In the past, the U.S. Navy has lagged behind the other branches of the military in large scale use of advanced carbon fiber composites in essential combat hardware. More specifically, its ships but this is about to change with the building of the first of a new family of advanced, multi-mission destroyers, the DDG-1000 Zumwalt class.
The challenge for the Navy is to meet fire-retardant/fire containment requirements, while reducing radar signature and weight, and yet control cost during construction of four upper decks of the destroyer’s deck house superstructure. To rise to this challance the Navy will use primarily flat carbon composite sandwich panels to simplify tooling, and core them with balsa, which burns more slowly than foam and better insulates the opposite sandwich skin from heat, optimizing fire safety.
The Zumwalt class had its begining in 2001, when the Navy announced the program would move forward after the DD 21 project. The goal is to release 32 new multi-mission destroyers by 2012. The Navy renamed it DDG-1000 and shrank it to three ships in 2008, although the DD plan originally called for 8 to 12 “advanced technology surface combatants,”. The new ships were to provide support for land attack and ground forces, unlike traditional destroyers, which primarily engage in offshore anti-air and undersea warfare.
The ship design includes an upper-section deck house (superstructure) constructed with panels and beams made of carbon fiber/vinyl ester skins with balsa wood and/or foam cores. The ship also features a composite helicopter hanger built of carbon composites, as well as a built in advanced composite ballistic screen. The composite deck house for the first ship (the Zumwalt) is in progress at Northrop Grumman’s shipbuilding facility in Gulfport, Mississippi, under the supervision of the Zumwalt’s main general contractor, General Dynamics based out of Maine.
The Navy first looked into expanding the use of advanced carbon fiber composites in its large capital ships fleet, just over 15 years ago. Glass fiber-reinforced polymers had been used but only in the construction of small river patrol boats, minesweepers and for various small components and hardware on the larger ships. The first large-scale application involved the USS San Antonio (LPD 17) and set the stage for the use of advanced composites in the DDG-1000 program.
Zumwalt has a “tumblehome” hull shape. Which is a design in which the hull slopes inward from above the waterline. This is because the ships will handle tactical landing operations, stealth has been a major consideration in the ship’s design. Such a shape drastically reduces the ship’s radar cross section. Official Naval Technology documentation describes the deck house as “fully EMC shielded with reduced infrared and radar signatures.” The all-composite dech house superstructure has helped the Navy fulfill its goals, as well as reduce top heavy weight and total ship weight.
The DDG-1000 project evaluated a ridiculous number of material combinations, generating more than 6,000 individual test pieces over 10 years. Ultimately, the engineering team settled on a sandwich construction, featuring balsa supplied from New Jersey, between skins made from Toray T700 12K carbon fiber, supplied by Toray Carbon Fiber America Inc of Texas, and the 510A vinyl ester resin from Ohio. Then a California based company wove the T700 fiber into three different types of fabric: a noncrimp ±45° stitched material at a weight of 410 g/m², a bonded unidirectional material at 680 g/m², and a plain-weave 0°/90° fabric at 300 g/m².
“The T700 fiber was chosen because it provided us with the required stiffness in whatever fabric we selected it in,” said Barry Heaps (project director). Navy engineers selected balsa core because they learned early in their testing of composite materials that “the mechanical properties of balsa as a core material compared to its cost are significantly better than any other material.” Navy tests also showed that a balsa-core sandwich contained fires better than those cored with foam or honeycomb.
For the most part, the composite panels are flat, an outcome dictated by a desire to keep down tooling costs. Tooling is typically coated with a release agent prior to the layup of the external reinforcing fabrics, balsa core and interior fabrics. A woven glass cloth peel ply also is usually placed on the tool or external side of the panel. The cloth is peeled off after infusion and cure, providing a clean, bondable surface for secondary bonding and assembly. A stainless steel mesh also is integrated into the fabric on the external skin, providing electromagnetic interference (EMI) shielding and a lightning ground in the otherwise nonconductive panels.
The unidirectional fabric is used in areas and structures where higher stiffness is needed in one single direction. “We have some very large beams that support a large open area in the helicopter hanger and those beams are made with the unidirectional material,” said Heaps, telling that the heavier material is significantly more difficult to infuse with the resins.
Although vacuum assisted resin transfer molding (VARTM) is exclusively used for the sandwiched panels on the Zumwalt, Heaps said the team is considering some pultruded sandwich panels in the construction of the second DDG-1000 ship. “Pultrusion has the potential to be cheaper, but there’s still some testing and work to be done to implement them in a deck house,”
via HPC via NGSB
Tags: boat, carbon, Carbon Fiber, Carbon fibre, navy, warship
