Cohesive failure in adhesive

P= primer A= additive AF= adhesional failure CF= cohesive failure in adhesive. 

P=primer A=additive AF= adhesional failure CF=cohesive failure in adhesive. 

AFG = adhesional failure to glass AFA=adhesional failure to aluminium CF-cohesive failure in adhesive. 

Figure 11.5 Silicone roller with primer - cohesive failure in adhesive base...

Figure 6.13 Different possibilities for the locus of failure in adhesion studies. Cases (b) and (c) (cohesive failure in adhesive/coating and in substrate) are more common than case (a) (adhesive failure). Reprinted from Myers (1991), with permission from John Wiley Sons, Ltd...

FIGURE 27.2 T-peel sfrength values of sulfuric acid-treated styrene-butadiene rubber (SBR)/polyurethane adhesive joints as a function of the immersion time in sulfuric acid. A = adhesion failure R = cohesion failure in the rubber. (From Cepeda-Jimenez, C.M., Pastor-Bias, M.M., Ferrandiz-Gomez, T.P., and Martm-Martmez, J.M., J. Adhes., 73, 135, 2000.)... 

Acidification of chloramine T with sulfuric acid produces the formation of dichloramine T (DCT) and hypochlorous acid (HCIO), species which react with C=C bonds of the butadiene units. The effectiveness of the treatment is ascribed to the introduction of chlorine and oxygen moieties on the mbber surface. A decrease in the pH of the chloramine T aqueous solutions produced more extended surface modifications and improved adhesion properties in the joints produced with waterborne polyurethane adhesive (Figure 27.9). The adhesive strength obtained is slightly lower than that obtained for the rubber treated with 3 wt% TCI/MEK, and its increases as the pH of the chloramine T solution decreases (Figure 27.9). A cohesive failure in the rubber is generally obtained. 

FIGURE 27.9 T-peel strength values of styrene-butadiene rubber (SBS) treated with chloramine T aqueous solutions with different pH/waterbome polyurethane adhesive/roughened leather joints, as a function of the pH value of the chloramine T aqueous solutions. A adhesion failure to the rubber, M cohesive failure in tbe rubber. (From Navarro-Banon, M.V., Pastor-Bias, M.M., and Martm-Martinez, J.M., Proceedings of the 27th Adhesion Society, Wilmington, NC.)... 

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. 

As presented in the introduction, two types of cohesive failure in shear were considered in the experiments. In the first case, the cohesive failure occurred in the bulk of the adhesive layer, while in the second case, failure occurred very close to the upper interface. The first failure case produced a highly rough cracked surface (Fig. 3(a).) associated with a large damage level throughout the thickness of the bond line. In the second case, however, fracture surfaces appeared mostly quite smooth and typical of brittle failure (Fig. 3(b).). In this case also, the crack grew without any deviation, very close to the upper interfece. 

Failure mechanism of the co-cured double lap joint imder cyclic tensile loads was a j artial cohesive failure in the thin adhesive layer. Figure 17 shows the relationship between the... 

The adhesion of PVAc adhesive was improved by the addition of calcite, alumino-silicate and starch but gains were rather small and the mode of failure was altered from cohesive failure in the unfilled sample to adhesive failures in the filled samples... 

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

Hamed [1982] also investigated the interfacial adhesion between EPDM and BR, as a function of the irradiation dose (Table 11.40). The modes of failure were attributed as follows (i) cohesive failure in BR at low doses (0-20 kGy) (ii) slip-stick behavior in which the locus of failure alternated between cohesive and adhesive (-30 kGy) and... 

Significant rates of transfer are observed when the initial interfacial adhesion is sufficient to develop cohesive failures in the polymer under the action of the transmitted shear stresses. If this transferred layer is weakly attached to the substrate, it is detached by the same tractions. Providing the geometry of the system allows, this material is displaced from the contact. More film is transferred to the substrate and a high equilibrium wear rate results typically in a polymer pin-on-disc configuration about 10 nm of polymer is removed from the pin during each cycle over the face of the disc. In confined or conforming contacts with... 

In order to elucidate the influence of the modifications occurring on the composite surfaces, two adhesives were used. The first one (adhesive X) shows a poor adhesion with the composites used, and the second (adhesive Y) provides high-performance assemblies, with a predominantly cohesive failure in the composite at the initial stage. 

In his test, a thin film of adhesive on a glass microscope slide or a metal coupon is cured and soaked in hot water until the film can be loosened with a razor blade. There is usually a sharp transition between samples that exhibited cohesive failure in the polymer and those which exhibited more of an interfacial failure. Since the diffusion of water into the interface is very rapid in this test, the time to failure is dependent only on interfacial properties and may differ dramatically between unmodified epoxy bonds and epoxy bonds primed with an appropriate silane coupling agent. The time to debond in the hot water for various silane primers differed by several thousandfold when used with a given epoxy. In parallel tests, a thick film of epoxy adhesive on nonsilaned aluminum coupon showed about the same degree of failure after 2h in 70°C water as a silaned joint exhibited after more than 150 days (3600 h) under the same conditions. 

Figure 4 shows failure types of the adhesive bonded joint under a pull strength test. The cohesive failures in Fig. 4(a) and (b) occur when fracture is developed either within the adhesive and substrate, while the adhesive failure in Fig. 4(c) separates the substrate and adhesive at the interface. 

Because there are so many geometries of adhesive joints encountered, and so many types of stress applied (tension, shear, torsion, thermal, etc.), the analysis of a given system must be tailored to meet the specific application. The processes of experimental design and data analysis, therefore, become quite complicated. It should also be kept in mind that flaws such as those often implicated in adhesive failure can also lead to apparent cohesive failure in the bulk material. 

If the bond failure occurs between the adhesive layer and one of the adherends, it is called adhesive failure (Figure 1.3a). A failure in which the separation occurs in such a manner that both adherend surfaces remain covered with the adhesive is called cohesive failure in the adhesive layer (Figure 1.3b). Sometimes the adhesive bond is so strong that the failure occurs in one of the adherends away from the bond. This is called a cohesive failure in the adherend (Figure 1.3c). Bond failures often involve more than one failure mode and are ascribed as a percentage to cohesive or adhesive failure. This percentage is calculated based on the fraction of the area of the contact surface that has failed cohesively or adhesively. 

Figure 1.3 Schematics of adhesive bond failure modes (a) adhesive failure (b) cohesive failure in the adhesive layer (c) cohesive failure in the adherend.

Table 1.3 Surface Chemical Analysis (ESCA) in a Cohesive Failure of Adhesive Bond...

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. 

This theory, due to Bikerman [15], is not a theory of interfacial adhesion. It states, in substance, that the streng (the breaking stress, the performance) of an adhesive joint is determined by the mechanical prc rties of the materials couprising the joint and the local stresses in the joint. It is not determined by interfacial forces, because clean failure "in adhesion" is a highly uncomnon occurrence. Failure is essenticdly edways cohesive, in the adherends and/or the adhesive or in some boundary layer. 

It is common practice, using various techniques, to attempt to determine the locus of failure ("in adhesion", "in cohesion", "mixed", etc.) in failed systems, e.g., adhesive joints, and to use the results to try to assign a cause of failure and to correct it. So far as the author knows, it is edways tacitly assumed in such studies that all material in the joint system is conserved during the failiire process. 

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