State of the Art

Basic Level

prepared by Asmond Broli, Hydro Aluminium Structures, Karmoy

Objectives:

−

To aquaint those who are unfamiliar with the use of aluminium in stressed

applications, with the past experiences and likely future developments

Prerequisites:

A general engineering background is an advantage but the subject matter is suitable for

most audiences concerned with transport and structural applications..

EAA - European Aluminium Association

Historical Development

Aluminium was relatively new when it was first introduced as a structural material. The

selection of alloys was limited and the fabrication techniques very primitive compared

with the situation today. Despite these facts, structural aluminium applications were

successfully introduced into many areas.

In this connection it is most relevant to group the applications into three main fields, and

to look at a few examples in

the Marine Industry,

the Transport Industry,

the Civil Engineering Industry.

TALAT 2201.01 3

Marine Industry

While the first steel ship was built in 1859, and only 11 steel ships were built in 1878,

aluminium came into use in marine applications interestingly soon after steel. Already

during the 1890s aluminium components were added to scores of ships and boats. But

the alloys and the fabricating techniques then available were unsatisfactory and

aluminium fell into disuse.

The 1922 Washington Disarmament Conference, which limited total naval

displacements, again spurred the thinking of naval architects toward aluminium. New

aluminium alloys were being developed to meet the strength and corrosion-resisting

requirements for marine constructions.

In 1928, the light cruiser U.S.S.

Houston was built with deckhouses of the then popular

structural alloy Duralumin. This ushered in a new era of warship construction. By 1940,

aluminium was used structurally for about 100 U.S. warships. More recently, the U.S.S.

Dewey

, a guided missile destroyer leader with aluminium superstructure, joined the

fleet.

The earliest applications to merchant ships were achieved in 1934 on three Mystic

Steamship Company colliers. One of these, a converted freighter, the S.S

. Glen White,

trimmed badly by the bow. The steel bulkhead between nos. 2 and 3 holds was replaced

by an aluminium alloy 6053 bulkhead which corrected the condition and permitted

carriage of 65 tons of extra cargo. When inspected 10 years later, there was no

indication of corrosion or excessive damage from coal handling. The adjacent steel

bulkhead, however, suffered from both.

Further development of alloys continued during the 1930s, a period which saw

aluminium used in additional merchant ship structural installations.

The higher-strength aluminium alloy 6061 containing magnesium and silicon as major

alloying elements, was under development prior to World War II. In 1944, as a result of

wartime experience, it replaced alloy 6053 for structural use, and was quickly adopted

for postwar merchant ships.

Aluminium construction received great impetus with the development of high-speed

welding techniques and other weldable alloys, particularly the Al-Mg 5000 series. Since

the early 1950s the majority of naval and merchant ship aluminium structures have been

welded.

As a consequence a total of more than 1000 merchant ships had been built with

aluminium superstructures in the beginning of the 1960s.

One of the best known ships with an aluminium superstructure is the S/S

United States

where the utilization of 2000 tons of aluminium resulted in a total weight saving of 8000

tons for the total vessel.

TALAT 2201.01 4

In addition to commercial ships and warships, aluminium is now used for tankers,

fishing vessels, personnel boats, ferries and hydrofoils (

Figure 2201.01.01).

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Training in Aluminium Application Technologies

Yangtze River Vessel with Aluminium Superstructure

Built 1948 2201.01.01

Transport Industry

In this context it is especially worth mentioning

the air transport,

the rail transport, and

the road transport industries.

In air transport the development and use of aluminium alloys is directly linked to the

development of that industry. It is clearly documentable that without the availability of

aluminium the civil aeroplane industry would still be in its infancy. Although titanium,

carbon fibre composites and stainless steel were used for military aircraft 70% of the

airframes of civil aircraft is aluminium alloy.

The use of aluminium in rail transport is another success story.

The railway industry took immediate interest in using aluminium when it became

available on an industrial scale around the turn of the century. Initially, the interest

centered on the light weight and corrosion resistant aluminium as a substitute for brass

fittings and wood or steel panelling in a coach structure, which was characterised by a

strong, load carrying steel underframe and a largely wooden superstructure.

During the twenties and thirties the design philosophy changed to enhance passenger

safety and reduce weight. The approach was to consider underframe and superstructure

as a load bearing entity. Steel panels riveted to a steel framework were used initially

TALAT 2201.01 5

followed shortly by aluminium sheet fastened to aluminium extrusions. This "sheet and

stringer" or "stretched -skin" design still persists to date for modern steel coaches with

the important difference that welding came in to replace the old-fashioned riveting and

that higher strength copper-bearing or stainless steels helped to improve the rustproblem

and to reduce weight.

A further recognisable change in the design of aluminium railway cars was dictated by

economic aspects. The significant increase in labour cost during the seventies spurred

the use of larger amounts of extruded sections with integrated functions. Together with

the availability of semi-automatic, multiplehead welding equipment, it became possible

to fabricate floor, roof and sidewall subassemblies with only a few longitudinal welding

passes on extruded shapes running the entire length of the car.

By using integrally stiffened extruded side and roof panels the rectification of distorsion

, which is inherently necessary in the stitch-welded or spot-welded "sheet and stringer"

design, was largely avoided. At the same time, labour-intensive finishing work and the

need for filler paste application preparatory to painting was reduced significantly.

In summary, the full application of the aluminium extrusion technology for the vehicle

body design resulted in cost reductions to such an extent that light-weight aluminium

coaches were and are being built at equal or lower costs than conventional steel coaches.

The all-extrusion design has consequently been applied in numerous modern railcar

projects all over the world (

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Training in Aluminium Application Technologies

All-Extrusion Design of High-Speed Passenger Train

Intercity Express, Germany. Built 1992. 2201.01.02

Source: VAW aluminium AG, Bonn

Aluminium alloys always have been used for automotive components including engine

parts, wheels, body panels and the structure frame since the beginning of the century. In

most cases the technical performance was satisfactory with significant weight savings

resulting. Often, however, the increased cost was not seen to be justified but this

situation is now changing with the demand for reduced fuel consumption and the need

to add safety and antipollution devices.

In trucks, trailers and tankers aluminium has been used for the past 40 years, the weight

advantages resulting in payload increase and for fuel savings which are more obvious

than in the automobile.

Civil Engineering Industry

During the 1930s a gradual introduction of aluminium applications into the civil

engineering industries took place. Special attention was directed towards various kinds

of roof structures, building systems, stairs, stairtowers, gangways, masts, silos, cranes,

pylons, towers, pedestrian bridges etc. (

Figure 2201.01.03).

In addition more recently a large number of structural military applications were

developed, e.g. transportable bridges, gun mountings, tanks etc.

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During the 1940s aluminium was introduced in road bridges, particularly in the USA.

Compared with the technology of today oldfashioned alloys and fabrication techniques

(riveting) were used. By 1963, approx. 20 road bridges had been built in the USA (the

longest being 100 m), and a total of approx. 40 worldwide.

Costwise these bridges were more expensive than equivalent bridges in steel, but the

expected lower life-cycle costs were planned to compensate for this difference. However

because of some deficiencies in design and fabrication this compensation was not

always achieved. While the general experience with many showed that they performed

perfectly over a 30 - 40 year period some corrosion problems occurred as a result of

incorrect alloy choice and/or wrong fastening methods.

While a great number of aluminium applications were developed and commercially

introduced during the first 6 - 7 decades of this century, not all of them can be reported

to have developed into substantial commercial success.

During the period 1970 - 1990 the following major trends can be identified:

•

In the traditional shipping industry a trend back to steel for hulls and

superstructures has been observed.

•

In some ships aluminium also has had a limited utilization, partly as a

consequence of the availability of new materials (GRP) and partly as a

consequence of a turn back to steel.

•

In fastgoing personnel boats, however, a very positive development has

taken place. The transition from 20 knots to over 35 knots speed levels,

introcuced by the catamaran concept, resulted in a need for all-aluminium

designs for reasons of fuel economy (

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•

Aluminium is still the preferred material in the civil aeroplane industry, and

had a very positive development in the rail as well as the road transport

industries.

In the civil engineering industry, aluminium has problems in maintaining its

position in many major applications, among those in bridge constructions

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The offshore industry is one new application for structural applications:

Helidecks

nd Perspectives

The present status of aluminium utilization in stressed structures can be summarized as

follows:

•

Despite the existence of good textbooks and codes of practice, the lack of

teaching material is obvious. As a consequence aluminium does not achieve

the status of an accepted structural material in engineering education (The

TALAT material will hopefully help to compensate this situation).

•

A lack of sufficient knowledge - often accompanied by prejudices- leads to

decisions against the use of aluminium.

TALAT 2201.01 12

•

Aluminium structures can mainly be found in applications like the rail and

road transport industries, speed personnel boats and aeroplanes where

weight saving is at a premium.

•

For those applications where traditional building materials like steel and

concrete are prevailing, aluminium is facing a stiff competition and

sometimes suffering set-backs.

The lack of formal education, competence and obvious commercial interests are

probably the major reasons for this situation.

Aluminium has a bright future as a structural material, but only based on following

prerequisites:

•

A comprehensive upgrading of the materials position at the educational

institutions.

•

The development of detailed cost studies for the respective potential

applications.

An example is the rapid and comprehensive use of aluminium in structural components

in the automotive industry. This development takes place as a joint development

between strongly motivated commercial interests, i.e. of the aluminium and the

automotive industries (

Figure 2201.01.12) and (Figure 2201.01.13).

Provided the required development regarding education and commerciality takes place,

aluminium has a great potential for making its way into new industries and applications

as well as regaining most of the lost positions.

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Training in Aluminium Application Technologies

Car Body Frame Used for Racing Cars

Built 1990 2201.01.12

TALAT 2201.01 13

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Training in Aluminium Application Technologies

An Aluminium Bodied Landrover Used

for Off Road Racing 2201.01.13

Criteria for Selecting Aluminium

All structural materials have different properties and technical characteristics, and

consequently differ in their suitability for a given application. For some obvious cost

reasons, aluminium will not become an alternative structural material in all cases, even

though its use would be technically possible.

In order to evaluate whether aluminium could be the right material in a specific

application some decision criteria must be considered:

•

Weight reduction

•

Maintenance aspects

•

Product costs

•

Load criteria

TALAT 2201.01 14

Lightweighting

Since, for all structural applications, aluminium will provide substantial weight saving

compared with traditional structural materials such as steel and concrete, all applications

where lightweighting has a commercial value are obvious candidates for aluminium

utilization.

Consequently, in the transport industry where fuel consumption is crucial for the

economy of a product, aluminium has a very strong position (aeroplanes, boats,

railways) as well as the greatest development potential (automotive).

A very often overseen effect of the lightweighting aspect is the downsizing effect. This

can be illustrated by focusing on a cable bridge where a substantial weight saving of the

bridge deck structure will also result in the possibility of downsizing towers, cables and

fundaments. A total application economy should therefore be introduced in order to find

the right solution for any structure.

Maintenance Aspects

Most aluminium alloys require low maintenance because of their good corrosion

resistance. This can be illustrated by

. Therefore, aluminium is an

excellent candidate for all applications where the benefit of freedom from initial

protection and maintenance yields a commercial benefit. A general problem in many

product developments is still the lack of life-cycle cost evaluations.

A tendency to select the cheapest alternative at the initial cost level could very well

result in higher life-cycle costs compared with other, initially more expensive solutions.

There is an increasing experience that life-cycle cost decision criteria will lead to

growing utilization of aluminium.

Product Costs

Aluminium is a more expensive material (per kg) than most alternative structural

materials. However, due to its low weight (resulting in cheap handling) as well as due to

modern joining technologies and the possibility of developing functional combinations

through utilizing especially shaped extrusions, labour costs become relatively low

compared to cheaper alternative materials.

Training in Aluminium Application Technologies

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General Corrosion Behaviour of Al and Steel -

a Factor of Maintenance Costs 2201.01.14

General Corrosion Behavior

Rate of corrosion in a marine environment: Steel: v

St = kSt ⋅ t

Aluminium: v

Al = kAl ⋅ t1/3

1 2 t (yrs)

Max. depth

Steel

Aluminium

1 2

t (yrs)

v

Steel

Aluminium

After 20 years in sea water:

Average corrosion rate/year: St52/Al 10-40/1

Or:

Consequence:

Virtually maintenance free construction

Training in Aluminium Application Technologies

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2201.01.15

1.2

1.0

0.8

40 50 60 70

Product cost

ratio Al/ Steel

Weight

reduction %

Primary structures

fabrication cost

Steel: NOK 15,- /kg

Outfitting

structures

fabrication cost

Steel: NOK 35,- /kg

1

2

1

2

Examples: Sture oil terminal: Pipe supports and access systems

Bridge structure: Connection bridge (105 m long) between 2 platforms

Capital Expenditures for Al-Structures Relative to Steel-

Structures (Effects of Weight an Maintenance are Included)

Capital Expenditures for Al-Structures

In

 an illustration of the consequences of this phenomenon is

presented.

 is developed based on competitive bidding of

aluminium applications in competition with equivalent steel alternatives. The diagram

shows that with a weight saving of 50% compared to steel in conventional outfitting

structures (stairs, stairtowers etc.), the aluminium alternative yields the same initial costs

as the steel alternative. If the aluminium product becomes more than 50% lighter,

aluminium is the cheapest material alternative - the lightweighting and maintenance

aspects having been considered.

For primary structures (bridges, etc), approximately 63% weight saving is required

before product cost equivalence aluminium/steel is achieved. If such a weight saving is

not achievable, secondary effects like lightweighting, downsizing and low maintenance

costs are needed to evaluate whether aluminium is an optimum material selection or

not.

Load criteria

Theoretical weight savings close to 70% compared with steel and 95% compared with

concrete are achievable. Consequently, aluminium has the potential of becoming the

cheaper alternative already on a product cost level.

Whether such weight savings are achievable or not depends on the load criteria. The

higher the dead load/live load ratio, the higher the weight saving which can be expected

By the example of a 105 m long bridge

Figure 2101.01.15 illustrates where the dead

load for the steel alternative represents 80% of the total load. By changing to aluminium

the dimensioning load was reduced resulting in 65% weight saving and product costs

10% less than for the steel alternative. Consequently, long span constructions especially

with high dead load/live load ratio are obvious candidates for aluminium utilizations.

Literature

Aluminium-Zentrale Düsseldorf (Editor):

Aluminium-Schienenfahrzeuge,

Entwicklungen-Technologien-Projekte. 174 pages, Hestra Darmstadt, 1992

Joliet, Hans (Editor):

Aluminium, die ersten hundert Jahre. 338 pages, VDI Düsseldorf

1988/89

Koewius, A., Gross, G. and Angehrn, G.:

Aluminium-Konstruktionen des

Nutzfahrzeugbaus. 358 S., Aluminium-Verlag Düsseldorf, 1990

Woodward, A.R.:

Gegenwärtige Probleme und zukünftige Bestrebungen bei der

Verwendung von Aluminiumlegierungen auf dem Bausektor. Aluminium,

Leoben 1968

Woodward, A.R. and Mc Laughton, B.D.:

The Fatigue Strength of Structure Joints in

Aluminium. Institution of Structural Engineers, Sheffield, July 1970 (12

References)

Woodward, A.R.:

The Use of Aluminium for Stressed Components. Institution of

Mechanical Engineers, Sheffield, Sept.1973 (22 References)

TALAT 2201.01 17

Woodward, A.R.:

The Future Uses of Aluminium Alloys, Sixteenth John Player

Lecture. The Institution of Mechanical Engineers, Sheffield, Feb. 1980

Woodward, A.R.:

Developments in Aluminium and Aluminium Alloys for Extrusion.

Designing with Aluminium Extrusions, Oct. 1983