Adhesive bonding is one of the oldest method by which materials can be joined to generate assemblies. Besides this, it continues to be used is an alternative to more traditional mechanical methods of joining materials, such as nails, rivets, screws, etc.
Use of adhesives is described in ancient Egypt and in the Bible, the early recipes for adhesives have been described as being formulated during Middle ages. One such recipe shows that people in the middle ages had an appreciation for the generation of composite materials as well as the use of drying oils: “Very Strong, Very Good Glue” Such a recipe sounded like this:
“Take clay roof tiles and grind them to a fine powder using a flour grinder. Add a similar amount of iron rust, also ground to a fine powder. Add live lime in an amount equivalent to the clay and iron rust and incorporate the mixture with linseed oil. Use immediately to glue what you want as this glue is better when used fresh than otherwise.”
Basically this recipe it’s not very much different versus how we make glue today. However A major step in adhesive technology took place in the early 1900s with the advent of synthetically prepared adhesives. Thus, widespread use of adhesives as a joining medium is a relatively recent phenomenon. All joining methods have their advantages and disadvantages and adhesive bonding is not an exception. Therefore in this article I will present you the most frequent positive and negative features of adhesive bonding as a joining method.
BASIC DEFINITIONS
For a clear understanding let’s quickly review the basic terms.
ADHESIVE BOND = An assembly made by the use of an adhesive is called an adhesive joint or an adhesive bond.
ADHERENTS = Solid materials in the adhesive joint, other than the adhesive.
ADHESION = The phenomenon, which allows the adhesive to transfer a load from the adherend to the adhesive joint. The actual strength of an adhesive joint is primarily determined by the mechanical properties of the adherends and the adhesive. The term we apply to the measured physical strength of an adhesive bond is practical adhesion.
ABHESION = The condition of having minimal adhesion. This property is important when an assembly is needed from which the adhesive can be removed on demand. Materials that exhibit abhesion are also known as release materials and they are used to make certain pressure sensitive adhesive constructions.
HOW ADHESIVE BONDING WORKS
One major differentiation between an adhesive joint and a mechanically fastened joint is that, in the latter a mechanical fastener must pierce the adherend in onder to execute an assembly. When a mechanical fastener pierces an adherend, or if the adherend is pierced before the installation of a mechanical fastener, a hole is created in the adherend.
This principle is sketched in Fig 1 where we have 2 examples of an adherent. As shown in Fig 1 (a) the adherent is intact. If a load was applied to the adherent the lines of force propagating through the adherent would be continuous. If instead the adherent has a hole in it (such as depicted in Fig 1.1 (b) the lines of force could not be continuous through the adherent and would have to go around the hole. Thus at the edges of the table, the force experienced by the material is much larger than the force experienced by the material remote from the hole. The edges of the hole not only have to support the force that is applied to those edges, but also must support the force that should have been supported by the material that would have been in the hole. On fracture mechanics this situation is known as a stress concentration.
A stress concentration can cause a decrease in many physical properties of the adherent as well as those of the mechanical joint. In contrast, if an adhesive is used to generate an assembly, no hole is generated in the adherent. Therefore; the physical properties of the adherent are maintained after the assembly has been created.
The use of mechanical fasteners in a joint can lead to several problems that are not present when adhesives are used. Here we can have 3 situations:
- 1st – the overall strength of the joint can be reduced.
- 2nd – the joint can experience early fatigue failure.
- 3rd – if either of the adherents is sensitive to shock, the act of applying the mechanical fastener could cause the assembly to fail.
ADVANTAGES AND DISADVANTAGES OF ADHESIVE BONDING
Adhesive bonds when executed in a properly designed adhesive joints do not exhibit high stress concentrations, so the properties of the adherents can be fully utilized. However adhesive joints do require a much larger arms of contact between the adherents and adhesive in order to carry the same load as a mechanical fastener.
For the most part, adhesives are polymeric materials that exhibit viscoelastic properties. Materials that display visco-elasticity have both a viscous character as well as an elastic character. Polymer-based adhesives absorb mechanical energy applied to the joint and dissipate that energy as heat. Hence, fatigue failures are delayed in comparison to mechanical fastening. As shown in table 1 the research results show that the fatigue life for joints made by a combination of mechanical fastening and adhesive bonding increases significantly.
Finally, many adhesives do not require input of mechanical energy to effect an assembly. Hence, shock-sensitive materials can be easily made into an assembly. For example, one would not consider joining of dynamite sticks with nails. However dynamite sticks can be easily joined by pressure-sensitive adhesive backed tape.
The primary disadvantage of adhesive bonding is that it relies on adhesion for the transfer of load through the assembly. Since adhesion is a surface phenomenon, it follows that the physical properties of the adhesive joint depend strongly on the character of the surface of the adherend and how the adhesive interacts with that surface. Thus, an adherend with an improper surface could lead to lower joint strengths than might be predicted from the mechanical properties of the adhesive and the adherend.
Surface problems are even more important when one tries to generate adhesive bonds that are durable in adverse environments. The need for a proper surface, and the fact that it is not always available, are disadvantages of adhesive bonding in comparison to mechanical fasteners, which are not affected by the state of the surface of the members of a joint.
However adhesives display several other advantages over mechanical fastening. One of these advantages is the reason for the widespread use of adhesives in the aerospace industry, specifically the ability of adhesives to not only form a joint, but also to seal the assembly in one step. Mechanical fastening often requires separate sealing steps to create a pressurizable assembly. Adhesives also allow galvanically dissimilar materials to adhere to one another without accelerating corrosion.
For example, the mechanical joining of steel and aluminum would be a disaster in the making. Aluminum would act as an anode to steel and corrode rapidly in corrosive environments. Since most polymeric adhesives are non-ionic and electrical insulators, a properly effected adhesive bond would electrically separate the members of the galvanic couple while still joining them structurally.
The potential weight savings inherent in adhesive bonding are also the major reason for their increased use both as automotive and aerospace technology advances. In addition, using a combination of adhesive bonding and spot-welding (sometimes known as “well bonding”), provides improvements in the noise and vibration characteristics of the automobile and aircrafts chassis.
Likewise in a home construction project, when floorboards are adhesively bonded to joints, the floor tends to be stiffer but quieter than when nails are used exclusively in a similar construction.
The main positive vs negative characteristics of adhesive bonding are summarized in table 2.
However, mechanical fastening does have a number of advantages over adhesive bonding. Once a mechanical fastener is applied, one certainly knows that it is there. Adhesives, by their nature, are internal to the joint. In most cases, it is not easy to determine (without destructive testing) whether the adhesive was properly applied. This lack of non-destructive quality control has led to entire studies on methods by which adhesive bonds can be inspected in a non-destructive fashion.
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