Лако-красочные материалы — производство

Failures range from minor flaws to catastrophes, and examples of failures can be found everywhere. Classifying an event as a failure implies that a deficiency has been identified, and that an improvement is sought. To prevent future failures an analysis is required, because one does not learn from the damage itself but rather from a proper failure analysis. Failure analysis almost always arrives at the conclusion that fundamental rules have not been observed. However, such as conclusion does not help to prevent future failures, because the complex interaction of relevant factors is only revealed by the current failure itself. Failure analysis is therefore a necessary and integral part of each developing technology, as it will only be accepted when failures can be explained and their recurrence prevented.

With regards to the discussion of failures occurring with adhesive bonding technology, a unique issue must be highlighted. Failures, which occur in welded, soldered, riveted and screwed joints are well known and well understood. The causes of these types of failure include crevice corrosion, heat-affected zones, stress concentrations, material incompatibilities and other factors. Based on a wide experience, failures can be explained rather quickly and conclusively, and no longer cause a designer to question the joining technique itself. With structural bonding, however, failures bring the technology itself under scrutiny. The reasons for this are twofold: (i) due to relatively little experience with these technologies there is a deficiency in, or a lack of, training in structural bonding; and (ii) there is an especially inadequate understanding of bonding as an interdisciplinary subject where rules must be applied. In practice, this situation becomes clear when, for example, following an adhesive failure, a designer requests a ‘more suitable’ adhesive. Here, simply trying to find an alternative adhesive misses the point — that the quality of the surface treatment, the construction, or other deficiencies within the process are more likely to be the cause. Yet normally, these points are not questioned. A lack of understanding becomes transparent by the quest for a ‘better’ adhesive, as this thinking ignores the main advantage of adhesive technology — which is to provide the capability of joining a wide range of different materials. The adhesive, in its low — viscosity phase, makes this possible since each time it adjusts itself to the energeti­cally most advantageous condition according to the adherent surfaces available. Therefore, bonding is a universal and uncritical joining technique where the adhesive is the ‘good-natured partner’. In adhesive technology, the majority of failures are caused by a breach of specific conditions during creation of the joint — quality defects of the adhesives themselves are hardly ever the cause of failure in bonded joints.

The following case studies ofadhesive bonding failures illustrate the aspects ofthe complex interaction of different factors, not only when dealing with failures and their causes, but also in presenting the successful and highly demanding adhesive bonding applications which normally show no failures and which have been widely adopted because of their trouble-free service.

In the following sections we will demonstrate first, that failure analysis is essential in order to prevent failure recurrence. Such analysis can be performed by examining the failed parts yourself, or by learning from failures reported in literature. Second, the case studies will show that the failure of adhesively bonded joints can be explained conclusively. Despite its basic function for structural adhesive bonding, the phe­nomenon of adhesion is rarely in focus in the case of failure. Many years of failure analysis of bonded joints can be summarized with the statement that, in about 70-80% of the cases, damage was due to design deficiencies and other types of overloading. The saying that. .only adhesive bonding can save a bad design” is not completely unfounded, and it reflects the lack of sensitivity for other essential parameters. This is because, in only a few cases was the damage due to deficiencies identified in the bonding process, while in some cases it was due to an inadequate surface preparation, to problems with contamination, or to failure of the adhesive itself.

7.8.1