Durability of bonded structure

The effects of water and temperature on the adhesive itself are also of utmost importance to the durability of bonded structures. In the presence of moisture, the adhesive can be affected in a number of ways, depending on its chemistry and how rapidly the water permeates through and causes significant property changes [51,86-88]. The potential efficacy of moisture penetration on the locus of failure of bonded joints has been discussed in the previous section. As expected, elevated temperature conditions tend to degrade joint strength at a faster rate. 

The chemical attributes of adhesion cover a much more subtle and wider range of interactions described as primary or secondary bridging. Primary bridging can result from direct chemical reaction between the adhesive and adherend, while secondary bridging relates to the residual electrical forces which surround the respective molecules. It is assumed that these fundamental energy relationships and their association with the wetting phenomena have been covered in the earlier chapters of this book. It should suffice to state that better wetting of the adherend by the adhesive correlates directly with superior durability of bonded structures in the actual service environments. 

TEST ENVIRONMENTS FOR DETERMINING THE DURABILITY OF BONDED STRUCTURES... 

This section describes processing procedures and some of the more important factors that impact on the quality and durability of bonded structure. Topics covered include (1) surface preparation, (2) proper storage and handling. 

From these brief opening sentences it is apparent that adhesion and adhesives play important roles in modern clinical dentistry. Current research covers many facets of the adhesion process, including surface pretreatment, formulation of adhesives, durability of bonded structures, and clinical effects of bonding agents and adhesive materials (Roulet and Vanherle 2004). The deployment of these substances in the mouth is not entirely... 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

Extensive information on the outdoor durability of bonded aluminum joints is available in the reviews of Minford.61,62,63 Hartshorn has also provided a catalog of references to outdoor weathering of structural adhesives by adhesive class and exposure conditions.64... 

In summary, bonded repairs are the preferred approach for manufacturing repairs to both honeycomb sandwich and monolithic secondary stmcture. However, for the FAA and European Airworthiness and Safety Administration, the main reason for withholding certification of bonded repairs for primary structure is the lack of certainty over bond quality as it is not possible to assess strength and durability of bonded joints without destructive testing. 

This article discusses briefly the use of adhesives on the construction site in the context of structural repair and reinforcement the requirements and practical dilficulties in the work on site with regtirds to the strength and durability of the rehabilitated timber structure and the consequent need for queility control. It also highlights the characteristics and requirements that must be fidfilled by structural adhesives and reinforcing materials factors affecting performance and durability of bonded joints tmd ways to improve adhesion and durability. Finally, it points out some research needs and future developments identified by the authors. 

Water is often regarded as one of the most distressing agents that can affect the bonded joint performance and durability. Most bonded structures when exposed to water or humidity will lose strength over a period of time and in rare cases they may collapse, although this effect is limited to extreme conditions. 

Cracking should be properly limited, since the presence of an excessive number of cracks or of too large cracks may reduce the durability of the structure, its functionality, its aspect, and cause damage to the bond performance of the FRP-... 

The ample in-service durability of urethane structural bonds has been demonstrated by their use in the trucking and RV industries for nearly 20 years. Extensive laboratory testing of these adhesives has also illustrated their very good durability as shown in Table VII. 

In contrast to the attention given to the bonding of aluminum and wood, very few studies have been reported dealing with the durability of other structurally bonded systems." A limited amount of sustained load durability data is available for steel bonded with epoxies and aluminum bonded with urethanesin recent years, reports have appeared dealing with the environmental resistance of modified acrylics and cyanoacrylates. Brockmann has also reported on the durability of steel joints exposed to hostile environments. " ... 

Bond enhancement is not required to increase initial strength, but where bonded joints may be subjected to repeated wetting and drying or long periods of exposure to water and high humidity (e.g., with outside applications of externally bonded FRP composites) it may be necessary to improve long-term durability of the structural bonded joint with primers, adhesion promoters and other surface treatments (Custodio et al., 2008, 2009b). 

EN 15190 Structural adhesives. Test methods for assessing long-term durability of bonded metallic structures. 

EN 14444 Structural adhesives. Qualitative assessment of durability of bonded assemblies. Wedge rupture test. 

Because most highly structural bonding applications in manufacturing involve the use of metals as one or both adherends, it should be important for understanding the durability of such structures to discuss the surface chemistry and physical characteristics of metal surfaces (see Chapter 8 by Clearfield et aL). 

The reader should bear in mind that the American Society for Testing and Materials (ASTM) has developed a number of standardized testing procedures which can be used for evaluating the long-term durability of bonded wooden structures. A partial listing of some of these can be found among the list of references at the end of this chapter. ( 2-86)... 

J. D. Minford, Durability of bonded aluminum structure, SAMPE J. 11, (July-Sept. 1975). 

A. W. Bethune, Durability of bonded aluminum structure, 19th Natl. SAMPE Symp., Los Angeles, Calif. (April 1974). 

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