25 Though this is typically performed only on one side of the plate, residual stress has been measured at depths from the surface of up to 10 mils. The improved micro structure and the altered stress state near the weld reduce the residual stress present after the weld cools and have been shown to prevent re-cracking [5]. From experience, this method requires careful planning and can induce cracking when not performed correctly or in locations where bending loads can reverse the stress state of the surface where cold worked stresses can add to the stress state. 3.3.3 Un-Weldable Material Guidance for 5xxx aluminum defines the mass loss welding limit as 60 mg/cm 2 . At or above this limit welding is not permitted, though through experience is often attempted and results in cracking at or in close proximity to the weld. When shipboard plating is not weldable the currently approved alternative methods are application of an E-Glass epoxy patch or a temporary adhesively bonded aluminum repair (BAR) kit applied by ship’s force. Both methods are designed to reinforce the region ahead of the crack tip, carry loads across the crack plane, and restore weather tight integrity of the plate. 3.4 Alternative Repair Methods Two primary alternative repair methods developed for U.S. Navy use since 2010 are the composite patch and bonded aluminum repair (BAR) kit. Composite patch repairs are the primary method for repairing non-weldable aluminum. Application requires trained and qualified personnel, the developed technique requires detailed surface preparation procedures, and advanced lamination techniques involving vacuum consolidation of the hand laminated composite. As experience grows there are composite patches that have seen over 5 years of service, continue to pass inspections, and receive approval for continued use. In parallel with composite patch research and implementation, BAR kits were developed to address emergent cracking while underway and can be applied by Navy sailors. These patches have been applied to areas experiencing low stresses during typical operation. 3.4.1 Bonded Aluminum Repair The current surface preparation and lamination procedures associated with composite patch repairs are not suitable for shipboard personnel, this is due to the level of training and experience required to achieve a consistent repair quality, and the expense of hazardous material handling and storage requirements. As a result, a Bonded Aluminum Repair (BAR) kit was developed to provide a temporary repair method that effectively mitigates crack growth, restores weather-tight integrity, and allows ship’s personnel to apply the repair while underway. BAR kits consist of malleable 11 gauge 5052 O-temper plates, methyl-methacrylate (MMA) adhesive cartridges with dispensing equipment, abrasive surface preparation tools, and disposables required for repair application. The steps required to install the kit are shown in Fig. 3.4. First, basic surface abrasion Fig. 3.4 BAR installation process: surface preparation ( a ), initial plate forming ( b ), adhesive application ( c ), final BAR repair step using weights to apply pressure ( d ), and BAR after 6 months in service ( e ) 3 5xxx Aluminum Sensitization and Application of Laminated Composite Patch Repairs
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