Adhesive joint adherends

KEYWORDS adhesive joints, adherends, shear stress, normal stress... 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Table 1 provides the approximate load bearing capabiUties of various adhesive types. Because the load-bearing capabiUties of an adhesive are dependent upon the adherend material, the loading rate, temperature, and design of the adhesive joint, wide ranges of performance are Hsted. 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

Samples constructed from adherends which had been alkaline cleaned, lubricated or left untreated exhibited similar joint strength values and durability trends (Figure 10). Adhesive joints placed in the room temperature control environment or the 23 C water bath retained lOOZ and 92% of initial joint strength, respectively. Failure remained cohesive within the adhesive for all of the control samples and for the first 20 days of exposure in the 23 C water bath. After 20 days, some failure began to initiate at both the primer/steel and primer/topcoat interfaces. The adhesive/topcoat interface proved to be more durable than those found between the substrate/primer/topcoat layers. Samples exposed to the more severe salt fog, 60 C water bath and cycle tests were able to retain 70% to 50% of their initial strength over a 60-day exposure period. 

Another chemical species which can be added to the interphase region in composites or in adhesive joints is the finish or primer . The finish is usually applied to the fiber before composite fabrication or as a primer to the adherend before joint formation. Both finish and primer are quite similar in that they consist of a resin rich... 

In all adhesive joints, the interfacial region between the adhesive and the substrate plays an important role in the transfer of stress from one adherend to another [8]. The initial strength and stability of the joint depend on the molecular structure of the interphase after processing and environmental exposure, respectively. Characterization of the molecular structure near the interface is essential to model and, subsequently, to maximize the performance of an adhesive system in a given environment. When deposited on a substrate, the silane primers have a finite thickness and constitute separate phases. If there is interaction between the primer and the adherend surface or adhesive, a new interphase region is formed. This interphase has a molecular structure different from the molecular structure of either of the two primary phases from which it is formed. Thus, it is essential to characterize these interphases thoroughly. 

Ion beams provide useful information either as a diagnostic tool or as a precision etching method in adhesion research. The combination ISS/SIMS method used along with other techniques such as SEM provides a powerful tool for elemental analysis of surface composition. These results, as well as earlier work in this laboratory, indicate that the surface composition can be significantly different from the bulk due to contamination, selective chemical etching and segregation. These same techniques also provide an analysis of the mode of failure in adhesive joints. Many failures classified as "adhesive" on the basis of visual inspection are frequently mixed mode failures or failures at a new interface containing elements of both adhesives and adherend. 

Residual stress resulting from thermal expansion or contraction is due to the differences in the thermal expansion coefficient between the adhesive and adherend and to temperature distribution in the joint due to differences in thermal conductivity. 

Strong chemical bonds between the adhesive and adherend help stabilize the interface and increase joint durability. Aluminum joints formed with phenolic adhesives generally exhibit better durability than those with epoxy adhesives. This is partially attributable to strongly interacting phenolic and aliphatic hydroxyl groups that form stable primary chemical bonds across the interface. 

For most adhesive bonded metal joints that must see outdoor service, corrosive environments are a more serious problem than the influence of moisture. The degradation mechanism is corrosion of the metal interface, resulting in a weak boundary layer. Surface preparation methods and primers that make the adherend less corrosive are commonly employed to retard the degradation of adhesive joints in these environments. 

An adhesive is a material capable of holding together solid materials by means of surface attachment. Adhesion is the physical attraction of the surface of one material for the surface of another. An adherend is the solid material to which the adhesive adheres and the adhesive bond or adhesive joint is the assembly made by joining adlierends together by means of an adhesive. Practical adhesion is the physical strength of an adhesive bond It primarily depends on the forces of adhesion, but its magnitude is determined by the physical properties of the adhesive and the adherend, as well as the engineering of the adhesive bond. 

---