Page 181 - Handbook of Adhesives and Sealants

Standard Test Methods 151

Figure 4.5 Modified lap shear specimens used to maintain axial
loading: (a) single saw cut specimen, (b) double lap specimen. 4

sion shear test apparatus. The compression shear design also reduces
bending and, therefore, peeling at the edges of the laps. Higher and
more realistic strength values are obtained with the compression
shear specimen over the standard lap shear specimen.

4.4.3 Peel tests
A well-designed joint will minimize peel stress, but not all peel forces
can be eliminated. Because adhesives are notoriously weak in peel,
tests to measure peel resistance are very important. Peel tests involve
stripping away a flexible adherend from another adherend that may
be flexible or rigid. The specimen is usually peeled at an angle of 90
or 180 degrees. The most common types of peel test are the T-peel, the
floating roller peel, and the climbing-drum methods. Representative
test specimens are shown in Fig. 4.6. The values resulting from each
test method can be substantially different; hence it is important to
specify the test method employed.
Peel values are recorded in pounds per inch of width (piw) of the
bonded specimen. They tend to fluctuate more than any other adhesive
test result because of the extremely small area at which the stress is
localized during loading. Even during the test, the peel strength val-
ues tend to fluctuate depending on the type of adhesive, adherend,