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Matthew Collette, University of Michigan Robert Sielski, Consultant

Aluminum Ship Structures

Abstract The U.S. Navy’s use of aluminum for vessel structures has been growing in recent years, with several classes of ships using aluminum as a structural material for their hulls or superstructure. Aluminum provides a significant weight advantage at a slightly increased cost over a comparable steel structure, which has advantages for high-speed craft and weight or stability restricted vessels. However, recent experience has underlined that aluminum is not simply a lighter version of steel, and constructing ships without considering aluminum’s unique properties can lead to extensive maintenance issues in service. This paper reviews recent work on aluminum, covering material issues including new alloys, sensitization and stress corrosion cracking, tensile and compressive limit state prediction, response under lateral load, fatigue initiation, crack propagation, and wider issues of design and lifecycle analysis. Future research and development needs are then presented based on the state-of-the-art. .

Introduction Aluminum has undergone a striking resurgence in the naval structural community over the past decade. At the present time, several classes of aluminum or partly-aluminum combatants and support vessels are now in service and under construction. Aluminum’s weight advantages over steel have opened up new possibilities for high-speed vessels which have been exploited by the U.S. Navy in both the Littoral Combat Ship (LCS) and Joint High Speed Vessel (JHSV) programs. The ability to extrude aluminum into complex profiles at the aluminum mill can offer significant construction advantages over traditional plate-and-stiffener structures. At the same time, supporting in-service aluminum structures on the FFG-7 and CG-49 class vessels has become increasingly

challenging as material difference from steel emerge that were not fully appreciated at the time of design. This decade of rapid development has resulted in a situation where pockets of different expertise on aluminum analysis, design, fabrication, and support exist. While focused technical papers have appeared on specific aluminum issues, integrating review papers have been notably lacking since the contribution of Sielski (2007). The objective of this paper is to review and assemble a high-level summary of these developments and lessons learned to help disseminate the applied state-of-the-art on aluminum structures for naval applications. In this work, naval applications and a few key references are presented in lieu of the more bibliographic-focused literature review of the traditional academic review paper. A key lesson from the experiences of the last decade is that aluminum is not lighter steel that can simply be substituted in place of steel in the design process — to fully take advantage of the material and to avoid potential pitfalls a more in-depth understanding of the material is required. The remainder of this work will review the material properties of aluminum, the strength and analysis tools currently available, the current approach for design and lifecycle analysis, and then present topics where future research is needed drawn from the previous areas. Current Materials Issues with Aluminum Sensitization of Aluminum The 5xxx-series alloys can be prone to reduced corrosion resistance from a condition known as aging or sensitization. Those alloys with magnesium content greater than 3 percent are particularly susceptible to sensitization. These high-Mg alloys include 5083, 5086, 5456, and 5383 alloys, containing 4 to 5 percent Mg, which is above the solubility limit in aluminum,

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about 2 percent at room temperature. During the process of rolling plate, the temperatures and degree of deformation during various stages is controlled so that the magnesium remains dispersed throughout the alloy. However, under some conditions, the magnesium in the form of Mg2Al 3 will precipitate at the grain boundaries. For marine structures, high temperatures from solar loading on decks and superstructures can lead to this sensitization, although the initial state of the material also appears to be a significant factor in how fast this process occurs. The Mg2Al 3 is highly anodic to the remainder of the alloy, and once at the grain boundary localized corrosion can occur. This type of corrosion is known as intergranular corrosion, because it proceeds along the grain boundaries of the metal. While the U.S. Navy was aware of the need to test as-received material for sensitization, the commercial industry had not addressed this issue until the last decade. Following the production of several aluminum commercial vessels with mill-sensitized 5083-H321 material, the industry developed a new marine-specific specification for high-magnesium sheet and plate material, ASTM B928. This standard defines the -H116 and -H321 tempers as marine-specific tempers and requires certification that the initial as-produced microstructure is sensitization-resistant. The acceptance criteria in ASTM B928 are currently undergoing extended validation testing under the auspices of ASTM. However, this testing is not designed to follow Navy-specific in-service thermal temperature profiles. Additionally, high-Magnesium 5xxx-extrusions, which are normally produced in either the -H111 or -H112 tempers, are not covered by ASTM B928. When specifying high-Magnesium 5xxx aluminum for marine applications that vary from the assumed temperature limits of B928, or involve tempers other than -H116 and -H321, the naval architect must be mindful to ensure the material will not suffer from sensitization. Given the unique nature of Navy deployments and the potential for in-service sensitization from elevated service temperatures, significant work on thermal profiles and resulting changes

in material properties has been undertaken by the U.S. Navy engineering community. Data collected under sponsorship of the U.S. Office of Naval Research (ONR) on the thermal profile on the deck of the X-craft Sea Fighter by Catherine Wong of Naval Sea Warfare Center, Carderock Division (NSWCCD) and reported by Sielski et al. (2012) was used to analyze two common 5xxx-series aluminum alloys for a tendency to sensitization. The result was that the estimated service life for alloy 5083 is 2 to 7 years and for alloy 5456 is 5 years. This evaluation of the tendency to sensitization is in line with a similar previous study that used temperature profiles of aluminum structure (Vassilaros and Czyryca, 1979) and concluded that, “…the use of AI-Mg alloys containing high magnesium content (i.e., greater than 3-percent Mg) for deck plate should be considered on the basis of the corrosion resistance of the sensitized mill product. Even the present specifications (QQ-A-50/19 and QQ-A-250/20), which limit 5086 and 5456 alloys to service temperatures below 150°F (65.6°C), may not be able to prevent the development of the sensitized microstructure condition due to aging which can be achieved via solar radiation alone.” Despite several decades of previous satisfactory service of high-magnesium alloys in ship structures exposed to solar heating, sensitization problems are occurring as documented through many reports, including (Schwarting et al., 2011) indicating that there is a definite problem on many ships with 5456-H116 alloy. It is possible that changes in fleet operations have led to more severe deck temperatures or that subtle changes in the processing of aluminum plate at the mills have brought about this increased sensitization in service. Whatever the cause, it is a problem that must be addressed. For now, designers specifying 5xxx-series aluminum for exposed deck locations should select the lower magnesium content 5454 alloy and its associated design allowables, which has greater resistance to sensitization. Although some have suggested that the current sensitization problems being seen in the CG-47 Class are due to longer service life of these ships, fleet experience does not support this

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assertion. The CG 69 was commissioned in 1992, and sensitization of the 5456-H116 plate was first observed in 2004, a period of only 12 years. The CG 16 class averaged 30 years of service and the CG 26 Class averaged 28 years of service, but no sensitization of the 5456-H116 plate on their deckhouses was reported. Although those and most other U.S. Navy ships designed prior to the 1970s had expansion joints in their aluminum deckhouses, that does not explain the sensitization of the CG 47 Class ships, as sensitization has been observed in rather short sections of the higher levels of the deckhouse that experience little stress from hull girder bending. Other Corrosion of Aluminum Sensitization is not the only corrosion mechanism that can affect aluminum. Early experience in the late 19th century and early to mid-20th century showed that many aluminum alloys, particularly those with higher copper content, were unsuitable for marine use. This led to the standardization of the 5xxx-series strain-hardened weldable alloys and the 6xxx-series heat-treated weldable alloys for marine use. However, these alloys are not completely immune to corrosion problems. The 6xxx-series alloys do have a relatively high copper content, and are particularly susceptible to corrosion by pitting. Therefore they are presently excluded by most classification societies and naval authorities for use in direct contact with seawater. The more modern 6082 alloy has a lower allowable copper content than the more widely known 6061, and in principle should have superior corrosion performance though long-term marine exposure data is currently lacking. Even with this limitation, corrosion of 6xxx-series aluminum is still seen in service. In one instance reported by Brady (2012) servicing of the strainers on some pumps caused seawater to flow on the structure and allowed to evaporate, eventually leaving behind several inches of salt residue, with severe pitting occurring beneath the salt. This may be an extreme case, but it matches anecdotal experience from commercial high-speed aluminum vessels that report pitting damage in areas exposed to spray-and-evaporate cycles in

superstructures. The presence of such locally aggressive environments demonstrates that aluminum does not have total resistance to corrosion, and preventive measures must be taken to avoid problems in service. The 5xxx-series alloys are not immune to corrosion, especially galvanic corrosion when immersed with metals having more positive electrical potential, particularly steel, copper, brass, or bronze. Generally, when such bimetallic conditions cannot be avoided, as with propulsion systems and rudders, a cathodic protection system is installed, either active or passive. In a recent instance, the tunnels for the water jets of a high-speed ship experienced galvanic corrosion because they did not have a cathodic protection system in place (Bloomberg, 2011). Even so, a breakdown of a cathodic protection system, particularly the malfunction of an impressed current active protection system can lead to rapid corrosion of the hull of an aluminum vessel. Another source of rapid corrosion can come if proper grounding is not used with a shore-based welding system used for repairs of an aluminum vessel when afloat. In the 1970s the 5456-H321 alloy then used for U.S. Navy aluminum on ships and craft began to experience a type of corrosion know as exfoliation, in which the exposed edges of the plate corrode in flaky layers. To overcome this problem, the alloy 5456-H117 alloy was developed (later replaced with the similar H116 alloy), with Navy acceptance criteria requiring the ASTM G 67 (NAMLT) test to determine an alloy's susceptibility to intergranular corrosion and the ASTM G66 (ASSET) test to determine susceptibility to exfoliation corrosion. This testing was also required for other higher-magnesium 5xxx-series alloys, including 5083. As discussed previously, both of these tests have now been incorporated into the commercial standard for high-magnesium marine sheet and plate, ASTM B928. The combination of a tendency for exfoliation in the alloys then in use as well as a breakdown of electrical insulation in the gap between the steel coaming welded to the deck and the edge of an aluminum deckhouse led to corrosion in these

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joints between steel and aluminum structure. To improve the situation, a bimetallic strip of steel either explosively or roll-bonded to aluminum was developed in the 1970s, and has seen generally good service when properly applied. However, the bimetallic joint succeeds only because it provides a smooth surface that can be easily coated to resist corrosion, but immersion in seawater, even intermittently, can lead to corrosion. Welded Material Properties The marine 5xxx-series and 6xxx-series alloys are considered to be weldable. However, unlike most steel alloys, there is a reduction in strength by 60 to 70 percent in the weld metal and in the associated heat-affected zone (HAZ) in aluminum welding (Sielski, 2007). This reduction in strength is allowed for in most design codes by basing the design strength on the lowest yield strength in the base metal, HAZ, or weld metal. This approach was shown to be overly conservative for aluminum structure in tension by Collette (2005) and for aluminum structure in compression by Paik et al. (2008). Collette indicates that for a typical panel of 5083-H116 aluminum where the yield strength of the HAZ is about 66 percent of that of the base metal, the combined yield strength of base metal and HAZ might be 85 percent of that of the base metal. Paik found that a weighted average between the material properties of base metal and HAZ based on the relative volumes of the materials gave good correlation with experimental results for welded aluminum panels in compression. However, beyond strength reductions, there is a corresponding reduction in the structure’s ability to achieve large plastic deformations that must also be considered. This is addressed in some detail in the section below on tensile strength. If additional analysis of and experimentation with welded aluminum structure can validate the use of a higher design stress than the strength of the HAZ and weld metal, there could be a 25 percent increase in allowable design stress, with a possible 25 percent decrease in the weight of the structure, depending on other considerations, including compressive and fatigue strength.

Historically, the approach to the reduced welded yield strength of aluminum has been to base tensile strength and yield strength on the testing of welded specimens with a 250-mm (10-inch) gauge length. Because the width of the weld and HAZ in these samples is about 20 mm wide, the majority of the specimen is base metal, and an averaging process occurs between the strength of the constituent materials. Recently, there has been a shift to the use of 50-mm gauge length welded tensile specimens, which are almost all weld metal and HAZ, with a welded yield strength about 25 percent less than from the 250-mm specimen. This change in specimens has caused confusion in some design codes, where the introduction of new alloys with properties based on the 50-mm gauge length testing has been hampered by a significantly lower design strength compared to other alloys. Additionally, misunderstanding in test requirements can lead to requirements for welds when tested with a 50-mm specimen to have the strength of the 250-mm specimen. This confusion in welded material properties for aluminum requires resolution and standardization as well as revision of design codes. For now, the designer should be aware of the differences and be certain what length specimen the properties being used are from and the basis for the design code being used. Fire Protection One disadvantage of aluminum is its low melting point, particularly when compared to steel. Aluminum will begin to soften and lose strength at temperatures as low as 100 OC (212 OF) and will melt at about 600 OC (1,100 OF) (Sielski, 2007). For this reason, special fire protection insulation is required for critical structure and to create fire zone boundaries. Requirements for fire protection are made by organizations such as the U.S. Coast Guard, International Maritime Organization, classification societies, and naval authorities. Generally, fire protection insulation is designed to prevent the average temperature on the unexposed side of a deck or bulkhead from rising by a specified amount in a period from 0 to 60 minutes, depending on the criticality of the fire zone boundary. The requirements for fire protection insulation have a cost impact on

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aluminum ships as well as a weight impact that partially offsets the weight reduction gained from the use of aluminum. In one example, Sielski showed that for a 43-meter 32-knot crew boat, the weight for structure for an aluminum variant would be about 56 percent of the weight of an equivalent steel hull, but when fire protection insulation was added, the aluminum variant weighed 62 percent of the equivalent steel hull.

Strength Prediction of Aluminum Structures Ultimate Compressive Strength of Plates, Panels, and Grillages Although there have been a number of highly successful ultimate strength methods proposed for stiffened steel structures, new methods have had to be developed for aluminum for several reasons. One of these reasons is the behavior of the material under load, particularly the shape of the stress-strain curve. Unlike most steel, alloys, aluminum does not have a pronounced yield point. Instead, there is a more gradual softening at higher stress levels as illustrated in Figure 1. For material characterization and design purposes, a yield stress, or proof stress is taken from the intersection of the stress-strain curve with a line parallel to the initial stress-strain curve but offset from it at 0.2 percent strain, as Figure 1 illustrates. The difference in stress-strain behavior makes the behavior of aluminum under stress, particularly in compression, significantly different from that of steel. Another significant reason is the weakening effect of fusion welding on marine aluminum alloys. As reviewed above, in the heat-affected zone (HAZ) near a weld bead the aluminum material is weakened compared to surrounding material, with reductions in material proof stress typically between 30 and 50 percent. This material inhomogeneity significantly complicates the structural response. Another important difference between aluminum and steel is that aluminum is easy to extrude, especially in the 6xxx-series. This opens up many different geometric alternatives for stiffened panels beyond rolled profiles welded to

Figure 1: Stress-strain curve for aluminum and steel.

flat plates, such as hat-shape stiffeners or double-sided extrusions, examples of which are shown in Figure 2. The ability to employ custom extrusions means that the naval architect must now check local buckling modes of the stiffening shape — a check traditionally performed by the material producer when rolling compact steel profiles for ships.

Figure 2: Extruded alternatives to conventional stiffened panel arrangements in aluminum.

Most of the methods for evaluating the strength of welded aluminum structure have come from methods developed for steel with modifications to account for these features. Such analyses often include a defined yield or proof stress, and are often semi-empirical formulations, derived from testing of steel structure. Figure 3 illustrates the differences, showing the collapse strength of steel columns compared by the plate slenderness ratio, β, and the column slenderness ratio,

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β= �� �σ0

� = � ��0

� where b = stiffener spacing, t = plate thickness,

a = stiffener length, σ0 = material proof stress, and E = material elastic modulus. Using β and λ normalizes the effect of the elastic modulus so that the behavior of different materials under load can be compared. In general the strength of the steel columns is greater than that of similarly-proportioned aluminum columns, as shown in Figure 3 (Benson et al., 2011).

Figure 3. Comparison of aluminum and steel column collapse (Benson et al., 2011).

There has been recent progress in developing improved methods for estimating the ultimate strength of welded aluminum structures in compression, including work sponsored by ONR by Paik et al. (2007) and Benson et al. (2011). The strength of aluminum structure considering the effects of welding, stress-strain behavior, residual stress, and initial imperfections can be determined using detailed nonlinear finite element analysis, which can be time consuming and costly because of the detail required of the models, which must have small element sizes to model the weld zones in a structure. A simplified methodology that uses orthotropic plate theory for stiffened panels is under development, and shows good correlation with the detailed finite element analysis, as shown in Figure 4 (Benson et al., 2011). These methods have successfully addressed the issue of aluminum’s unique stress-strain curve

and the impact of HAZ near welds. However, much more research is needed, especially in the consideration of the effects of supporting

Figure 4. Collapse strength of stiffened panels computed with semi-analytical method using orthotropic plate theory (Benson et al., 2011).

transverse structure in large grillages and in the effects of bi-axial loading, particularly for multi-hulled ships. Further experimental validation of these approaches on representative structures would also be beneficial. Additionally, the above-mentioned methods have been designed for conventional plate and stiffener structures, and entirely new forms are required for extruded panels such as those shown in Figure 2. An initial method for considering extrusions in compression was presented by Collette (2011); while the method showed promise it was clear that additional work is required before the method can be used in design. Ultimate Tensile Strength of Aluminum Structures Unlike steel structures, welded marine aluminum structures cannot normally be idealized as having homogenous material properties. Fusion welds in marine alloys, such as the 5xxx or 6xxx series alloys, lead to a region of reduced strength near the weld, which is often referred to as the heat-affected zone (HAZ). For common marine alloys, the reduction in proof strength in this region is often on the order of 30 – 50 percent, and the HAZ

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normally extends between 10mm and 30mm from the centerline of the weld. Thus, fusion welds are marked by pronounced material inhomogeneity in material strength, and this inhomogeneity occurs at a much smaller length scale than what characterizes the other dimensions of the structure. Careful use of extrusions and friction-stir welding can significantly reduce the number of fusion welds in an aluminum vessel; however, at the present time it is not practical to remove all fusion welds from the structure. Furthermore, allowances must be made for in-service repair, where it may not be possible to use friction stir-welding on the final completed vessel. One feature of the response of a structure with under-matched welds is that the plastic flow of the structure in the post-elastic regime is concentrated in the under-matched region. This region tends to be small; often 1–3 times the thickness of the plate in the structure in size, and the dynamics of this region can be complex. A sample load-extension curve for a 5083 butt weld between two flat plates is shown in Figure 5, showing that the response of the joint falls between the all-weld and all-base response, and that the local geometry of the HAZ significantly affects the load-extension property of the joint. For loads that are applied quasi-statically, there is an apparent increase in strength above that of the HAZ and weld metal alone. However, for high strain-rate dynamic loads, such as from weapons effects, less energy can be absorbed in the narrow HAZ and weld zone, reducing the resistance to these loads. Recently, more advanced finite element models have been proposed including explicit material failure models (Wang et. al 2007). A key challenge in modeling the tensile behavior of aluminum structures is how to effectively model the HAZ in grillage and vessel-size models where it is simply not feasible to mesh the entire structure with a mesh fine enough to capture all the details of the HAZ. Various approaches have been discussed, including cohesive zone modeling or abstracting the weld behavior into large elements, but there is not a publically-available standardized or experimentally-validated method at the current time for large

Figure 5. Numerically-simulated stress-strain curves for 5083 butt welds with differing HAZ widths, HAZ (Collette, 2011a).

structures. Additionally, at this time the number of experimental programs that can be used to validate these approaches is very limited. More analyses and testing under quasi-static and dynamic loading are required to establish validated design criteria that reflect all of the material properties. Deformation and Limit States under Lateral Loads for Aluminum Structures Similar to the tensile response of aluminum structures, the response of aluminum structures under lateral loads is also strongly impacted by the presence of undermatched HAZ by welds. However, the ability to extrude aluminum does give the freedom to locate many of these welds in low-stress areas, or to increase the local thickness of the material by the weld to compensate for the reduced post-weld strength. Many of the same modeling issues exist under lateral load as they do for tensile response. Similarly, the existing experimental collapse tests for lateral load are very limited at the moment and very few are marine specific. Abildgaard, Hansen, and Simonsen (2001) studied a series of 150mm square and 150-mm x 300-mm rectangular plates with and without HAZ at the plate boundaries. The presence of HAZ at the boundary of the plate reduced the allowable load for a given amount of permanent set by 10 – 25 percent, depending on the size of plate and load level. These plates were not tested all the way to rupture, however, Dobmeier et al.

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(2001) and Siwowski (2009) studied 6063-T6 and 6005A-T6 extruded bridge decks until failure and noted that the HAZ often played a central role in the final failure of the structure. Moen, Hopperstad, and Langseth (1999) investigated the rotational capacity of welded and unwelded 6082-T6 beams, noting that the failure mode of the welded beams was rupture in the tensile HAZ near the weld. Similar to the tensile response discussion previously, a central concern in laterally-loaded welded aluminum structures is ensuring that they can develop sufficient plastic response to be compatible with the assumptions of steel-based Navy design methods for laterally-loaded structures. Fatigue Cracking Compared to steel, aluminum has significantly reduced resistance to fatigue crack initiation, as indicated by Figure 6 (Sielski, 2007). In the figure, the number of cycles (N) required for fatigue crack initiation are shown as a function of alternating stress range, which has been normalized for the yield strength of the material. The figure shows that a typical aluminum structural detail characterized by Eurocode 9 as 23,3.4, has a fatigue strength about 20 percent less than a typical steel structural detail characterized by British Standard 5400 as Class F.

0.1

1.0

10.0

1.E+04 1.E+05 1.E+06 1.E+07

Cycles (N)Str

ess

Ran

ge

/ Y

ield

Str

eng

th

BS 5400 Class F

EuroCode 9 - 23, 3.4

Figure 6. Comparison of 5086-H116

aluminum and ordinary strength steel fatigue strength normalized by yield strength

(Sielski, 2007).

Furthermore, when a crack initiates in an aluminum ship structure, it will grow at a far greater rate than an equivalent crack in a steel hull. This is illustrated in Figure 7 for a typical 43.9-m 32-kt service craft, where a 24-mm crack will grow after 24 months of service by about 5mm in a steel hull, but by about 26mm in an

equivalent aluminum hull, an increase in crack growth that is about 4 times greater (Sielski, 2007).

20

25

30

35

40

45

50

55

60

0 6 12 18 24

Months Service

Cra

ck L

eng

th (

mm

)

5083 Aluminum H 36 Steel

Figure 7. Predicted crack growth for a 43.9-m 32-knot craft (Sielski, 2007).

Because of the weakness of aluminum to fatigue cracking, continuous repair of cracks in aluminum structure have been part of the history of aluminum when used in larger ships, particularly naval vessels (Sielski, 1987). This problem has particularly plagued ships with aluminum deckhouses that were designed in the 1970s before the development of today’s fatigue analysis procedures and before finite element analysis was available for routine use in ship design. These fatigue issues as well as the sensitization of exposed aluminum are both problems for the U.S. Navy’s CG 47 class, which has experienced both types of problems, resulting in continuous monitoring and repair of the aluminum structure and in many cases replacement of large areas of structure (Schwarting et al., 2011). Largely because of the maintenance problems with aluminum structure, the U.S. Navy stopped using aluminum for deckhouse structure of combatant ships in the 1980s, although continued to use it for smaller craft. The high-speed larger vessels such as the LCS-1 Class with its aluminum deckhouse and the all-aluminum LCS-2 Class and the JHSV are just beginning to see service. These ships have been designed using advanced fatigue analysis methods so that there should be less chance of fatigue cracking. They have been fabricated with either 6xxx-series aluminum or the lower-

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magnesium 5454 alloy for exposed topside structure, and therefore should not experience sensitization problems. However, given the number of structural connections on a large ship, and the novelty of these structures, some amount of fatigue cracking in service is possible. Of great concern is the possibility of rapid crack propagation should one occur in an all-aluminum hull, as there is no inherent method within the structure to stop the growth of a fatigue crack, which could lead to catastrophic loss of a ship. Service experience will show if there have been errors in the fatigue analyses used in design that have permitted poor structural details to be used in critical locations, but research is needed to develop means of arresting a rapidly growing fatigue crack in service to avoid the loss of an aluminum-hulled ship. The propensity of aluminum for fatigue crack initiation and propagation is an important factor in design. Fatigue analysis methods have been under development for several decades, and although still requiring further development, particularly for the structural details typical of ship structures, existing guidance, including Sielski (2007) should provide the basis for a more robust ship design. The impact of consideration of fatigue in design will include an increase in the weight of structure. Sielski showed that for a 43-meter, 32-knot crew boat, a quite significant increase of about 12 percent in the weight of structure occurred. This illustrates that fatigue analysis is an essential part of the design process, and must be undertaken at the earliest stages of design in order that the impact can be factored into the total design. Waiting until the later stages of design can result in ship performance requirements not being met because of unplanned structural weight increases, or could otherwise result in a reduced safe operating envelope where higher sea states and speeds must be avoided to prevent premature fatigue failures.

Current Maintenance Issues with Aluminum Fatigue cracking of aluminum remains the greatest maintenance issue, particularly for ships

that were not designed using proper fatigue analyses. Fatigue cracks are often difficult to detect, and may possibly grow to dangerous lengths before they are detected. This can especially be an issue where the aluminum structure is covered with insulation or other systems that make routine visual inspection impossible for large sections of the structure. One way of addressing this problem that is being pursued is through damage detection technology, by which various sensors are installed on the ship structure and by detecting an anomaly in signatures can infer the existence of damage, prompting further visual inspection (Sielski et al., 2012). Repair of fatigue cracks in aluminum can be difficult, as grinding out a crack and rewelding it introduces residual tensile stresses that will accelerate recracking. Replacing the cracked structure can provide longer life, but the best solution is to redesign the structural detail using fatigue analysis to significantly reduce the chance of further fatigue crack initiation. An alternative method that is being pursued in current research programs, including work sponsored by ONR, is reinforcing a crack with a bonded patch of either aluminum or composites. This has been done successfully by the Royal Australian Navy for their FFG-7 Class frigates with aluminum superstructures (Grabovac, 2003). The recently completed Co-Patch project in Europe (Co-Patch, 2012) has investigated composite patches for steel structures, and research in bonded structures has been reported by Det norske Veritas (Weitzenböck, 2012). Repair of sensitized aluminum plate is far more difficult as unlike fatigue cracking, which originates in localized structural details, entire areas of plate are subjected to sensitization. If repairs are made to a cracked or otherwise damaged area of sensitized plate, recracking will quickly occur, sometimes from the residual stress of the weld repairs. Repair requires cutting out and replacing large areas of plate, which can become very expensive, particularly when ship systems have to be removed and replaced as part of the repair effort. To provide a temporary solution, experimental work is underway under sponsorship of ONR to overlay

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areas of sensitized plate with glass-reinforced plastic that is laminated in place. These overlays provide watertight integrity and sufficient strength so that walking on decks with sensitized plate can be safely done. Some prototype overlays have been made on ships in service, and await the results of service experience to determine how effective they have been. Lifecycle management of aluminum ship structures can be enhanced in the future through the development of effective structural health monitoring (SHM) systems (Sielski et al., 2012). Current research by ONR is attempting to identify and model operational and environmental actions that challenge the integrity of a ship structure throughout its life, to identify and model the failure mechanisms and failure modes that result from these actions, and to quantify the performance of the ship structural system given these actions using reliability-based performance measures. The operational and environmental actions that are considered include wave loads and structural and material degradation due to the marine environment. Thus, SHM is an umbrella effort over all efforts to predict and maintain structural integrity. The systems engineering approach taken in this program brings together experts in hydrodynamic loading, structural strength, fatigue, fracture, structural health monitoring, inspection, repair, production, and structural materials. Each of these elements affects the structural system performance and must be included to support rational decision-making with regard to the structural system from cradle to grave.

Design, Optimization, and Lifecycle Prediction Issues with Aluminum For design optimization, the current central challenge is to provide computationally-efficient objective functions for the unique structural response of aluminum vessels. To take full advantage of concepts such as extruded profiles or specific weld locations requires strength methods that can predict the strength as these design aspects vary. As reviewed above, there are currently holes in our toolset to fully predict the tensile, compressive, and lateral response of

such structures which limits the ability to apply rigorous optimization. A further challenge is estimating the construction and maintenance cost of aluminum structures, where differences from steel such as the production and use of extrusions, friction-stir welding, and extensive fire-protection requirements must all be included. This is especially true if a comparative optimization to a steel structure is to be made. As extrusions give more topological freedom to design with aluminum compared to steel, the resulting optimization problems are likely to be larger in terms of independent variables than many steel structural optimization problems. A major complication with lifecycle analysis of aluminum structures, especially when the structure is being compared to a steel structure, is that the lighter all-up weight of the aluminum structure results in a significantly different overall design for the platform. Even after including the additional weight of the increased fire insulation, the overall weight savings is often enough to reduce platform dimensions, engine size, or fuel capacity. Thus, it is often difficult to make valid lifecycle comparisons with truncated analysis. Lamb and Beavers (2010) estimated that for a 91-m frigate, the total displacement would be reduced from 2,627 tons to 2,122 tons, a reduction of 19 percent. This weight reduction would lead to a savings in fuel alone of 2,840 ton per year, which at 2010 prices of $450 per ton, would equate to an annual savings of $1.3M, or $32M over a 25-year lifetime. The estimated acquisition cost (without the cost of the combat system) would increase from $356M to $383M, or $27M, which is offset by the decreased fuel costs alone, even greater if today’s prices continue to grow.

Future Needs in Aluminum New Aluminum Alloys In recent years, newer aluminum alloys have been developed, particularly in the 5xxx-series. The 5059 and similar 5383 alloys were developed to have a higher welded yield strength than the 5083 alloy, but at a lower cost than the 5456 alloy. These alloys have seen increased use, although the confusion as to welded yield strength from 50-mm and 250-mm specimen

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testing has been somewhat of an obstacle, because if strength of the new alloys determined from the smaller specimen is compared to an older alloy, such as 5083 with the strength determined from the larger specimens, the newer alloys will be judged to be weaker. Adjudication and standardization of welded strength through a systematic testing program to establish rational design and failure strength of welded structure in tension is needed to resolve this issue. For whatever reason, whether it be higher temperatures in service or changes in mill processing procedures that have led to less stable magnesium, an improved alloy with the strength characteristics of 5456-H116 but greater resistance to sensitization is needed. In addition, new standardized testing procedures are needed to ensure that these new alloys will be resistant to sensitization (Bovard, 2004). Needed Improvements in Design and Analysis of Aluminum Structures

Validated method for determining the strength of welded aluminum structure in tension. A large weight penalty is imposed by designing aluminum structure for the strength of the weld metal or HAZ when evidence such as that cited above indicates that the actual strength of the fabricated structure is somewhere between that of the base metal and weld. An experimental program is needed to fabricate sample structures of different aluminum alloys and test them in tension to determine the actual strength. Such a test program would be accompanied by finite element analyses of sufficient detail to model the HAZ and weld metal, with the analyses validated by the experiments. In this way more rational criteria for design would be developed, and the properties from 50-mm tensile tests made more applicable to design. Validated means of estimating the collapse strength of large grillages including the effects of biaxial and lateral loading. Work is continuing in developing simplified means for the design of large grillages, particularly the effect of the stiffness of transverse members on collapse strength. An experimental testing

program is needed to validate this analytic work and develop appropriate design criteria. Improved database of fatigue strength of typical aluminum ship structure details. A test program at NSWCCD sponsored by ONR is developing data on fatigue strength of a typical structural detail that combines an extruded 6xxx-series extrusion with 5xxx-series plate. However, more structural details that are also typical of aluminum ship structure require testing. This is particularly important because in many cases the fatigue strength of structural details determines important scantling criteria, including hull girder section modulus, and therefore significantly affects the weight of structure. Validated methods for predicting fatigue crack growth of aluminum ship structures under random loading. Because of crack closure effects as well as the effect of residual and complex three dimensional stress fields, it is difficult to estimate fatigue crack growth rates in aluminum structure, which could vary by a factor of eight, depending on assumptions made. An experimental test program of realistic, large-scale aluminum structures is needed to validate the assumptions made in analysis. This is important because overestimating the significance of a crack can lead to over-conservative design and possible unnecessary repairs, but underestimating the significance can lead to structural failures that could be catastrophic, especially for an all-aluminum hull. Methods for arresting fatigue crack growth in aluminum ship structure. Unlike steel hulls, where tougher strakes of plating can be placed in critical locations such as the gunwale and bilge, there are no particularly tougher aluminum alloys to serve the same purpose of arresting a growing crack in an aluminum hull. The practice of seams fastened by mechanical means such as rivets of similar fasteners could be used, but at a high fabrication expense and increase in maintenance costs. Alternative means, such as inserting thicker sections in the way of possible cracking need to be pursued. Most importantly, analyses should be made to determine how critical this problem is before a major catastrophe occurs.

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Validated long-term sensitization data (5xxx alloys high in Mg). Long-term testing to validate concerns for sensitization of high-magnesium content alloys is needed. Although alloys such as 5083 and 5086 may be subject to sensitization because of their magnesium content, verification of their risk of (or lack of) susceptibility to sensitization needs to be made before major failures occur that would require replacement of large portions of the structure of ships. Corrosion data for 6xxx-series alloys for design. These easily-extruded alloys are being used increasingly in aluminum ship structure, particularly for integrally stiffened extrusions and sandwich panels. Their use is limited because of concerns for pitting corrosion due to the higher copper content of some alloys. A long-term corrosion study that includes exposure to seawater and salt spray is needed to determine if the lower copper content alloys can perform as well in a marine environment as do 5xxx-series alloys. If such a determination can be made, more extensive use of the lower weight panels can be made.

Conclusions and Recommendations The application of aluminum for weight and stability critical vessels has been growing significantly. With aluminum applications in CG-47, FFG-7, LCS-1, LCS-2, and JHSV classes, the U.S. Navy will be heavily involved in the analysis and support of aluminum structures for the foreseeable future. Previous experience has indicated that aluminum cannot be treated simply as a lighter version of steel if the best lifecycle performance is to be achieved. Unique considerations, including sensitization and resulting inter-granular corrosion, rounded stress-strain curves, reduced strength by fusion welds, the ability to extrude aluminum into complex topologies among others all demand the attention of the naval architect. Recent work by the marine structures community has made significant progress in developing engineering methods and standards addressing these issues. However, a number of challenges still remain. These challenges have been presented as a series

of future improvement needs, covering material, analysis and design topics. Resolving these challenges will likely lead to further lifecycle performance gains for aluminum structures and the aluminum vessels in the Navy’s fleet. References Abildgaard, P. M., P. W. Hansen, and B. C. Simonsen, “Ultimate Strength of Welded Aluminium Structures,” in Proceedings of the Conference on High Performance Vehicles, Hamburg, Germany, 2001, pp. 4-18. Benson, S., Downes, J. and Dow, R.S., “Structural Performance of Aluminium Naval Vessels,” Final Report to the Office of Naval Research, Newcastle University Document ID: ONR‐UNEW‐20‐1, June 27, 2011. Bloomberg.com/news/2011-06-17/navy-finds-aggressive-corrosion-on-austal-s-combat-ship-1-.html Bovard, Francine S., “Sensitization and Environmental Cracking of 5xxx Aluminum Marine Sheet and Plate Alloys,” Electrochemical society Proceedings, Vol. 2004-14, pp. 230–243. Brady, Thomas, private communication with the authors, 2012. Collette, Matthew C. Final Report for N00014-10-1-0193 Aluminum Structures Research Program, University of Michigan Marine Structures Design Laboratory, Ann Arbor, Michigan, 2011a. Collette, Matthew C. “Hull Structures as a System: Supporting Lifecycle Analysis”, Naval Engineers Journal, Vol. 123, No. 3, pp 45–55, 2011. Collette, Matthew C. “Rapid Analysis Techniques for Ultimate Strength Predictions of Aluminum Structures”. In Advances in Marine Structures. MARSTRUCT 2011. Hamburg, Germany: CRC Press, pp. 109–117, 2011.

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Collette, Matthew C. Strength and Reliability of Aluminium Stiffened Panels, PhD Dissertation, University of Newcastle, 2005. Co-Patch News 2012: Issue 2, www.co-patch.com Dobmeier, J. M., F. W. Barton, J. P. Gomez, P. J. Massarelli, and W. T. McKeel Jr., “Failure Study of an Aluminum Bridge Deck Panel,” Journal of Performance of Constructed Facilities, vol. 15, no. 2, pp. 68-76, 2001. Grabovac, Ivan. “Bonded Composite Solution to Ship Reinforcement,” Composites: Part A, vol. 34, 2003 pp. 847–854, Elsevier. Lamb, Thomas and Nathaniel Beavers, “Aluminum Applications to Naval Ship Design and Shipbuilding,” Proceedings of the 2010 SNAME Annual Meeting and Ship Production Symposium, SNAME 2010, Vol. 2, pp. 68–77. Moen, L.A., O.S. Hopperstad, M. Langseth, “Rotational Capacity of Aluminum Beams Under Moment Gradient I: Experiments”, Journal of Structural Engineering, Vol 125. No. 8 910-920. 1999. Paik, Jeom Kee, A K Thayamballi, J Y Ryu, J H Jang, J K Seo, S W Park, S K Soe, C Renaud, and N I Kim, Mechanical Collapse Testing on Aluminum Stiffened Panels for Marine Applications, Ship Structure Committee Report SSC-451, U.S. Coast Guard, www.shipstructure.org, 2008. Schwarting, Richard, Greg Ebel, and T. James Dorsch, “Manufacturing Techniques and Process Challenges with CG47 Class Ship Aluminum Superstructure Modernization and Repairs,” presented at the ASNE Fleet Maintenance and Modernization Symposium 2011 held August 30–31, 2011 in San Diego, California. Sielski, Robert A. “The History of Aluminum as a Deckhouse Material,” Naval Engineers Journal, Vol. 99, No. 3, pp. 165–172, May 1987.

Sielski, Robert A., Aluminum Marine Structure Design and Fabrication Guide, Ship Structure Committee report SSC-452, 2007. Sielski, Robert A., Ken Nahshon, Liming W. Salvino, Karlin Anderson, and Robert Dow, “The ONR Ship Structural Reliability Program,” presented at ASNE Day 2012 held February 9–12, 2012 in Arlington, Virginia, American Society of Naval Engineers, Alexandria, Virginia. Siwowski, T. W., “Structural Behaviour of Aluminium Bridge Deck Panels,” Engineering Structures, vol. 31, no. 7, pp. 1349-1353, 2009. Vassilaros, Michael G. and Ernest J. Czyryca. Natural Aging and Sensitizing of 5000-Series Marine Aluminum Alloys, David W. Taylor Naval Ship Research and Development Center report DTNSRDC/SME-78/106, June 1979. Wang, T., O. S. Hopperstad, O.-G. Lademo, and P. K. Larsen, “Finite Element Analysis of Welded Beam-to-Column Joints in Aluminium Alloy EN AW 6082 T6,” Finite Elements in Analysis and Design, vol. 44, no. 1-2, pp. 1-16, Dec. 2007. Weitzenböck, Jan R. Adhesives in Marine Engineering, Woodhead Publishing, 2012.

Acknowledgements Much recent research in the reliability of aluminum ship structures has been performed under the sponsorship of the U.S. Office of Naval Research and directed by Dr. Paul Hess. His Ship Structural Reliability Program aims to provide the Navy with a quantitative ship structural life-cycle performance prediction capability to support design, operation, recoverability, and maintenance decisions. The authors appreciate his continuing support for this vital work.

Author Biographies Dr. Matthew Collette joined the Naval Architecture and Marine Engineering Department of the University of Michigan in

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2009 as an Assistant Professor. His research focuses on the application of numerical methods to design and operational support, with a focus on lifecycle structural response including lightweight structures, structural optimization, and stochastic methods. He is currently an associate editor for Ocean Engineering. Before joining Michigan, he worked at SAIC as a Senior Naval Architect supporting a variety of governmental and commercial research projects covering hydrodynamic analysis, optimization, and computational technologies for design and operation, and analysis tools for lightweight structures. Dr. Collette started his career in Boston working for John W. Gilbert Associates as a naval architect. He is a 1999 graduate of Webb Institute and completed his PhD at the University of Newcastle in the United Kingdom in 2005. Robert A. Sielski is a naval architect specializing is ship structures. He was formerly Director of the Structural Integrity Group of the Naval Sea Systems Command, chair of the Chair of Panel HS-3, Stress Analysis and Strength of Structural Members of The Society of Naval Architects and Marine Engineers and was a member of Panel HS-4, Design Procedures and Philosophy, as well as the SNAME Hull Structure Committee. Dr. Sielski was a member of the International Ship and Offshore Structures Congress, serving on Committee II.1, Design Methods. He is a member of the American Society of Naval Engineers, the Association of Scientists and Engineers of the Naval Sea Systems Command, and is a member of the editorial board of the international journals Marine Structures, Ships and Offshore Structures, and Shipping and Ocean Engineering. He received the degrees of BSE in Naval Architecture and Marine Engineering from the University of Michigan, MEA in Engineering Administration and DSc in Structural Engineering from the George Washington University,