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

Equation (2):-50-Hu1l SMa~um = Hull SMsteel (as built) x‘steel‘aluminumIt is noted that the actual S.M. of the steel hull : s used rather than the“effectiveT’S.M., reduced for corrosion allowance, since the conversion oflife-cycle moment to stress was based upon the actual S.M.Considerable investigation is required to establish a general lifecycle histogram of hull girder stresses for a bulk carrier, consideringcombinations of still water and wave bending moments, anticipated servicejNorth Atlantic versus Pacific, etc., loading conditions, including per centof time in ballast, operational profile and others. The scope of this studyis not sufficient to investigate this problem in detail, although a generalapproach has been established which is sufficiently accurate to demonstratefeasibility. The results of this study are summarized in Appendix A.Figure 13 illustrates the application of the foregoing criterion to theM/V CHALLENGER, where S083 alloy is being used in lieu of mild steel. Thisfigure indicates that the allowable stress for the aluminum hull would varyfrom 2.1 KSI (still water bending stress) at 108 cycles to 13.s KSI (extrapolated)at 10° cycles. The corresponding values for the steel hull areS KSIand 19 KSI. The area under the steel and aluminum life-cycle stresscurves between 10° and 108 cycles are 6.7S x 108 KSI and 3.S6 x 108 KSI respectively,resulting in a required ratio of hull girder section moduli of1,90,,0~.”.– ;; :: . 0 ; : : m : . ~\..-. —1 ...... ,.~.~—.. .’—1.. I xl’~- ..~l~.~-~.-j–”lo~:–_::- ‘:-T i iJ~[~ l=”;;:;-”% ili~ ---! ---+~NoTES: S-N CURVES OF STEELANDAL131!INUM13ASED 1 :..; .,::+,.:-,. . ~ .1‘,1 ONWELDEDIIATERIAL, WITH13ElD ON,USINGi r—.$‘, .,. –.- AVG.OFR=Ofl“:=::.. -.”/-.”’ ”;l::!: ”:””.’~’1-”’ IILNDR‘-l,FROMFIGURE8.LIFECYCLE BENDING STRESS FORSTEtLHULL ~IS FIASEOJON IDMENTS FROM APPENDIX A ANDA HULL GIRBER S.M. OF 67IIB 1N2 FT.30,!p.”:. w—.. . —. .... .,,.,~,,,, ,,.’.~-~-~dd,.~-l–.l..~d, },l.l.l..—;—l-;..l ,,,;1---------- J ..11I,—,—–,—L.. 1-1 --; -—:--,-!-, :l---,--F? ,,”.,102,03 , ~4 1$ 106NU!DJ!EOF CYCLES,07 1OaFIG. 13 Relationship Between 5-N Curves and Life Cycle HullBending Stress for steel and Aluminum Bulk Carrier