For nearly a decade, a team of engineers at the U.S. Naval Surface Warfare Center’s Carderock Division in Potomac, Maryland, USA, has worked to develop a non-welded repair for stress corrosion cracking in highly sensitized aluminum, which was occurring on the Navy’s Ticonderoga-class cruisers.
The engineers from Carderock’s structures and composites division first received funding for the project in 2010.1 Since then, by leveraging international cooperation and previous experience from the Royal Australian Navy (RAN), they designed a fracture-mechanics-based bonded repair to address the cracking issue on the cruiser’s superstructure.
The three-person team of engineers includes Daniel Hart, John Noland, and Bruce Wells.1 According to Hart, the original reason for the composite patches was aluminum sensitization and the corresponding degree of sensitization, caused by the aluminum alloy being exposed to heat and a corrosive environment.
Four Levels of Sensitization
The four basic levels of sensitization are: unsensitized; sensitized, but weldable; sensitized and weldable, but requires some type of cold work; and, above a certain level of sensitization, the metal is unweldable.
“Most of the plates we’ve come across have been on the high side of sensitized, but weldable with cold work to unweldable,” Hart says. “Welding aluminum plate is a detailed and technical process, made more difficult when the material has sensitized.”
The first ship the Carderock team installed the repairs on in late 2010 was the USS Port Royal (CG 73). Hart said they had some issues on that first repair, but they were able to learn quickly from the problems.
“We learned a lot about our surface prep and discovered a couple of things, not only about the way we were treating and abrading, but also about the effects a shipboard environment has on our original surface preparation chemicals,” Hart says.
In a traditional stress crack on a ship, welding might be the best solution. However, Hart says that this composite patch allows them to make repairs to stress-corrosion cracks in difficult places. According to the team, composite patches typically only require access from one side of the structure and generally do not require the removal of equipment, wires, ventilation, insulation, plumbing or weapons systems.
By contrast, welding requires not only the removal of the equipment and cracked plate, but also removal of enough highly sensitized plate to reach weldable plate. Welders then have to install the replacement metal and post a fire watch throughout the welding process.
Hart estimates that the composite-patch repair method has saved the Navy anywhere between $1 million and $4 million per repair for several of the larger repairs.
Official Temporary Repair Procedure
One such repair was an emergency repair on USS Normandy (CG 60) to stabilize a 10.5-ft (3.2 m) crack and four cracked longitudinal stiffeners.
Normandy was the third Ticonderoga-class cruiser assigned to a Baltic Sea exercise in 2012, with no alternative asset available after two sister ships in the class were unable to support the mission. Before the ship could deploy, a repair was needed to restore structural integrity to a cracked deck.
As a direct result of the research done at Carderock, the composite patch team designed and installed a hat-stiffened composite patch in 21 days at a cost of $140,000. The weld repair estimate required more than six weeks at a cost of $1.5 million.
From 2010 to 2015, the investment in composite-patch science and technology was about $6.2 million, and it has resulted in greater than $30 million of maintenance cost savings across on 15 Ticonderoga-class cruisers and one Harpers Ferry-class dock landing ship, according to the Navy.
This process is now an official temporary repair procedure approved by Naval Sea Systems Command (NAVSEA). Starting in 2017, the Carderock team began transitioning the composite-patch repair work to the Navy’s Regional Maintenance Centers (RMC) for broad fleet use.
“We have been training the RMCs to install the composite patches, and that allows us to get back to focusing on the research side of it,” Hart says. “That’s really our goal for this transition.”
In October 2018, the team was called to help the Southwest RMC do an emergency composite-patch repair on USS Mobile Bay (CG 53) with only three weeks to get the repair done, including designing it and getting structural approval. The damage and design process was very similar to the USS Normandy effort in 2012, however, this was the RMC’s first attempt at a repair.
“That was a really fast turnaround, and it was a complicated repair,” Hart says, adding that team members Wells and Anna Bernal, an engineer with Carderock’s non-metallic materials research and evaluation branch, went to San Diego, California, USA, to provide oversight and support for the repair.
Installation by Regional Maintenance Centers
About 100 repairs later, Hart said they went back to Port Royal in 2019 during a maintenance availability at the Pearl Harbor Naval Shipyard. This time, they taught the RMC there how to install the repair. Composite-patch installers from both Southwest RMC and Pearl Harbor’s fleet maintenance facility-surface (FMR) worked together to perform large and logistically complicated repairs, as well as complete their installer qualifications.
That repair effort qualified seven RMC and FMR installers under the newly developed composite patch qualifications developed by Carderock and NAVSEA. Hart says the team now has memos with Commander Navy Regional Maintenance Centers and individual RMCs outlining the requirements for installer qualification, inspection procedures, and maintenance of the composite-patch repair, which includes attending a training course co-developed by Carderock and Gougeon Brothers (Bay City, Michigan, USA). Gougeon Brothers is the manufacturer of the epoxy resin used for repairs.
The Carderock team is now working with several RMCs around the United States, as well as those forward deployed in Yokuska, Japan, and Rota, Spain. “Now the RMCs are building the capability to install NAVSEA-approved composite patches,” Hart says. “Once certified, the RMCs can install basic patches, and we can provide guidance and oversight for more complex repairs. With the RMCs on the waterfront, it’s a lot easier for them to work with the incoming ship’s schedule and respond quickly to emergent needs.”
Composite-patch related research and development is continuing with a goal of transitioning the repair method to other ship classes, such as Harpers Ferry, Wasp, littoral combat ships, and other military branches. “We are also working on the verification of the analysis and design tools required to extend patches to structural repairs, modifications, and ship alterations,” Hart says.
According to the Navy, further research is aimed at better understanding the bond-line behavior; the ability to bond to corroded and contaminated steel; advanced numerical-analysis tools; methods necessary to design bonded reinforcement solutions for structural reinforcement; and the experimental mechanics required to inform those models and generate accurate predictions.
Source: U.S. Defense Video Imagery Distribution System, www.dvidshub.net.
Reference
1 “Carderock’s Composite-Patch Technology is an Alternative Repair Method for Sensitized Aluminum,” Defense Visual Information Distribution Service News, Aug. 12, 2019, https://www.dvidshub.net/news/335567/carderocks-composite-patch-technology-alternative-repair-method-sensitized-aluminum (Sept. 16, 2019).