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

-46-DNV would consider fatigue in establishing hull girder section modulusby comparing the life cycle histogram of the stress level for combined waveand still water bending versus the number of cycles, with the fatigue strength(S-N) curve of the material. The relative areas under these curves wouldestablish a safety factor which would be the same for both steel and aluminum.Relative to hull deflection, DNV noted the possibility of resonance betweenthe natural frequency of a flexible hull and period of wave-inducedforces. They also noted the possibility of problems with flexibility of thedouble bottom, particularly if its frequencies are in resonance with thoseof the hull. The effects of cargo mass and antrained water must be consideredwhen determtiing these frequencies.Registro Italiano Navale - RIN experience relating to aluminum is presenti~ylimited to deckhouses and large LNG tanks. Eoweve~, they indicated that thefollowing considerationswould apply todesigfing alarge aluminumbulk carrier:(Ei)(b)(c)(d)(e)Deduct a 10 per cent corrosion allowance from the steel hullgirder section modulus before converting to aluminum.For the hull girder, fatigue would be considered b compartigrelative strength of steel and aluminum at 702, 1& and 106cycles. The combined wave and still water bending stressesover the life of the hull would be compared to the fatiguecharacteristics for both materials. Both ultimate tensilestreng%h and welded yield strength should be considered.Unstable propagation of a fatigue crack in aluminum shouldnot occur. Therefore, crack arresting should not be required.Limitations on hull girder deflection for an aluminum shipmay not be necessary though greater permissible deflectionsrequire more careful consideration of low-cycle fatiguebehavior.Pa~tTcular attention is requtred to ensure proper welding, - .with no undercutting, and to prevent excessive weld distortion.Bureau Veritas - The Bureau Veritas suggests a 10 per cent reduction insteel thicknesses for corrosion, and conversion of effective steel scantlingsto aluminum on the basis of yield strength ratios unless the field strengthexceeds 0.6 tines the ultimate strength, in which case ultimate strength mustalso be considered. Unwelded characteristics of aluminum alloys should beconsidered as long as the welded connections are well checked, and butts inthe sheer strake, bottom and deck plate are staggered.Bureau Veritas recommends that the deflection of the aluminum not exceedthat of a steel hull with a length-depth ratio of 16.1, which is the maximumthey permit.U.S. Navy - The Navyrs general specifications.proviclea working stressbased upon the following equation for design of secondaxy structures su~jecttO normal loading such as wave slap, deck loads,-etc. on al~inum deckhouses:lfa~m= :Welded Y.S. of Aluminum ~ Welded UTS of Aluminum[F.S. on Y.S. of Steel F.S. on UTS of Steel 1