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Paint Cues Scientists to Bridge Stress

Thursday, January 12, 2012

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Some people watch paint dry. Materials scientist Mark Iadicola watches it move—speck by speck.

Iadicola’s work—capturing and analyzing 3D images of infinitely subtle paint movement—is part of a critical federal-level research effort now focusing on the integrity of bridges and other steel structures.

Photos: NIST

Federal Highway Administration engineer Justin Ocel points out slight changes in painted steel to engineer Mark Iadicola of the National Institute of Standards and Technology. The changes can signal changes in structural integrity.

Led by Iadicola’s employer, the National Institute of Standards and Technology (NIST), and detailed in a video by NIST, the research was spurred by the catastrophic 2007 collapse of the I-35 bridge in Minneapolis.

Component Failure Cited

About 1,000 feet of the bridge’s main deck truss collapsed, dropping a section 108 feet into the Mississippi River. Thirteen people died, and 145 were injured.

Federal investigators traced the collapse to the failure of a critical bridge component called the gusset plate, a flat, heavy piece of steel bolted in pairs to connect the ends of the steel members that make up the bridge truss.

The National Transportation Safety Board determined that, as a result of a design error decades before, the gusset plates in the I-35 bridge were about half as thick as they should have been.

The collapse highlighted the fact that gusset plates were not included in load ratings, said engineer Justin Ocel, of the Federal Highway Administration (FHWA).

That's because it was long assumed that the plates were properly sized to be stronger than the members they connect, according to NIST.

Developing Data

Working with the American Association of State Highway and Transportation Officials (AASHTO), FHWA then tried to develop guidance on how to load-rate gusset plates.

Finding little data on failure modes of the plates in major bridges, however, the team was forced to develop its own.

The current research was prompted by the catastrophic collapse of the I-35 Bridge over the Mississippi River in Minneapolis in 2007.

So engineers decided to build full-scale models of bridge gusset plate joints and pull them apart with a huge hydraulic test machine at FHWA’s Turner-Fairbank Highway Research Center in Virginia.

The goal: to determine what makes gusset plates fail, under normal and extreme conditions.

Studying Speckles

Enter Iadicola, a mechanical engineer who was there to watch what happened as the plate stretched and failed. He covered the plate with an irregular pattern of paint speckles and then trained a pair of carefully calibrated, high-definition digital cameras on it.

Using a digital imaging technique known as photogrammetry, researchers can watch the failure of gusset plates in what NIST calls “exquisite detail.”

The cameras take repeated images of the plate and send them to a computer. Custom software then compares each image to the previous one and calculates which of the paint spots have moved, in what direction, and by how much, according to NIST.

“Using two cameras allows the computer to ‘see’ the plate in three dimensions, so it can tell if points on the surface move in or out as well as up, down or sideways,” said NIST.

Digital Image Correlation

The changes in the images offer researchers a method of measuring the force of a stressor and its effect. Tracking the subtle movement of the paint helps identify the correlating effect as stress is applied. The eventual goal, says Ocel, is to develop equations that can predict those behaviors.

“The NIST digital image correlation method is a good complement to the FHWA measurement methods,” Iadicola explains.

“Their techniques—strain gages and photoelasticity—are very good for the normal range of stress, in which the plate will stretch and spring right back to its original shape.

“Our method can tell you a little about that, but it really shines in showing you what happens past that point, when the plate starts permanently deforming and finally rips apart. The failure modes.”

Lessons Learned

After more than a year of experiments, Ocel says, the FHWA has learned a lot about how to predict what loads will cause a gusset plate to fail.

Now, FHWA and AASHTO are working to translate those findings into language that can be adopted into the AASHTO Bridge Design Specification and Manual for Bridge Evaluation, used nationwide to design and load-rate bridges.

Digital imaging is a growing presence in protective coatings. Just last week, U.S. Navy researchers unveiled a digital coatings inspection system for use on military and civilian vessels.

At NIST, the FHWA project is just one of a range of applications being studied for digital image correlation, Iadicola says.

“We’ve been using it in looking at sheet metal forming—you have very high strains during the forming process—and we’ve used it at very small scales, looking at targets with an optical microscope.”

More information: www.nist.gov or inquiries@nist.gov.