N161-069 TITLE: Effective Crack Arrestors for On-Board Fatigue Crack Repair of Aluminum Ship Structures

TECHNOLOGY AREA(S): Ground/Sea Vehicles

ACQUISITION PROGRAM: Naval Sea Systems Command - Surface Warfare (NAVSEA 21)

OBJECTIVE: Develop and implement an effective method to incorporate crack growth stopping features which can be performed by ships force. Develop a repair selection toolkit for design and performance evaluation of repaired aluminum ship structures that can be utilized by ships force for emergent cracking issues.

DESCRIPTION: New high speed aluminum vessels have been recently constructed and are now in operation by the US Navy. The structure has been designed to preclude crack initiation during the operational life of the ship, however past experience with welded aluminum structures has demonstrated plate cracking will occur and will grow. Research in ship structures has shown that not only will aluminum cracks grow, they will grow three to four times faster than cracks in steel plate at the same fatigue stress level [Sielski, 2007]. The reason for faster growth rate is the lower fracture toughness of aluminum, compared with steel. Unlike steel structures, there has been very little R&D on crack-arrest of welded aluminum ship structures.

The current state of the art for marine aluminum plate cracking repair includes standard grind out and welding, simple drill stopping of the crack tips, interference bolt to apply compressive stress to the hole edge, welded insert, welded doubler plates, and composite patched plate. Composite patches have demonstrated fatigue life benefits [Noland et. al. 2013], however the current Navy method cannot be applied by ship�s force. The interference bolt method provides an increase in fatigue life, however it requires careful drilling and reaming of the hole so that an evenly distributed stress can be applied to the hole�s edge [Callinan et. al. 1998]. Effectiveness of simple compression bolts for aluminum structure has not been well documented and other options involve welding, which increases the risk of new cracking due to welding defects, workmanship, and general sensitivity to heat input and cooling.

The aerospace industry continues to investigate methods to predict and arrest crack growth in thin aluminum plate. Marine industry aluminum plating is typically thicker and does not behave in a simple 2D plane stress manor. Bonded reinforcement and cold working have both been studied for their effects on crack growth behavior. Jones and Dunn�s (2009) research on crack growth near cold worked holes demonstrated the need to account for the through thickness stress profile created during processing and evolution under fatigue; traditional 2D plane stress analysis could not capture the complex stress profile and required 3D analysis methods. A combined experimental and 3D numerical study is essential to develop innovative crack-arrest methods and features such as special fasteners or bonded reinforcement.

PHASE I: Develop concept(s) for crack arresting methods for welded aluminum ship-type components and demonstrate performance and production feasibility at lab scale under low cycle tension-tension fatigue loading conditions. Coordination with government to determine stress levels and loading rates will be required. The Phase I performance demonstration can be accomplished using ship scale flat plate (10-mm thickness) under uniaxial loading conditions. The test specimen should be similar to the one developed by Kosai et al (1996) and allow for both uniaxial and biaxial load conditions. The flat cruciform specimen was an alternative method for simulating pressurized fuselage crack kinking in the presence of tear strips. Demonstrating crack growth rate and directional behavior under uni-axial loading conditions and in the presence of crack arresting technology facilitates the down select of promising lab scale crack arresting methods for further development under Phase II.

PHASE II: The goal of Phase II is to refine the top performing methods to arrest crack growth developed in Phase I. The low cycle fatigue testing and analysis efforts should be expanded to capture higher stress levels and increased stress state complexity with constant and random amplitude fatigue spectrum loading. Coordination with government engineers will be required to determine stress state. Specimen loading should represent the combination of crack tip fracture energy typical of a standard welded aluminum detail flaw. The loading scenario should be developed through detailed numerical analysis with test response behavior used to confirm the predictions. Numerical capabilities should facilitate the estimation of a fatigue lifetime limit and demonstrate the ability of the technology to arrest crack growth or significantly decrease the crack growth rate and increase the load cycles to failure of the repaired plate. The Phase II Option, if awarded, should demonstrate crack arresting capabilities for full scale structural components under fatigue loading. This demonstration can be accomplished using a fixture and specimen similar to the large ductile tearing fixture.

PHASE III DUAL USE APPLICATIONS: Coordinate with NAVSEA 21 to design, simulate, and demonstrate crack arresting capabilities for existing damage that resulted from fatigue loading. This will be a shipboard demonstration on a Navy ship with aluminum structure, or other appropriate welded aluminum structure, such as high speed ferry, passenger train, or tractor trailer. Analytical and design tools should be packaged, demonstrated, and transitioned to develop effective crack arresting technology that can aid in cost effectively managing life cycle performance and maintenance costs. Effective crack arresting technology for aluminum would be applicable across commercial maritime industry where life cycle management drives business operating costs and profits. Lightweight aluminum structures also exist in the energy and land transportation industries. Aluminum structures from tractor trailers, shipping containers, passenger trains, to high speed ferries can benefit from development of this technology.

REFERENCES:

1. Sielski, R. A., "Research Needs in Aluminum Structure", American Bureau of Shipping, 10th International Symposium on Practical Design of Ships and Other Floating Structures, Houston Texas, 2007.

2. Noland, J.M., Hart, D.C., Loup, D.C., Udinski, E. P., Sielski, R.A., "Initiatives in Bonded Ship Structural Repairs", American Society of Naval Engineers� Fleet Maintenance and Modernization Symposium, San Diego, CA, August 26-27, 2013.

3. Callinan, R.J., Wang, C.H., Sanderson, S., "Analysis of Fatigue Growth from Cold-Expanded/Interference Fitted Stop Drilled Holes", DSTO Aeronautical and Maritime Research Laboratory, DSTO-TR-0704, July 1998.

4. Jones, K. W., Dunn, M. L., "Predicting Corner Cracking Fatigue Propagation From Cold Worked Holes", Engineering Fracture Mechanics, Vol 76, pg 2074-2090, Elsevier Ltd., 2009.

5. Kosai, M., Shimamoto, A., Yu, C.-T., Kobayashi, A. S., Tan, P., "A Biaxial Test Specimen for Crack Arrest Studies", Experimental Mechanics, Vol 36 Issue 3, pg 277-287, Springer US, 1996.

KEYWORDS: Crack Arrest; Aluminum; Fracture; Fatigue; Marine Structure; Ship Structure; Compression Bolt; Cold Working; Composite Patch

TPOC-1: Paul Hess

Email: [email protected]

TPOC-2: Daniel Hart

Email: [email protected]

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