Cohesive failure in the adherend

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.

FIGURE 4.6 The failure of a joint can be in any or all of three sites, (a) Adhesive failure, where there is a clean separation between the adhesive and adherend. (b) Cohesive failure in the adhesive, where the adhesive breaks or flows (see Figure 2.7). (c) Cohesive failure in the adherend (object), where the substrate breaks. The most common mode of failure in conservation is (d), where the adhesive bond, the object and the adhesive all fail. Figure redrawn from an original of Jane Down (Canadian Conservation Institute). 

A larger number of failure modes is thought to be possible with composite materials bonded by adhesive joints than with equivalent metal joints. Failure in the adherends can be tensile, interlaminar or transverse. The last two can be either in the resin or in the interface. Finally, cohesive failure can occur in the adhesive. 

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. 

Cohesive failure of the adherend takes place when the adherend fails due to loads in excess of the adherend strength. In laminate structures the failure typically initiates from the matrix between the laminae as a result of out-of-plane loads or interlaminar shear loads. Other types of failure initiation are also possible, especially for FRP composites that do not have a layered structure. 

The predominant failure mode in bonded joints was cohesive failure of the adherend. Only in those adhesively bonded joints with the polyurethane adhesive (1897) was a cohesive failure of the adhesive encountered. No adhesive failures were detected, indicating that the surface treatments performed had been sufficient. The quality of adhesive layers was in general satisfactory. However, in joints of laminates with a woven roving or a uni-directional surface layer (Table 7) the adhesive layers were thick and of poor quality, having large voids. With laminates of a uni-directional surface layer, this... 

Cohesive failure Cohesive failure is the loss of cohesion within the body of a material. Cohesive failure can take place in the adhesive, termed cohesive failure of the adhesive, or in the adherend, termed cohesive failure of the adherend. 

Adhesive joints may fail adhesively or cohesively. Adhesive failure is interfacial bond failure between the adhesive and adherend. Cohesive failure occurs when the adhesive fractures allowing a layer of adhesive to remain on both substrates. When the adherend fails before the adhesive, it is known as a cohesive failure of the adherend. The various modes of possible bond failures are shown in Fig. 7.4. 

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. 

Based on the failure mechanisms and stress distributions at the interface between steel and composite adherends of the co-cured single and double lap joints, tensile load bearing capacities of the two joints were evaluated. Since failure started at the edge of the interface between steel and composite adherends, it is important to consider the failure criterion using interfacial out-of-plane stress distributions at the interface. Three-dimensional Tsai-Wu and Ye-delamination failure criteria were used to predict partial cohesive failure or interlaminar delamination failure in the co-cured single and double lap joints. 

Failure, cohesive—Rupture of an adhesive bond in such a way that the separation appears to be within the adhesive. Rupture of an adhesive bond in which the separation appears visually to be in the adhesive or in the adherend. 

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. 

Another observation which can be made in this test and in all the adhesive tests which follow is the type of failure which may occur. Failure can occur by three different modes. Cohesive failure occurs when the specimen fails within the adhesive layer. Adhesive failure occurs when the specimen fails at the interface between the adhesive and adherend. In some cases, failure of the adherend itself can result. It is recommended that for every sample, the percentage of cohesive and adhesive failure be recorded. Any samples which fail in the substrates should be discarded since such cases do not constitute a test of the adhesive material. 

Failure in an adhesive joint can occur in one of two ways (1) adhesive failures that occur at the interfaces between the adhesive and adherends, and (2) cohesive failures, which occur either in the adhesive or in the adherends. The determination of the strength, failure, and reliability of an adhesive joint requires both an understanding of the mechanisms of adhesion and a knowledge of deformation and stresses in the joint. The mechanisms of adhesion are closely related to chemical and physical properties of the adhesive polymers. The deformation and stress states can be determined once the geometry, loading, boundary conditions, and mechanical properties of the constituent materials of the joint are known. The mechanical properties of the adhesive and adherend materials enter the stress analysis via constitutive models, which relate strains, temperature and moisture gradients, and density to stresses and fluxes in the joint. The chemical, physical, and mechanics aspects of the constituent materials enable the formulation of appropriate constitutive models for adhesive joints. The determination of stresses allows the prediction of the strength, failure, and reliability, in a macromechanics sense, of adhesive joints. 

---