Flow-formed, thin-walled aluminium cylindrical shells as substructures for aerospace applications

Due to their high load-bearing

capacity in combination with minimal

structural weight, thin-walled shell

structures made of aluminium are

frequently used in the aerospace

industry. Non-reinforced shell

structures are sensitive to

imperfections, i.e. even minor

deviations from the geometrically

perfect structure, although these are

still within production tolerances,

cause a significant reduction in the

buckling load of the shell. The

geometric imperfections vary and are

unknown during the design phase, so

that a reliable buckling load forecast

is only partly possible.

In connection with current research

projects at the Institut für

Strukturmechanik und Leichtbau of

RWTH Aachen (Institute for Structure

Mechanics and Light Construction at the

RWTH Aachen University (Rheinisch-

Westfälische Technische Hochschule

Aachen)), methods and procedures for

the design of the shell structures of

launch vehicles such as e.g. the

European civilian launch vehicles of the

ARIANE and VEGA series are further

developed. In particular, the influences

of the installation condition and the type

of load application, which so far have

only been partly taken into account, on

the structural-mechanical characteristic

behaviour of the shell structure are to be

investigated numerically as well as

experimentally. For experimental

preliminary investigations, test bodies

are to be produced that are to meet the

following requirements:

• High precision, i.e. minimal

geometric imperfections of the

shell structures

• Minimal wall thickness of the

shell so that the load to be

applied in the laboratory is

within a manageable range

• High strength properties of the

aluminium, in order to avoid any

plastic buckling of the shell

structure

In order to be able to realize a scaled

structure representative for aerospace

applications, the radius of the shell was

selected to be 160 mm and the wall

thickness of the shell to be 0.2 mm. The

length of the shell was defined to be 160,

so that the influence of boundary

conditions on the buckling load can be

considered to be negligible. This

structure was examined numerically,

with various different imperfections

taken into account, and geometric non-

linear static calculations as well as

dynamically explicit calculations being

carried out with finite elements

software. The buckling course occurring

as a result is shown in Figure 1. With an

increasing load (see Figure 1 a. to f.),

initially a local dent forms which then

causes a global failure of the structure.

This and additional theoretical

verification actions are to be compared

with the results of structure-mechanical

tests, carried out in the structure

laboratory of the Institut für

Strukturmechanik und Leichtbau

(Institute for Structure Mechanics and

Light Construction) of RWTH Aachen, in

order to validate the numerical models.

Figure 1 Course of buckling in the reference

structure - dynamically explicit finite elements

calculation

Furthermore, a device was designed

which is able to record with high

precision the geometric imperfections

and wall thickness variations. The data

record measured can then be

transferred to structural-mechanical

models, and in this way it is then

possible to improve the forecast

precision for the theoretically

determined buckling loads. In addition

the insights gained thereby can be used

for the further development of the

production process.

The above-mentioned demanding

requirements for the test bodies can be

realized by flow forming.

Flow forming is probably the most

interesting forming process for the

production of highest quality,

rotationally symmetric components. It

provides for a high utilization of

material, offers extreme geometric

freedoms and almost completely

exhausts the strength potential of the

materials. EMS (formerly Pronexos)

offers additional value with customized

special solutions in the sectors

aerospace, medical technologies or

automotive.

This very flexible, non-cutting

production process, which allows thin-

walled, seamless pipes, cylinders and

other axially symmetric components to

be produced with maximum precision,

in a wide area of formable material

applications, offers an extremely high

wall thickness reduction from the initial

state, up to prefabricated components

with a wall thickness of just a few tenth

of a mm.

Figure 2 shows a high precision

aluminium pipe with a wall thickness of

0.5 mm (internal diameter ID= 160 mm),

which corresponds to a total pre-

product forming of 87.5%.

The principle of flow forming is a cold

massive forming, causing the initial

strength of the materials to be increased.

The high degree of forming and the

extreme reinforcement, i.e. the increase

in specific strength, are possible only

due to this process being based on

forming by compressive stress. Other

processes act by tensile stress.

Figure 2 Preliminary product right (length 200

mm, wall thickness: 4mm)

In the case of this non-cutting process, a

workpiece is formed by using rollers at

high pressure. A mandrel is used as a

mould and represents the internal

geometry of the finished component.

Thus, in order to obtain the shell

required – in principle, a pipe section -,

the preliminary form shown in Fig. 2,

made of aluminium (AlMg2,7Mn, Fig. 2

right), is applied to the mandrel which

also features a cylindrical form. In

general, the components can be

cylindrical but also “flat” or pot-shaped.

By means of a so-called driver, which is

formed beyond the mandrel, this

mandrel sets the preliminary product in

rotation. By means of this counter-

direction process it is therefore possible

to produce very long precision pipes as

the forming is fully complete before the

pipe leaves the mould. The rollers

themselves are not driven; they run in a

counter-direction to the workpiece.

Schematically, the process of the so-

called three-roller flow forming is

shown in Figure 3.

Figure 3 Schematic of the three-roller flow

forming in front view

For the material, initially, this means a

plastic tensile stretching, than a stress

reversal up to compression under the

feed angle of the roller, and then it runs

back to the stress-free area under the

radius of the roller, where the stored

elastic energy can be removed. This

process is effected several times

underneath a roller, before the next

roller is used.

Figure 4 shows the finished flow-formed

aluminium pipe in the vertical flow

forming machine.

Figure 4 Finished product AlMg2,7Mn in the

vertical flow forming machine

The final wall thickness of the pipe after

flow forming was 0.5 mm, which, for the

purposes of this test, was then reduced

to 0.2 mm by turning and shortened to

the corresponding length of the shells.

Due to the forming process, the

aluminium pipe now features strengths

(Rm) of up to 366 MPa. Also, the yield

strength (Rp0,2) is comparatively high

with values up to 345 MPa.

Following manufacturing, the shells

were prepared by EMS for initial

preliminary tests. In order to ensure an

even load introduction, the upper and

lower edge of the shell are moulded with

resin in a rigid aluminium ring, and in

this way the usual boundary conditions

for shell buckling tests are realized, see

Figure 5.

After the shell buckling tests as well as

the steps described above have been

carried out, scaled structures taking into

account adjacent structures and the

connection technology representative

for launch vehicles are to be designed,

numerically examined and finally tested,

in order to provide for an improved

understanding of the structure-

mechanical behaviour of shell structures

and their design.

Figure 5 Moulded shell prepared for the shell

buckling tests

The production of the second set of test

bodies will again be effected by means of

a cooperation between the Institut für

Strukturmechanik und Leichtbau

(Institute for Structure Mechanics and

Light Construction) of RWTH Aachen

and EMS. For it is particularly for this

type of application that the advantages

of flow forming are obvious:

• Low production costs • Maximum material utilization • Very minor mechanical (re)-machining

of the components • Complex geometries • Very thin wall thicknesses • Very exact internal and external

surfaces • Very good surface quality • Cold forming possible even with

maximum strength materials (pressure forming)

Authors:

M.Eng. Linus Friedrich, Research Associate at the Institut für Strukturmechanik und Leichtbau (Institute for Structure Mechanics and

Light Construction) RWTH Aachen University

M.Sc. Maike Blankartz, Marketing Engineer at EMS (formerly Pronexos)

This article was published in Aluminium Praxis, 10/2015

www.ems-evolves.com