Page 190 - Handbook of Adhesives and Sealants

160 Chapter Four

terial do not have time to slip by one another and absorb a great deal
of the energy. Special, high speed impact tests have been developed
for certain applications. These tests generally use chemical explosive
force or electromagnetic energy to establish the high speeds required.

4.4.7 Creep tests
In practical joints, adhesives are not always loaded for short periods
of times. Often the application requires that the adhesive joint survive
continuous loading or stress. The dimensional change occurring in a
stressed adhesive over a long time period is called creep. Creep data
are seldom reported in the adhesive supplier’s literature because the
tests are time-consuming and expensive. This is very unfortunate,
since sustained loading is a common occurrence in adhesive applica-
tions. All adhesives tend to creep, some much more than others. With
weak adhesives, creep may be so extensive that bond failure occurs
prematurely. Adhesives have also been found to degrade much more
rapidly when environmentally aged in a stressed rather than an un-
stressed condition. This phenomenon will be investigated more closely
in Chapter 17.
Creep-test data are accumulated by loading a specimen with a pre-
determined stress and measuring the total deformation as a function
of time or measuring the time necessary for complete failure of the
specimen. Depending on the adhesive, loads, and testing conditions,
the time required for a measurable deformation may be extremely
long.
ASTM D 2294 defines a test for creep properties of adhesives util-
izing a spring-loaded apparatus to maintain constant stress. With this
apparatus (Fig. 4.16) once loaded, the elongation of the lap shear spec-
imen is measured by observing the separation of fine razor scratches
across its polished edges through a microscope. A typical creep curve
of an adhesive bonded lap shear specimen is shown in Fig. 4.17. There

Figure 4.16 Compressive spring with tension creep-shear specimen. 4