US2634700A

US2634700A - Monolithic metal structure

Info

Publication number: US2634700A
Application number: US61256A
Authority: US
Inventors: Aaron B Bagsar
Original assignee: Sun Oil Co
Current assignee: Sunoco Inc
Priority date: 1948-11-20
Filing date: 1948-11-20
Publication date: 1953-04-14
Anticipated expiration: 1970-04-14

Description

April 9 1953 A. B. BAGSAR 2,634,700

MONOLITHIG METAL STRUCTURE Filed NOV. 20, 1948 I INVENTOR. AARON B, BAGS/4R ATTORNEYS Patented Apr. 14, 1953 UNITED STATES gm ()FFICE MONOLITHIC METAL STRUCTURE Aaron B. Bagsar, Drexel Hill, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey 4 Claims.

The development of fusion welding has resulted in its successful application to various monolithic steel constructions. An early application was to structures such as pressure vessels, after which the shipbuilding industry adopted this method of construction as a substitute for the riveted type of construction. The welding method has also been applied to a certain extent to bridges and other massive structures.

The known advantages of the welded over the riveted construction include: considerable weightand material-saving effected by eliminating lapped seams; appreciably stronger seams obtained by welding, since the riveted construction requires drilling or punching through the structural members; no maintenance problems involved similar to riveted seams, such as leaks occurring through rivet holes due to corrosion or excessive flexing, etc.; obtaining a flush job,

thereby simplifying internal drainage and painting problems and obtaining a stream-line construction, thereby reducing frictional loss.

However, in certain all-welded constructions of monolithic type, certain difiiculties have developed that were not foreseen and which in some cases have had serious consequences, such difiiculties being characteristic of those monolithic steel structures which are subject to multiaxial and severe stresses. For example, in ship construction serious fractures were encountered, because a normally ductile steel would exhibit a brittle fracture in a. welded ship, due to complexity of the stresses imposed by construction and by the service conditions, these fractures mostly occurring in the plates and not in the weld itself. In order to minimize these difficulties it was later proposed to insert in certain critical sections riveted seams in order to break up the continuity of the plates in the welded structures and also to obtain a certain amount of flexing, the purpose being to at least arrest the extent of cracking. It has also been proposed to add numerous structural members to the ships structure, mainly internally,in order to relieve the welded shell of the ship from excessive stresses, with the object of thereby preventing cracking of the hull and decking.

Both the above remedies have distinct limitations and disadvantages. They may reduce, but do not eliminate, the danger of losing a ship by fracturing of the hull.

Insertion of riveted seams in critical sections of the ship is distinctly objectionable, since riveting requires drilling of the main shell at these sections in order to install rivets, thereby seriously reducing the strength of the section. This modification is also objectionable for the reason that most of the stresses are thrown onto these riveted seams, because the remainder of the shell and decking is continuous by virtue of the fact that the plates are welded together and do not therefore flex as much as the riveted joint. Still another objection to this expedient is that the rivets in these insertions leak badly after a short service; which may be due to the excessive flexing and stressing that are induced in these seams, since the remainder of the shell is welded and continuous. These stresses are therefore more severe than in a wholly riveted shell, since in the latter such stresses are more evenly distributed over a number of riveted joints.

The insertion of independent members in the ship to stiffen the ships structure is also objectionable because of the difficulty of installing the sections in such a manner and with such rigidity as to make these additional sections share proportionately the stress simultaneously with the remaining members of the ships structure. In the circumstances, the hull fails first before the stress is transmitted to these additional stiffeners, or in other cases the stiffeners fail first and transmit the stress to the ships shell, thereby rendering the stiffeners ineffective.

It is known that riveted seams are not as susceptible to crack propagation as welded ships; of monolithic type, and that cracks may be ar-- rested in these riveted seams, although failure: of riveted seams themselves by cracking is not. altogether uncommon. It is believed that the: virtue of the riveted scam in this respect over the welded construction is attributable to the fact that no excessive stress concentrations are.

possible because of the relative flexibility of the riveted seam, and also to the inherent geometry of the riveted seam in which the adjoining plates: are not made integral with each other, as in: welding, but are held together mechanically by" rivets and butt straps or by the lapping arrange-- ment.

In welded bridge and other massive constructions, also, similar defects exist and similar serious fracturing has resulted, which, in the opinion of some engineers, make the application to bridges and other massive structures of fusion welding impracticable, notwithstanding the recognized advantages of such type of construction. Even pressure vessels of the welded monolithic type, such as gas storage tanks, are not immune to the dangers above outlined, which are not satisfactorily remediable-by any of the above specified expedients that break up the continuity of the plates. In fact the application of buttwelding, as distinguished from riveting, to structural steel generally, notwithstanding its pronounced advantages over riveting, has been restricted and, in some proposed applications, prevented, by the special defects above set forth.

Regardless'of all thecare exercised to eliminate welding imperfections and to improve workmanship, some irregularities will be present in the completed welded construction, such as im perfect weld penetration, misalignment, iZIGIH-f sions in the welded seams, weld undercutting, and certain discontinuities in the steel -plates them= selves which are difiicult such as scratches, arc craters, serrations or laminations. The loci of such imperfections act as focal points from which cracks may start, and unless some means is provided to arrest-them, these cracks will assume serious proportions. In the-case of a, ship a'single" crack may. wholly' incapacitate itor, even cause it to break into" sections.

The object oi -my' invention is to arrest'the extension of shearing or' tearing fractures: of

local originwhile avoiding: the limitationsan'd disadvantages of: both'the "riveted and welded construction and of" the expedients heretofore adopted having thesameobjeot. I have "found' that a metal plateofathickness less than that30 of conventiona-l plates.employed in the fabrication' of welded" ships and'other welded struc; tures is more resistant. to the propagation of a. crack than is the thicker plate; and further, that the thinnertheplate the more: resistantit is to such fracturing. I have;found,iurther, that; a metal unit com-posedof'a nu.-ber of such thin special plates superposed one upon'anoth'e'rf presents sogreat a resistance to cracking'thal'. it may be depended upon,;if applied to a mono- 40 lithic structure composed :ofconventional steel" plates, to eftectually resist a crack originating in a conventional plate;jhoweverfar removedythati proceeds in the-direction ofand-encounters such a unit. The plates forming'such a special unit should preferablybe at least three in number.:' Their-combinedthickness may slightly exceed," but should not exceed, substantially, the thick-- ness of a conventional plate; but if thespecial unit is interposed betweenconventional plates and butt-welded theretoit is prefer-able thatsuch unit shall be of the same thickness as -the con-' tiguous conventional plates. The thickness of no oneof the several plates should be substane tially over half thethickness of the conventional'* plate.

It will be. understood. that by conventionak plates I mean to define plates, usually of plain; carbon steel, each of the thickness of the shell to be fabricated and, when connected togetherby welding, forming the shell or at least the main body of the shell; suchplates being those cus tomarily employed in the-construction of welded" ship hulls and decks. The term plain carbon-1 steel is to be understood'in its ordinary: and accepted meaning, in that, besides carbon an'dt' usually small proportions of manganese and silicon, itis not alloyed with substantialproportiiims" of other. alloying ingredients; as in so-call'ed'-'al-- loy steel.- V I Instead-of- -'interposing 'such special} unitb tween; andbutt-welding-*-'it to," conventional plates, it -may"be-app-liedto"the interiorbr I terior of -one 'or*-more-conventional plates"and" welded -'-'thereto. So"-applied,-it will "resist the "37 to eliminate altogether; 15'

" sheets is 'of a thickness less thanfh-alf thickthat the thinner a sheet of"nieta;l. mayfheIfth propagation of a crack as efiectively as if it were inserted between, and butt-welded to, adjacent conventional plates.

The plates of any special unit may be of the same composition as that of the conventional plates, or one of the plates of the unit, preferably an interior plate, as for example a plate enclosed between two other plates, may be of a special composition that insures maintenance of ductility at the lowest temperature which the ship or other structure may be expected to encounter in'normal service. Thus, for example, such enclosed plate may be of brass, copper, or 'nic'kel co'pper; or it may be composed of one of the many alloy'steels which, unlike ordinary carbonsteel, maintains its ductility at such low temperatures, say below minus 50 F.; examples of which are chromium steel, nickel steel, nickelchromiu-m steel and molybdenum bearing steels. Plates of such materials are resistant to cracking regardless of" theirthickness. The several plates of a unit -may be*welded"together, although-this is not necessary. h

Units-of the structure last described are pref erable; since they afford a double protection against' cracking, with an insurance: against' crack propagation that'is practically absolute, regardless of the degree of stress or strain that originatesand tends to propagate the fracturey The special units' above describedv should be sorsha pedand so distri-buted over the area: desired to be protected that any crack, in what ever direction it proceedsywill necessarily 'en counter a special unit of'the described construe tion' and thus have its progress arrested. v

The invention may be better understood by reference-to the following description in connec tion with the annexed drawings, in which:

Fig. 1 is 'a plan view'of part of a ship deck:

Fig. 2 is a cross-section on the line 2-2 of Fig. 1 er on any line'extending transversely through the illustrated deck; W N

Fig. 3 is a sectional view of oneembodimentof my-invention in which the "special unit is intere posed between contiguous conventional: plates andbutt-welded thereto. H

Fig. l is a sectional view of another-embodii; ment-ofmy invention in whichthe special: unit is applied to one face of a. conventional plate.-

InIFigsl Z and '3, the plates marked a are-Q conventional plates of carbon/ steel, which, atlow temperatures, lose theirductility andacquire a a degree of brittleness that makes them, com paratively non-resistant to cleavage v fracture. Another figure, like Fig. 1, illustratingthe appli'ca'tion of the special units. tothefaceofthe hullglwould be identical with Sig. 1,. which there; fore may be'assumedi tov sr wspeein units asso ciat'ed' in. either'way with the conventional. plates. The platesor stripsb are special units inter-f "posed between, and butt-weldedto, the conven l 31 Eacli spc iall.

tional plates, as shown unit is composed of a number of'thin sheet,.pr' f erablyhotlessthan' three'i'n' number. (I, 'g and 'h), superposed one upon'fanother." Each'of such; thin.

ness'of' the'conve'ntional' plates. rereramy; combined thicknessfof"all'the sheets islapproximat'ely the thickness "of the i convention piste Thiscon'struction'has its-"basis in the. discliv *y-mt e never with. fine mer resistant; it 'is to'"an gextensipn: offa. "crack; or fracture through"andjbeyondit;" i-ere r thegre'ater thef'number 'btsheet d thitlii of a crack originating in some other area and directed toward a special unit of such construction. Moreover, such a unit is not an element of weakness, but is in fact stronger than a unit composed of a single plate of corresponding thickness.

I have found that if the described special strips extend, as indicated in Fig. 1, longitudinally of the ship and across the area to be protected, but occupying a very minor width of the hull and deck, the resistance to cracking or fracturing in the direction of the arrow is effectual; that is, the crack will extend through a conventional plate or plates until it meets a special unit, when it encounters a barrier through and beyond which it cannot proceed. Such a crack or fracture usually starts at or near the angle between hull and deck, but wherever it starts it cannot extend entirely across the hull or deck.

While two long special strips I) applied to the deck (if the deck is to be protected) and two long strips 2) applied to the hull afford adequate protection, it is desirable to apply additional long strips 0, especially to the hull. It is not necessary, however, that each special unit shall be a long strip extending entirely along the protected area. For such long special strips may be substituted, or to such long special strips may be added, a number of relatively short units, marked 01, so distributed (preferably staggered as shown) as to extend along a plurality of parallel lines extending longitudinally of the ship across the area to be protected and thus adapted to intersect any line extending transversely of the ship within that area. While in Figs. 1 and 2 these short units are shown as applied only to the deck, the protection of the hull from cracking or fracturing is of primary importance. These short strips are not in these figures shown as applied to the hull in order to avoid the confused showing that would result from overlapping of full and broken lines.

Instead of interposing such special units 1), or c, or d between the conventional plates and buttwelding them thereto, special plates or strips 6 of the special multiple sheet construction described may be applied to the face (exterior or interior) of the hull and (or) deck and united thereto as shown in Fig. 4. Such special units may be distributed as indicated by reference letters b, c and d in Fig. 1 and as described in specifying the distribution of inserted special butt-welded units. These special units may overlap, like a butt-strap, adjacent conventional plates (see for example the plates b in Fig. 1), but this is not necessary, and in fact is not desirable, especially in the construction of new ships, since separate conventional plates, with the special units applied thereto, may be fabricated before they are welded in place.

The thin plates constituting any special unit may be of carbon steel, that is, of the same composition as the conventional plates, but the units may be made even more highly resistant to progressive fracturing r cracking by utilizing as one of the thin plates, preferably a plate 9 confined between two other plates 1 and h, a plate of a special composition, as described in my application filed of even date herewith, Ser. No. 61,255. Such special composition may be that of any metal that, unlike carbon steel, retains its ductility under the lowest temperature that the ship may be expected to encounter in service, e. g. minus 50 F. In fact all three (or more) plates comprising the special units may be of such special composition, provided the composition is such 6 that the plate exposed to air and water will have the qualities required to enable them to be as resistant to corrosion or deterioration as ordinary steel, and provided also that they do not induce accelerated corrosion in an adjacent plate.

What I claim and desire to protect by Letters Patent is:

1. A construction for the shell of a ship the major area of which comprises a multitude of conventional plates of plain carbon steel and welded joints by means of which said plates are secured together to form the main body of a monolithic shell, in which shell cracks that may develop tend to extend in one general direction, namely, transverse to the longitudinal axis of the shell, said construction also comprising a number of special units and welds uniting said units to certain of said plates, each of said spe-, cial units comprising a number of strips arranged one above another, said special units extending longitudinally of the axis of the shell and thus transverse to said general direction and being of a length that is short relative to the length of that area of the shell that is designed to be protected from crack extension, said units being arranged along a number of lines extending longitudinally of the axis of the shell and that are transversely spaced apart, said units of each line also being spaced apart, said special units being of such length that they are transversely opposite spaces between special units extending along other lines and are also transversely opposite ends of such special units extending along other lines and are thus in staggered relation, whereby any line transverse to the longitudinal axis of the shell within such area designed to be protected intersects one or more special units and is thus adapted to arrest, and prevent the extension of, any crack that may spread in such general direcion.

2. The construction specified in claim 1 in which the special units are applied to one face of the shell.

3. The construction specified in claim 1 in which the special units are of a combined thickness the same as the thickness of the conventional plates and are inserted in the shell between conventional plates and in which opposite faces of each unit substantiall align with the opposite faces of adjacent conventional plates and are along their edges butt-welded to adjacent conventional plates.

4. The construction specified in claim 1 in which the strips of a special unit are at least three in number and in which a strip interposed between other strips of a special unit is of a composition, other than that of plain carbon steel, that maintains ductility at a temperature substantially below that at which plain carbon steel loses its ductility.

AARON B. BAGSAR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,290,091 Cole Jan. '7, 1919 1,869,208 McMillen July 26, 1932 1,904,253 Rydbeck Apr. 18, 1933 2,041,519 Altgelt May 19, 1936 2,419,218 Jannsen Apr. 22, 1947 2,481,614 Redmond Sept. 13, 1949 FOREIGN PATENTS Number Country Date 159,914 Great Britain 1921