design considerations for aluminum hull structures - Ship Structure ...

111-FATIGUE STRXNGTHAs noted earlier in this report, there are a number of factors relatingto alloy fatigue strengths which require further clarification and testing.Foremost among these is the question of fatigue strength in the presence ofsalt spray. This requires an extensive test program, incorporating thefollowing variables: intensity of salt spray, effect of fillet and buttwelds, alloys (plate and extrusion tempm) bead-on versus bead-off. Additionaltesting of extrusion tempers would also be advisable, in both thewelded and unwelded condition.FRACTURE TOUGHNESSFurther testing is required to determine the relative quantitativefracture toughness of aluminum and steel for comparison with anticipatedstress levels. These tests should evaluate the following variables:directionality (transverseversus longitudinal), welding and other fabricationprocedures, environment (sea water versus salt spray), effects ofrepeated.loads, and alloys (plate and extrusion tempers).CORROSION AND ABRASIONThe etioliation resistance of ~083 alloy should be tested to determineif an HI17 temper is required. I?wcthertesting on the relative abrasionresistance of aluminum and mild steel is also required.DESIGN DETAILSA study should be initiated to standardize design details for aluminumship structures, both to facilitate fabrication and to prevent excessiveresidual stress build-up and subsequent cracking. This is a vital stepwhich must be taken before a large aluminum hull can be built, if structuralfailur@s are to be avoided. Specific areas to be detailed would ii-icludeend connections of intercostal stiffeners, connection of continuousstiffeners to web frames or other supports, stanchion endings, proportionsof stiffeners, relief of hard spots and other stress raisers, requiredclearances for proper welding. The required size and continuity of filletwelds requires further study, as there is presem~ly a significant di~ferencebetween Navy and commercial requirements.VI.CONCLUSIONS AND RECOMMENDATIONSCONCLUSIONSThe conclusions to be de~ived from this Feasibility Study for anAluminum Bulk Carrier are summarized below:1. General - The construction of a bulk carrier utilizing aluminumalloy for the hull structure is technically feasible witiin the presentstate-of-the-artin shipbuilding, but is not economically justified indirect competition with a steel vessel of equivalent capabilities.2. Review of Aluminum Alloys - The present S000 series aluminumalloys being considered (s0s2, 5083, 5086, 5154, 5454 and 5456) havesufficiently high welded mechanical and physical properties for the proposedapplication, though additional research is required in the area of