment and Testing Field Computer Camera Viewing a Detail

29. Field Test Result Large dynamic stresses due to heavy truck loads at connection plate web weld termination

30. Poll Question # 4 • 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

31. Summary • 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.

32. Concluding Remarks • 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.

33. Acknowledgements FUCHS CONSULTING INC.

34. Questions?