Chapter 8.4.2 - Examination of the Joint in the Field

The eye is the first “instrument” that should be used to determine the cause of adhesive failure. It can be very informative and can help focus attention on the areas to be tested. A trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area even if the substrate is not transparent or translucent.

When examining a failed joint, one should study both the failed area as well as unfailed areas. Often the joint appearance around an unfailed joint will provide information concerning the processing of the joint. Unfailed joints can also be subjected to rather simple proof tests to determine their level of strength retention. It is important to determine whether these unfailed joints have degraded and are just hanging on or if the problem is limited only to the failed joint. It may be necessary to perform a tap or proof test the unfailed joints to determine if the cause of the problem is widespread. These test methods were described in Sec. 8.3.1.

The previous section and Table 8.4 discussed the characteristics of joints that can be detected nondestructively. Table 8.9 shows visual characteristics of failed joints, their cause, and remedy.

The most important visual observation will be to look at the mode of failure of the failed joint. Joints may fail in adhesion or cohesion or by some combination of the two. Adhesive failure is an interfacial bond failure between the adhesive and adherend. Cohesive failure occurs when the failure is such that a layer of adhesive or sealant remains on the adherend. When the adherend fails before the adhesive, it is known as a cohesive failure of the adherend. The various modes of possible bond failures were shown in Fig. 1.2.

Cohesive failure within the adhesive or one of the adherends is sometimes considered a preferred type of failure, because the maximum strength of the material in the joint has been reached. However, failure mode should not be used as a sole criterion for a useful joint. A cohesive type of failure does not necessarily insure a practical or economic assembly or one that will even survive the expected product life. The ultimate strength of the joint is a more important criterion than the mode of joint failure. However, an analysis of failure mode can be an extremely useful guide to determine if the failure was due to a weak boundary layer or improper surface preparation (see Table 8.10).

An inference to the nature of the failure can be made by examining the failure mode. For example, if the adhesive failure is interfacial, the bulk cohesive strength of the adhesive material can be assumed to be greater than the intermolecular strength of adhesion. If the overall joint strength is not sufficient, the user needs to address the “weakest link”—probably the surface condition of the substrate. Improving cohesive strength of the adhesive or sealant would not add to the quality of the joint in this case.

If, on the other hand, the failure exhibits a cohesive mode (either the adhesive or the adherend), then the bond strength is stronger than the forces holding the bulk together. To improve the characteristics of this joint, one must look at the bulk adhesive or sealant material. In practice, however, one usually cannot improve the adhesive and keep the interphase area untouched.

The eye is the first “instrument” that should be used to determine the cause of adhesive failure. It can be very informative and can help focus attention on the areas to be tested. A trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area even if the substrate is not transparent or translucent.

When examining a failed joint, one should study both the failed area as well as unfailed areas. Often the joint appearance around an unfailed joint will provide information concerning the processing of the joint. Unfailed joints can also be subjected to rather simple proof tests to determine their level of strength retention. It is important to determine whether these unfailed joints have degraded and are just hanging on or if the problem is limited only to the failed joint. It may be necessary to perform a tap or proof test the unfailed joints to determine if the cause of the problem is widespread. These test methods were described in Sec. 8.3.1.

The previous section and Table 8.4 discussed the characteristics of joints that can be detected nondestructively. Table 8.9 shows visual characteristics of failed joints, their cause,...