ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
When aluminum is used as a superstructure on a steel hull, the stresses in the aluminum
are generally lower than in the steel hull because of the lower elastic modulus of aluminum.
However, the reduced fatigue strength of aluminum means that care must still be taken in design
1.8 Residual Stresses and Distortion
The lower elastic modulus of aluminum compared to steel has benefits and drawbacks.
Residual stress from welding is lower, but the stress that does occur causes greater distortion and
buckling of thin structure. The ability to predict distortion in steel structure is still limited today,
even with very involved finite element analysis. The prediction models rely to an extent on
experimental data, and there is far less data for aluminum, so that the state-of-the-art in
prediction of distortion in aluminum has not advanced. Rules of thumb for such things as weld
sequencing are available, but experience with fabricating similar structure is the best guide
today.
Because of the greater distortion that generally occurs in aluminum structure, fabrication
tolerances are greater than for similar steel structure. However, the impact of these increased
tolerances on strength has not been well addressed.
1.9 Fatigue and Fracture Design and Analysis Procedures
The low fatigue resistance of aluminum is primarily due to the faster fatigue crack growth
rates in aluminum, which are about 30 times faster for the same stress level with the same size
crack. The primary method of reducing the risk of failure from fatigue cracking is to prevent
crack initiation. This is done by using fatigue analysis during design and ensuring that the stress
levels and structural details used will not result in cracks initiating during the service life of the
vessel.
Fatigue analysis requires knowledge of the loading history that will occur during the
lifetime of the vessel. Analytical methods exist for doing this, but the high speeds and unusual
hull forms associated with many aluminum vessels sometimes exceed the capabilities of all but
the most advanced methods, which are expensive and time-consuming to use.
Aluminum has good tolerance to resist fracture from single overloads, such as
unexpected events or weapons effects. However, the fracture resistance is not as great as for
marine-grade steel. The greatest risk of hull girder fracture comes from fatigue crack
propagation, but means to arrest a growing crack have not been developed.
1.10 Fire Protection
Aluminum has a relatively low melting point, and must be insulated to protect the
structure from softening or melting in a shipboard fire. Requirements for fire zone boundaries on
commercial vessels are established by international convention and by the regulations of the U.S.
Coast Guard. The procedures for designing the insulation to meet these requirements for
aluminum structure were established in the 1970s by the Society of Naval Architects and Marine
Engineers, and have not significantly advanced since that time.
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