A review in design and manufacturing of stainless steel sandwich

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A review in design and manufacturing
of stainless steel sandwich panels
Jukka Säynäjäkangas and Tero Taulavuori, Outokumpu Stainless Oy, Finland
INTRODUCTION
The demand for bigger, faster and
lighter moving vehicles, such as ships,
trains, trucks and buses has increased
the importance of efficient structural
arrangements. In principle two
approaches exist to develop efficient
structures: either application of new
materials or the use of new structural
design. A proven and well-established
solution is the use of composite materials and sandwich structures. In this
way high strength to weight ratio and
minimum weight can be obtained.
The sandwich structures have potential to offer a wide range of attractive
design solutions. In addition to the
obtained weight reduction, these solutions can often bring space savings,
fire resistance, noise control and
improved heating and cooling performance.
Laser-welded metallic sandwich panels offer a number of outstanding
properties allowing the designer to
develop light and efficient structural
configurations for a large variety of
applications. These panels have been
under active investigations during the
last 15 years in the world. Outokumpu has been participating in several collaborative projects in this area.
In Finland the research related to all
steel sandwich panels was initiated in
1988 in the Ship Laboratory of
Helsinki University of Technology.
The first study focused on the application of sandwich panels in the shell
structures of an icebreaker. Since then
in a considerable number of research
projects in Finland, such as Shipyard
2000, Weld 2000 and the Kenno –
Light Structures Technology Program, manufacturing, design and
optimization of steel sandwich panels
have been investigated. The work is
based on several R&D projects driven
jointly with VTT Industrial Systems,
technical universities in Finland,
www.stainless-steel-world.net
stainless steel manufacturer Outokumpu Stainless Oy as well as
Finnish sandwich panel manufacturers.
In this article the results of the earlier
mentioned R&D work in steel sandwich structures and applications is
summarized from the stainless steel
material point of view. The research
related to design and design optimization of steel sandwich panels has been
summarised by Romanoff and Kujala
(2003).
SANDWICH STRUCTURES
The most common type of sandwich
structure consists of two thin, stiff and
strong sheets of dense material separated by a low density material which
have a lower stiffness and strength
compared to the materials used as top
and bottom faces. As a rough guide to
the proportions, an efficient sandwich
is obtained when the weight of the
core is close to the combined weight of
the both faces.
Sandwich panels having both the top
and bottom plates as well as the core
made of steel are called steel sandwich
panels or steel sandwich structures.
Steel sandwich panels can be divided
according to the core structures: I-core
with straight webs, O-core with rectangular beams as a core, Vf/V-core
with hat or corrugated sheets as a core
and X-core with two hats as a core,
Fig. 1. Other types of the profiles such
as C, U or Z can also be used as a core.
By using sandwich structures, it is possible to obtain high strength to weight
ratio, i.e. the sandwich structures were
found to be 30 – 50 % lighter than the
conventional steel applications (Kujala
et al, 2003).
Figure 1. Five different core geometries suited for steel sandwich structures.
Structural
Bending
stiffness
strength
Weight
1.0
1.0
1.0
7.0
3.5
1.03
37.0
9.2
1.06
Figure 2. A comparison of the strength to weight ratio of sandwich panels and a solid plate
with equal structural properties by prof. Dan Zenkert 1998
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DESIGN PRINCIPLES
Engineering materials have characteristic range of values to be used in
designing. The performance of the
structure is maximized and in loadbearing component this means maximum stiffness to strength ratio for a
given weight. The performance of a
structure depends on the type of loading (tension, compression, bending,
torsion or a combination of them), on
the shape of the section (solid, tubular,
I-section, etc.) as well as on the properties of the material (modulus, strength,
toughness, etc.).
The design formulations for steel sandwich panels are presented in the report
“Formulations for the strength analysis
of all steel sandwich panels” (Kujala
and Romanov, 2002). The panel
geometries considered in this report
were I-core panel, O-panel and Vf/Vcore panel. A brief description of how
to design formulations of all steel sandwich panels was also given. Some comparison between the calculations,
strength tests and 3-dimensional finite
element models were also presented in
that paper.
The strength, fire-resistance, fatigue
etc. properties of the metallic sandwich
panels can be considerably improved
by implementing filling materials such
as PU-foams, metallic foams, light concrete or mineral wool. The type of filling material used in an application
largely depends on the desired properties, fabrication method and price.
The correct design of sandwich panel
constructions has similar importance
as the analysis of deflections, stresses
and buckling loads. Furthermore
designing the joint of one sandwich
panel to another or to other structures
is one of the key elements in the practical applications of these structures.
DESIGN TOOLS
A Finnish company Componeering
Inc. (www.componeering.com) provides a software application for analysis and design of metal sandwich panels. The ESAComp software,
originally developed for the analysis
and design of composite structures,
has recently been enhanced with an
add-on module for metal sandwiches.
The theoretical input for this develop-
ment has been provided by the Ship
Laboratory of Helsinki University of
Technology. The work has been conducted as part of a technology programme supported by the Finnish
Technology Agency TEKES and within the “Advanced Composite Sandwich
Steel Structures” project supported by
the European Commission.
ESAComp software allows definition
of metal sandwich cross sections having I, O, V, C, Z type web configuration. A filling material may also be
included in the panel. The equivalent
stiffness properties of a panel are computed using analytical approaches
from which the structural components
like beams and rectangular plates
under given loads can be analysed.
Resulting panel stresses are taken back
to the level of the face sheets and the
web, thus allowing detailed failure
assessment including modes like yield
and local instability. ESAComp further integrates with finite element
(FE) software such as ABAQUS,
ANSYS and NASTRAN. The analyses of the complex structures are possible with relatively simple shell based
FE models as ESAComp brings the
details of the metal sandwich behaviour into the analysis loop.
ESAComp software has an easy to use
graphical user interface that allows
definition of metal sandwich configurations, Fig. 3. The results of a plate
analysis can be viewed as contour plots
indicating critical areas of the plate.
MATERIALS
Figure 3. Views of ESAcomp software
Figure 4. V-core stainless steel sandwich panel with hat sheets as a core. (Photo:
Mizar Oy)
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The results of the studies have indicated that the austenitic stainless steel
grade 1.4301 (AISI 304) can be used
for laser welded sandwich panels with
good mechanical and general corrosion behaviour in different applications. To increase the corrosion resistance and to improve the resistance to
pitting and crevice corrosion the
austenitic stainless steel grade 1.4401
(AISI 316) can be used.
The use of higher strength austenitic
stainless steels like grade 1.4318 (AISI
301LN) or even hard cold rolled
materials was shown to be good or
reasonable when substantial weight
reduction of load bearing structures is
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desired. Especially point loading resistance can be increased. The use of hard
cold rolled stainless steels for sandwich
panels is still under investigation.
CORROSION
The corrosion resistance of welded
stainless steels in salt spray chamber
tests and road conditions has been
studied in several projects (Alenius et
al 2002). The salt spray chamber tests
and the field tests consistently showed
that the use of proper post-weld cleaning method is of great importance
when subjecting welded stainless steel
parts unprotected to de-icing salt environments. Pickling was demonstrated
to be the most effective post-weld
cleaning method. Other methods were
clearly less effective.
Large laboratory and field corrosion
test programs were performed in the
ECSC funded project “Stainless steels
in bus constructions” (Report EUR
20884 EN 2003) which was coordinated by Outokumpu Stainless Oy. These
tests consisted different stainless steel
grades and joint types.
The aim of the study was to compare
the laboratory test results with the
results of the field tests. Remarkable
slighter corrosion took place in the
field tests in Rome, Gibraltar and
Madrid compared to the test results
obtained in Helsinki where de-icing
salt is used on the roads during the
winter season. Corrosion resistance
was classified in ascending order:
1.4003 (Cr12) - CrMn16-7 – 1.4301
(AISI 304) – 304sp (Mo-alloyed).
Grade 1.4301 (AISI 304) managed well
in these tests and it could be stated as a
preferred grade regarding corrosion
resistance. Joints and crevices are stated as the critical points.
In general it can be said that a visual
evaluation gives a good general view of
the surface, but a closer microscopic
examination is needed to check the
severity of the corrosion. That is the
case for example to identify existence
of crevice corrosion.
PRODUCTION PROCESS
Various manufacturing techniques, such
as resistance and spot welding, weldbonding and adhesive bonding are
www.stainless-steel-world.net
Figure 5. In the special container for gravel truck the stainless steel sandwich panels as
side walls are welded to wear resistant plates in the bottom of the structure. A Moalloyed stainless steel grade 1.4432 (AISI 316L) has been used. (Photo: Kennotech Oy)
applied for sandwich panels with metallic core. Laser welding combines high
productivity and low heat input with
excellent fatigue properties.
Austenitic stainless steel is ideally suited
for laser welding thanks to the low heat
input and high welding speeds. Laser
welding minimizes metallurgical
changes that could impair the corrosion
resistance of the weld metal. Another
advantage of using laser welding
austenitic stainless steels is the small
weld distortion produced by the low
heat input and the laser weld shape.
Figure 6. Stainless steel sandwich panels
made of austenitic stainless steel grade
1.4301 (AISI 304) used in the wood drying
tunnel. (Photo: Kennotech Oy)
RECENT APPLICATIONS OF STAINLESS
CONCLUSIONS
STEEL SANDWICH PANELS
Laser welded stainless steel sandwich
panels have big potential in wide range
of attractive design solutions. The correct design of the details of the sandwich constructions is of great importance as well as the analysis of
deflections, stresses and buckling
loads. Joint of sandwich panel to other
sandwich panels or to other structures
is one of the key elements in the practical applications of these constructions.
The results of the studies have indicated that austenitic stainless steel grade
1.4301 (AISI 304) can be used in laser
welded sandwich panels offering good
mechanical properties and corrosion
Steel sandwich panels have been used
for commercial applications during the
last 15 years. Stairs and staircase landings, bulkheads and decks are the main
application areas of metallic sandwich
panels in cruise ships and in other
marine applications.
In recent years a wide variety of applications of stainless steel sandwich panels are used in civil and mechanical
engineering as well as in other industrial sectors. These include floors of
buses, walls and floors of elevators,
working platforms in industrial applications and balconies of shipyard
(Kujala et al, 2003).
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▼
resistance. The use of higher strength
austenitic stainless steel as sandwich
panels was shown to be reasonable
when substantial weight reduction of
load bearing structures is desired.
In addition to laser welding the development of resistance and spot welding, adhesive bonding and weld-bonding processes will increase the variety
of efficient techniques in manufacturing of stainless steel sandwich structures in the future.
M., Ruostumattomat kotelo- ja kennolevyt
- Stainless steel in bus constructions, Final
kuljetusvälineissä (title in English: Stainless
report, European Comission, technical steel
steel frame and sandwich structures in trans-
research, Special and alloy steels, Report EUR
port vehicles), VTT Industrial Systems,
20884 EN, Bryssels 2003
Research Report VAL22-023283, 46 p., 2002
- Kujala,P., Romanoff, J., Salminen, A., Varis, J.,
About the author
Vilpas, M., Teräksiset kerroslevyrakenteet
(title in English: All Steel Sandwich Panels),
Metalliteollisuuden keskusliitto, MET, 2003
- Pimenoff, J., Manufacturing Sandwich Structures by Means of Superplastic Forming,
Helsinki University of Technology Laboratory
of Engineering Materials, TKK-MTR-3/00,
Espoo, 2000
REFERENCES
- Romanoff, J., The Effect of a Filling Material
- Alenius, M., Hänninen, H., Fabrication and
to the Local Ultimate Strength of an all Steel
properties of stainless steel sandwich struc-
Sandwich Panel, Helsinki University of Tech-
tures, Helsinki University of Technology Labo-
nology Ship Laboratory, M-256, Espoo, 2000
ratory of Engineering Materials, Publication
TKK-MTR-2/99
- Romanoff, J., Kujala, P., Formulations for the
Strength Analysis of All Sandwich Panels,
- Alenius, M., Kyröläinen, A., Vilpas, M., Hänninen, H., Corrosion Resistance of Welded
Helsinki University of Technology, Ship Labo-
Mr Jukka Säynäjäkangas
ratory, M-266, Espoo, 2002
Mr Jukka Säynäjäkangas, M.Sc. (Eng.), is
working as a Senior Research Engineer at
Stainless Steels in Salt Spray Chamber Tests
- Sopanen, A., Talonen, J., Kujala, P., Corrosion
and Road conditions, Helsinki University of
tests for Sandwich Structures, in Finnish (Kor-
Tornio Research Centre of Outokumpu
Technology Laboratory of Engineering Mater-
roosiokokeet kerrolevyrakenteilla), Helsinki
Stainless Oy. His earlier professional expe-
ial, TKK-MTR-3/01, Espoo, 2001
University of Technology, Ship Laboratory, M-
rience consists of both industrial and
252, Espoo, 2000
educative duties over a 12 years period at
- Alenius, M., Talonen, J., Hänninen, H., Vilpas,
Metso Paper Machinery Inc. Currently he
works in the field of product development
Facts about the Finnish manufacturers of steel sandwich panels
and he is responsible for the construction-
Currently there are two sandwich panel manufacturers in Finland.
al applications made of stainless steels.
Kennotech Oy
Mizar Oy
(http://www.kennotech.fi)
(http://www.mizar.fi)
The industrial production is based on
Mizar is specialized in laser welding
the latest development of laser weld-
of steel sandwich panels. The compa-
ing technology, strength calculation
ny has highly automated production
methods and new high strength steel
equipment and the business idea is
materials.
to produce laser welded sandwich
panels and other sheet metal struc-
The customer segments are
tures for construction industry, ship
• Transportation
building industry and other industry.
• Construction
• Shipbuilding
Equipment consists from three pieces
• Process industries
of gantry type laser welding stations:
Mr Tero Taulavuori
• 1 station equipped with 5 kW laser
Mr Tero Taulavuori, M.Sc. (Eng.), is the
The company optimises structural
for cutting and welding
configurations for different applications achieved by varying the thick-
Research Manager of Tornio Research
• 2 stations equipped with 8 kW laser
for welding
ment. He is responsible for the develop-
ness of cover plates and cores, material and the core shape.
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Centre in the field of product development of new innovative products based
The maximum size of the sandwich
on the new and existing production facili-
panels produced is 17 m x 4 m x 0,5
ties in the field of stainless steel. His eight
m and sheet thickness range is
years experience at Outokumpu consists
between 0,7 - 6 mm. Utilization
of both technical customer service and
of bigger thickness is also possible.
research duties.
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