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

-21-parallel to the direction of load are presented in References (6), (7)and (13). The fatigue strength of longitudinal weld specimens isapproximately equal to that of specimens with transverse welds. However,the weld region covers the entire width of test specimens, but only anegligible width in full-scale structures. Further investigation isrequired to determine whether fatigue cracks will form at welds parallelto the direction of load in full-scale structures. The effects of temperatureon the fatigue behavior of aluminum alloys are not included in thisStudy. Test resfits at elevated te~pe~atures are presented in References(1) and (5), and results at cryogenic temperatures are presented inReferences (l), (6), (10) and (12).Fabrication, welding and inspection requirements for steel andaluminum ship hulls are defined in References (24) and (25). The criticalvariable relative to inspection is weld defects, which are more criticalfor aluminum than steel due to the inability of aluminwm welds to develop100 per cent efficiency. The effects of weld defects; cracks, incompletepenetration, lack of fusion, slag inclusions, porosi~, etc. on the fatigueproperties of aluminum, as yet undetermined, should be incorporated ininspection specifications. Weld defects relative to acceptance standardsare discussed in References (26), (27) and (28).Figures 3 through 9 present curves corresponding to ratios of minimumto maximum stress that vary from 0.5 to -1. Endurance limits for O.~stress ratio are reasotibly higher than for zero stress ratio, whichachieves higher eudurance limit values than -1 stress ratio (completereversal), as expected. Where available, S-N curves aye presented forcomplete stress reversal obtained by rotating beam and reversed beambending test procedures. The rotating beam test uses specimens withcircular cross-section which subjects several fibers only at the top andbottom of the c,ross-sectionto maximum stress. The reversed beam bendingtest uses specimens with rectangular cross-section which subjects allfibers along the top and bottom edges of the cross-section to maximumstress, thus achieving lower fatigue strength values. The few availabletest results for complete stiressreversal obtained using axial tensioncompressiontests give fatigue strength values approximately equal to thoseobtained from reversed beam bending tests. Since axial tension-compressiontests stress all fibers equally, it is probable that the reversed beambending test specimen has inherent stress concentrations or that incipientfatigue Tailure of beams occurs at the fatigue strength of the outer fibers.The test values reported in the references are obtained from smallspecimens that are fabricated and tested in the laboratory and do notsimulate full-scale structures that are fabricated in shipyards and subjectedto marine environment. Pertinent variables, the evaluation of which isbeyond the scope of this study, include the size, shape and configurationof full-scale structures, shipyard welding procedures, Reference (2),slow surface corrosion together with slow or rapid discontinuous fatigueloading, plate shear fatigue and section-on-plate bsnding fatigue, saltwater and salt air environment such as waves, wake, barnacles, etc.,protective coatings (References (15) and (23)), and cumulative fatigue.The effects of each of these factors on the static and fatigue strengthsof unwelded and welded alloys may be reasonably greater than normal testscatter. Evaluation of these variables is required for accurate predictionof the structural behavior of alloys. References (33), (34), (35) and (36)define procedures for evaluating cumulative fatigue damage. Considerationof varying stress ratios, maximum stress magnitudes, number of cycles at