Page 191 - Handbook of Adhesives and Sealants

Standard Test Methods 161

Figure 4.17 Typical creep-time curve for a lap shear adhesive joint bonded with a visco-
elastic adhesive. 4

is a substantial increase within the first time period followed by a
gradual increase with time. Once the load is removed at the conclusion
of the test, there will be a gradual recovery of strain with time. Since
the capacity of adhesive to recover will vary, the basis on which to
judge and compare creep behavior should not be the total strain at
the conclusion of the test, but the irrecoverable strain or permanent
set. The strain of an adhesive increases with thickness of film, and
hence the creep per unit thickness is the recommended measure of the
creep of an adhesive.

4.4.8 Environmental tests
It is desirable to know the rate at which an adhesive bond will lose
strength due to environmental factors in service. Strength values de-
termined by short-term tests do not always give an adequate indica-
tion of an adhesive’s performance during continuous environmental
exposure. Laboratory-controlled aging tests seldom last longer than a
few thousand hours. To predict the permanence of an adhesive over a
20-year product life requires accelerated test procedures and extrap-
olation of data. Such extrapolations are extremely risky because the
causes of adhesive-bond deterioration are complex. Unfortunately no
universal method has yet been established to estimate bond life ac-
curately from short-term aging data.
Adhesives and sealants may experience many different and exotic
environments. Laboratory environmental aging is accomplished by ex-
posing a stressed or unstressed joint to simulated operating condi-