AER-56-7

Stainless Steel in A irc ra ft C onstruc tionBy FR ED RIC FLADER,1 BUFFALO, N. Y.

The charac teristics of a new m a te ria l, com m only know n as sta in less steel, are considered, w ith fac ts and conclu ­sions concerning i ts ad ap tab ility an d su itab ility fo r use in a irc raft co n stru c tio n . T he type o f steel discussed is called 18-8, by th e p roportions o f ch ro m iu m an d nickel used. T h is m a te ria l is selected because i t is a u s te n itic in ch arac te r and is m ore readily p rocurab le th a n som e o f its k indred alloys. The s tren g th -to -w e ig h t ra tio s o f 18-8 stain less steel are first considered in re la tio n to com parab le values from a lu m in u m alloy, w hich is th e m ost widely used m eta l in a irc ra ft s tru c tu re s . T his com parison is m ade using four c rite ria as rep resen ting th e value of a m ateria l for s tru c tu ra l uses. These c rite ria are (1) tensile stren g th , (2) th e s tren g th of co lum ns, (3) th e s tre n g th of m em bers in bending , and (4) th e deflection or stiffness charac teristics. In th is connection th e finishes w hich are necessarily em ployed on a lu m in u m alloy as corrosion preventives are charged ag a in st th a t m a te ria l.

THE arrival of a new material in the construction of air­craft has been attended by considerable discussion regard­ing its merits as a structural material and its acceptability with respect to manufacturing costs. The characteristics of this

material will be considered in this paper with the view of present­ing such facts and conclusions as are available to permit an opin­ion to be formed as to its adaptability and suitability for use in aircraft construction. The discussion is a report of the develop­ment which has taken place up to this time and is only intended to represent a study of the art in its present condition. The material under consideration is the corrosion-resistant chrome- nickel alloy commonly known as stainless steel. There are many types of chrome and chrome-nickel irons and steels pro­duced for a wide variety of purposes, many of the uses being of a highly technical and specialized nature. The particular grade chosen for aircraft work is known as 18-8, in reference to the proportions of the principal alloying constituents, chromium and nickel, used in its manufacture. This material is selected for the following reasons: I t is the most common of the stainless steels, is most easily handled in its processing, and is therefore produced in larger tonnages and is available from a number of reliable sources of supply. I t may be hard-rolled to a very high tensile strength. I t is austenitic and non-magnetic, which prop­erties make it a satisfactory material for use in and around pilot’s cockpits because of its neutral effect on navigating and electrical

instruments. I t is satisfactorily corrosion-resistant for aircraft work without painting. This quality is of particular value in flying-boat and seaplane hulls because of their continual contact with salt water.

An examination of the strength-weight ratios of 18-8 stainless steel will first be considered in relation to comparable values for other materials tha t are most widely used in aircraft structures at the present time. Four criteria are considered as representing the value of a material for structural uses. These criteria are (1) tensile strength, (2) the strength of columns, (3) the strength of members in bending, and (4) the deflection or stiffness charac­teristics. In this connection the weight of the finishes which are necessarily employed on aluminum alloy as corrosion pre­ventives is charged against tha t material. I t is considered tha t the use of paint on stainless steel is unnecessary except for external color requirements.

Fig. 1 shows a representative stress-strain curve taken from a specimen of hard-rolled 18-8 stainless-steel sheet material. Un-

1 Aeronautical Engineer, Curtiss Aeroplane and M otor Company, Inc. Fredric Flader received his education a t Carnegie Institute of Technology, having the degree of B.S. in M.E. He was for five years with the Air Corps at McCook Field, on designing, stress analy­sis, and research, and was Air Corps representative a t the Cox-Klemin plant during the design and construction of the A -l ambulance air­plane. He was designer for the Buhl Verville Aircraft Corporation, Detroit, Mich., producing the “Airster;" designer for the Consoli­dated Aircraft Corporation and project engineer on the Consolidated Commodore flying boat; assistant chief engineer, Keystone Aircraft Corporation; design engineer a t the Curtiss Aeroplane and Motor Corporation, a t present working on stainless-steel construction re­search and other U. S. Navy projects.Contributed by the Aeronautic Division and presented a t th e Semi- Annual Meeting, Chicago, 111., June 26 to July 1, 1933, of T he A m e r i c a n S o c i e t y o p M e c h a n i c a l E n g i n e e r s .Note: Statements and opinions advanced in papers are to be understood as individual expressions of their authors, and not those of the Society.

F io . 1 R e p r e s e n t a t i v e S t r e s s - S t r a i n C u r v e o f H a r d - R o l l e d 1 8 - 8 S t a i n l e s s - S t e e l M a t e r i a l

(U ltim ate tensile strength , 190,000 lb per Bq in .)

like ordinary steel, this material does not have a definite yield point. The limit of proportionality is rather low, after which the material yields along a uniform curve of decreasing slope. In de­fining a yield point for a material of this character it is necessary to select a point a t which the stress-strain curve has deviated a definite amount from the straight line of Hooke’s law. The amount of this deflection is somewhat arbitrary, but not entirely so. An attem pt is made to define a value for the yield point so tha t when structures are designed using this value, the design load will be sustained without a permanent set of any of the mem­bers of the structure. A definition of such a value is taken a t tha t unit stress under which the test specimen shows an exten­sion of 0.002 in. per inch in excess of tha t which will be computed from Young’s modulus of elasticity and the formula:

Unit stresses = Young’s modulus X Unit deformationFrom reliable test data a minimum yield point of 140,000 lb

per sq in. is obtainable from 18-8 hard-rolled material. This value may be used in design work and is believed sufficiently conservative to take care of material variations and an allow­ance for discrepancies between test results and actual failing stresses in built-up structures.

Fig. 2 indicates the cost in weight due to paint. Curve 1 shows the relation between the weights of painted and unpainted aluminum-alloy surfaces. Some 25 to 30 per cent is added to

295

296 TRA NSA CTION S OF T H E A M ER IC A N SO CIETY OF M ECH A N IC A L E N G IN E E R S

F ig. 2 C o m p a r i s o n o f T e n s i l e S t r e n g t h o f P a i n t e d A l u m i n u m A l l o y W i t h U n p a i n t e d S t a i n ­l e s s S t e e l

(Tensile strength of alum inum alloy, 55,000 lb per sq in .; tensile strength of stain less steel, 175,000 lb per sq in .)

the weight of the gages of sheet usually employed on skin- covered structures such as monocoque fuselages and ta il surfaces. Curves 2 and 3 show the relationships between painted alumi­num alloy and stainless steel. The foregoing da ta indicate clearly th a t a saving in weight is possible when using stainless steel in so far as tensile strength m ay be used as a criterion of s truc tu ra l efficiency.

In Table 1 are computed some values of the factor e defined by the form ula given w ith the table.

This factor shows the relative streng th of various sizes of alum inum -alloy tubes compared to stainless-steel members of the same weight when tested as col* um ns. L / p values vary suffi­ciently to cover bo th the straight line and Euler ranges for the a lu m in u m - a l lo y tubes. The variation of strength of stainless- steel columns is no t clearly de­fined by either a stra igh t line or a Euler relationship. The factor e indicates a definite ad ­vantage in the use of stainless steel in short columns up to an L / p of about 50. Above this value stainless steel is inferior toalum inum alloy. This is simply because the weight ra tio of stainless steel to alum inum alloy is 2.83, while the ratio of their moduli of elasticity, which is the governing factor in long col­umns, is only 2.48.

Two beam sections, one of aluminum alloy 1 f t long and one of stainless steel 20 in. long, were made and tested in compression. Both specimens were of similar design and were made to repre­sent sections of a beam for the same airplane. The aluminum- alloy member weighed 0.57 lb per f t and sustained a unit stress of 40,500 lb per sq in. in compres­sion. The stainless-steel section weighed 1.098 lb per f t and held a un it stress of 128,000 lb per sq in. The factor e (see Table 1) computed from these tests based on equal weight is 1.21, indicating approximately a 20 per cent advantage in favor of stainless steel, even though this specimen was the longer one.

Beam members of similar de­sign to the compression specimens previously described b u t of longer lengths were made and tested in bending. The aluminum-alloy beam is an efficient design as evidenced by the development of 47,700 lb per sq in. stress in bending. Owing to a shear fail­ure experienced in the stainless- steel beam test, no ultim ate bend­ing stress was obtained. The

ratios of actual deflection to calculated deflections in the two beams were alm ost the same (about 0.90), which indicates th a t the same load would have been held by the stainless-steel beam th a t was sustained by the aluminum-alloy beam had not the shear failure occurred first. D a ta on the strength of stainless-

TA B LE 1Painted W eightsurface per 100Area W eight per 100 in.Size alum inum - of per in. painted P / A alum inum P / A stainlessalloy tube tube 100 in. length tube alloy steel e

Vs 0 .0 3 5 0 .05113 0 .5 1 1.09 0 .5 6 8 f 44,000 \ 1 .3601 0 .0 4 9 0 .1464 1 .4 7 2 .1 8 1 .627 L / p =10 \

1 44,000 ( 152,000 1.352IV 2 0 .0 6 5 0 .2 9 3 2 .9 3 3 .2 7 3 .1 6 5 1 44,000 ( 1 .3202 0 .0 9 5 0 .5 6 85 5 .6 8 4 .3 6 5 .9 9 4 { 44,000 ) 1.288Vs 0 .0 3 5 0 .05113 0 .5 1 1.09 0 .5 6 8 L / p = 50 |[ 28,000 ) 76,000 1 .0702 0 .0 9 5 0 .5685 5 .6 8 4 .3 6 5 .9 9 4 1 28,000 ) 1.013Vs 0 .0 3 5 0 .05113 0 .5 1 1 .09 0 .5 6 8 L / p - 100 |i 10,330) 25,500 0 .9732 0 .0 9 5 0 .5 6 85 5 .6 8 4 .3 6 5 .9 9 4 [ 10,370 } 0 .921N o t e s :

Weig

e —

2

;ht of painte1 coat red o?2 coats navy 1 coat alumiT o ta l..W p

W P / A Wp = wei W = wei = wei . 83 = rati

d surfaces: ride primer. • gray enam num bitumi

P / A steel . dural X 2 ght of alum ght of alum ght of stain o w eieht of

astic pain

.83inum alio inum alio less steel stainless

t (inside)

y painted y unpainisteel to

bedweight of alu minum aHoy

0.0070 .0 4 00 .0 2 50 .072

lb per sq i lb per sq i lb per sq ilb per sq 1

itititit

steel beams in bending and in combined bending and compression are too meager to be of value in drawing any definite conclusions. W ithin the near future, however, a rather system atic series of tests along this line will be completed.

AERONAUTICAL ENGINEERING AER-56-7 297Data which have been obtained a t the present point in the re­

search program, while not fully conclusive, do indicate that a saving in structural weight of 10 to 15 per cent is possible when using stainless steel except in long columns. Owing to the slightly lower modulus of elasticity of stainless steel—26,000,000 as compared with 29,000,000 for other steels—structures made from stainless steel may be expected to deflect about 11 per cent more than equivalent members of chrome-molybdenum steel or aluminum alloy. Recent research has indicated tha t by special processing it will be possible to improve the modulus of elasticity and to raise the limit of proportionality of stainless steel. From information gained, it appears tha t the structural characteristics may be improved as more is learned about the material.

The remaining factor to be considered by the designing engi­neer is the matter of making joints and fastenings. Practically we are confined to riveting and bolting or welding. A riveted or bolted-up structure of stainless steel, aside from the cost of the rivets and bolts and their insertion, would be very expensive because of the difficulty in drilling holes. Acetylene welding and electric-arc welding are as a general rule not suited for use in fabricating joints which are strong and resistant to vibrations, because of their detrimental effects on the structure of the mate­rial. Electric-resistance welding of stainless steel is now an estab­lished and a reliable process.

A brief discussion of the physical characteristics of 18-8 stain­less steel and the adaptation of electric spot welding to its peculi­arities will be given. This alloy is normally austenitic, which means that the carbon content of the material is dissolved in the iron somewhat as salt may be in solution in water. The main­tenance of this condition is necessary for the material to have its normal physical properties and to be resistant to fatigue and corrosive attack.

Microscopic examination of a sample of austenitic metal shows that its structure consists of rather uniform crystals of rectangular section and that they are bounded by sharp, well- defined lines. When the carbon in this metal is merely mixed with the iron and is not in solution, the alloy is said to be non- austenitic. In this condition the crystals are somewhat separated by rather blurred and indistinct lines wider than in the austenitic metal. The material between the crystals is supposed to be carbon which has separated from the iron. This change in the character of the material gives rise to the term “carbide precipi­tation,” analogous to the crystallization of salt from a salt-water solution. Apparently this change from an austenitic to a non- austenitic material takes place when the metal is heated to a temperature between 950 F and 1550 F and is held a t this tem­perature for a sufficient length of time.

At about 950 F the carbon is completely dissolved in the iron. Upon raising the temperature, carbon begins to precipitate. This precipitation takes place a t an increasing rate until a tem­perature of 1200 F is reached. Above 1200 F the rate of carbide precipitation becomes less, until a t 1550 F it stops entirely. If the temperature is carried on up above 1550 F, the carbon gradu­ally goes back into solution and the alloy returns to its normally austenitic condition. If the metal, once more in its most desirable state, can be cooled sufficiently rapidly, it will pass through the critical temperature range just described without carbon leaving the solution again. This quality is of very great importance in selecting a method of welding the material.

Acetylene or arc welding, as all know, is a process of fusing two pieces of metal a t the points where they are to be joined in such a way tha t the metal is melted and caused to flow together. During these processes a considerable mass of material is melted and cooled rather slowly. During this cooling an ideal condition is produced for carbide precipitation to take place. Electric spot welding is a more desirable method, since only sufficient,

metal is melted to form the spot. Under ideal conditions a weld is made by pressing the two or more pieces of metal together and a t the proper time allowing just sufficient current to pass between the electrodes to bring the metal to the fusing point. At this instant the pressure applied causes the fused metal to flow to­gether, thus forming a homogeneous and firm bond between the plates. Because of the small amount of metal which has been melted and also because of the fact that the adjacent metal has not been heated, the molten spot cools very rapidly, so that the carbide precipitation does not have a chance to form.

To obtain welds of suitable quality and reliability for air­craft production it is necessary to make use of equipment some­what more advanced in its development than the average power- driven spot welder used in commercial work. In spot welding there are three main variables:

1 Pressure of the electrodes on the work2 The amount of current3 The time of current application.

The pressure can be adjusted to the amount required by varying an electrode compression spring, which is ordinarily furnished with a machine. The current is easily adjusted by changing the setting of a control, which also comes with the welding machine. The timing control is more difficult and requires a special de­vice. I t is necessary to make resistance welds in as short a time as possible for several reasons:

1 High-speed production2 Minimum oxidation or scaling3 Prevention of carbide precipitation, distortion, and

warping.As previously explained, carbide precipitation, as well as dis­

tortion and warping, is held to a minimum when the heat is confined to the actual welding area, and this is possible only if the welding time is very short. If welded in a short enough time,, it is theoretically possible to generate the necessary heat a t the location of the weld, fuse the surfaces together, and allow the heat to be conducted rapidly away by the surrounding metal and elec­trodes after the weld has been completed without heating the outer surface of the sheets to an injurious temperature. I t is not only necessary to complete the weld in a very short space of time, but it is also necessary to synchronize the make and break of the weld with points on the alternating cycle curve which pass through zero, or a t the times when no current is flowing owing to a reversal of the cycle.

The reason for beginning and ending a welding operation a t times of zero current flow is to avoid arcing when the electrodes are making or breaking a contact. The formation of an arc is a cause of oxidation and burning of a weld.

Each cycle of the sine wave representing a 60-cycle current takes a time of 0.0167 sec. If it is possible to weld in even cycles or half cycles, then the question of timing control is solved both as to accuracy of the duration of each weld and the fulfilment of the requirement of starting and stopping of a weld a t exact instants when no current is flowing.

Actually with suitable equipment it is possible to make welds in one-half-cycle increments. Theoretically it is best to use this amount of time for all welding and vary the amount of current in direct ratio to the thickness of the parts being welded. Practi­cally, however, a machine of normal capacity is not large enough to supply the needed current for welds on heavier gages, so tha t it is necessary to increase the time, using several cycles, so that the total amount of electrical energy dissipated a t the weld is approximately the same as would be the case with the shorter welding time and more current.

The most satisfactory device for controlling the application of

298 TRANSACTIONS OF T H E A M ER IC A N SO CIETY OF M ECH ANICAL E N G IN EE R S

F i q . 3 W e l d i n g P e r i o d s i n T e r m s o p C y c l e s o p C u r r e n t (Oscillograms illustrating range of control to be obtained w ith G .-E. CR7503-B2 thyratron-tube control for resis­tance-welding m achines. A, 2 cycles on, 1 cycle off; II, 1 cycle on, 2 cycles off; C, 3 cycles on, 2 cycles off; D, 4 cycles on, 1 cycle off; E, 9 cycles on, 19 cycles off; F , 13 cycles on, 2 cycles off; G, 97 cycles on, 7 cycles off.)

He only needs to know the strength per spot for each gage of material. The strength of a spot varies in a fairly definite relation with the thickness of the thinnest of the material being welded. In stainless-steel sheet-metal construction equiva­lent to th in aluminum alloy such as wing and tail-surface covering material, the strength of a spot weld is about double th a t of the most efficient size rivet in aluminum alloy. This advantage gradually diminishes on the heavier gages in which the strengths of welds and rivets are comparable.

M a n u f a c t u r in g C o s t sI t is not possible with avail­

able da ta to present any accurate figures to show the difference between the costs of aluminum- alloy and stainless-steel construc­tion. However, it is possible to make some comparisons between the various processes involved so th a t a fairly definite conclusion may be drawn as to the relative merits of the two methods.

Normal forming and cutting operations are much more diffi­cult and costly on stainless steel than on other commonly used materials. For this reason such operations are simplified and avoided as far as possible in designing stainless-steel aircraft parts. For the most part, shapes are made by a drawing process producing simple forms, but conforming to known principles

current to a weld a t the proper instant, definitely measuring a predetermined tim e in even cycles of current, and stopping the weld a t the proper tim e, is an electrical rectifier known as the thy ra tron contactor. The figures and microphotographs illus­tra te some of the results obtained w ith a device of this kind ap­plied to a spot-welding machine as the time-control medium. Fig. 3 illustrates welding periods in term s of cycles of current. The sym m etry and uniformity of these records show clearly the accuracy of control. Fig. 4 shows the results obtained by using long and short periods of dwell. The center spots were welded with 60 cycles or 1 sec time. A zone around these welds has been heated to the critical tem perature a t which carbide precipi­tates, so th a t these are not good welds. A t the top of the photo­graph are three welds m ade with 12 cycles. These welds are more uniform in quality and appearance due to the more rapid heating and cooling periods. A t the lower left side of the picture are four welds made in 1 cycle each. These welds are of excellent quality , w ithout carbide precipitation or discoloration of the metal. Fig. 5 is a section of one of these welds magnified 500 diam. The excellence of the grain structure is noteworthy.

W hen using electric spot welding, the designer does no t need to th ink in term s of rivet-shearing strength and bearing area.

F iq . 4 W e l d i n g R e s u l t s O b t a i n e d b y U s i n g L o n g a n d S h o r t P e r i o d s o p D w e l l

AERONAUTICAL ENGINEERING AER-56-7 299for their structural efficiency.

Because of the high density of the material (the same as ordi­nary steels) it is necessary to use it in thin gages in order to build structures of comparable weights to wood and aluminum alloy.A thin sheet inherently lacks stiffness normal to its own plane.For this reason it is considered necessary in order to take fulladvantage of this metal as a structural m aterial to resort in general to the principle of using frequent stiffeners in such a m an­ner as to keep the flat-sheet element widths down to low values, and further to make use of frequent supports along the lengths of the members to keep the column lengths down to low values.

Owing to this method of design with additional formers and stiffeners, it is believed th a t probably four times the num ber of spots will be used compared with rivets in aluminum-alloy con­struction. The cost of drilling, inserting, and heading operations on aluminum-alloy rivets has been compiled from actual time records for the three operations shown in Table 2. A comparison of the cost of spot welding is made based on the rate of speed in welding which has been obtained in work of a similar nature to th a t which employs the rivets.

This comparison indicates a great advantage in favor of spot welding considering only the present application of the method. The possibilities of spot welding m ay be more fully realized by considering what m ay be done using production methods. I t is within the realm of possibility to produce 75 ft of linear welding per minute, making four spots per inch, which am ounts to 3600 spots per minute. At such a speed it would of course be neces­sary to properly feed and guide the work through the machines. This is and will be accomplished to an increasing degree of ef­ficiency by indexing and feeding devices operated in synchro­nism with the welding operation.

Figures compiled from the parts lists of a large airplane which has recently been produced in limited quantities show th a t 142,- 000 rivets were used in each airplane. This corresponds to a cost of $ 6 3 9 0 a t 4 '/ 2 cents per rivet, including overhead. Assum­

TA B LE 2 C O M PA R ISO N OF R IV E T IN G A N D SP O T -W E L D IN G COSTS

Operation Fastening bottom sheet on boat hull Fastenings on truss type wing beam Fastening skin on m onocoque fuselage

a 3/i6 rivets.b Approximately 70 per cent are 3/ie r ivets and 30 per cent x/ \ rivets. c V6 rivets.

D rilling Inserting Man-hr Man-hr R atio of(holes and per for four cost ofper heading rivet Cost Spot spot welds Cost rivetsm inute (rivets per drilling, per welds equivalent of four andone m inute inserting, inch, per to one spots, costman) tw o men) and heading cents minute rivet cents of spots0 .9 1 5 1 .0 7 “ 0 .0493 4 .9 3 40 0 .00166 0 .1 6 6 2 9 .70 .9 5 8 0.646 0 .0 6 94 6 .9 4 10 0 .00664 0 .6 6 4 10 .50 .8 5 0 1 .33c 0 .0446 4 .4 6 40 0 .00166 0 .1 6 6 2 6 .8

F i g . 5 S e c t i o n o f O n e o f t h e W e l d s S h o w n i n F i g . 4

ing th a t 568,000 spot welds would be used in this same machine in stainless steel, the cost of welding would be only $471.44, taking the average of the figures given in Table 2. The saving per plane would be $5918, or in this instance a to ta l of $147,950 on a production of 25 planes.

Stocking and handling costs of stainless-steel m aterial are much less than these costs of alum inum alloy. In m any cases it is necessary to store aluminum-alloy sheets between alternate pieces of oiled or waxed paper to avoid scratches when sheets are pulled from the racks. I t is also necessary to take care to pre­vent abrasion and scratching of the m etal while in process of fabrication, and the use of sharp m arking tools is prohibited. This care of alum inum alloy is im portant because abrasion of the mill surface provides places for corrosion to set in, particularly on parts which are to be used near salt water. The same care is not required when using stainless steel, since this m aterial is much harder and less susceptible to damage of this kind. An­other im portant point is the difficulty of distinguishing between annealed and heat-treated aluminum alloy. One instance has re­cently been experienced of a very costly m istake in which improp­erly heat-treated rivets were in some way used in production. The visual differentiation between annealed and high-strength stainless steel is pronounced enough to avoid confusion, as an­nealed stock m ay be purchased with a dull pickled finish, whereas hard-rolled m aterial is bright.

The physical properties of 18-8 steel are derived from cold working, and i t does not respond to any sort of heat treatm ent except normalizing. Therefore this phase of manufacturing when using this m etal is elim inated except for such normalizing as is

considered to be necessary.In the au thor’s experience a

very costly item in aluminum- alloy construction has been the rejections and reworking made necessary by wrongly drilled holes, elongated holes, and bad rivets. The use of stainless steel and the spot-welding proc­ess obviates these difficulties to a large extent as no holes are required and a lesser degree of care is required in spacing the welds th an is the case with rivets.

Next in im portance to the saving effected by replacing the riveting process by welding is the practical elim ination of the elaborate finishing operations on alum inum alloy and other steel construction. Anodizing a t approxim ately 25 cents per square foot and sand blasting and painting a t approximately

X500 20 cents per square foot are

300 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERSalmost entirely eliminated when using the stainless steel.

Summarizing the probable advantages of stainless-steel con­struction from the airplane manufacturer’s viewpoint, the method should yield lighter structures for equal strength and should ma­terially reduce construction costs. The outstanding factors in

costs are the use of electric resistance welding and the reduction of costly finishing processes. From the operation standpoint the foregoing considerations lead to greater operating economy through increased pay loads and reduced maintenance costs, par­ticularly the item of continually renewing protective coatings.