Cohesive-type failure

The adhesive failures can be categorized as adhesive type and cohesive type. In adhesive type, the failure occurs along the interface of the adhesive and the adherent in, while in cohesive type, failure is contained within the adhesive layer. Cohesive failure occurs more commonly in the heterogeneous materials than neat polymer adhesives. 

The use of Auger and XPS analytical techniques enabled the locus of failure to be determined. This was found to be interfacial between the metallic oxide and the epoxy adhesive at the edges of the joint, becoming cohesive within the adhesive layer towards the center of the joint. The relative amounts of interfacial and cohesive type failure depended on the length of time that the joint was immersed in water. 

Unstable damage development of thick bond-lines (lower plastic strain development) resulted in adhesive cracking in multiple locations with a cohesive-type failure and lower failure strengths. 

Before pursuing the implications of this point further, consider first the related case of fracture between two dissimilar materials. In contrast to the former case in which the fracture separated the same material, thus leading to a cohesive type failure, this debonding of different materials may be described as adhesive fracture. From the point of view of continuum mechanics, the problem and energy balance are essentially the... 

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-... 

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. 

The COD can be obtained by measuring the crack opening displacement between two points (one above and one below) at the location of the crack tip in the unloaded state [176]. Fuchs and Major [177] described that cohesive stresses can be evaluated by taking the first derivative of J-integral with respect to COD (5) for Mode-I type failure. 

It is for this reason that the discovery by Ulrich was of significant importance to the successful development of acrylic PSAs. He found that by copolymerizing polar monomers, such as acrylic acid, one could greatly increase the cohesive strength of the polymer allowing PSA articles coated with this type of material to sustain a load without premature shear failure. These polar monomers commonly... 

Knife test (KNF) The test is done by making two intersecting scratches through the paint film to the substrate with a sharp steel knife. Adhesive or cohesive failures are evaluated by peeling the coating from the intersection point and outwards. Common for the three adhesion evaluation methods are that the test is performed on immersed and non- immersed panel-half (referred to as respectively "wet" and "dry" adhesion). The type of rupture is reported, and the severity is judged on a scale from 5 (perfect) to 0 (poor). 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

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. 

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