Research Needs in Aluminum Structure
Robert A. Sielski
Naval Architect—Structures
Indio, California, USA
Abstract
The technology required for the design, fabrication,
operation and maintenance of aluminum structures for
ships and craft are reviewed to assess the needs for
improvements in that technology. The areas reviewed
are: material property and behavior, structural design,
structural details, welding and fabrication, joining
aluminum to steel, residual stresses and distortion,
fatigue design and analysis, fire protection, vibration,
maintenance and repair, mitigating slam loads and
emerging technologies.
Keywords
Aluminum ship structures; Aluminum high-speed
vessels.
Introduction
Aluminum has been used for the construction of ships
and craft for more than a century. In many cases, the
vessels have served well for several decades of use
without any serious structural problems. However,
there have been some difficulties. From the very
beginning, and continuing up to very recently, there
have been instances where the selection of the wrong
aluminum alloy has led to corrosion problems so severe
that the vessels had to be scrapped within a few years of
construction. Aluminum has been used for the
deckhouse structure of US Navy combatant and
amphibious ships for more than 70 years. Those ships
served well, but the aluminum structure was the source
of significant maintenance problems, mostly from
fatigue damage. Serious concerns for survival in
shipboard fire and for maintenance reduction led to the
discontinuation of the use of aluminum for major US
Navy combatant ships in the 1980s.
Towards the close of World War II, some merchant
ships built in the U.S. had aluminum in their
deckhouses, and this practice continued after the war,
primarily in the superstructures of passenger ships.
Aluminum began to be adopted worldwide for
fabrication of the superstructure of passenger ships, a
practice that continues today. Aluminum began to be
used in the 1940s for pleasure craft and for workboats,
the size of which has increase greatly over the years.
The use of aluminum for the hulls of high-speed
merchant vessels began in the 1990s with increased
construction of high-speed ferries. These vessels have
become so technologically advanced that they have
surpassed the capabilities of many naval vessels; many
navies today are adapting derivatives of these high-
speed vessels to combatant craft.
The interest in aluminum structure has increased greatly
over the past decade. Evidence of international interest
in aluminum ship structure is the International Forum on
Aluminum Ships, the fifth of which took place in Tokyo
in 2005. The Ship Structure Committee (SSC) has
recently completed a number of projects concerning
aluminum ship structures:
SSC-410, Fatigue of Aluminum Structural
Weldments
SSC-438, Structural Optimization for Conversion
of Aluminum Car Ferry to Support Military
Vehicle Payload
SSC-439, Comparative Structural Requirements
for High Speed Craft
SSC-442, Labor-Saving Passive Fire Protection
Systems for Aluminum and Composite
Construction
SR-1434, In-Service Performance of Aluminum
Structural Details
SR-1446, Mechanical Collapse Testing on
Aluminum Stiffened Panels for Marine
Applications
SR-1447, Fracture Mechanics Characterization of
Aluminum Alloys for Marine Structural
Applications
SR-1448, Aluminum Marine Structure Design
and Fabrication Guide
SR-1454, Buckling Collapse Testing on Friction
Stir Welded Aluminum Stiffened Plate Structures
Additionally, the US Navy’s Office of Naval Research
has begun a multi-year research program, Aluminum
Structure Reliability Program, aimed at improving the
technology for design construction, operation, and
maintenance of high-speed aluminum naval vessels.
10th International Symposium on Practical Design of Ships and Other Floating Structures
Houston, Texas, United States of America
© 2007 American Bureau of Shipping
This paper is based on the aluminum guide and on the
ONR program. Research needs will be discussed in the
following areas:
Material property and behavior
Structural design
Structural details
Welding and fabrication
Joining aluminum to steel
Residual stresses and distortion
Fatigue design and analysis
Fire protection
Vibration
Performance metrics, reliability and risk
assessment
Maintenance and repair
Structural health monitoring
Emerging technologies
Material Property and Behavior
There is vast experience with aluminum alloys in both
the 5xxx and 6xxx-series; the most commonly used
marine alloys. However, there are still fairly large
knowledge gaps in basic knowledge on these alloys.
The most common gap is in fatigue properties and
fracture toughness, particularly dynamic fracture
toughness, with much of the existing fracture data
coming from non-standard tests with invalid data.
An important discrepancy in basic material properties is
the variation among different sources on the strength of
welded aluminum. Table 1 illustrates this discrepancy
for several alloys. The differences come in part from
different standards for determining the yield strength
from a “dog bone” sample cut across the weld in a plate.
Some use a 50-mm gage length that measures only weld
metal and heat-affected zone (HAZ), but others use a
250-mm gage length sample that includes base metal.
Perhaps even more important than the difference in
yield strength is the manner in which this property is
used in design. The welded strength is typically 30 to
50 percent of the strength of the base metal, and this
reduced strength is used for most design calculations.
There are indications that the approach is overly
conservative. In studies on the compressive strength of
welded panels, Paik et al (2006) used a weighted
average based on the relative volumes of base metal,
weld metal, and HAZ. In a simplified analysis of a
welded panel in tension, Collette (2005) found that the
yield strength of a 5xxx-series welded plate was close to
the yield strength of the base metal, although for a
6xxx-series alloy, the strength was closer to that of the
HAZ. Research in this basic material property could
result in significantly increased allowable stresses and
reduced weight.
Table 1:Yield Strength of Some Alloys as Specified
by Different Authorities (MPa)
AuthorityAlloy
ABS DnV Aluminum
Association
AWS
Hull
Welding
US
Navy
5086-
H116
131 92 95 131 152
5083-
H116
165 116 115 165
5383-
H116
145 140
5456-
H116
179 125 179 179
6061-
T6
138 105 105 138
Aluminum alloys, particularly those of the 5xxx-series
have shown excellent corrosion resistance in service,
with some bare hulls operating for more than 30 years
without discernable corrosion. However, there is a
general reluctance to place 6xxx-series alloys in similar
service. Indeed, classification societies and the U.S.
Navy prohibit most uses of 6xxx-series alloys in contact
with seawater. A review of available corrosion data
fails to provide any experimental basis for this prejudice
against 6xxx-series alloys.
The 6xxx-series alloys generally have excellent
resistance to general corrosion over the surface, but
compared to 5xxx-series exhibit more localized pitting.
Most of the corrosion testing of aluminum in seawater
occurred in the 1950 through the 1960s. Goddard et al.
(1967) report that the maximum pit depth in three 5xxx-
series alloys (5052, 5056, and 5083) was 0.18 and 0.86
mm after five and ten years of immersion in seawater,
respectively. In the same tests, 6061-T6 had 1.30 and
1.65 mm of pit depth when samples were removed after
5 and 10 years of immersion. Although the 6061 had
twice the depth of pitting in this test, the rate is not
necessarily unacceptable. Other 6xxx-series, such as
6082 have less copper that 6061 and should have better
corrosion resistance, although data is lacking
The 6xxx-series are beginning to be used more
extensively in integrally stiffened deck panels, and there
is a desire to use these light panels for general hull
structure. A systematic comparative test of different
alloys in corrosive environments will demonstrate if
more extensive use of the 6xxx-series alloys is possible
and if this can be safely done will result in significant
weight and cost savings.
While the 5xxx-series alloys have generally shown to
have excellent resistance to corrosion, there is concern
that material is becoming sensitized over time to
intergranular corrosion and stress-corrosion cracking,
particularly when subjected to higher service
temperatures on exposed decks. An accelerated test that
would be based on the thermal profile of the decks of
operational ships must be developed to screen the
material.
There are also indications that there are reductions in
corrosion resistance in the heat-affected zones of welds
in the 5xxx-series alloys such as shown in Fig 1. The
standard ASTM G 67 NAMALT test is designed to
measure weight loss in a relatively large surface area,
not in the narrow band of the HAZ of a weld. A
standard for weight loss in the HAZ should be
developed to allow comparison and optimization of
welding methods.
Fig. 1: Corrosion at a weld in 5xxx-series plate.
Testing in accordance with ASTM G 67 as well as the
ASTM G 66 G66 (ASSET) test to determine
susceptibility to exfoliation are required for marine-
grade 5xxx-series aluminum alloys ordered in
accordance with ASTM specification B 928, which was
developed following extensive stress corrosion cracking
that was experienced in 5083-H321 material ordered in
the late 1990s (Bushfield et al, 2003). However, recent
experience (Kieth and Blair, 2007) showed some 5083
H321 that had been ordered to ASTM B 928 to have
considerable excess magnesium precipitating as a
secondary phase, Mg
2
Al
3
or -phase, in the grain
boundaries of the metal. The -phase is an
electrochemically active phase. When the -phase forms
as a continuous and complete network on the grain
boundaries, the material becomes “sensitized” or
susceptible to intergranular forms of corrosion. The
5083-H321 may have had more than 15 mg/cm
2
mass
loss in ASTM G 67. This experience indicates that
further research into sensitization of higher-magnesium
5xxx-series alloys may be needed.
Structural Design
Methodologies for computing the compressive strength
of plates and welded panels are well established and
validated for steel structure. For welded aluminum
structure, that is not the case. The work of Paik and by
a few others such as Rigo et al. (2003, 2004) represents
a good start on this validation, but many questions
remain such as the effect of transverse welds and the
effect of localized heat-affected zones resulting from
welded attachments. There is limited guidance on how
to incorporate such welds into finite element models. A
conservative approach is to treat such a panel as all-
HAZ material; however this may incur a large weight
penalty and does not shed any light on strain
concentration and other effects from the differences in
material properties over the panel.
Other design issues include:
Ultimate strength of plates and panels undergoing
combined loading such as biaxial compression in
multi-hull cross decks, or a combination of in-
plane compression and lateral pressure in the
slamming zone of high-speed vessels.
The effect of initial imperfections and residual
stresses on the strength of common aluminum
structures needs further validation, including
guidance on how to incorporate these values into
finite-element models for ultimate strength along
with simplified methods able to incorporate a
range of imperfection magnitudes in ultimate
strength prediction.
Guidance on how to incorporate HAZ effects on
ultimate strength calculations needs to be defined,
including techniques for incorporating HAZ into
finite element models, including estimates of the
effect of strain concentration on tensile ultimate
strength, and estimates of the effect of various
types of HAZ (GMAW, Laser, FSW) on in-plane
and lateral loads of panels.
Simplified methods for predicting the load-
shortening curves of plate and panels under
combined loading for use in overall hull-girder
ultimate strength calculations.
Ultimate strength methods for advanced
extrusions, where the plate thickness may not
remain constant.
Structural Details
Although many structural details that have been used
over the year with steel ships are acceptable in
aluminum construction, many are not, particularly
because of concern for fatigue strength. Likewise,
many details that boatbuilders have used for years on
smaller craft are not acceptable on larger craft because
of longitudinal strength and fatigue concerns. The Ship
Structure Committee has sponsored many projects over
the years on the suitability of different steel structural
details and their fatigue strength, but has not yet
produced significant guidance for aluminum details.
Fig. 2 shows one of the new types of details being used
in aluminum structures today—a detail of questionable
strength.
Lightweight
Deck Extrusion
Deck Beam
Flange
Deck
Plating
Flat Bar
Flat Bar
Fig. 2: Detail with lightweight deck extrusions.
Joining Aluminum to Steel
Many ships combine an aluminum superstructure with a
steel hull. The standard for performing the joint
between the metal is the roll-bonded or explosively
bonded bimetallic (actually trimetallic strip), which has
to be at least four times wider than the thickness of the
plate that it joins. If 10-mm aluminum plate is to be
joined to similar thickness steel plate, the bimetallic
strip has to be about 40 mm wide, which is unsightly
and somewhat difficult to paint, and it must be painted
to avoid galvanic corrosion. Kimapong and Watanabe
(2004) explored a simpler method to use friction stir
welding to join 2-mm 5083 plate to mild steel of the
same thickness. This work should be continued to
produce a less expensive and cleaner joint between the
two dissimilar metals.
Residual Stresses and Distortion
The residual stresses and distortions associated with
welding aluminum structures have advantages and
disadvantages compared to steel. The elastic modulus
of aluminum is one-third that of steel, but the coefficient
of thermal expansion is about twice as much. This
means that the strains that occur from the cooling of the
welds and surrounding areas will produce lower residual
stress in aluminum. However, the reduced elastic
modulus means that when residual stresses do occur,
they will tend to produce greater distortion than in steel
structure. Because aluminum conducts heat anywhere
from 2.5 to 9 times faster than steel, the area heated
during welding processes is greater but not as intense.
In general, welded aluminum structure tends to exhibit
greater overall distortion than steel structure, and
tolerances for ship construction reflect this, with greater
allowance for distortion being permitted in aluminum
structure than in comparable steel structure. Although
there has been much research done on the residual
stresses and distortion of steel ship structure,
particularly by the National Ship Research Program,
much comparable work is needed for aluminum.
Fatigue Design and Analysis
Design of aluminum structure to resist fatigue damage is
severely limited by a lack of information on the fatigue
strength of typical structural details used in aluminum
high-speed vessels. Several organizations have
compiled databases relating to the fatigue strength of
aluminum structural details and have published design
codes. The most recent of these codes is Eurocode 9,
which was developed by merging data from most of the
other sources and developing new data from testing of
medium-scale specimens typical of the details used in
civil engineering structures. These codes all assume that
the fatigue strength of welded details is the same for all
aluminum alloys, and that mean stress effects are not
significant.
The data from which these design codes were developed
does not reflect many of the structural details currently
used or proposed for use in construction of high
performance aluminum marine vehicles. Rather, they
are for the structural details used for civil engineering
structures such as buildings and bridges, for which
aluminum is sometimes used. Comparison of the
limited data that is available for the structural details
used for ship structure with the Eurocode 9 standard
shows that the international standard is far more
conservative than the data for ship details indicate. A
testing program is needed to address these deficiencies.
Some of the details used today for which no data is
available include joints in extruded aluminum sandwich
panels and other complex details commonly used with
other lightweight extruded panels.
A deficiency in Eurocode 9 is illustrated by Fig.3,
which has experimental data for a common ship-type
aluminum structural detail compared to the more
conservative fatigue strength of Eurocode9. In the
figure, the dotted line represents the lower 5 percent
limit of the data, and the solid line represents a
Eurocode 23, 3.4 fatigue classification, which would
apply to a detail of this sort. The Eurocode 9 standard is
considerably more conservative than the data would
suggest. This illustrates the need for more fatigue data
on specific ship structural details.
10
100
1,000
1.E+04 1.E+05 1.E+06 1.E+07
Cycles (N)
Stress Range (MPa)
EuroCode 9 - 23, 3.4 All Specimens
Fig. 3: Fatigue data for stiffener intersection compared
to Eurocode 9.
To perform fatigue analysis during structural design, an
accurate fatigue-loading spectrum is needed, which is
typically developed in the latter stages of design through
the use of hydrodynamic analysis or model testing.
However, fatigue considerations frequently control
many of the scantlings of aluminum vessels, including
hull girder strength and methods of performing fatigue
analysis in the early design stages are needed. Although
the format of fatigue allowable stress levels developed
by classification societies for initial guidance in the
design of steel ships for fatigue has some merit, the
method is too restrictive for the design of most
aluminum vessels because of their high speeds and
sometimes unusual hull forms. Rather, a simplified way
to develop a fatigue-loading spectrum is needed,
perhaps tied to the various methods of estimating hull
girder loads during early design stages.
The different multi-axial loadings occur at different
phases during a loading cycle, and a means of
combining all of the different loads to assess the fatigue
strength of structural details is needed. There is
currently no universal parameter for correlating cyclic
multiaxial stress/strain with fatigue life for marine
structures. Very few methods have been investigated
for welded joints as a group, particularly in aluminum,
and additional validation efforts are required before they
can be recommended for application to marine
structures. Potentially useful tools for extrapolating the
responses of aluminum structural details from one stress
state to another and for life correlation in high cycle
multiaxial regimes include the use of maximum shear
stress for crack initiation and maximum principal stress
for crack growth.
Aluminum has a crack propagation rate under fatigue
loading that can be as much as 30 times greater than that
of steel under the same applied stress intensity factor
range. Fig. 4 illustrate a fatigue crack growth
calculation for a steel hull and an aluminum hull that
were designed to the ABS HSC guide, with the section
modulus of the aluminum hull increased significantly
over minimum rule requirements because of a fatigue
crack initiation analysis. An initial 24-mm crack in the
aluminum propagated to 50 mm in 24 months of
service, but the same size initial crack in the steel
propagated to only 30 mm. When a crack reaches
appreciable size in an aluminum hull, it can grow
quickly and lead to catastrophic hull failure.
In steel hulls, placing significantly tougher grades of
steel in critical areas to serve as crack arrestors reduces
the chance of catastrophic hull failure from fast fracture.
Recognizing that that fatigue crack growth resistance
and fracture toughness are entirely different
metallurgical phenomena, an effective means must
found to arrest a crack in an aluminum hull before
catastrophic failures occur.
20
25
30
35
40
45
50
55
60
0 6 12 18 24
Months Service
Crack Length (mm)
5083 Aluminum H 36 Steel
Fig. 4: Predicted crack growth for a 4.39-m 32-knot
craft.
Riveted seams may be an answer to crack arrest, but
they represent a significant maintenance problem in
aluminum structure because they can lead to crevice
corrosion and are prone to leakage. Solutions such as
welding thicker bars of aluminum that will temporarily
reduce the stress intensity may be effective, but the
concepts need to be analyzed and experimentally
verified prior to use.
Fire Protection
The structural insulation requirements for aluminum are
more extensive than for steel because the aluminum
structure itself must be protected from the heat of the
fire by using fire protection insulation to prevent the
aluminum from softening or melting during the fire.
The goal of aluminum fire protection insulation is to
prevent the aluminum being heated to more than 230
°C.
Table 2 shows the results of two comparative studies
that were made of the weight of aluminum structure.
The first is a 42.7-m, 32-kt crew boat and the second is
the deckhouse of a naval combatant. In the first study
the aluminum structure alone weighed 56 percent of the
weight of the steel structure, but the total weight of
aluminum structure and fire protection insulation was
62 percent of the weight of the steel structure. In the
second study the aluminum structure alone weighed 49
