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

-23-As noted previously, salt spray significantly reduces the fatiguestrength of steel and, to a greater degree, aluminum. Howe~er~ thiseffect is not being considered in this study for several reasons. First,the highly-stressed portions of the hull girder would be subjected to directsalt spray during relatively small percentage of their operating life. Thebottom, for example, is totally immersed, while the deck wofid experiencespray in the highly stressed midship portion only a small percentage ofthe time. Current salt spray fatigue data is based upon continuousexposure, and it is probable that the effects of salt sPraY are exponential~For a given reduction in exposure time, the reduction in strength degradationwould be far less. Secondly, the relative depth of surface pittingand loss of thickness of thin test samples for a given period of exposurewould be far greater than for the thick plates of a b~k carrier h~l~which may reduce the net section loss in area. In conclusion, itdoes not appear that the salt spray data in Figure 9 is applicable totypical ship structures in a normal life-cycle sea environment. However,it is not intended to minimize the problem. As shown in Figure 9, asufficient concentration of salt spray can effectively destroy the stresscarryingcapabilities of aluminum alloys at a large number of cycles.Thus this problem warrants considerable future attention.Figure 10 indicates that the S-N curves of the various aluminum alloyshave approximately the same shape, with initial strength corresponding tothe bead-on values of welded ultimate tensile strength of Table 4 reducingto between 6 and 9 ILSIat 108 cycles. Based upon the curves of Figure 10,the gross area under the S-N curves of the aluminum alloys relative tothat of mild steel are as follows:6050l,-!;!,,30 ~ ,Ii ...—- -’*-I10II (,,, !1 1 ,,, 11,! 1!’ 1 >,, 1 - ,—–L. .––.1! ‘- - .. . .. ..1’ II.. L-..—L. ...-.1 11, ‘. I l.. 1...-....11 .’..1 ...$. .. ..OJ102II ,, J; I 1,,,, ,,,, ) 1’ 1 ,1! I Ill, 11,.3’ :IOLI ‘ ‘ 105 ‘,“ .~06‘ 10’ 108L ‘,NUhBFEOF CYCLES(N)FIG. 10 Recommended S-N Fatigue Curves for Welded 5000 SeriesAluminum Alloys and Mild Steel forDesign of Ship Structure