Composite to Steel Joints - Developed for the Next Generation Surface Combatant
Larry Brown - Edison Welding Institute
The Edison Welding Institute is located in Columbus, OH and was formed in 1983 with an emphasis on applied Research & Development in the area of joining material. Over time it has been found that pure research is better left to the Universities, and the Institute has tended to specialize in development and validation of innovative joining techniques toward application in industry.
The specific program to be discussed tonight is a project of the Navy Joining Center which is one of the US Navy's Centers of Excellence and is operated by Edison Welding Institute. The specific project was to develop a superior and cost effective method for joining an "all composite" deckhouse to the steel hull of the DDG 1000 (this nation's next generation of Navy destroyer). The conventional joining technique is bolting, which has become quite expensive, and a more cost effective system was sought.
This evening we will be looking at the problem, the objective, and benefits of a new joining system developed for this project as well as an overview of the present status, implementation, and future activity associated with it.
The DDG 1000 is the next generation Navy destroyer. It is the "electronic ship". It will be operated by a crew of 105 (as opposed to 300 sailors on a present day destroyer) and is 100 feet longer than today's destroyer. The composite deckhouse will provide a low hard-to-spot profile on the ocean. The deckhouse surfaces are to be carbon fiber composite which are to be joined to the traditional steel hull of the ship. There is a strong desire to get away from bolting for providing this joint driven largely by high costs and very tough service requirements.
The Navy Joining Center was put together by its contractor Edison Welding Institute using partnership with several other organizations including NSWC at Caderock, The Composites Manufacturing Technical Center, Boeing Aircraft, and Applied Research Laboratory of Penn State. The Navy has shifted many ship design responsibilities from its own engineering facilities to the Shipyards, to is was imperative that the shipyards be included in all phases of the development.
A study of legacy systems soon revealed that a new "joining system" would be needed for this task. Existing systems for shipbuilding were costly bolting. Looking at aerospace applications, many joining systems were found involving adhesive bonding with titanium and aluminum, but all of these involved very close tolerance fit-up (on the order of a few thousandths of an inch), where as shipbuilding tolerances at more like +/- .040", and the aerospace solutions simply did not apply. Also etchants used on titanium and aluminum for surface preparation proved to be completely ineffective for steel. Beyond this were rather unusual requirements for the joint. The Center proposed that two benchmark measurements be used to judge the joints effectiveness. These were expressed as a Factor or Safety of 4 above "normal operational" requirements for the joint. and a factor of safety of 1.1 for "combat load" requirements which included a nearby nuclear blast.
Given the above requirements the Center looked at shipyard capabilities in the areas design, performance, fabrication, loads and Environmental Safety.
A joint was developed (Mr. Brown had a specimen of the joint to display) which involved a several inch wide balsa wood core surrounded by carbon fiber composite. This fit into a flat bottomed steel shoe (machined from a structural I-Beam section) with slightly tapered sides in the lower several inches of engagement. An adhesive (based on the same resin as the graphite composite) forms the actual bond between the steel shoe and the outside faces of the composite material. Studies of this design showed that any failure due to stresses occurred within the composite material itself, and not in the adhesive joint. The design also maintains the joint in compression during all loading sequences. Fatigue tests were performed to simulate a 35 year service using 1.1 million cycles of load. It was found that the joint did not suffer any fatigue, and had the same failure load values after the fatigue cycles as it had before the test.
Beyond this the Center was able to boast of the following advantages for the joint:
A weight reduction: 63 lbs/linear foot for new design vs. 109 lbs/linear foot for bolted design.
A cost reduction: $250/linear foot for new design vs. $667/linear foot for bolted design.
Based on the promise of this design, a challenge was set up by a Navy Admiral for the design to go head-to-head in tests against bolted joints. The tests involved both wave slap loads and compression tests. In all of the testing the adhesive bonded joint performed without failure, while only one of four bolted samples was able to pass. Things were looking very bright for the new joint.
Inspectability of the joint became an issue. A round robin test block of the new joint design was developed featuring several types of flaws including embedded steel rods, tapes, O-rings, and applications of patterned masking agents to encourage "non" to "kissing" type bonds within its geometric shape. Conventional NDE (Ultrasonic) was able to resolve all of the defects except the masked area. A newly developed DSSS (Direct Spread Spectrum Processing) of the UT information from was applied by ARL to the program, and with this technology, the "kissing bonds" present in the masked area were readily identified within the bond surface closest to the UT probe. It is expected that with further tuning of the technique, the bond on the far side of the composite can also be evaluated.
Despite all the promise of this technology, in the end the shipyards did not buy off on it, and the DDG 1000 will be produced using bolted bonding techniques. The decision was mostly political and had nothing to do with the merits of the technique itself. The Navy Joining Center failed to meet its primary objective which was to transition the new technology into production. Cost overruns of the entire DDG 1000 project has forced the Navy to reduce production quantities from 15 destroyers down to only 4.
One use has been found for the technology in the private sector. A foreign billionaire is converting an old warship into a yacht (called "Swift Gigayacht"), and wants to put a futuristic all composite cabin on the steel hull. The joint will be used to accomplish this task.
During the question and answer period Mr. Brown provided the following responses:
The adhesive material used is a two part adhesive used by Boeing. While it can develop full strength at room temperature over time, it was found very effective to cure it at about 140 F.
A surface treatment for the steel was developed by EWI chemists. After grit blast cleaning, the surface treatment is applied by rollers (paint style) prior to application of the adhesive and bonding.
The service temperature for the joint is qualified up to 250 F. When pull tests produce failure within the composite, the failure typically occurs at the second or third layer of composite layup.