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

-15-Tes’cvalues of static ultimate and yield strengths for the parentmaterial are generally higher than the minimum values in Tables 2 and 3.The following exceptions were noted during the evaluation of the referenceddata. Yield strengths for 10 specimens of ~4~6-H321 alloy are approximately10 per cent below the minimum value, Reference (9). Although significant,this is not critical since aluminum does not have a yield point and theyield strength is arbitrarily defined. Reference (10) lists yield strengthsfor 2 specimens of 5086-H3~l,alloy (0.06L inch thickness) that are approximately3 per cent below the minimum value. The elongation of S083-HI13alloy which is identical to H321 temper, is abou’t’~per cent less than theminimum value, Figure 25 of Reference (13). tie specimen of S086-H32 a~~oYhas a yield strength in the transverse direction 2 per cent below the rninim.mvalue and one specimen of JLL56-H321has yield strengths both longitudinaland transverse about 7 per cent below the minimum value, Reference (21),Th~se limited cases do not modify the conclusion that the propertiespresented in Tables 2 through 4 are considered satisfacto~ for generaldesign purposes.Fatigue StrengthFigures 3 through 9 present typical S-N fatigue curves for 5000 seriesaluminum alloys and structural steel. The curves are based on average datafrom available references. Some of the curves have been verified by mawtests while others were obtained using few specimens. Ranges of testscatter are not presented, and the curves are used to develop relativetrends ofly. Generally, specimens with weld defects are not included inthe development of the curves. Fabrication variables evaluated includebutt-welds and weld bead. Envirormlentalvariables evaluated include stressratio R, test loading procedure, notches and wate~ spray.Figure 3 describes fatigue curves for unweld~d (parent) alloys subjectedto zero and complete stress reversal. The values of endurance limit (EL)are the same, within normal experimental scatter, for all ~000 seriesaluminum alloys evaluated, although the static strengths vary from 33 to51 KST. Complete stress reversal (R = -1) reduces the endurance limit bySO per cent from the value for zero stress reversal (R = O). The endurancelimit (EL) of mild steel is higher than that of ~~oo series aluminum byapproximately the same ratio as that for the average static skre~~ths.Fatigue curves are not presented for anmealed (O temper) alloys. However,the endurance limit of a~ealed alloys is the same as that of temperedalloys, References (4), (5) and (7).Fatigue curves for butt-welded alloys in the as-welded condition arepresented in Figure ~1.for zero and complete stress reversal. As with theunwelded mat.~rial,all butt-welded ~QOO series aluminum alloys approachthe same endurance limit, although the static strengths vary from 29.9 to46.8 lsxc. Complete stress reversal reduces the endurance limit by 40 percent from the value for zero stress reversal. The endurance limit of buttweldedannealed alloys is the same as that of butt-welded tempered alloys,References (6) ati (16). A significant observation from Figure 4 is thatthe fatigue strength of butt-welded S000 series aluminun is less thanhalf that of butt-welded structural steel, whereas the fatigue strength ofunwelded aluminum is 70 to 80 per cent that of unwelded structural steel.Also significant is the magnitude of the fatigue limit of butt-weldedaluminum subjected to complete stress reversal. The value, 6-7 HI,leaves little room for the safety factors that are required because ofenvironmental conditions, water spray, corrosion, notches.