In the future, Army tanks, troops and material may cross ravines, ditches and minefields on twin spans of polymer matrix composites. ``Use of advanced composites makes this development possible,'' said John Kosmatka, a structural-composite designer and associate professor at the University of California, San Diego, School of Engineering.
``The bridge must withstand the long-term pounding of the Army's Abrams main battle tank, be no more than 24 inches deep at midspan and cost less than ones made of aluminum,'' Kosmatka said.
The Defense Advanced Re-search Projects Agency and an industry consortium fund a UCSD-administered bridge infrastructure renewal program.
As a longtime developer of fielded bridges, the Army Tank-Automotive and Armaments Com-mand in Warren, Mich., is looking for a more durable, lighter and cheaper model.
Existing Army inventory includes a mid-1970s British-made aluminum medium girder bridge, a 1960s U.S.-produced aluminum, armored vehicle launch bridge and a World War II-era British-made steel Bailey bridge.
For more than 12 years, the Army has been developing configurations of a new Wolverine heavy-assault bridge that is scheduled to be fielded by the year 2000. Prime contractor General Dynamics Corp.'s land systems division of Sterling Heights, Mich., along with MAN GHH of Gustavsburg, Germany, will produce the nearly 79-foot bridge of aluminum with steel connectors.
By comparison, work on the composite bridge is just beginning.
``We want to apply the technology if composites are mature enough for us to incorporate them in our program,'' said Brian Hornbeck, Army assault bridging team leader.
Aerospace engineer Bill See-mann consulted with the UCSD-led team on the low-cost manufacturing requirement.
The solution involves unidirectional carbon fiber in large volumes and the inventor's inexpensive way to make large composite parts, the Seemann Com-posites Resin Infusion Molding Process.
Each of two treadways measures 46 feet by 5 feet and may be the largest carbon-fiber, vacuum-resin-transfer-molded fabrication of a single piece, according to George Tunis, president of joint-venture Hardcore DuPont Composites LLC.
Structural-composite designer Frederick Policelli agreed.
``The low-cost RTM fabrication process has never been used before on carbon-fiber parts this large and complex,'' he said.
Hardcore DuPont vacuum infused the hull, which is as thick as 11/4 inches, with Shell Chemical Co.'s Epon 862 epoxy bisphenol epichlorohydrin resin in a 45-minute wetout time at 130§ F.
The resin penetrated 184 plies of 12,000-strand polyacrylonitrile-based carbon fiber. Amoco Corp.'s Performance Products line supplied T300C, and Hercules Inc.'s composite products division, now Hexcel Corp.'s fibers business unit, AS4D. Johnston Industries of Phoenix City, Ala., stitched four-ply sets together, creating 46 fiber-packs for easier assembly.
Seventeen bulkheads contain at least 600 pounds of composites. Stiffeners, weighing some 200 pounds, keep side panels from buckling.
The 4-inch-thick laminated top deck looks like a sandwich: 22 plies of carbon fiber split into three layers and balsa wood in two layers of 2 and 1 inches. Three workers laid up the deck in a single day at Hardcore DuPont's facility in New Castle, Del., said engineer Mike Grunza.
Composite separator bars, 2 feet long, link the treadways in field use.
Lemke Manufacturing of San Diego fabricated 0.125-inch-thick, 2-foot-wide aluminum skins that overlay and underlay the carbon fiber on each treadway's vulnerable entrance-exit ramps.
Hardcore DuPont shipped two treadways Aug. 28 to UCSD's Powell Structural Research Laboratories.
``We will perform tests in September,'' Kosmatka said.
Using an Army load fixture, technicians will apply loads of 40 tons and beyond, intentionally fracturing one treadway to determine maximum capacity.
Working with the other treadway in November, an Army lab at Fort Belvoir, Va., will conduct full- cycle fatigue tests equaling 1,200 crossings.
Each Abrams tank weighs at least 75 tons.
Beginning in 1994, DARPA funded development of the prototypes, one of 12 tasks in the program's $11 million Phase I. Cor-porations match those funds, often through in-kind contributions.
DARPA is evaluating a Phase II proposal to build prototype bridges for field tests.
An Army existing launcher would carry two 46-foot treadways and separator bars for a single bridge.