Story by Brooke Marquardt and Kelley Stirling on 08/12/2019In a research lab such as Naval Surface Warfare Center, Carderock Division, a "win" is to see one's research applied in real life. For engineers Daniel Hart, John Noland and Bruce Wells, not only have they been involved in the development and application of their research, but they have also transitioned it to the fleet.
In 2010, this team out of Carderock's Structures and Composites Division received funding to find a non-welded repair for stress-corrosion cracking in highly sensitized aluminum that was happening on Ticonderoga-class cruisers. 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.
According to Hart, the original reason for the composite patches was aluminum sensitization and the corresponding degree of sensitization, which is caused by the aluminum alloy being exposed to heat and a corrosive environment. There are four basic levels of sensitization: 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 said. "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 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 said.
In a traditional stress crack on a ship, welding might be the best solution. However, Hart said that this composite patch allows them to make repairs to stress-corrosion cracks in difficult places. 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. 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.
All of this adds up to a lot of money. Hart estimated that their composite-patch repair has saved the Navy anywhere between $1 million and $4 million per repair for several of the larger repairs.
One such repair was an emergency repair on USS Normandy (CG 60) to stabilize a 10-1/2-foot 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, negotiated structural technical warrant holder (TWH) approval 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.
This process is now an official temporary repair procedure approved by Naval Sea Systems Command (NAVSEA), and starting in 2017, the Carderock team began transitioning the composite-patch repair work to the 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 said. "That's really our goal for this transition."
Last October, 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 the structural TWH approval. The damage and design process was very similar to the USS Normandy effort in 2012, however, this would be the RMC's first attempt at a repair.
"That was a really fast turn-around, and it was a complicated repair," Hart said, adding that team members Wells and Anna Bernal, an engineer with Carderock's Non-Metallic Materials Research and Evaluation Branch, went to San Diego to provide oversite and support for the repair.
Somewhere in the neighborhood of 100 repairs later, Hart said they went back to Port Royal in 2019 while it was in a maintenance availability at Pearl Harbor Naval Shipyard. However, this time, they taught the RMC how to install the repair. Composite-patch installers from both Southwest RMC and Pearl Harbor's fleet maintenance facility-surface (FMR) worked together to do some 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 said they have memos with Commander Navy Regional Maintenance Centers (CNRMC) and the individual RMCs outlining the requirements for installer qualification, inspection procedures and maintenance of the composite-patch repair, which included attending a training course co-developed by Carderock and Gougeon Brothers. Gougeon Brothers is the manufacturer of the epoxy resin used for the repairs.
"We had worked with them (Gougeon Brothers) under some TIPS (Technology Insertion Program for Savings by Office of Naval Research) funding to set up what the course would look like, and ensured it covered all the aspects that we thought were important," Hart said, adding that they still provide oversite of the RMCs on the actual shipboard repairs. "Class is one thing, but having to coordinate all your materials, schedule, timing, get all the stuff on the ship, that's a whole other animal."
The Carderock team is working with RMCs in Norfolk, San Diego, Pearl Harbor and Mayport, Florida, 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 said. "Once certified, the RMCs can install basic patches, and we can provide guidance and oversite 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 continues with transition 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 said.
Continued research is focusing on further 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.