Page 213 - Handbook of Adhesives and Sealants

184 Chapter Five

part will be loaded in service, but it should also be designed so that
the geometry and mass does not depart too severely from the actual
production part. The test specimen is then either tested immediately
after bonding or after a simulated environmental cycle.
After testing, the joint area and mode of failure should be examined
closely. This generally leads to clues that are indicative of problems.
The causes and remedies for faults revealed by such mechanical tests
and subsequent visual inspection are described in Table 5.2. Similar
visual techniques have been developed to determine the causes of con-
tact adhesive failures. 3
Test specimens such as those mentioned above are often used to
verify the quality of the first article through the production line and
then to periodically test articles for conformance. This type of testing
will detect discrepancies affecting the entire lot, but it cannot evaluate
factors that affect individual joints or specific areas of a particular
joint.

5.3.2 Non-destructive testing
Non-destructive testing (NDT) is usually far more economical than
destructive test methods, and every assembly can be tested if desired.
However, nondestructive testing primarily offers qualitative informa-
tion reguarding the strength of the joint. Several non-destructive test
methods are used to check appearance and quality of structures made
with adhesives or sealants. The main methods are simple ones like
visual inspection, tap, and proof testing. More advanced nondestruc-
tive monitoring such as ultrasonic or radiographic inspection is also
used in critical applications. The most difficult defect to find are those
related to improper curing and surface treatments. Therefore, great
care and control must be given to these processes.

5.3.2.1 Visual inspection. A trained eye can detect a surprising num-
ber of faulty joints by close inspection of the adhesive around the bond
area even if the substrate is not transparent or translucent. Close
examination of the visually apparent adhesive or sealant (generally
around the edges of the joint) can lead to useful conclusions. Unfilled
areas and voids can sometimes be detected by noting lack of adhesive
or sealant material. Misalignment of parts are readily visible. The
texture of the adhesive around the edges of the joint can also be a clue
to the effectiveness of the curing process and whether air was en-
trapped in the adhesive. The adhesion of the flash to the substrates
can be qualitatively measured by attempting to pry the flash away
from the substrate. This can indirectly be a measure of surface clean-
liness. Table 5.3 lists the characteristics of faulty joints that can be
detected visually, their cause, and possible remedies.