Cohesive failure in the adhesive layer

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.

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. 

Similar considerations control the resistance to debonding of adhesives. In some cases, when the interface is very strong, the process is identical to that in bulk materials. The joint fails by cohesive failure within the adhesive layer. However, even when the crack propagates along an interface between an adhesive and a substrate, the ability of the system to resist the propagation of such a crack depends on the ability of the adhesive to absorb energy by a yielding deformation near the crack tip. 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

As discussed in previous chapters, the locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or... 

Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to study the nature of the failure interface and the failure mechanism in a PVC film/ adhesive/glass system. The failure mode was found to be mostly interfacial, occurring at the glass/adhesive interface, and also partially cohesive, located in the adhesive layer very close to the interface. The results are discussed in relation to the chemical nature of the interface and the failure mechanism. 11 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. Accession no. 773793... 

Although distinct "metal" and "adhesive" sides were apparent upon visual examination of the debonded surfaces treated with 100 ppm NTMP, SEM analysis showed the presence of an adhesive layer on the "metal" side. XPS analysis indicated low A1 and 0 and identical high C levels on both debonded sides, confirming a failure within the adhesive layer (cohesive failure), i.e., the best possible performance in a given adherend-adheslve system. This result is similar to that obtained using a 2024 A1 alloy prepared by the phosphoric acid-anodization (PAA) process (16) and indicates the importance of monolayer NTMP coverage for good bond durability (Fig. 4). 

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... 

Cohesion fracture Failure of a bonded joint due to fracture in the adhesive layer. 

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. 

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... 

With vacuum treatment of the specimens (at 200°C, 103 Pa) for 10 days, the content of the monomer in the adhesive layer decreased and the adhesion strength increased. The character of adhesion failure of the joint changed to cohesive type. The data allow a more complete explanation of the so-called latent period— the period of achieving the maximum adhesion strength. It is evident that the strengthening of some adhesive-bonded joints in the course of time is caused by volati-... 

If the adhesive layer was always to represent the weakest link in the joint, the quality of the assembly would be limited to an assessment of the cohesive properties of that layer. Unfortunately it is the interface which tends to be of greater concern because it is here that joint failure is more commonly encountered, particularly after ageing. The most useful tests for indicating interfacial quality are in fact pre-bonding surface inspection procedures, which can nevertheless suggest potential joint performance. The tests used for assessing cohesive properties of the adhesive layer must necessarily be post-bonding activities. 

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... 

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

Two aspects of the effects of environmental aging deserve mention here. The first concerns the locus of joint failure, i.e., the path followed by the fracture surface during the breaking of the joint. If structural adhesive joints are prepared correctly, then failure invariably occurs by cohesive fracture through the adhesive layer. However, it is commonly found that after environmental exposure the locus of failure is at, or very close to, the adhesive/adherend interface.As pointed out by Kinloch, this change in... 

In this case, cracks run along the interface between two materials due to interactions between the stress field in the adhesive layer and spatial variations in fracture properties. The cracks are not generally free to evolve as mode I cracks, as was the case for cohesive cracks, and mixed-mode fracture concepts (combinations of tension and shear) have to be considered. Mode II or shear components are induced, even in what appear to be nominally mode I loadings, due to differences in moduli about the interface. Again, if the presence of the adhesive layer is being ignored and the adherends are dissimilar, then a crack appears to be adhesive (i.e. an adhesion failure) on the macroscopic scale. 

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