Adhesive joint strength, maximum

The adhesive stresses of a single lap joint reach maximum at the edges, which are important in assessing the adhesive joint strength. The expressions of maximum adhesive stresses obtained using different analytical models are summarized in Table 24.1. 

When the above criteria are used to assess strength of adhesive joints, the maximum stresses need to be determined. The adhesive stresses in symmetric lap joints are given in Sects. 24.2 and 24.3 of this chapter. The analytical approaches for asymmetric and unbalanced adhesive joints are discussed in Sect. 24.4, and their closed-form solutions are normally complicated. If the adhesive is thin and the overlap is long enough, for example, c > 20(fi + + ty), the... 

This conclusion Is further supported by Fukushlma s work on Joining silicon nitride rods. When a low purity adhesive sheet was used the maximum strengths achieved matched those of the adhesive material. Without the use of such an adhesive. Joint strengths were limited to only 70-90% of the Intrinsic strength of the high purity rods. 

Petterson13 has shown that the use of only 50 percent of the stoichiometric amount of hexamethylenediamine imparts to a cured DGEB A adhesive the maximum butt-joint strength, bulk tensile strength, and flexural moduli, whereas higher proportions of the diamine give lower properties. 

The application of corrosion-resistant primers has become standard practice for the structural bonding of aluminum in the automotive and aerospace industries. The adhesive-primer combinations are chosen to provide both maximum durability in severe environments and higher initial joint strength. Improved service life is typically achieved by establishing strong and moisture-resistant interfacial bonds and protecting the substrates surface region from hydration and corrosion. 

The amount of the applied adhesive and the final bond line thickness must be monitored because they can have a significant effect on joint strength. Curing conditions should be monitored for pressure, heat-up rate, maximum and minimum temperatures during cure, time at the required temperature, and cool-down rate. The primary concerns are to ensure the following ... 

Thermodynamic Work of Adhesion. One other important aspect of surface energetics (71, 72) is the use of surface free energy to calculate the maximum reversible work of adhesion, Wad, which has been correlated to the adhesive strength (41, 44) and should not be equated to the strength of an adhesive joint (6). Since neither wetting nor adhesion is controlled purely by thermodynamic factors, we should use the maximum reversible work of adhesion, Wad on the basis of an idealistic approach. When all other variables are equal, we can use Wad to compare the effectiveness of adhesives for a specific substrate. 

Acetal Copolymer (CELCON ) Two types of non-solvent adhesives are used, structural and non-structural. Most structural adhesives are based on thermoset resins and require the use of a catalyst and/or heat to cure. This type of adhesive is normally used in applications which require maximum bond strength and minimum creep of the adhesive joint under sustained loading. Many structural adhesives can be used continuously at temperatures up to 350 F, which is higher than the... 

In experiments on the adhesion of a powder, HOPE modified by oil-soluble contact Cl containing salts of organic amines or sulfur acids with either carboxyl or hydroxyl groups has been used. The composition of HDPE - - Cl has been applied to a preliminary cleaned and degreased 100—pm-thick aluminum foil at a temperature of T = 155 5°C and a pressure of p = 7—10 MPa. The splices have then been subjected to thermal treatment in an oven at 200° C for one hour, which corresponded to the maximum strength of the adhesive joints. The strength has been estimated by delamination tests under a constant deformation rate. Not less than 20 samples were tested in each point to show 10-30% variation factor at a confidence interval of 0.95. 

Figure 15.7. Two types of lap joints ta) thick adhesive is compliant and weak (b) zero adhesive thickness, i.e. molecuiar adhesion, gives the maximum stiffness and strength.

Kinloch(4) observed that the selection of appropriate failure criteria for the prediction of joint strength by conventional analysis is fraught with difficulty. The problem is in understanding the mechanisms of failure of bonded joints, and in assigning the relevant adhesive mechanical properties. Current practice is to use the maximum shear-strain or maximum shear-strain energy as the appropriate failure criterion. However, the failure of practical joints occurs by modes including, or other than, shear failure of the adhesive. This difficulty has led to the application of fracture mechanics to joint failure. 

Adhesive joints may fail adhesively or cohesively. Adhesive failure is an interfacial bond failure between the adhesive and the adherend. Cohesive failure occurs when a fracture allows a layer of adhesive to remain on both surfaces. When the adherend fails before the adhesive, it is known as a cohesive failure of the substrate. Various modes of failure are shown in Figure 1.3. Cohesive failure within the adhesive or one of the adherends is the ideal type of failure because with this type of failure the maximum strength of the materials in the joint has been reached. In analyzing an adhesive joint that has been tested to destruction, the mode of failure is often expressed as a percentage cohesive or adhesive failure, as shown in Figure 1.3. The ideal failure is a 100% cohesive failure in the adhesion layer. 

Powder adhesives are generally one-part epoxy-based systems that require heat and pressure to cure. They do not require metering and mixing, but must often be refrigerated to obtain maximum shelf life. Because coating uniformity is poor, large variations in joint strength may result with these adhesives. 

In the EUROCOMP design procedure the maximum calculated loading is assumed to be proportional to the square root of the adhesive layer thickness. Thus, when the adhesive layer thickness is increased from 0.5 mm to 0.8 mm (60%) the maximum calculated loading is increased from 87 kN/m to 110 kN/m (26%). However, the actual joint strength, i.e. the measured ultimate joint loads, does not increase. This indicates that the EUROCOMP design procedure does not take into account the effect of the adhesive layer thickness realistically. It should also be acknowledged that the applicability limits of the theory presented in the EUROCOMP Design Code are exceeded when thick adhesive layers are analysed. 

In material characterization, shear strength is the maximum shear stress a material is capable of sustaining. In testing, the shear stress is caused by a shear or torsion load and is based on the original specimen dimensions. Shear strength is rarely a factor in molded and extruded plastic products because of their relatively thick wall sections. However, it can be important in film and sheet products and in adhesive joints. 

The resultant strength of the examined hybrid interference fitted-ahdesive bonded joints was not always a result of the sum between the adhesive resistance and the interference contribution. The experiments carried out at laboratory level showed that different adhesive types provided dissimilar behaviors in the hybrid joint. Indeed, a greater influence of the adhesive on the maximum strength attainable for the hybrid joint was observed. Rheology of adhesive played a relevant role in determining how much adhesive was spewed away from the joint. If a few quantity of adhesive remained inside the... 

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