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Hybrid Structures for Weight Management

June 8, 2010

Combining composites into metal structures has been practiced in the aerospace industry for decades. Modern design methods produce the best stress management at the lowest added weight. Many of those structures are riveted, but adhesive bonding is increasingly attractive for further weight reduction and elimination of point loads in favor of distributed load management.
The same approaches, offering benefits of weight reduction and stress management, can be brought over into the heavy manufacturing sector where structures are more massive. Added benefits include trading structural mass for more payload, improved fuel economy, and lower center of gravity.

Recently, EWI worked with Logistics International in Abu Dhabi to develop a completely bonded interfacial joint for the composite-steel deck attachment of the four-story composite superstructure. The Swift 141 program employed a worldwide engineering task force, including EWI, to produce the ‘gigayacht’ atop the hull of a 141-meter former Dutch frigate. The featured all-composite superstructure employed completely bonded interfacial deck joints for exterior and interior walls and did not include structural mechanical fasteners.

Having been contacted by the naval architects, EWI first developed the test plan. Joint performance was predicted with FEA and verified with mechanical performance tests. Test samples of the adhesive in composite-metal test joints, with included EWI-selected surface treatments for corrosion control, provided verification of the FEA design along with projected joint performance derating based on corrosion cycles, temperature, and humidity. The outcome was the desired DNV approval of the fully bonded joint for use on the vessel. EWI also provided Swift 141 with a detailed step-by-step manufacturing crawler, including surface preparations. Although the ship was being built thousands of miles away, onsite engineering support was not necessary, as the shipwrights were familiar with bonding techniques.

This integrated design-manufacturing approach is transferred readily to other heavy structures outside of the marine sector. A base joint design is selected. Test specimens are produced to verify adhesive performance and corrosion protection. Larger sub-element joints are fabricated to verify the scalability of the FEA design. Finally, full-sized test articles verify the full-up joint structure performance using static and spectrum fatigue testing. NDE techniques require special attention for hybrid joint designs and those are verified. Full documentation is then provided, as was the case for the DNV certification.
Efficient combinations of materials and performance verification are within reach. EWI has the in-house capability to provide this complete engineering service for bonded structures.