Load-bearing structures, bonding

Grade I = life depending primary structures, Grade II = secondary structures and Grade III = non-load-bearing structures/bonding. 

Abstract. In the 60ties engineers started to combine short glass fibers with a concrete matrix. Today engineers combine endless fibers with the concrete matrix. Hence, the so called textile-reinforced concrete offers more and more opportunities to manufacture consumer goods like desks or chairs and to build face elements for houses and even load-bearing structures. This revolution is only feasible because a lot of research work has been done - into textile, into concrete, into the combination and the bonding. 

EN 302-1 2004, Adhesives for load bearing structures — Test methods — Part 1 Determination of bond strength in longitudinal tensile shear. 

A typical joint cross-section is shown in Figure 2.1, where a clear distinction is made between the bulk of the adherends and their surfaces. Chapter 4 discusses the importance and nature of a component s surface and methods of improving its structural strength and increasing its suitability for bonding. However, it must be borne in mind that sophisticated surface preparation is often necessary only for major load-bearing structures - often using some form of lap joints - which are exposed to severe environments. 

Adhesives are used extensively by the aerospace industry for bonding structural components of aircraft (both military and commercial), missiles, and satellites. Sealants are used in joints around windows, in fuel tanks, etc. hot melts and pressure sensitive adhesives are utilized in aircraft interiors (primarily in fabricating decorative panels) while thermosetting adhesives are used to bond load bearing structural components. 

The combination of core (aluminium, aramid, paper, carbon, glass or balsa), skins (aluminium, steel, titanium, FRP, melamine or wood) and adhesive (film or paste, epoxy, phenolic, PUR, PI, cyanate ester, acrylic or thermoplastic) gives a sandwich panel which is an integrally bonded, load-bearing structure. 

To form a strong, integrally bonded, load-bearing structure, the surface of the adherend should be pretreated before application of the adhesive this is vital if good environmental or thermal durability is required. Such a procedure ensures that the surface is in as clean a condition as possible, removing weak boundary layers which could adversely affect the performance of the resultant joint. 

The heart of any bond is, of course, the adhesive. Most bonding in the aerospace industry relies on the so-called structural adhesives, i.e. adhesives based upon monomer compositions which polymerise, or cure, to give high modulus, high strength bonds between relatively rigid adherends, such as those discussed above, so that a load-bearing structure is produced. 

One of the most important requirements of a structural adhesive bond is durability that is, the ability to retain a significant portion of its load bearing capability for long periods of time under the wide variety of environmental conditions which are likely to be encountered during service life.CjJ Unfortunately, the poor durability of metal/adhesive bonds in wet, hostile environments has proven to be the major obstacle to widespread development and practical usage within many industries. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

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