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Camera Is Faster than a Speeding Bullet

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Daniel C. McCarthy

To understand how things work, engineers must first comprehend how things fail. But what happens if the event of failure occurs with such force and speed that it is nearly impossible to view, much less document?

Researchers at the University of Rhode Island's Dynamic Photomechanics laboratory study catastrophic events that most hope never to see, such as the detonation of explosives, bullets penetrating bulletproof vests or the failure and collapse of building materials. Since most of the useful data from these events are almost immediately obliterated, the researchers capture the moment of failure with the Imacon 200 high-speed camera from DRS Hadland. "A lot of physics happens very early in the event, so the first microseconds are important," said Arun Shukla, a professor of mechanical engineering and director of the lab.

The camera exceeds its predecessor in both speed and the number of exposures. It can capture 200 million digital frames per second and incorporates eight double-gated CCDs to provide two images per exposure.

The study of dynamic mechanics has benefited from high-speed imaging. Here, a 0.308-caliber armor-piercing bullet strikes a ceramic-Kevlar composite armor specimen at 750 m/s. The time between each frame is 10 µs. The top left image shows the bullet just before striking the plate. The bottom left image follows immediately after impact and shows the copper jacket peeling away. In the top right image, the steel core bores into the ceramic. The last frame shows catastrophic failure as the frame begins to fracture. Courtesy of the University of Rhode Island.

Spark in the dark

Before Shukla purchased the camera -- through a $457,000 grant from the National Science Foundation -- he relied on a multiple-spark camera that he built in 1981. He still works with that instrument, which uses 30,000 V to create a 1/4-in.-long arc that serves as the camera's flashbulb. The spark, channeled through a fiber optic cable, lasts 500 ns.

"In 500 ns, things still move a little, so the image can appear blurry," Shukla said. A bullet striking a target, for instance, causes stress waves that can travel at 5000 m/s. The Imacon has a 5-ns exposure time, which can produce images in which the bullet appears frozen in space.

"The biggest problem," he said, "is [that the multiple-spark camera] is very dangerous and you have students working with it." Also, the homemade camera nearly fills the room. The optics alone are cumbersome, measuring 14 in. in diameter to collect more light. The Imacon camera, by contrast, is the size of a computer and captures images digitally.

The Imacon's 1280 × 1024-pixel chips deliver comparatively high resolution for a high-speed imager but, Shukla notes, the camera captures only 16 images before the disc is full. He selected a digital medium for his research because developing the film on which most high-speed cameras rely can be extremely time-consuming.

"It used to take us days and days to process the images, and the analysis was very cumbersome," he said. "Digital gets results simultaneously.

Photonics Spectra
Nov 2000
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