Menu

Structural Adhesive Bonding in Aerospace

Posted on 1/7/2013 10:09:30 AM By Sandy Niks
  

Adhesives have been used extensively for structural bonding applications in aerospace. I assume everyone is aware that the tiles on the space shuttles were attached using a room temperature vulcanizing (RTV) silicone adhesive. (See below for some sample links.)

Structural bonding of aircraft has been very important in the history of the industry. Aluminum has been a major substrate for the outer skins, being lightweight while retaining strength and durability. Combining adhesive bonds and rivets has been a typical assembly construction to further optimize weight savings, strength, and durability. Adhesively bonding the metal components has spread loads over larger areas. Adhesives used in these applications include epoxy-based materials, often applied as a film. The surfaces are cleaned and primed prior to bonding and cure is typically accomplished in an autoclave.

Sandwich structures have further increased strength and durability, while minimizing weight. They consist of outer skins of aluminum or a composite such as fiberglass or carbon fiber with an internal honeycomb structure, often made of aluminum. The most effective assembly method is again adhesive bonding. Foamed cores could also be used. There is a link below to an example of a next generation sandwich structure created from several layers of dissimilar materials.

SAE International, is the home for the majority of the standards for aerospace, with over 8000 current standards. Of those, about 170 are for adhesive and sealants, including material specifications, test methods, and other instructional documents. The standards are managed by multiple committees within the Aerospace Council, including AMS G9 Aerospace Sealing Committee, AMS P Polymeric Materials Committee, AMS P17 Composite Materials Committee, and AMS G8 Aerospace Organic Coatings Committee. Unlike the standards from the Automotive Adhesives & Sealant Committee, where the majority of the standards are test methods, the adhesive and sealant standards developed within the Aerospace Council are mostly material specifications.

ASTM International also has aerospace standards developed and maintained by committees including F07 on Aerospace and Aircraft (about 50 standards), E21 on Space Simulation and Applications of Space Technology (about 60 standards), as well as some newer committees on small aircraft. However, there appears to be no specific “aerospace adhesive” standards. The adhesive standards from Committee D14 on Adhesives would be used. For example, ASTM D 3762, “Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test)” and commonly known as the ‘Boeing wedge test’; ASTM D 3933 “Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodizing)” and ASTM D 6465 “Standard Guide for Selecting Aerospace and General Purpose Adhesives and Sealants” which provides an extensive list of standards.  International Organization of Standards (ISO) also has some aerospace standards and there are probably a few found in other organizations around the world. The aircraft manufacturers and military will also have additional standards that haven’t been shared through standards developing organizations.

There is a wealth of information available on this topic. Hopefully, this short article will inspire the reader to investigate further. I have provided some references as a starting point.

Space shuttle tile bonding links:

  • Wikipedia – Space Shuttle Thermal Protection System
  • NASA—The Shuttle’s Therman Protection System (TPS)
  • Article on “Adhesive bonding of aircraft structures,” by A. Higgins

Image showing where adhesive is used:

 airplane, cross-bonding, bonding, aerospace

 

  • Example of cross-section construction of an aircraft wing

References:

  • Adhesive Bonding of Aluminum Alloys, edited by Edward W. Thrall, Raymond W. Shannon, Marcel Dekker, Inc., 1985, ISBN: 0-8247-7405-1
  • Handbook of Adhesive Bonding, edited by Charles V. Cagle, McGraw-Hill Book Company, 1973, ISBN:  0-07-009588-4, Chapter 24, “Adhesives for Space Systems”, by George Epstein.
  • Joining Technologies for the 1990s, edited by John D. Buckley, Bland A. Stein, Noyes Data Corporation 1986, ISBN:  0-8155-1095-0, pp 340-349, “Adhesives for Aerospace”, by L. E. Meade

As mentioned in a previous article (October 17, 2012) a meeting of the SAE Adhesives and Sealants Committee was being planned. The date has finally been set. If interested in the meeting, you can view the announcement here.



comments powered by Disqus