A new cost-effective way of inspecting welds on nuclear-powered submarines may soon be a reality, thanks to a partnership between the U.S. Navy Metalworking Center (NMC) and the Lawrence Livermore National Laboratory (LLNL).
The Laboratory, which is based in Livermore, CA, will be working with the NMC to develop a non-destructive method of inspecting welds on the submarines to find any potential defects. Hull inspections currently require the removal and reinstallation of the coating in order to examine the welds underneath. This process is costly and time-consuming.
“We’re investigating a cross-platform technique that allows us to interrogate the welds without doing destructive work on that treatment,” said LLNL Materials and Engineering Section Leader Karl Fisher, a principal investigator on the project. “[Submarine maintenance] is a major undertaking. A lot of things can happen and anything that impedes the schedule is bad. Our proposal is to reduce the cost, but it is a costly process.”
The new non-destructive testing method they’re investigating is based on a technology currently used to search for improvised explosive devices buried underground. It uses acoustical structural excitation along with ultra-wide-band radar technology. Fisher says the technology isn’t guaranteed to work on submarines, but it could help inspectors narrow the area where a crack or defect may have occurred in the welding.
“We would use acoustics and vibration excitation to get the hull moving, and then watch and listen for changes in motion,” Fisher said. “It’s analogous to a wineglass—if you tap it and the glass is solid, it will make a ‘ding.’ If there’s a crack in it, you’ll hear a dull thud. In theory, the defect will radiate differently and have a different mechanical response, and we could scan it and find out where.”
LLNL researchers will use samples of submarine hulls to test the inspection process at their labs. Two other institutions will also be investigating non-destructive testing methods using terahertz and phased array approaches. The Navy will review the results before the end of 2017, before making a decision on whether to move ahead with a prototype system.
“Each modality has its shortcomings; there’s no one technique that will address the problem,” said engineer and co-principal investigator John Chang. “To me it highlights the Lab’s uniqueness in multidisciplinary research and tapping into experts in their fields to generate new capabilities.”
The Navy estimates that reducing the amount of coating that has to be removed and reinstalled on a Virginia Class submarine during inspections could reduce costs by as much as $1.2 million per hull per inspection cycle, or $6 million over a five-year period.