Factors affecting joint strength

The strength of a bonded joint will be determined by the strength of its weakest component, whieh is generally designed to be the adhesive. It follows that the requirements for satisfactory joint performance eu-e (a) good contact between the adhesive and substrate (b) absenee of weak layers in the joint (e) that the adhesive should possess appropriate mechanical properties. These basic requirements are implieit in the essential elements of bonding, and are related to many faetors which affect the performance of bonded assemblies as summarised in Table 4.2. 

Bonding conditions temperature of substrate ambient temperature humidity 

Testing conditions strain rate cyclic frequency temperature 

Given that the adhesive itself should determine the strength of a bonded joint, the stress required to rupture a joint is, nevertheless, not a well-defined materials constant. When two materials are bonded, the resultant composite has at least five elements, namely the adhesive itself, two adhesive/adherend interfaces, and two adherends. If a primer is applied to both substrate surfaces, the number of elements increases to at least nine. These elements involving a metallic adherend are depicted schematically in Fig. 3.2. Note that the adhesive (or primer in this case) is in contact with the metal surface oxide layer, and not with the metal itself. 

Kinloch(8) suggests that the measured bonded joint strength almost always reflects the value of two parameters  

---

In the discussion of factors affecting joint strength and durability, it was seen that the presence of water was responsible for many of the problems. Most bonded structures, if exposed to water or high humidity, will lose strength over a period of time and in extreme cases will fail completely. 

Tamrin K, Nukman Y, Zakariyah S. Laser lap joining of dissimilar materials a review of factors affecting joint strength. Mater Manuf Process 2013 28 857-71. 

The reader will appreciate that a large number of factors can affect joint strengths, and hence the caveat advanced in the introduction that extreme care must be exercised in the interpretation of bonded joint performance. 

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. 

Besides the environmental factors mentioned above, the materials involved in a structural joint also influence bond strength and durability. The factors in the material category include the adherends the adhesive the design of the joint absence of surface contamination (including contamination with wood extractives) stability of the adherend surface the ability of the adhesive to wet the surface, and entrapment of air/volatiles. Thus, the condition of the adhesive/adherend interface becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint [31]. 

The condition of the adhesive/adherend interface then becomes a decisive factor affecting the initial bond strength as well as the long-term durability of the bonded joint. 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

A measure of the damages affecting the bonded joints is the loss of strength of the aged test pieces in contrast to the nonaged test pieces, which may be indicated in the form of reduction factors (Section 10.2.2). Only property testing of bonded joints under these complex stresses composed of mechanical and environmental influences enables an extensive statement on the behavior in practical application. 

Adhesives exposed outdoors are affected primarily by heat and humidity. Thermal cycling, ultraviolet radiation, and cold are relatively minor factors. Structural adhesives, when exposed to weather, rapidly lose strength during the first six months to a year. After two or three years, the rate of decline usually levels off, depending upon the climate zone, polymer, adhesive, and stress level. The following are important considerations when designing an adhesive joint for outdoor service ... 

---