Bonded joints adherends

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

Other opportunities for stress concentration in bonded joints that may be aggravated by low-temperature service include trapped gases or volatiles evolved during bonding, residual stresses in adherends as a result of the release of bonding pressure, and elevated-temperature cure (i.e., shrinkage and thermal expansion differences). 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

With a few exceptions (e.g., body-in-white manufacturing), the production of bonded joints requires surface treatment of the adherends as an additional production stage. 

Adherend surface The glued surface or surface to be glued of an adherend or a bonded joint. 

Bonded joint Joint of adherends, obtained by an adhesive. 

There are also polymers that can be brought to melting by heat supply. They are applied to the adherends in molten and solvent-free form. When the adhesive melt has cooled down, a bonded joint develops. Such adhesives, processed by melting and cooling down, are called hot-melt adhesives (Section 5.1). 

Since the chemical reaction of the two components A and B begins in a pot immediately after mixing, this ready-made adhesive mixture requires speedy application. Otherwise the reaction for the formation of the AB polymer (the adhesive layer) will have developed to such an extent already prior to the application to the adherends that the expected strength of the bonded joint is impaired. Between the mixing of the adhesive mix and its application to the adherends and their fixing only a certain time span is allowed - which may vary for the individual reactive adhesives. This time is called the pot life. Depending on the reactivity of the A and B monomers, pot life can lie in the range of minutes or even hours. 

Bond the adherends as long as the adhesive is wet, then fix the bonded joint under pressure. 

Adhesive tapes are also suitable as fixing aids for the manufacturing of bonded joints to secure adherends against moving (in the case of heat-curing adhesives, however, applicable only to a limited extent). 

Figure 6.1 Positively bonded joints of adhesive layer and adherend.

On the assumption of a clean and, respectively, pretreated surface, the next step will be the application of the adhesive. It has to be ensured, however, that in areas with adhesive forces emanating from the adherend surface, the adhesive molecules are really able to get closer. Only then can the adhesive distribute itself on the surface, that is, wet the surface despite a, more or less, existing roughness. Furthermore, sufficient flowability of the adhesive is important. A complete and equal wetting of the surface to be bonded is therefore an indispensible prerequisite for the production of a strong bonded joint. Figure 6.5 demonstrates the difference between a low-viscosity and high-viscosity adhesive. 

Following the surface preparation, it is the task of surface pretreatment to generate the adhesive forces on the adherend surfaces required for the development of a strong bonded joint. Since almost all materials interesting for bonding have the property to cover the surfaces with impurity layers (oxides, rust, dust, greases), those layers have to be completely removed prior to adhesive application, since otherwise failures in the development of the adhesive forces will occur (Figure 7.5). 

The strength values mentioned above depend on the respective construction conditions as well as on the stress duration. An example A plastic hook fixed to a tile by means of a pressure-sensitive adhesive and exposed to stress can come off in the course of time due to a failure of the adhesive layer (creeping). In the case of dynamic stress, the hook can break inside, the bonded joint remains unaffected. In this case, the adherend is the weaker link in the strength chain . 

So-called white glues are particularly important. The chemical basis is polyvinyl acetate (PVA), the bonded joints - or traditionally adhesive gluings - are characterized by very high adhesive strengths, which usually result in adherend fractions after destructive testing. In order to avoid possible tensions, it is recommended to let the bonded joint set under even pressure (clamp) for more than the indicated... 

Tensile shear strength (adhesive strength), in the sense of this standard, is defined as the maximum force Fmax at the break of the bonded joint in relation to adherend surface A. The adherend surface A results from the test piece width h (25 mm) and the overlap length lu (12.5 mm) ... 

If bonded joints have to last when stressed by forces, the choice of the adequate adhesive is just as important as the bonding-compatible arrangement of the adherends in the glueline. Here, some basic rules have to be observed ... 

In order to enable the transfer of forces by means of a bonded joint, a sufficient adhesive surface must be provided between the adherends. This requirement is emphasized by Figure 11.4 ... 

In the case of metal adherends with their high strength, the adhesive layer is the weakest link in the strength chain under tensile stress in case (a). Stressed by force F, the adhesive layer in such a bonded joint will break. In case (b), under tensile shear stress, the adhesive layer can be expanded to a certain extent (see Section 10.2.2) by enlarging the overlap length lu, and a bigger force F can be transmitted. In general, bonded joints require sufficient adherend surfaces. 

In general, bonded joints shall provide sufficient adherend surfaces. 

Adherend fracture Failure of a bonded joint under mechanical stress in the adherend material, thus, outside the adhesive layer. Indicates that the bond strength is higher than the adherend strength. 

Assembly time (closed) Period of time during which a bonded joint has to be fixed until it is strong enough to resist a displacement of the adherends by external force effects. 

Scanning electron microscopy (SEM) was used to investigate the morphology of the primed adherends and the fracture surfaces of the bonded joints. 

The same effect as on aluminum adherend was also found with magnesium and PEI adherend. The optimum concentrations of each of the primers studied were applied to PEI (Ultem 1000) and magnesium alloy (AZ-91). Table 15.6 summarizes the single lap shear strengths of the resulting bonded joints. The results show that PAMAMs are effective in improving lap shear strength on... 

ISO 11343, 1993, Adhesives—Determination of dynamic resistance to cleavage of high strength adhesive bonds under impact conditions- Wedge impact method The method is mainly aimed for the characterisation of metal substrates suitable for automotive applications. An instrumented impact testing machine (pendulum type) of 50-300 J and. 3-5.5ms maximum capacity is required for this test. A blunt wedge of approximately 1 mm tip radius and included angle of 8 is impact driven into a bonded joint at 3 ms" for aluminum alloy adherends and 2 ms" for steel adherends. The impact event should be... 

Figure 9 Overview of fracture surface of phenol-formaldehyde bonded joint showing exposed adhesive layer (A) without preexisting cure-shrinkage cracks and surface of lower adherend (B). The sloped test fracture surfaces (arrow) characteristic of phenol-formaldehyde and toughened urea-formaldehyde adhesive layers show where the crack jumped from one interphase to the opposite as the crack traveled in the fiber direction.

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