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

-35-(d)Quality assurance procedures, particularly for checking welds,must be developed. It will be necessa~ to determine the levelof weld porosity which can be accepted as degrading the strengthof the weldment to a lesser degree than would a repair.Alloy Material CostThe final factor which must be considered in selecting an alloy is theraw material cost. A survey of aluminum manufacturers indicates that thebase price of all common ~000 series plate or sheet alloys in largequantities will vary from about $o.~0 to $0.~~ per pound, depending uponwidth, length, thickness and manufacturer. However, all manufacturersquestioned indicated that their individual base prices for ~000 seriesplates are identical.The base price of all S000 series extrusions except SLS6-HIII willvary from about $0.62 to $0.66 per pound, with SlLS6-Hill costing about@.04 per pound or about 6 per cent more. Since extrusions generallyrepresent less than Is per cent of the weight of an aluminum hull, this6 per cent differential-for S4S6-HIII allo~ has a negligible effect on theselection of alloys.In conclusion, alloy material costs do not have a measurable effect onselection of alloys for the construction of a large bulk carrier. It isnoted that specific instances will arise in which a fabricator can obtainlow-priced plates or shapes of a particular alloy from a specific manufacturerswarehouse, ba~ed upon ~tilizing currekt invento”~ rather thanplacing a special order. This showld not be a factor in this study,however, since the quantities, thicknesses, widths and lengths requiredfor constructing a bulk carrier womd warrant a direct shipment from themill to the fabricator.Selection of AlloysThe selection of an alloy or series of alloys for use in designing analuminum bulk carrier, or, for that matter, aqy aluminum hill, is a verydifficult process, particularly if the availability and basic cost per poundof the alloys is identical. This is due primarily to the fact that eachalloy’s advantages in a particular area are usually balanced by disadvantagesOr some type. For example, the alloys with high magnesium content such as5083 amlS4S6 have high strength and would thus produce a lighter hull withlower material procurement cost and life cycle operating cost. However,-these alloys-present more problems.in.weldi~, cold working and corrosionthan do the..low-rnagnesi@alloys such”as 5052 and 5086. -Achieving a properbalance between these factors is difficfit, since it depends upon theindividual designers assessment of their relative importance.In the process of selecti~ an alloy or series of alloys for the designof a bulk carrier hull, an ass~ed relative importance has been establishedfor each of the factors considered in evaluating the alloys: staticstrength, including buckling, fatigue strength, corrosion and abrasionresistance, toughness, weldability and workability. Alloy costs were notincluded because of their similarity, and weight was not included directlysince this factor is directly related to material strength, and is thusimplicitly incorporated in the evaluation.