5xxx Aluminum Sensitization and Application of Laminated Composite Patch Repairs

Abstract

Sensitization can occur in 5xxx aluminum alloy utilized in welded ship structures. Cumulative exposure of plating to elevated temperatures causes magnesium in the aluminum to migrate to the grain boundaries and develop a corrosion cell. As a result, the sensitized plate is susceptible to intergranular corrosion, which when exposed to salt water and stress leads to stress-corrosion cracking (SCC), typically at stress levels below nominal plate strength. SCC typically occurs at a level of sensitization that renders welded repairs impractical, requiring an alternative methodology. One alternative to traditional welded repair of marine structures is the application of a composite patch. Composite patch repairs have seen significant use by the aerospace community on thin aluminum aircraft skins to address stress related cracking while only limited use on thicker marine structures. Patches for marine structures have demonstrated their ability to mitigate crack growth, inhibit water ingress, and reinforce damaged or degraded structure for commercial and military vessels. The use of composite patches for repair of SCC onboard U.S. Navy ships began in December of 2010. Patch design and installation are supported by ASTM material testing, environmental conditioning, large center crack tension fracture mechanics specimens, and experience with a full scale plate ductile tearing test. Patch repairs patches are installed in-situ with vacuum consolidated hand laminations and have demonstrated the ability to increase static plate strength, fatigue life, large displacement capacity of the crack plate, and durability for in-service applications.