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

-45-These criteria with some modifications have been adopted into Lloyd’slatest Rules for AluminumYachts, Reference (52), which require a Is per centincrease in plate thickness and a 70 per cent increase in stiffener sectionmodulus when substituting aluminum alloy for steel. These rules apply toaluminum alloys with a 0.1 per cent proof stress of 8 tons/in.2, an ultimatetensile strength of 17 tons/in.2 and an elongation in a 2 inch and 8 inchgage length of 12 and 10 per cent respectively.The Lloyd’s Rules relative to aluminum deckhouses require the followingincreases in scantlings:Fronts, sides, aft ends and unsheathed decksSheathed deckBeams and stiffenersScantlings of small isolated houses20 per cent10 per cent70 per centO per centThese requirements are consistent with those discussed previously, and reflecta consideration of ratios of ultimate strength.Recent discussions with Lloydls relating specifically to the AluminumBulk Carrier project resulted in the following observations:(a)(b)(c)(d)(e)(f)A reduction in steel section modulus of ~ per cent should beaccepted as a basis for converting to aluminum.Consideration must be given to low-cycle fatigue properties inrelation to the hull’s life cycle stress spectrum.Extensive radiographing of welds will be required to ensureproper reliability. Butts should be staggered as much aspossible.Notch toughness is not considered a problem, and no crackarresting riveted seams are required.Hull girder deflection should generally not exceed that of asteel hull of equal length but with an L/D ratio of 16, i.e.,r = SMsteelalumx .95 x 3 x half-depth of ship. This requirementmight be modified for this speciffc case, though both wave andstill mater deflection must be considered.Deflection of local aluminum beams may be SO per cent greaterthan that of equivalent steel section.Det Norske Veritas - DNV has no rules ralating diractly to aluminumstructure at this time. However, they indicate that the procedures usedin selecting high strength steel scantlings would be applicable to aluminum.These requirements’convertmild steel scantlings to high strength steel bya factor which is based upon the ratio of yield strengths. Ultimate tensilestrengths do not enter into the conversion directly, though lower limits areplaced on the ratio of ultimate tensile strength to yield strength. Foraluminum alloys, they would consider both ultimate and yield tensile strengthratios, assuming welded strengths of aluminum alloys, and the entire areaunder the stress-strain cuwe. They would deduct an appropriate corrosionallowance frcm the steel scantlings before converting, and would not requirea corrosion allowance for aluminum. They would also not require rivetedcrack arresting seams for an aluminum hull.