Adhesive and bonding for composite materials - Summary
Adhesives are polymers used for the connection of two parts. Curiously, when dealing with composite materials, the adhesives are sometimes with thermosets, because these are based on epoxy. However, adhesives are epoxy resins with different ingredients in order to bond the surfaces in contact. There are two basic properties of an adhesive, the chemical structure and the subtract interface. Basically, they indicate the affinity of the adhesive with the materials that will be bonded. Particularly, the substrate interface is the most important since it is very difficult to preview it with high accuracy. The reason is that this depends on the wettability and the surface energy of the components in contact. The higher the wettability, the better the bonding performance due to the high surface energy. If the wettability is low, the surface energy is also low, thus the bonding performance is reduced.
However, this problem can be mitigated by surface preparation, which for composite materials can be mechanical, chemical or pressure methods. The mechanical surface preparation is based on grinding or sand blasting. The chemical surface preparation methods are mainly the acid etching. The pressure surface preparation is based on the contact between the two components under a specified pressure. Some of these methods are performed for specific situations. For instance, pressure surface preparation is used when it is necessary to have high control on the adhesive layer thickness. The mechanical and chemical methods can be applied when in cases of metals that develop an oxide layer. Hence, these can be removed without damaging the main material.
Types
Adhesives are usually characterized with respect to the mechanical properties and glue type. In the first case, they can be structural or high elasticity adhesives. The structural adhesives are characterized by its high strength, which fits well for applications exposed to high stresses. However, this comes at the cost of a very low elasticity. On the other hand there are the high elasticity adhesives. These, as the name suggests, have a high elasticity, which makes them useful for applications where the components are very exposed to high temperatures and they are made from different materials. In this situation, these will expand at different rates, which results in a thermal stress on the adhesive layer. Then, the stress distribution at the substrate changes, it becomes asymmetric. High elasticity adhesives can deal with this difference without impact on the part operation. However, these are not strong as the previous one, they are considered low-medium strength adhesives. When characterizing the adhesives regarding the glue type, these can be liquid or pasty. Clearly, liquid adhesives have a good wettability since they can be easily spreaded over the surfaces. However, they are problematic when dealing with vertical surfaces due to its very low viscosity. In those cases, pasty adhesives are suggested. These have a high viscosity, which allows them to be applied in vertical surfaces with a very good uniformity.
Working principle
The working principle of an adhesive is based on two important concepts, the adhesion and the cohesion. Basically, the first is the friction obtained by the microroughness contact, while the cohesion is the friction obtained from the macroroughness contact. Therefore, an adhesive can not work well when only adhesion or cohesion is governing the bonding, both mechanisms must act together. This principle is also the source of adhesive problems and failures. Actually, adhesive can fail by four failure modes, adhesion, cohesion, mix and substrate delamination. The first is when the adhesion mechanism is very weak, which causes the complete detachment of the adhesive. The cohesion failure occurs when the adhesive layer fails, then rupture occurs on it. The mixed failure is a combination of the previous ones. Basically, the adhesion failure is caused when the adhesion forces are very low when compared to the cohesive ones. The cohesive failure occurs when the forces exerted by the adherents exceeds the maximum joint strength. The substrate delamination is failure mode that begins from a local point that exhibits a very low resistance with respect to others across the adhesive surface.
Types of joints
The joints are how those adhesives are used to bond two or more surfaces together. There are several types of joints. The main ones are listed below:
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The overlap is the most basic and used joint. It is based on tensile forces exerted on the adherents that motivates shear stresses on the joint. This configuration has three layers, two adherents separated by an adhesive layer. The main advantage of this kind of joint is the adhesive thickness, which can be very small. However, it is exposed to load misalignment due to the point of application of the forces. The double lap joint is a double overlap joint. It basically transforms the double lap joint into a double shear one. The main advantage is its high strength, because the thickness increases significantly with respect to the overlap joint. The scarf or beveled joint is considered the ideal joint. It is based on the complete tapering of the adherent and the adhesive in order to exhibit a sharp edge. This reduces the stiffness mismatch between adherents and adhesive. However, this kind of joint is very expensive and time consuming to be built. The cylindrical joint is adopted to deal with shear stresses and torsion, which are common components as tubes and shafts. Its problem is that the bonding surface is, in some cases, at the same orientations of the forces. The double joint is based on two adherents of different stiffness separated by the adhesive. It is similar to a double joint, but stiffness mismatches between the adherents are purposely designed in order to balance the joint. The butt joint is based on an adhesive that separates two surfaces exposed to forces that are normal to the adhesive. The peel joint is mostly used for experiments that evaluate the adhesive performance under tension and shear stresses. The adherent has an angular tab which is under a force, the angle splits this in two components.
Design rules
When a part, component or entire machine is being designed with bonded parts, this will not follow the usual procedure. Actually, in this case, these are designed for bonding. Hence, the parts made to be assembled through the bonding process have different aspects with respect to the ordinary ones, which are fixed by fastening. There are three ultimate rules for bonded components design, which are to improve bonding surface, reduce peel/cleavage stresses and avoid forces perpendicular to the bonding surface. In the first rule, the region of the fixture is increased. However, studies suggest that an overlap length excessively long weakens the bonding performance. For overlap joints, the length is about 25-40 mm. The reduction of peel stresses is provided by the reduction of the misalignment and of loading conditions perpendicular to the adhesive surface. Even though the misalignment is not completely avoided, it can be reduced by aligning the forces. This is done by a design that motivates the load concentricity when the joint is under stress. It is a rather difficult task since some simulations are necessary to preview the behavior of the adherents and adhesive. The adherents' deformation might help to improve load concentricity. This is also connected to the geometry smoothing, which is basically the tapering and chamfering of the adherents. The objective is to reduce the adherent thickness at specific regions, usually at the extremities, in order to reduce the stiffness mismatch between it and the adhesive. In some applications the adhesive is also tapered. The objective is always the same, improving the joint balance.
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