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

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Gradient Image

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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

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Raw Data

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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?