Application and use of aluminium alloys in ship construction

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Application and Use of Aluminum

Alloys in Ship Construction

BY Y[. G . FORREST, ]V~EMBER 1

During the last twelve years tremendous strides have been made in the development and use of new aluminum alloys in the marine field. These alloys have been used for secondary structures in both merchant and naval construction [1]. 2 Their examination under service conditions at various times has indicated t h a t the materials are fulfilling their design purpose, which is to satisfy , the requirements for a light-weight, non-corrosive

material for seagoing service.

Experimentally, the subject of the corrosive resistance of aluminum base alloys to marine exposures has been covered amply in other papers presented to the Society [2] and in other written mat- ter listed in the Bibliography and only the latest pertinent results will be incorporated in this paper. In addition to their corrosive resistance, their physical properties have been improved and m a n y tests have been made to determine the efficiency of the material. B u t here again reference will be made only to the pertinent results.

Experience with a third factor, fabrication, both ashore and afloat, has been such t h a t it is now considered possible to take the next step in its use which is the design and construction of seagoing vessels using these alloys for the ship's structure as a whole, and for all those components for which they are suitable, based on research and experience.

T h e purpose of this paper is to furnish the naval architect, who m a y be interested in the design of such a vessel, with information and data on the development of the material and its application at present and to suggest future possibilities.

i Senior A s s i s t a n t N a v a l A r c h i t e c t , G i b b s & Cox, Inc., N e w York, N . Y . Mr. F o r r e s t received t h e d e g r e e of B a c h e l o r of Science in N a v a l A r c h i t e c t u r e a n d M a r i n e E n g i n e e r i n g f r o m t h e U n i v e r s i t y of M i c h i - g a n in 1927, a n d has been w i t h G i b b s a n d Cox. Inc., f r o m 1928 to

date. H e was a m e m b e r of t h e S u b - C o m m i t t e e on W a t e r t i g h t In- t e g r i t y , S t a b i l i t y , L o a d Lines, etc., of t h e T e c h n i c a l C o m m i t t e e on S a f e t y a t Sea which p r o d u c e d S e n a t e R e p o r t No. 184. H i s w o r k i n the field of n a v a l a r c h i t e c t u r e has i n c l u d e d special d u t y w i t h t h e N a v y a n d special w o r k for t h e Office of N a v a l Research. He is also a m e m b e r of a s u b - c o m m i t t e e of t h e S h i p s ' S t r u c t u r e s T e c h n i c a l a n d

R e s e a r c h C o m m i t t e e of T h e S o c i e t y of N a v a l A r c h i t e c t s a n d

M a r i n e Engineers,

2 N u m b e r s in b r a c k e t s indicate references in B i b l i o g r a p h y a t t h e

end of t h e p a p e r .

Only those phases of ship design which are di- rectly affected b y the use of aluminum in place of steel will be covered here, as the field is too broad to do otherwise. I t is hoped t h a t a t a later date there will be an o p p o r t u n i t y to cover the construction of a ship having an aluminum alloy hull and superstructure, etc., at which time the practical working out of the design can be described.

D a t a collected from various sources have been included in this paper and it is believed t h a t the Bibliography will prove of value to those who de- sire to investigate further.

General. One justification for the use of alumi-

num in place of steel would be, in special cases, the ability of the metal to resist corrosion caused b y special liquids. Also, although the metal itself is less corrosive than steel when subject to marine exposure, the extent of the gain due to this factor cannot be ascertained until vessels are built and in service for some years. T h e basis for the design is generally the application,of the same thickness corrosion factors for aluminum as for steel.

• T h e primary justification at the m o m e n t for the use of aluminum in place of steel is the saving in weight t h a t can be obtained. This in turn will be reflected usually in an increased deadweight carrying capacity, an off-setting item to be considered when evaluating the higher initial cost of the base metal and the higher fabrication cost.

I t is found t h a t the proportionh of aluminum vessels will follow, in general, those of steel vessels except t h a t wherever possible an increase in the depth to beam ratio and a decrease in the length to depth ratio are advisable.

More than the usual particular attention must be paid to details, so t h a t extremes are avoided which would require unusual scantlings beyond the rolling and extrusion limits of aluminum.

In the layout of the form, e v e r y reasonable a t t e m p t must be made to keep the n u m b e r of furnaced plates and shapes at a minimum. If required to be heated, in order to be formed, they

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506 A L U M I N U M ALLOYS I N S H I P C O N S T R U C T I O N

FIG. 1.--SPECIMEN ]FOR EDGE COMPRESSION TEST OF A FLAT SHEET SUPPORTED ALONG THE EDGES AND STIFFENED WITH TRANSVERSE STIFFENERS, LONGITUDINAL LAP SEAMS AND LONGITUDINAL INTERCOSTAL STIFFENERS

s u b s e q u e n t l y m u s t b e h e a t t r e a t e d . I n t h e s e c a s e s t h e d i m e n s i o n s a r e i m p o r t a n t b e c a u s e of t h e l i m i t a t i o n s of t h e he, a t - t r e a t i n g f u r n a c e s . W h e r - e v e r p o s s i b l e , t h e d e s i g n s h o u l d b e s u c h t h a t c o l d f o r m i n g w i l l b e a l l t h a t is n e c e s s a r y , o T h e v e s s e l t o d a y , a n d p r o b a b l y f o r s o m e t i m e i n t h e f u t u r e , w i l l b e r i v e t e d .

CHARACTERISTICS OF THE MATERIAL B a s i c a l l y , t h e s u i t a b l e a l l o y s a r e 5 3 S - T a n d 6 1 S - T f o r p l a t e s , s h a p e s a n d e x t r u s i o n s , 220 o r 356 f o r c a s t i n g s , 5 3 S f o r r i v e t s , a n d A 5 1 S - T f o r f o r g - i n g s . T h e 6 1 S - T a l l o y , b e c a u s e of i t s h i g h e r s t r e n g t h a n d g o o d c o r r o s i o n r e s i s t a n c e , is u s e d a l m o s t e x c l u s i v e l y . T h i s a l l o y is a c o m p o s i t i o n o f :

Per cent Per cent

M a g n e s i u m . . . 0 . 8 - 1 . 2 Manganese,

Silicon . . . 0 . 4 - 0 . 8 max . . . 0.15 Chromium . . . . 0 . 1 5 - 0 . 3 5 Zinc, max . . . 0.20 Iron, max . . . 0.70 Other elements,

Copper . . . 0 . 1 5 - 0 . 4 0 max . . . 0.15 Titanium, Aluminum . . . Remainder

max . . . 0.15

and its guaranteed minimum physical properties

a r e :

Plates

Modulus of elasticity, psi . . . 10 × 106 Tensile strength, psi . . . 42,000 Yield strength, psi . . . 35,000 Elongation in 2 in . . . 10% Bend diameter N for 180 deg . . . 7

Shapes

Tensile strength, psi . . . 38,000 Yield strength, psi . . . 35,000 Elongation in 2 in . . . 10%

Rivets 53S Driven at 1030-1050 degrees F

Shear strength, psi . . . 24,000

The physical properties of steel to American Bureau of Shipping and United States Navy requirements are also listed for ready reference:

American Bureau U . S .

of Shipping N a v y

Plates and Shapes

Modulus of elasticity,

psi . . . 29 X 10 G 29 >( 106 Tensile strength, psi.. 58,000--60,000 60,000 Yield, 0.5 tensile,

psi . . . 29,000-30,000 33,000 1,500,000 In 2 in. Elongation in 8 in . . . . _{tensile str.}-- 2 5 . 8 - 2 5 % _{23 %}

Rivets

Tensile strength, psi.. 55,000-65,000 58,000- 60,000

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ALUMINUM ALLOYS IN SHIP CONSTRUCTION 307

FIG. 2.--REVERSE SIDE OF SPECIMEN SHOWN IN FIG. 1

Yield, 0.5 tensile,

psi . . . 29,000-30,000 30,000

Shear strength, psi... 48,000 48,000

Elongation in 8 in . . . . 1,500,000 27.3-25% In 2 in.

tensile str. 22%

STRENGTH

T h e procedure used in arriving a t the strength of an a l u m i n u m ship is in general t h a t used for a steel ship, when due consideration is given to the properties of the material.

• F r o m these properties and experience to date, it is reasonable to assume a design stress for the a l u m i n u m material of 15,000 pounds per square inch. This figure has been applied to a design working from first principles, and checks v e r y closely with elements of the steel sections obtained from the American Bureau of Shipping rules using a conversion factor of 1.30 for plates and 1.40 for shapes. These factors compare with the following:

R a t i o of the plate thicknesses as a measure of the rigidity:

~

29 X 106 -- 1.'43 >< l0 s

Ratio of the plate thicknesses for the same factor of safety on the ultimate strength:

58,000/42,000 = 1.38

Ratio of the plate thicknesses for the same factor of safety on the yield strength:

30,000/35,000 -- 0.86

Ratio of the mean values of the ultimate

s t r e n g t h yield strength:

1.38 X 0.86 = 1.187

T h e section, as developed b y the basic ratio or conversion factor, requires a check of its various elements against critical stress in compression. T h a t portion of the material between the transverse frames behaves, generally, as a round-ended column. E x p e r i m e n t a l results show t h a t it can be calculated conservatively on this basis.

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308 A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N m o d e l s e c t i o n of the d e c k of a t r a n s v e r s e l y f r a m e d s h i p w a s set u p [3]. T h i s was t e s t e d b y e d g e l o a d i n g first w i t h o u t l o n g i t u d i n a l s e a m s in t h e p l a t i n g , t h e n w i t h s e a m s a n d t h e n w i t h s e a m s a n d i n t e r c o s t a l l o n g i t u d i n a l stiffeners, t h e r e s u l t s b e i n g a s follows: (1) W i t h t h e t r a n s v e r s e stiffeners alone, t h e c r i t i c a l s t r e s s in a p a n e l 2.5 i n c h e s X 16.4 inches' X 0.088 i n c h w a s f o u n d to b e a b o u t 9500 p o u n d s p e r s q u a r e inch. (2) W i t h t h e ' t r a n s v e r s e stiffeners a n d d o u b l e - r i v e t e d l a p s e a m s m a k i n g t h e l a r g e s t p a n e l 2.5 i n c h e s X 6.73 i n c h e s X 0.088 inch, t h e c r i t i c a l s t r e s s w a s f o u n d to b e a b o u t 13,500 p o u n d s p e r s q u a r e inch. (3) W i t h i n t e r c o s t a l a n g l e stiffeners b e t w e e n t h e l a p s e a m s m a k i n g t h e l a r g e s t p a n e l 2.5 i n c h e s X 3.36 i n c h e s X 0.088 inch, t h e c r i t i c a l s t r e s s was f o u n d t o b e a b o u t 15,500 p o u n d s p e r s q u a r e inch. I t is b e l i e v e d t h a t in t h e a c t u a l s h i p t h e r e will b e a n i n c r e a s e in t h e l o a d - c a r r y i n g c a p a c i t y of t h e s t r u c t u r e , since, if t h e p l a t e y i e l d s , t h e l o a d will b e t r a n s f e r r e d t o t h e s t r o n g p o i n t . T h e t h i c k n e s s of t h e o u t s t a n d i n g leg of s u c h a n g l e stiffeners s h o u l d b e n o t less t h a n o n e - h a l f t h e t h i c k n e s s of t h e p l a t e .

P h o t o g r a p h s of b o t h sides of t h e final t e s t speci- m e n a r e r e p r o d u c e d in F i g s . 1 a n d 2, w h i c h also s h o w t h e S R - 4 e l e c t r i c r e s i s t a n c e wire s t r a i n gages. W E I G H T C O M P A R I S O N T h e final s e c t i o n in a l u m i n u m will s h o w a w e i g h t s a v i n g o v e r a s i m i l a r steel vessel of a p p r o x i - m a t e l y 44 p e r cent. T h i s will b e m a d e u p of t h e f o l l o w i n g s t r u c t u r a l c a t e g o r i e s : Shell plating . . . Framing, longitudinal and transverse . . . Deck plating . . . Main structural bulkheads . . . Inner-bottom plating . . . Superstructures . . . Saviug. per ceut 46 32 50 41 45 50 W h e n t h e o t h e r i t e m s i n c l u d e d in t h e s h i p ' s w e i g h t a r e t a k e n i n t o c o n s i d e r a t i o n , t h e t o t a l s a v - i n g will b e a p p r o x i m a t e l y 38 p e r cent, m a d e u p a s f o l l o w s : Saving,

E q u i p m e n t and outfit per cent

Cargo handling and rigging . . . 33

Miscellaneous fittings . . . 37

Hull piping . . . 45

Electrical plant . . . . . . . 18

Deck machinery and miscellaneot*s . . . 12

Total for complete equipment and outfit.. 27

Machinery, total . . . 24

Total u~eight saving, complete light ship . . . 38

, t 0'563"P1" ~ 1 0'75"P1" }

-~ + + + + + + u) + + + + + + + + "co I

: . . .

~' Iq~vels - 4~ D~orne~ers )

]"Rive}s-4 D~omefers E~,~icle~ey 4&G Per Cenf

E~iciency 4l.q Per Cen~ S t e e l A l u m i n u m F I G . 3 . - - T Y P I C A L I ~ I V E T I ~ D S H ~ L L S E A M - - S T E E L A N D A L U M I N U M .RIVETING PRACTICE I t will be seen f r o m t h e p r o p e r t i e s of t h e m a - t e r i a l t h a t t h e s h e a r v a l u e of t h e 53S a l u m i n u m 4-4 0,~5" PL t t l , + • + + ÷ t + + + + + ~- ÷ +

:]

4 - + + + + + + + ~ + + + + + 4- + -I- + - F 4- 4- + + + + + + + -I- ~ -F + + + + + + + ~- + + + + + + + + -e + + + + -k- + + 4 - , I + + l Single Sfrap 25'~0.g38" l" Rivets Spoced 3~ & 4~ OiomeCers i - ÷ q8'-' N t + l [

+÷z

+__d

* ÷ [ ++l

I

++1

++I

i E{{'tc]enc~es Plebe Tear . . . . "/4.9 Per Cen~r R~vefs Shear--] 5.4 PerCenf Sfrop Tear---'/3.3 Per Cen~

S t e e l

~

8'!

m + ₊ 1.00"9l. ~ - + ' [ + + * + + + + + + + + + + + [ + + + + + + + + + + 4- + + + + + + + + + + + 4- + -I- 4- + + + + + + l + + + + + + , + , + 4 - + + + + + [ + + + + + + + + + + + -V + + + + ~_*_ . . . + Ooub e S~r~ps ZS×O.~Z5 - + I

+ I I"R~ve{s Spaced 4 &5 O;orne•ers F_ + ++1 + + 1 *+] t ~1 ++I +_+~

~+,+

Ef#iciendes Plafe Tec~r--- q'/.q Per Cen+ Rivers Shemv--91.2 Per Cenf Skrop Tear--- 82.1 Per Qen#

Aluminum

F I G . 4 , - - - - T Y P I C A L S T R I N G E R P L A T E B U T T S T R A P - - S T E E L A N D A L U M I N U M

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M a i n plates

(1) (2) (3) (4) (5) (6)

T A B L E I . - - S T A T I C T E S T S OF R I V E T E D H U L L PLATE JOINTS ULTIMATE LOADS AND RI~LATI~D DATA

R i v e t s Areas, sq in. Predicted failure A c t u a l failure

o . . • - ~ ~ ~ ~ . - • ~ ~ ~. .~ ,: ~'~ ~'~ ,~ - (12) (13) (14) (15) (16) (17) (18) (19) (20) > (7) (8) (9) (I0) (11) LARGE JOINTS 1 Single H.T.S. 0.227 9~-~ M.S. 5/~ 1A Single H.T.S. 0.227 9 ~ M.S. 5A~ 2 Single M.S. 0.275 9 ~-~ M.S. 5/~ 2A Single M.S. 0.275 9 ~ M.S. 5/~ - 3 Double H.T.S. 0.227 75/~ M.S. ~ 3A Double H.T.S. 0.227 75/~ M.S. ~,~ 4 Double M.S. 0.275 7:~ M.S. ~,~ 4A Double M.S. 0.275 75/~ M.S. ~,~ 5 Single 53S-T 0. 479 1 3 a / g 53S-W 7/~ 5A Single 53S-T 0.479 1 3 3 / ~ 53S-W 7/~ 6 Double 53S-T 0.479 I 1 1 ~ 2 53S-W a/~ 6A Double 53S-T 0.479 1115/~2 53S-W 9 2 . 1 6 1.52 3.044 74,670 44,570 P l a t e 113,300 P l a t e 106,000 6 5 . 7 110160 2 . 2 3 t'~ 9 2 . 1 6 1.52 3 . 0 4 4 74,670 44,570 P l a t e 113,300 P l a t e 104,500 6 4 . 9 11,000 2 . 2 0 _©t'~ 9 2.61 1.87 3 . 0 4 4 64,380 44,570 Plate 120,400 R i v e t 127,200 7 5 . 7 13,400 2 . 6 8 9 2.61 1.87 3.044 64,380 44,570 P l a t e 120,400 R i v e t 124,000 7 3 . 8 13,050 2 . 6 1 r~0 9 1.73 1.29 3 . 9 9 0 74,670 47,010 P l a t e 96,300 P l a t e 98,000 7 5 . 8 12,860 2 . 5 7 ~-* 9 1.73 1.29 3 . 9 9 0 74,670 47,010 P l a t e 96,300 P l a t e 96,500 74.6 12,660 2 . 5 3 9 2 . 1 0 1.57 3 . 9 9 0 64,380 47,010 Plate 101,000 P l a t e 109,100 8 0 . 6 14,300 2 . 8 6 9 2 . 1 0 1.57 3 . 9 9 0 64,380 47,010 P l a t e 101,000 P l a t e 110,000 8 1 . 3 14,430 2 . 8 9 ~:~ 16 6.41 4 . 5 4 10.320 38,430 18,170 P a n d 173,400 R i v e t 182,500 7 4 . 0 13,650 2 . 7 3 *'* R ~ 16 6.41 4 . 5 4 10.320 38,430 18,170 P a n d 173,400 R i v e t 182,800 74.1 13,670 2 . 7 3 C) R a O 13 5.49 4 . 0 6 12.464 38,430 17,410 Plate 156,000 P l a t e 163,400 7 7 . 4 14,280 2 . 8 6 _St) 13 5.49 4 . 0 6 12.464 38,430 17,410 Plate 156,000 P l a t e 165,000 78.1 14,400 2 . 8 8 ,-~ SMALL JOINTS ~:~ 7x Single H.T.S. 0.143 63/~ M.S. ~ 6 9 0 . 9 6 5 0.694 1.553 84,590 48,390 Plate 58,700 P l a t e 67,100 8 2 . 3 9,940 2 . 9 8 C) 8x Single M.S. 0.217 6 ~ M.S. 7/~ 6 9 1.465 1.085 1.553 71,570 48,390 R i v e t 75,000 R i v e t 75,300 7 1 . 8 11,170 3 . 3 5 ~.] 9x Double H.T.S. 0.149 4a/~ M.S. 5/~6 . 9 0 . 7 0 8 0.522 1.670 84,590 43,850 Plate 44,100 P l a t e 47,900 8 0 . 0 10,080 3 . 0 2 ,-, 10x Double M.S. 0. 193 43/~ M.S. 5/~ 6 9 0.917 0.674 1.670 71,570 43,850 Plate 48,200 P l a t e 51,950 79.1 10,920 3 . 2 8 l l x Single 53S-T 0.334 91/~ 53S-W ~ 16 3.173 2.267 5.411 41,100 19,460 Plate 93,200 P a n d 95,700 7 3 . 4 10,080 3 . 0 2

R d

12x Double 53S-T 0.331 75/~ 53S-W ~ 13 2 . 5 2 4 1.839 5.764 41,100 19,010 P l a t e 75,500 P l a t e 79,450 7 6 . 4 10,420 3 . 1 3 a Calculated according to A.R.E.A. Specifications. using nominal hole diameter.

b Calculated from nominal hole diameter (~'~2 inch larger than rivet diameter)• 0 Calculated from values in various preceding columns as follows: Efficiency (per cent) 4 Combination plate and rivet failures, involving the plate and two rivets.

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X I00. (9) X (12)

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Joint (1) C1 C2 C3 C4 Rivets M a i n plates

, - - - ~ - ~ N u m - Area, sq in. Tensile

Actual bet s t r e n g t h Shear

thick- Diam- in each N e t of s t r e n g t h Width, ness, eter, half of Gross ten- m a i n of

in. in. in. j o i n t tensile sile" Shear plate rivets (2) (3) (4) (5) (6) (7) (8) (9) (10) 13.375 0.504 0.875 13 6.631 4.782 8.386 41,500 24,130 13.375 0.504 0.875 11 6.631 4.116 7 . 0 9 5 41,500 24,130 9.50 0.327 0.625 13 3.106 2.212 4 . 3 9 7 36,750 25,240 9.50 0.327 0.625 11 3.106 1.907 3.721 36,750 25,240

T A B L E 2 . - - S T A T I C TESTS OF RIVETED HULL PLATE JOINTS

U L T I M A T E L O A D S A ND R E L A T E D D A T A

R I V E T E D J O I N T S I N 53S-T P L A T E S , 53S R I V E T S D R I V E N A T 1 0 7 0 D E G R E E S F

Properties of materials

U l t i m a t e

Predicted failure Actual failure Effi- load, lb Load ciency per in. F a c t o r per lb

Load, Load, of of of splice

T y p e lb b T y p e ib j o i n t • w i d t h safety d material (11) (12) (13) (14) (15) (16) (17) (18) Plate 198,000 R i v e t 209,000 76.2 15,650 3.13 42,000 P1 a n d R ~ 170,900 R i v e t 163,700 5 9 . 5 12,250 2 . 4 5 37,700 P l a t e 81,500 Plate 87,300 76.4 9,190 2.76 . . . P l a t e 70,000 Plate 76,000 6 6 . 6 8,000 2 . 4 0 . . . a N e t t e n s i l e a r e a e q u a l s p r o d u c t of n e t w i d t h a n d t h i c k n e s s less t h e c r o s s - s e c t i o n a l a r e a r e m o v e d iv c o u n t e r s i n k i n g . T h e n e t w i d t h f o r a n y c h a i n of h o l e s e q u a l s t h e g r o s s w i d t h m i n u s t h e s u m of t h e hole d i a m e t e r s p l u s t h e q u a n t i t y s2/4g f o r e a c h s p a c e , w h e r e s = p i t c h b e t w e e n a n y t w o c o n s e c u t i v e h o l e s , a n d g = g a g e of t h e s a m e t w o holes. b C a l c u l a t e d f r o m v a l u e s in p r e c e d i n g c o l u m n s as f o l l o w s : P r e d i c t e d l o a d = (7) X (9) o r P r e d i c t e d l o a d = (8) X (10) w h i c h e v e r is s m a l l e r . (14) c C a l c u l a t e d f r o m v a l u e s in p r e c e d i n g c o l u m n s a s f o l l o w s : E f f i c i e n c y ( p e r c e n t ) ffi ~ X 100. d B a s e d on a d e s i g n l o a d of 5 0 0 0 l b p e r i n of w i d t h f o r l a r g e j o i n t s a n d 3 3 3 0 l b p e r in. of w i d t h of t h e s m a l l j o i n t s . • B a l a n c e d d e s i g n , e i t h e r p l a t e or r i v e t s m i g h t f a i l a t t h i s l o a d .

T A B L E 3 . - - S U M M A R Y OF RESULTS OF R E P E A T E D LOAD T E S T S OF R I V E T E D JOINTS

R a t i o of fatigue Fatigue s t r e n g t h R a t i o of Fatigue s t r e n g t h Fatigt~e s t r e n g t h s t r e n g t h : of joint, lb per in. s t r e n g t h s :

of joint," psi of material, psi Material -- j o i n t of w i d t h Static fatigue -- static M a x i m u m

Single or :~Iaterial s t r e n g t h n u m b e r

double a t a t a t a t a t a t a t a t of joint, b a t a t of cycles Joint b u t t Main 50,000 1,000,000 50,000 1,000,000 50,000 1,000,000 50,000 1,000,000 lb per in. 50,000 1,000,000 a t nominal

No. straps plates Rivets cycles cycles cycles cycles cycles cycles cycles cycles of w i d t h cycles cycles design load

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) 7 Single H.T.S. M.S. 18,200 8,100 50,000 37,400 2 . 7 4 4.61 2700 1180 9,940 0.272 0. 119 23,000 8 Single M.S. M.S. 14,900 6,400 31,700 26,000 2 . 1 3 4 . 0 6 3270 1410 11,170 0.293 0.126 45,000 9 Double H.T.S. M.S. 22,700 11,800 50,000 37,400 2 . 2 0 3.17 3290 1710 10,080 0.327 0.170 48,000 10 Double M.S. M.S. 22,000 17,100 31,700 26,000 1.44 1.52 4300 3330 10,920 0. 394 0. 305 1,0O0,000 11 Single 53S-T 53S-W 10,000 5,200 23,100 17,300 2.31 3 . 3 2 3350 1730 10,080 0. 332 0. 172 52,000 12 Double 53S-T 53S-W 10,800 6,700 23,100 17,300 2 . 1 4 2 . 5 8 3630 2230" 10,420 0. 348 0. 214 96,000 a B a s e d on gross a r e a of m a i n p l a t e s . b T a k e n f r o m T a b l e 1, C o l u m n 19, s m a l l j o i n t s . > > 0 :Z r~ 0 H ¢b ~H 0 :Z

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A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N 511 Driv- ing temp., deg F 970 ° 1070 o 1070 °

TABLE 4.--SUMMARY COMPARISON OF RIVETED 53S-T HULL PLATE JOINTS

SINGLI~ BUTT STRAP J O I N T S

Rivets Static test results Fatigue test results

(Joints with 7/8-in. rivets) (Joints with 5/8-in. rivets)

N u m -

b e r Ulti- Fatigue Fatigue Maximum

in Slip at mate strength strength number

each Num- Average design load, in. load, at at of

half ber shear . . . lb per Effi- 50,000 1,000,000 cycles at

of of strength, Perma- in. of ciency, cycles, cycles, design

joint rows psi Total nent width % psi psi load

16 5 1 8 , 6 0 0 0.041 0.008 13,660 74.1 10,000 5200 52,000

13 4 2 4 , 5 7 0 0.035 0.006 15,650 76.2 11,200 4500 65,000

11 3" 2 4 , 5 7 0 0.040 0.008 12,250 59.5 10,700 3900 54,000

a This joint contained no e x t r a r i v e t s in the outside r o w .

rivet material is only 57 per cent of the u l t i m a t e strength of the 61S-T base metal, whereas the shear value of the steel rivet material is approxi- m a t e l y 80 per cent of the u l t i m a t e strength of the base metal. Particular attention, therefore, m u s t be paid to the design of the riveted joints in an a l u m i n u m ship. Generally, where it has been necessary in steel to use only single-strapped b u t t joints, double-strapped b u t t joints m u s t be used in aluminum. Also, it frequently will be found necessary to a d d an additional row of rivets.

Preliminary tests of new a l u m i n u m rivet alloys of i m p r o v e d strength h a v e given v e r y encouraging results and it is hoped t h a t these rivets soon will be available commercially.

I n general, the standards for the design of riveted joints in steel m a y be used for the design of riveted joints in aluminum, following the standards as set up b y the American Bureau of Shipping a n d th e United States N a v y . If the United States N a v y standards for a l u m i n u m riveted joint design are used, it should be k e p t in mind t h a t alloy 61S-T has a b o u t 50 per cent more bearing strength t h a n alloy 53S-T on which the N a v y Specifications are based.

F o r comparison, typical joints in steel and a l u m i n u m are shown in Figs. 3 and 4, each having been developed for the same efficiency as required b y the American Bureau of Shipping Rules.

Considerable experimental work has been done on riveted joints in aluminum, and direct comparison m a d e with similar joints in m e d i u m steel and high tensile steel, all designed for equal loads per inch of width [4], [5], [6], [7]. C o m p a r a t i v e de- formations, static strengths and fatigue stgengths were measured. T h e results indicate t h a t in actual service the a l u m i n u m alloy riveted hull construction should prove to be equally as satisfac- t o r y f r o m a strength s t a n d p o i n t as the steel riveted construction so successful in the past.

T h e results of the tests from which the foregoing conclusions were reached are given in Tables 1, 2,

3 and 4. Tables 1 a n d 3 describe the first series of tests driving the rivets a t 970 degrees. T a b l e 2 describes a second series of similar tests driving the rivets a t 1070 degrees. T h e joints driven with rivets a t the higher t e m p e r a t u r e s show a higher shear value of the rivet m a t e r i a l when driven. This has the effect of reducing the total n u m b e r of rivets required in each joint.

T h e reports on these tests are v e r y complete. Those participating in the tests, and others, con- sider t h a t t h e y offer conclusive evidence. These reports should be studied carefully before proceed- ing with a l u m i n u m ship design.

D E F L E C T I O N

T h e fact that riveted construction is e m p l o y e d requires that consideration be given to the deflection of the ship as a w h o l e a n d to the effect of this deflection o n the ability of the riveted joints to preserve the tightness of the structure. "

T h e a m o u n t of deflection in a n a l u m i n u m ship as c o m p a r e d with a steel ship is in inverse proportion to their respective m o m e n t s of inertia times the m o d u l u s of elasticity. ;Fhe result of this will s h o w generally the de.flection of the a l u m i n u m ship to be twice that of the c o m p a r a b l e steel ship.

T h e r e h a v e b e e n m a n y tests in steel a n d a l u m i n u m o n the strength of riveted joints b u t the author k n o w s of n o n e to date o n the tightness of these joints in either a steel or a l u m i n u m structure loaded hydrostatically at the s a m e time that it is subject to b e n d i n g tests. S u c h tests are u n d e r consideration at the present time, a n d possibly, b y the time this p a p e r is presented, c o m p a r a t i v e results m a y be available.

O n e of the closest e x a m p l e s of such a structure is riveted t a n k cars of 5 3 S - T a l u m i n u m . Certain of these cars h a v e b e e n in service for eight years a n d the results indicate a m i n i m u m of leakage u n d e r severe rail conditions.

W h i l e m o r e deformation c a n be expected in the a l u m i n u m ship, it is not expected that difficulty

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512 A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N

will be experienced due to the lower designed stresses.

Experience has indicated t h a t in the heavier weights of material an increase in the a m o u n t of calking is required to insure tightness of the joints under the large deflections. This is not necessary or desirable in the joints of the lighter weight material. A compound or tape can be applied to these faying surfaces if found necessary.

NOTCH SENSITIVITY

T h e question of notch sensitivity of ship plate material has become of great importance in recent years and no consideration of a new material for ship construction would be complete without an investigation of this quality. Information should be available as to whether or not the aluminum alloys show a difference in the types of fracture of notched tensile specimen under static loading at the extremes of temperatures likely to be encoun- tered.

Considerable preliminary work has been done "on the notch sensitivity of aluminum alloys under static fatigue loadings at room temperatures and a s t u d y over a wide t e m p e r a t u r e range is presently being undertaken. T h e preliminary tests made were on specimens cut from 0.091-inch thick 61S-T sheet. T h e tensile properties of this sheet were determined with standard 1/~-ineh wide specimens. All these tests were made at room temperatures. All the specimens failed b y shear fracture.

These tests indicate t h a t no serious embrittle- m e n t of this material will result and t h a t it will not be as notch sensitive as steel. This will be shown ~nore conclusively, however, when tests similar to those made at the David T a y l o r Model Basin on steel plates [8] are completed.

In addition to the usual tests for tensile and shear properties of the material, impact tests at various temperatures are planned.

ALUMINUM ALLOYS AT ELEVATED TEMPERATURES

T h e loss of strength of aluminum alloys at ele- v a t e d temperatures up to 400 degrees F need not be considered a factor in the design of an alumi- n u m ship [9]. 61S-T is a precipitation harden- able alloy previously aged at elevated temperatures. I t is the 61S-W alloy aged 8 hours at 350 degrees F or 18 hours at 320 degrees F. T h e effect of continued exposure to elevated temperatures would be overaging, which maintains generally the strength characteristics of the material. This alloy can be exposed to a temperature of 400 degrees F for 20 minutes and very little loss in strength results. Above 450 degrees F the loss in strength increases rapidly. Because of this, the alloy is not used for items which are expected to be

exposed for considerable periods to high temperatures. T h e modern American vessel with its fire- proof materials is well suited for a development of this nature.

STABILITY

One interesting point becoming apparent in the use of aluminum for hull construction is in the restoration of the balance between the weight and vertical center of gravity of the machinery plant in relation to t h a t of the hull.

As we know, the weight of the machinery p l a n t has been getting smaller and this in conjunction with the usual steel hull weight has had the effect of raising the over-all vertical center of gravity. M a n y of our approximations used in basic design in years past have had to be adjusted in recent years to take account of this and in order to avoid adding weight at a low level in order to get the required GM in the operating conditions of the finished ship.

T h e use of aluminum for the hull and for all those parts for which it is suited will permit the return to those design values in use heretofore.

In a normal cargo vessel of medium size, the vertical center of gravity of the aluminum ship will be at least 6 inches lower t h a n in a similar steel ship in the light ship condition.

PAINTING

It is generally known that the type of corrosion which m a y take place in these aluminum alloys is in the form of pitting r a t h e r t h a n scaling as is evi- dent in mild steel. Such pitting loss, which is v e r y small, occurs v e r y p r o m p t l y and qloes not increase to any material extent during the life of the metal.

A recent inspection of the Alumette, the experimental aluminum alloy hull [10], has confirmed the foregoing statements and given a good indica- tion of the proper painting procedure to be used to obtain the maximum protection of the metal.

When received in the stockyard the metal should be given a coat of zinc chromate primer, after first being thoroughly cleaned and treated with a water solution of phosphoric acid and grease solvents. T r e a t m e n t s of this type are described in United States Army Corps of Engineers' Specification T l 1 8 4 - D . Suitable zinc chromate primers for use on aluminum are covered b y United States N a v y Specification T T - P 6 5 6 b and United States Maritime Commission Specification 52-MC-29. Primers containing lead pigments are not recommended for use on aluminum since these primers have been found to cause corrosion in salt water immersion.

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T A B L E 5 . ~ R E S u L T S O F T E S T S O F 61S S H E E T A N D P L A T E T U N G S T E N - A R c - W E L D E D I N A N A R G O N A T M O S P H E R E T e n s i o n t e s t s B e n d t e s t s P u l l s e c t i o n s p e c i m e n s ( 2 ) . R e d u c e d s e c t i o n s p e c i m e n s (3) . Alloy Subseo J o i n t a n d q u e n t yield E l o n g a t i o n in T e n s i l e t e m p e r T h i c k - t h e r m a l T e n s i l e s t r e n g t h , ~ - - - ~ effi- T e n s i l e

before ness, P a n e l t r e a t - s t r e n g t h , (offset ffi 2 in., 8 in., c i e n c y , L o c a t i o n of stren.gth, L o c a t i o n of

welding in. N o . m e n t (1) psi 0 . 2 % ) , p s i % % % f r a c t u r e (4) ps~ f r a c t u r e (4)

6 1 S - T 6 1 S - W 61S-O 6 1 S - T 6 1 S - W 6 1 S - O 6 1 S - T 61S-W 61S-O 6 1 S - T 61S-W 61S-O ~ 12A 13A 14A 16A ~ 15A 12B H T & A 46,300 40,000 1 4 . 0 . . 13B H T & A 44,800 38,100 1 6 . 0 . . 14B H T & A 45,400 30,200 1 9 . 0 • • 1 6 B A 46,100 40,300 1 8 . 0 . . ~ 1 5 B H T 35,900 18,400 2 8 . 0 . . .~ 12A . . . 33,200 13A . . . 31,000 /a~ 1 4 A . . . 28,300 ~a~ 16A . . . 29,700 /3~ 15A . . . 18,200 12B H T & A 46,000 13B H T & A 43,900 8/~ 14B H T & A 44,300 a~ 16B A 32,000 a/~ 15B H T 34,300 F r e e G u i d e d " ~:~ b e n d b e n d Brinell h a r d n e s s (5) ~'~ e l o n g a - e l o n g a -

tion, t i o n , A t A d j a c e n t D i s t a n c e f r o m weld, in.

% % weld to weld ~-~ 1 1 ~ 2 3 8 O R I G I N A L SHEET AND P L A T E , W I T H O U T W E L D S (6) • . . 45,600 41,800 1 1 . 0 . . . . • . . 44,200 39,800 1 5 . 0 . . . . • . . 45,700 40,400 1 6 . 0 . . . . • . . 41,200 28,400 2 3 . 0 . . . . • . . 18,200 8,000 3 0 . 0 . . . . S H E E T AND P L A T E , W I T H O U T W E L D S , A F T E R S U B S E Q U E N T T H E R M A L T R E A T M E N T S A S - W E L D E D S H E E T AND P L A T E

24,300 5 . 5 1 . 4 7 3 ~ in. f r o m w e l d 33,100 ~ in. f r o m weld 14 ( 7 ) . . 62 61

22,100 7 . 7 1 . 8 70 I n weld; ~ in. 30,300 ~ in. f r o m weld 13 (8) . . 61 57

f r o m weld

18,900 8 . 0 1 . 6 62 I n weld; ~ in. 28,700 ~ in. f r o m weld 7 (9) 9 (9) 59 57

f r o m weld

20,000 1 0 . 0 ' 3 . 4 72 I n weld; ~.~ in. 29,600 I n w e l d ; ~ in. 9~(9) 9 (9) 59 59

f r o m weld f r o m weld

8,000 9 . 2 1 8 . 8 100 2 in. f r o m weld 18,700 ~ in. f r o m weld 7 (9) 7 (9) 64 36

WELDED S H E E T AND P L A T E D A F T E R S U B S E O U E N T T H E R M A L T R E A T M E N T S

39,000 8 . 0 1 1 . 8 99 2 in. f r o m weld 46,800 ~ in. f r o m weld 29 (7) . . 90 84

37,700 5 . 2 6 . 0 98 I n weld 45,800 I n weld 10 (8) 95 88

42,500 9 . 8 7 . 3 98 I n weld 42,400 I n weld 7 (9) 4"(9) 94 90

25,500 7 . 0 1 . 9 69 ~ in. f r o m w e l d 32,400 ~ in. f r o m weld 4 (10) 4 (9) 81 80

18,200 1 2 . 7 1 7 . 8 95 I n weld; 3 ~ in. 33,100 I n weld 9 (9) 9 (9) 67 62

f r o m weld ( 1 ) H T - - s o l u t i o n h e a t t r e a t m e n t ; A - - p r e c i p i t a t i o n h e a t t r e a t m e n t , o r a g i n g . (2) V a l u e s for w e l d e d s h e e t a n d p l a t e a r e a v e r a g e s of t h r e e t e s t s . (3) V a l u e s a r e a v e r a g e s of t w o tests. (4) D i s t a n c e s m e a s u r e d f r o m c e n t e r l i n e of weld. (5) 500-kg load on 1 0 - m m ball. . (6) R e s u l t s of s i n g l e t e s t of ~ in. w i d e t e n s i l e s p e c i m e n . (7) 0 . 1 4 in. g a g e l e n g t h . (8) 0 . 3 0 in. g a g e l e n g t h . (9) 0 . 4 6 in. g a g e l e n g t h .

(10) 0 . 4 0 in. g a g e l e n g t h ; f a i l e d ~ in. f r o m weld.

. . . O 82 90 90 92 88 87 ~¢) 70 88 88 88 88 88 59 80 91 94 03 02 60 75 76 74 74 75 ~'~ 33 34 34 33 34 33 O 91 02 86 82 88 88 ~'~ 93 91 93 90 91 90 91 92 90 92 01 92 66 86 103 96 96 93 6 1 61 61 62 61 62 ~'~ ©

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514 A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N

paints which have been found to give satisfactory service on aluminum. T h e United States N a v y cold plastic anti-fouling paint 65-5 series has also been found to retard effectively the a t t a c h m e n t of marine organisms. When applied over one or two coats of the zinc chromate primer, no corrosion of the aluminum occurred despite the fact t h a t the toxic ingredients in these paints are corrosive to aluminum.

Topsides, the same zinc chromate primers previously described, followed b y finish paint of the desired color, will be found satisfactory. Certain areas above the waterline, which it is desired to have left in natural finish, should be finished, after proper surface preparation, with clear lacquer based on methacrylate resins or t h e y m a y be main- tained b y frequent waxing, if desired, depending upon the item and its location on the shi~.

During fabrication a/1 faying surfaces should be coated with zinc chromate primer and a touch-up coat of the same priming paint should be applied over all driven rivets.

DISSIMILAR METALS

Since aluminum is anodie to most other metals, e x c @ t zinc and magnesium, it will be found t h a t the practice of using zincs for cathodic protection of the aluminum should be applied to the alumi- n u m vessel as to the steel vessel.

C a d m i u m is either slightly cathodic or has a potential equivalent to these aluminum alloys and for this reason can be used with ahiminum without corrosion resulting. Where medium steel must be used in contact with aluminum, electro-de- posited coatings of cadmium on the steel will prove effective against" the corrosion of the aluminum. Zinc used in place of cadmium will be corroded off and will have to be replaced.

Tests of aluminum alloys in contact with stainless steel show a t t a c k on the aluminum alloys in proportion to the a m o u n t of the stainless steel to the a m o u n t of aluminum alloy [11], [12]. This is especially true of alloys such as 17S-T, whereas it is m u c h less in the case of the 53S alloys whose behavior in salt water and salt atmosphere is similar to t h a t of 61S-T. For this reason and because of the corrosion of 17S-T alloy in salt air and salt water even without the presence of stainless steel, the 17S-T alloys are not recommended for marine use. This is unfortunate, as higher strengths are obtainable in this alloy and would be especially desirable in rivet material.

T h e combination of nickel alloys with the aluminum alloys should be avoided. T h e resulting corrosion of the aluminum is bad with all nickel alloys and is the worst with copper base alloys.

WELDING

At the present time, the welding of aluminum alloys b y the usual means is not recommended for marine work where strength is involved. T h e re- suiting efficiency of the joint is quite low, as compared with a welded steel joint. Because of this, welding is used only as a sealing bead and for minor attachments.

T h e latest major development in the field of in- dustrial welding, the Argon-gas tung~ten-are process, shows great promise for the welding of aluminum. B y this process, it is possible to obtain a higher degree of joint efficiency t h a n by the other welding processes, and it is believed t h a t continued use and wider application will see this process used more extensively in the welding of aluminum.

No flux is required, and the weld can be made readily in any position. T h e absence of weld spatter and the neat appearance of the joint a r e noticeable in this work.

A series of tests made on 52S, 53S and 61S material [13] welded b y this process to determine the tensile, bend and hardness properties of sev- eral tempers and thicknesses indicates t h a t promis- ing joint strengths can be made during the usual course of work.

T h e tensile and joint yield strengths of the as- welded panels varied inversely as the thickness of the sheet or plate, regardless of the alloy. T h e r e was no consistent difference of this kind, however, when the 53S and 61S panels were re-heat-treated and aged after welding.

Only the s u m m a r y of results for the 61S material are included in Table 5 and from these it will be seen t h a t the joint efficiencies of the as- welded 61S-T specimens ranged from 62 to 73 per cent. Higher effieiencies, up to 100 per cent, were obtainable b y subsequent heat-treating and aging, which, of course, would be possible only in small sub -assemblies.

As a further step in the consideration of joints in aluminum welded b y this method, fatigue tests are under way which it is believed, based on fatigue tests of welded joints made b y the usual metallic arc-welding methods, will show t h a t this m e t h o d will be satisfactory for the cycles likely to be expected during the life of the ship [14].

~{ACHINERY AND ELECTRICAL

Machinery units, pipe, valves and fittings of aluminum alloy are already in use in shore installa- tions for fresh-water, salt-water and fuel-oil systems. Their application to marine use will be quite similar except t h a t additional care must be taken to use the correct alloy, insulate from dis-

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A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N 515

similar metals and to provide the necessary protection against electrolytic action.

T h e y must be limited to applications which are not subjected to temperatures over 400 degrees.

At the moment, their proposed application to pipe valves and fittings is for services where the pressure is not over 150 pounds per square inch and the t e m p e r a t u r e not over 200 degrees.

T h e piping systems are made of tubing of aluminum-clad material, generally alloy 3S-1/,j hard. This, in conjunction with castings for valves and fittings of alloy 356-T6 or

220,

should give satisfactory service. Stainless steel is the most satisfactory metal for the trim of aluminum valves. T h e small area of the trim in relation to the valve b o d y results in little corrosion.

Such items as all aluminum condensers, boiler air casings, pumps, etc., are a distinct possibility. A t the present time, it is considered t h a t aluminum is suitable for m a n y marine electrical applications where steel or copper is normally used.

T h e p r i m a r y application is for electric cables, not only for the outer braid, b u t for the conductor as well. Even with the reduced current-carrying capacity per circular rail of aluminum conductors, compared with copper, it is estimated t h a t a 30 per cent weight saving will be realized. This can be a substantial a m o u n t as cables and bus structure m a y constitute as much as 40 per cent of the weight of the entire electric plant.

T h e cable weight saving is slightly less than the ratio of the weights of aluminum and copper for equal current-carrying capacity, since the diameter of the aluminum conductor is about 25 per cent larger tlhan t h a t of the equivalent copper conductor. Accordingly, more weight of insulation is required for the aluminum conductor than for the copper one. These aluminum conductors are hard-drawn, since annealed aluminum for this use is too soft and would not have the required tensile strength. T h e hard-drawn aluminum wire has about the same ductility and strength as the soft copper wire.

Other applications would be for the enclosing cases of distribution panels, m o t o r controllers, connection and junction boxes, cable hangers, lighting fixtures and appliances, etc.

SUMMARY AND CONCLUSIONS

Aluminum is a metal which can be alloyed readily and the addition of small amounts of various elements has the effect-of producing alloys suitable for m a n y different purposes. F o r this reason, it can be anticipated t h a t alloys for marine use will be forthcoming in the future which will be even more suitable than those in existence today. T h e standard of corrosive resistance now reached in the 53S and 61S alloys is sufficient for the purpose intended and this corrosion standard plus improved physical properties will usher in a more widespread use of aluminum for those applications which can justify iche increased cost.

T h e problems involved in the design of aluminum alloy ships have been investigated, or the in- vestigations are on the verge of being completed, and the results to date justify the construction of vessels of this material up to a b o u t 450 feet. Pres- ent practical limitations in sizes of plates and shapes would seem to preclude going farther at the moment.

Superstructures would not suffer from this limi- tation and, in addition, t h e y have already attained a certain a m o u n t of service experience.

M a n y other applications of aluminum in ship- building are not listed; where t h e y are justified economically, care need be t a k e n only to use the correct alloy for the purpose intended.

I t is hoped t h a t this paper is sufficiently comprehensive so t h a t it will be of help to those who have occasion to design ships of this aluminum alloy which offers such great promise.

T h e author is indebted to the Aluminum Com- p a n y of America for the tests they have carried out, the United States N a v y Department, Bureau of Ships, and others who have permitted the results of their work to be incorporated in this paper.

B I B L I O G R A P H Y

[1 ] United States N a v y Department, Bureau of Ships, Washington, D. C., Technical Bulletin No. 1 : " T h e Use of Aluminum Alloys in Vessels of the United States N a v y , " b y L i e u t e n a n t Com- mander S. N. Pyne, U.S.N., L i e u t e n a n t P. W. Snyder, U.S.N., and Mr. H. D. McKinnon, De- velopment Division, Aluminum C o m p a n y of America, September 1940.

[2] "Resistance of Aluminum-Base Alloys to Marine Exposures," b y R. B. Meats and R. H. Brown, T h e Society of Naval Architects and Marine Engineers,

Transactions,

Volume

52,

1944. [3] " E d g e Compression T e s t on Sheet with Edge Supports and Transverse Stiffeners," b y M. Holt of Aluminum C o m p a n y of America, Alumi- num Research Laboratories, Engineering Design

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516 A L U M I N U M ALLOYS IN S H I P C O N S T R U C T I O N

Division, Report Number 12-46-12, April 25, 1946. (Unpublished.)

[4] "Static and Repeated Load Tests of Aluminum Alloy and Steel Riveted Hull Plate Splices," by R. L. Templin and E. C. Hartmann, Aluminum Company of America, Aluminum Re- search Laboratories, Technical Paper No. 5, 1941. [5] "Riveted and Pin-Connected Joints of Steel and Aluminum Alloys," by L. S. Moisseiff, E. C. Hartmann and R. L. Moore, American Society of Civil Engineers, Paper Number

2233,

reprinted from

Transactions,

Volume 109 (1944), page 1359.

[6] "Static and Repeated Load Tests of Aluminum Alloy Riveted Hull Plate Splices," by M. Holt and J. R. Leafy, Aluminum Company of America, Aluminum Research Laboratories, Engi- neering Design Division, Report Number 12-46- 16, May 29, 1946. (Unpublished.)

[7] "Fatigue Tests of Riveted Joints, Prog- ress Report of Tests of 17S-T and 53S-T Joints," by E. C. Hartmann, J. O. Lyst and H. J. Andrews (Aluminum Company of America), National Ad- visory Committee for Aeronautics. NACA ARR Number 4115, September 1944.

[8] "Notes on the Conditions of Fracture of Medium Steel Ship Plates," by D. F. Windenburg and Captain W. P. Roop, U.S.N.; Supplement to

the Journal

of the American Welding Society, Welding Research Council, Volume X, Number 11, November 1945.

[9] "Room Temperature Tensile Properties of Aluminum Alloy Sheet Following Brief Ele-

vated Temperature Exposure," by J. T. Lapsley, A. E. Flanigan, W. F. Harper and J. E. Dorn, University of California, Project NRC-548,

Journal

of the Aeronautical Sciences, published in March 1947.

[10]

"Alumette--Experimental

Aluminum

Hull After Ten Years' Exposure, Fourteenth In- s p e c t i o n - M a y 7, 1946," by R. I. Wray, Alumi- num Company of America, Aluminum Research Laboratories, Paint Finishes Division, Report Number 6-47-85B, February 7, 1947. (Unpub- lished.)

[11] "Tidewater and Weather-Exposure Tests on Metals Used in Aircraft--II," by W. Mutchler and W. G. Galvin (National Bureau of Stand- ards), National Advisory Committee for Aero- nautics, Technical Note Number 842, February - 1942.

[12] "Marine Exposure Tests on Stainless Steel Sheet," by W. Mutchler (National Bureau of Standards), National Advisory Committee for Aeronautics, Technical Note Number 1095, Febru- ary 1947.

[13] "The Mechanical Properties of Tungsten- Arc Welded

52S,

53S and 61S Sheet and Plate," by C. F. Babilon, Aluminum Company of America, Aluminum Research Laboratories, Mechanical Testing Division, Report Number 9-46-17, June

18, 1946. (Unpublished.)

[14] "Static and Fatigue Tests of Arc-Welded Aluminum Alloy 61S-T Plate" by E. C. Hartman,

Marshall Holt and A. N. Zamboky,

Welding

Journal,

March 1947.

DISCUSSION

CAPTAIN SCHUYLER N.

PYNE, U.S.N.,

Member:

I have read Mr. Forrest's paper with much interest and wish to congratulate him on a comprehensive summary of the general problems which face the designer of aluminum alloy ships and shipboard fittings. In fact, I believe that this paper is the first general report in the field for many years.

This paper has brought us up to date in the progress that has been made in the basic investigation of "seagoing" aluminum. It is noted that a number of new studies have been made since 1940, at which time I collaborated in the preparation of a Bureau of Ships Bulletin on the subject but because of wartime duties I was forced to give up my detailed interest in the subject. These

studies have not been as numerous as had been hoped for. Probably during the war years the aircraft industry "stole the show" in aluminum developments and the reasons for using aluminum in ships were not sufficient, economically or otherwise, to ckeate much competition and thus permit advancement.

From reading the paper and from information already on hand, it is concluded t h a t 53S and 61S plates and shapes, using properly designed 53S-T riveted joints, are suitable for hull construction; that the problems of strength, deflection, corrosion, and dissimilar metals have all been bested. But I had hoped that we were ready for a design of the hull of an aluminum ship. It would be inter-

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A L U M I N U M A L L O Y S I N S H I P C O N S T R U C T I O N 317

esting to see or hear some detailed information on the scantlings of a fair-sized riveted aluminum vessel. A discussion of hull framing with sketches of typical examples tossed in for examination would also be of interest. T h e alloys for and the design of stem and stern castings together with the attachments of these castings to the shell plating would likewise be interesting. There are, in addition, other details I would like to hear about. I trust t h a t these items will be covered in the not- too-distant future.

I t is hoped t h a t developments in the use of these alloys in ships will proceed at an accelerated rate and t h a t before long we shall have a full-scale seagoing aluminum ship which will p a y its way.

MR. S. A. VINCENT, Council Member: Under the heading " S t r e n g t h , " the author mentions a design stress of 15,000 pounds per square inch for aluminum, which seems fairly reasonable for most details, as well as the standard L/20 wave main hull girder stress for moderate-sized vessels. Im- mediately following, under the same heading, he also mentions an experimentally determined critical compressive stress of 15,500 pounds per square inch for plating having seam laps and intercostal stiffeners. This, whether intended or not, seems to imply t h a t 15,500 pounds per square inch critical for 15,000 pounds per square inch design stress is acceptable. If 15,000 pounds per square inch is used as a designed stress for members in compression, the critical stress should, in m y opinion, be substantially greater to provide a reasonable factor of safety such as is usual in steel construction. In service, both local and main hull girder stresses m a y occasionally exceed the as- sumed condition designed stresses.

If, in this particular case, the stiffeners are not spaced closer than in the test sample, thus increasing the Critical stress, it would appear necessary to lower the designed stress in compression to well below 15,000 pounds per square inch. For the main hull girder, closer spaced stiffeners, or the equivalent to raise the critical substantially, would ordinarily seem the better choice. T h e author mentions the possibility of loads being transferred " t o the strong point," if overstressed plating yields. This m a y be true to at least some extent b u t it is also equally true and probably more so for steel construction. Because of the particular strength characteristic ratios of aluminum, it seems to be an even more risky assump- tion for aluminum design than for steel. As I see it, for the time being at least, the designed factor of safety for tension, compressiola, and shear members should in each case be not less than is usual for a comparable modern steel vessel using

ultimate, yield, or compression critical strength, whichever will provide the most conservative design basis. T h e per cent elongation and the relatively small difference between yield and ultimate of aluminum should influence the comparative acceptable factor of safety. In a new development such as this, scantlings and design should preferably err on the safe side, avoiding all foreseeable risks.

If the hydrostatically loaded test specimens mentioned under the heading "Deflection" are available in time, t h e y would be a valuable addition in the printed volumes. T h e a u t h o r has condensed years of methodical research into a few pages t h a t will no d o u b t serve as a valuable Chap- ter 1 for those investigating this new field. He is to be congratulated.

MR. D. P. BROWN, Member: T h e r e has been so much speculation and, in m a n y cases, erroneous information regarding the modern alloys of aluminum that, in m y opinion, this paper will serve a very valuable purpose and those of us who are concerned with the structural problems in the design of ships are fortunate in having the a u t h o r make available in such complete y e t concise form the results of the v e r y extensive review which he has been required to make of the large a m o u n t of data on these alloys which have been accumulated since t h e y were first developed on a commercial basis. T h e problems which must be considered in the adaptation of the aluminum alloys to ships' structures are m a n y and varied and not the least among these are those which arise in a t t e m p t i n g to translate the knowledge which we have acquired from our experience with steel structures into equivalent terms as required to evaluate properly the requirements for a hull of comparable strength using aluminum alloys. I t is m y belief t h a t inso- far as the yield point and ultimate strength alone are concerned we can approach this problem with a fair degree of confidence b u t the problems which m a y arise from the relatively low modulus of elasticity are those for which we have v e r y little background of knowledge and which m a y prove to be of considerable seriousness.

When the American Bureau was first consulted in connection with the design of the all-aluminum- alloy vessel to which reference is made in this paper, we expressed some concern over the possible effects of the low modulus of the material and recommended t h a t if at all possible the design should be developed with both the depth and the breadth relatively large in proportion to the length. I t was felt t h a t b y so doing it would be possible to develop the scantlings purely on a comparative strength basis and still keep the de-

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518 A L U i ~ I I N U M A L L O Y S I N S t t I P C O N S T R U C T I O N

flection of the hull, vertically, transversely, or in a n y inclined position within the limits as m i g h t be expected under the same conditions in a steel ship of the same length b u t having the least favorable ratios of b e a m a n d d e p t h to length with which satisfactory experience h a d been obtained. There is v e r y little knowledge of the actual effects of the over-all deflection of hull girders and from a strictly theoretical standpoint it m i g h t be argued t h a t flexibility of the hull could be of a d v a n t a g e in reducing the stresses due to the t e n d e n c y to conform more nearly to the contour of waves and thus re-distribute the supporting forces more nearly in line with those in smooth water. On the other hand, the m a g n i t u d e of the deflection m a y h a v e an appreciable effect on the d y n a m i c forces, concerning the importance of which there is practically no knowledge b u t considerable speculation. However, in the ~tesigns which were actually selected for development, other conditions dic- t a t e d t h a t the proportions of length to d e p t h should be practically up to those which are considered acceptable for steel ships and it is quite possible t h a t , if a vessel f r o m one of these designs is actually constructed, it m a y prove to be fortunate t h a t such was the case, since we m a y be able to relieve ourselves of a n y fears concerning this particular phase of the problem in future applications.

Another problem arising f r o m the low modulus of elasticity, a n d which m a y prove to be of more importance, is t h a t of the behavior of the riveted joints under stress. Irrespective of the magnitude of the over-all deflection of the hull girder, the deformation in w a y of the individual rivet holes, or w h a t m i g h t be described as the change in geome- try, can be expected to v a r y with the stress in the plating and inversely with the modulus of elasticity. Accepting for purposes of comparison t h a t on the basis of the relative ultimate strengths and yield points the section modulus of an a l u m i n u m alloy hull should be a b o u t 40 per cent greater t h a n t h a t of a corresponding mild steel hull, it will be seen t h a t the ratio of the stress to the modulus of elasticity in the a l u m i n u m structure wilt be ap- p r o x i m a t e l y double t h a t in the steel structure. I n other words, the actual effects of a n y given set of conditions on the riveted joints in the a l u m i n u m hull m a y be the same as those obtained in the steel hull under conditions twice as severe. A v e r y extensive search was m a d e in an effort to uncover a n y d a t a regarding the limits to which riveted joints could be stressed w i t h o u t leakage but, as mentioned b y Mr. Forrest, no reliable information along these lines was found for the types of joints a n d within the range of thicknesses of material which will be required to be used in even the more

moderate size of ships' hulls a n d as a result the American Bureau suggested t h a t it could only be considered proper prudence to c a r r y out the tests which are planned to be made and the results of which we hope will be made available in the near future.

Perhaps of more immediate concern to the naval architect t o d a y are the possibilities of the use of the a l u m i n u m alloys for the long deckhouses fitted above the main steel hull of large passenger liners. T h e increasing d e m a n d s for lifesaving equipment, safety devices, fire protection, etc., in our modern passenger vessels all have a tendency towards increasing topside weights and in this re- spect t h e y are diametrically opposed to the increasing requirements for stability to w i t h s t a n d flooding so t h a t the designers of passenger vessels are being forced into a search for light-weight m a - terials which are suitable for the construction of the long deekhouses. T h e a l u m i n u m alloys offer m a n y a t t r a c t i v e possibilities for these applications and, due to the low modulus of elasticity which would allow these long structures to conform to the flexure of the steel main hull with c o m p a r a t i v e l y low unit stresses, it should be possible to develop these houses with quite m o d e r a t e scantlings even without expansion joints and still w i t h a reasonable degree of assurance t h a t the unsightly cracks which are not u n c o m m o n in the long steel houses m a y be avoided.

T h e r e is a possibility, however, t h a t the substi- tution of this low modulus material m a y have some adverse effects on the main hull girder. T h e r e have been developed v e r y highly technical m e t h - ods for assessing the requirements for the longitudinal material in the hull girders of the large passenger vessels b u t it should be a p p r e c i a t e d t h a t in the final analysis these are all purely c o m p a r a t i v e . We often t a l k in t e r m s of stress b u t the figure which we designate as stress is merely a ratio between two values, the first being called a bending m o m e n t and the second a section modulus. Both of these values can be calculated in a n u m b e r of different ways but, as long as the ratio compares with those of other successful vessels of similar t y p e obtained in the same manner, the designer can feel t h a t he is on p r e t t y safe ground. I n all of the large liners f r o m which such c o m p a r a t i v e d a t a can be developed there h a v e always been fitted long steel deekhouses which m a y h a v e some effect on the hull girder itself and, while it is not even intended to suggest t h a t these houses con- t r i b u t e to the strength and stiffness of the m a i n hull girder in a n y t h i n g like their theoretically calculated effect, nevertheless such actual stress ob- servations as have been made on vessels of this t y p e under controlled conditions of loading indi-