Cohesive failure object

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). 

An adhesive system should enable the parts of the object to be separated without damage. The adhesive should be strong enough to hold the object together when subjected to the predicted stresses in its application. Ideally, the joint should fail, either in service or during reversal, by cohesive failure within the adhesive or at the adhesive interface. For many applications, this is best achieved using a thermoplastic material that can be dissolved in solvents or melted by... 

The objective of surface treating is to obtain a joint where the weakest link is the adhesive layer and not the interface. Thus, destructively tested joints should be examined for mode of failure. If failure is cohesive (within the adhesive layer or adherend), the surface treatment is the optimum for that particular combination of adherend, adhesive, and testing condition. However, it must be realized that specimens could exhibit cohesive failure initially and interfacial failure after aging. Both adhesive and surface preparations need to be tested with respect to the intended service enviroiunent. 

The objective of this test method is to measure the cohesive stress and the time to failure of a crystalline polymer craze layer under rapid, uniform extension. The method is an impact variant of the Full Notch Creep test used by Fleissner [12], Duan and Williams [13], Pandya and Williams [14] and others. The specimen (Fig. 2), a square-section tensile bar, is injection moulded. At the mid-plane of the gauge length a sharp, deep circumferential notch reduces the cross-section to about one fifth of its original area. This notch plane is formed by a moulded-in, hardened steel washer. Specimens were injection moulded at 210°C into a warm (100°C) mould and air cooled to 40 C using a hold pressure of 45-50 bar. 

In the gouging process, a painted plastic part is hit by or hits a foreign object (often another painted part). Failure occurs cohesively within the sub-... 

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