Steven Chase1, Yaw Adu-Gyamfi2 and Paul Fuchs3 1&2Department of Civil Engineering, University of Virginia 3Fuchs Consulting Inc. Explain why this is an important advance in bridge inspection Introduce thermoelasticity Provide overview of the project Answer any questions Have you driven over a highway bridge recently? ◦ Yes ◦ No Root cause of many collapses and failures The current practice is to rely on hands on visual inspection ◦ ◦ ◦ ◦ Tedious Expensive Dangerous Unreliable Damage already exists 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. 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 The relationship between the temperature change and the strain in the object is expressed as: α ∆ = + 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 α ∆ = − 0 Neglect if stress change occurs fast enough ∆ = −Δ(1 + 2 ) – Thermoelastic constant, α 1 , 2 – principal stresses. – 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 Initial project funded by MAUTC Stress Concentrations Simulated TSA response 50 50 100 100 150 150 200 200 250 250 50 100 150 200 250 300 • • • • 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 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. 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 Uncooled micro-bolometer camera Frame rate: 60 Hz Resolution: 256 by 324 pixels 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 3450 10 3440 10 20 3430 20 30 3420 40 30 3410 40 50 3400 50 60 3390 60 70 70 3380 80 3370 80 10 20 30 40 3360 90 20 40 60 Crack 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 Computer Camera Viewing a Detail 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 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. 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.