When your million-dollar racing yacht falls from a crane, how do you evaluate unseen damage? Thomas Jones, vice president and senior engineer at Industrial Quality, has developed a creative solution to just such a problem. Using space heaters and an infrared camera from Inframetrics Inc., Jones is able to map defects in the hulls of boats to serve as a guide for repairs.
Infrared imaging revealed a stair-step crack (arrow) in a yacht's foam core hull made when a broken lifting strap dropped the vessel onto concrete. Courtesy of Thomas Jones.
The boats that Jones examines are frequently expensive racing or pleasure yachts measuring 50 ft or longer. Although this application of infrared thermography is somewhat new, he said his method provides accurate estimates of hull damage. He can detect delamination of the skin, fractures or water in the foam core and disbonds between the core and the skin.
Jones begins the procedure by removing most of the interior structure to get access to the hull's inner surface. Using space heaters, he heats the interior spaces to 30° to 40° above the ambient temperature. Once the interior is heated, he scans the exterior with an infrared camera. Problems with the hull show up as hot spots or cold shadows.
Jones said that other nondestructive testing methods, such as ultrasound, often prove ineffective at examining the composite materials that make up boat hulls. In a typical composite hull evaluation, an inspector surveys the boat visually and then may use a tap hammer and a moisture meter for more localized tests. For repairs, technicians often cut away damaged material until they find sound material, assuming that they have found the extent of the damage. In reality, problems can lurk in the composite materials.
In one case, Jones surveyed a boat that had been submerged. In the process of his examination, he heated the interior, then vented individual compartments and scanned the bulkheads separating them. He found moisture trapped in a bulkhead because it held heat longer than a dry bulkhead.
Jones has been using long- and short-wavelength infrared cameras from Inframetrics in his work. He explained that the long-wavelength cameras, which are sensitive around 8 to 12 µm, work better because they are better matched to room temperatures.
"The camera needs to be a reasonably sensitive IR camera, but the specific detector type and camera design aren't critical," Jones said. "The biggest problem with midwave band for boat inspection is glare and reflection from the hull's paint, which is high gloss."
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