N151-052 TITLE: Temporary Crack Repairs for Aluminum Structures on Surface Ships

TECHNOLOGY AREAS: Ground/Sea Vehicles

ACQUISITION PROGRAM: SEA21, PMS400F Surface Combatant Program Office

OBJECTIVE: Develop a novel temporary repair solution for both sensitized and stress-cracked aluminum ship structures that arrests/retards crack growth, restores watertight boundaries, and which can be performed by the ship�s force.

DESCRIPTION: There is a need for improved temporary repair technologies for United States (U.S.) Navy surface ships, available to the ship�s force or shipyard maintenance for immediate application to cracks identified on aluminum structures. Currently, permanent repairs of cracked and sensitized aluminum in naval ship structures requires damaged material to be cut out. Replacement material is then welded back into the cutout shape. Permanent repair can be too costly and time-consuming for the limited repair times available, or if damage is identified during a deployment. In these cases, temporary repair methods are often used. Current ship-force temporary repairs are designed to keep the interior dry, but do not prevent additional crack growth. Improved temporary repair technology would ideally help minimize permanent repairs by preventing additional crack growth.

Cracking in aluminum marine structures is often a result of fatigue or weld defects (Reference 1). Additionally, several classes of U.S. Navy ships use marine aluminum alloys for structures that are susceptible to sensitization. Sensitization can lead to stress-corrosion cracking (Reference 2). Permanent repairs of a cracked aluminum structure are expensive, and replacement of sensitized aluminum is even more expensive due to the additional quality controls implemented in fabrication, welding and inspection of repairs (Reference 3).

Several temporary repair methods approved for use include fiberglass composite patches, polysulfide, doubler plates, or compression bolts. Each of the current methods has drawbacks that limit the utility as a repair option. Fiberglass composite patches are costly due to installation and non-recurring engineering costs for each application. The current fiberglass resins cannot be stored shipboard nor are they feasible for ship�s force to apply properly. Polysulfide is usable by ship�s force but only re-establishes the watertight boundary. Doubler plates come in two varieties - welded or adhesive bonded. Welding adds residual stress that can start new cracks around the new weld joint, and adhesive bonded plates can be applied by ship�s force repair to reestablish water-tightness but does not arrest crack growth. Compression bolts have been proven effective when used on fatigue crack tips to prevent additional crack growth, but they cannot be implemented when the cracks end in non-planar areas and they do not restore watertightness.

Research is needed to develop a temporary repair technology that is deployable by ship�s force, not limited by geometry, and that can provide structural support to prevent crack growth and provide a watertight boundary for marine aluminum structures. The solution needs to be compatible with aluminum from a corrosion perspective.

PHASE I: The company will define and develop a temporary repair concept for cracked aluminum structures. The company will demonstrate the feasibility of the repair concept for applications in both primary hull and deckhouse structures. The company will also establish that the repair concept can be developed into a useful solution for the Navy. Material testing and modeling will be used to establish feasibility.

PHASE II: Based on results of Phase I, the company will develop a prototype repair concept for cracked aluminum structures for evaluation. The prototype will be evaluated to determine its capability in meeting the performance goals and Navy requirements for suitability as a structural repair method. Performance will be demonstrated through prototype testing over parameters relevant to end use. Performance results will be used to refine the repair concept prototype into a documented repair procedure that will meet Navy requirements. The company will prepare a Phase III development plan to transition the technology to Navy use.

PHASE III: Based on results of Phase I and Phase III development plan, the company will refine the repair concept for evaluation. The repair concept will be evaluated to determine its capability in meeting the performance goals defined in Phase III development plan and the Navy requirements for suitability as a structural repair method. The company will assist the Navy in transitioning the repair procedure for Navy use.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Aluminum cracking and sensitization likely affect other marine vessels and structures using 5XXX series aluminum. This technology may be applicable for use in repair of these commercial items. Repairs employing elements of epoxy-based polymers will likely have immediate benefit in commercial hull applications.

REFERENCES:
1. Kramer, R.K.; Rampolla, B.; and Magnusson, A. SSC Report 410 "Fatigue of Aluminum Structural Weldments". Ship Structures Committee. May 2000. Accessed 23 APRIL 2014. http://www.shipstructure.org/pdf/410.pdf

2. Cormack, Emily. "The effect of sensitizitation on the stress corrosion cracking of aluminum alloy 5456." Naval Postgraduate School Thesis. June 2012. Accessed 3 APRIL 2014. http://www.dtic.mil/dtic/tr/fulltext/u2/a562767.pdf

3. Schwarting, R; Ebel, G, and Dorsch, TJ. "Manufacturing Techniques and Process Challenges with CG47 Class Ship Aluminum Superstructure Modernization and Repairs." American Society of Naval Engineers. 2011. Accessed 3 APRIL 2014. https://www.navalengineers.org/SiteCollectionDocuments/2011%20Proceedings%20Documents/FMMS2011/Papers/Schwarting.pdf

KEYWORDS: Sensitization of aluminum structures; stress corrosion cracking of aluminum; structural repairs of ships; marine aluminum fatigue and stress cracking; crack growth prevention on ship structures; watertight repairs of aluminum structures while underway

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