A call to shipbuilders for a new approach to composite hull fabrication
Currently an independent marine composites consultant based in Coronado, Calif., U.S.A., Richard N. Blackington is a member of the Materials Panel of the U.S. National Shipbuilding Research Program and holds a B.S. in marine science from Massachusetts Maritime Academy. He has more than 20 years experience (in
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Composites have inaugurated a new generation of change in the field of shipbuilding, giving naval architects their first opportunity to design load-bearing marine structures in the integrated manner afforded to them by these new materials. Advanced composites bring to bear on the construction of hulls and other large structures an advantageous combination of material and process capabilities, allowing designers to unify previously separate parts and functions into single, multi-functional structures. Part/function consolidation leads to highly efficient use of these materials, lowering overall costs. Not only can composites deliver superior performance, they also enhance the performance value of the total ship system.
As the use of composites multiplies in naval construction, the impact of advanced structural composites on the shipbuilding industry could be dramatic. Marine composites, in fact, have the potential to help shipbuilders leap into a bright and profitable future. But that assumes that those who make decisions about shipbuilding materials and construction processes understand and appreciate that potential.
Unfortunately, in seagoing vessels and structures, U.S. shipbuilding has not kept pace with the potential for composite materials. Composites can be a precursor to a technically innovative shipbuilding base, be they offer the special opportunity to apply the "tailorability" of these advanced materials to the construction of load-bearing marine structures. But shipbuilders must overcome the inherent reluctance to change found in most mature industries and move beyond the current concepts of prefabricated, ship-unique production methodologies to an emerging philosophy of adaptive manufacturing. Advanced shipbuilding must have the flexibility to quickly move from one design to another and to rapidly insert new designs and technologies into existing designs. It is vital that shipbuilders accept the composite mantle: Composites alone offer shipbuilders the potential for significant parts consolidation and a much higher level of automation as well, resulting in unprecedented affordability.
Glass, basalt and even carbon fibers can compete successfully with traditional shipbuilding materials in load-bearing marine structures when fabricated with processes that maximize the materials' merits while minimizing the impact of their costs. Currently, however, most large composite sections or modules are fabricated by labor-intensive hand layup. This typically involves the manual placement of sequential layers of fiber on a mold surface, followed by infusion of the resin, or in the case of prepreg by heating under pressure to cure the structure. Although automation is speeding up some processes, production rates for these hands-on methods are still slow and costly for the volume production rates associated with shipbuilding. The selection of a cost-effective advanced process must assess the cost/performance advantages of each process and select that process that offers the most competitive result. This competitive position will depend, in large part, on the judgement of a select few decision-makers at shipyards in the Pentagon, who must examine the various processes for cost and performance advantages in the fabrication of ships of varying sizes as well as the process impact in the market place.
While vacuum-assisted resin transfer molding (VARTM) offers lower tooling costs and large component size, the process requires the use of low viscosity resins thus compromising mechanical properties especially in resin-starved areas. Further, medium-to-high labor costs are associated with VARTM as it is currently practiced, and the process does not lend itself easily to automation. Today, however, relatively low-cost equipment is already available for robotic, multi-axis placement of prepreg (see CT's sister publication, High-Performance Composites November 2003, p. 30). Other processes such as automated tape laying and pultrusion offer positive control over resin and fiber content, eliminate almost all touch labor from reinforcement handling, and deliver higher fiber volume ratio and, as a result, optimum structural properties. On a cost-per-pound basis, one study reported that a fire-resistant glass/polyurethane pultrusion process introduced by Creative Pultrusions (Alum Bank, Pa., U.S.A.) could produce structures at half the cost of VARTM ($5.40/lb vs. $10/lb).
The future demands that shipbuilders employ automated methodology. An essential prerequisite will be the selection of that process that best achieves the goal of lower costs and higher performance. As indicated above, the opportunities for composites lie not in the part-for-part replacement of metal, but rather in a process that affords the shipbuilder a method of fabrication that can incorporate many parts and functions into a single component. The need for more affordable hull technology has long been acknowledged, but the shipbuilding revolution that would result from meeting that need would require an enormous commitment to innovation. And shipbuilding still awaits the type of leaders who would lead the industry to make that commitment.