Page 189 - Handbook of Adhesives and Sealants

Standard Test Methods 159

Figure 4.15 Impact test specimen. [From ASTM D
950]

ent strength also increases. 16 This suggests that a portion of the en-
ergy required to rupture the bond is absorbed by the adhesive layer
and is independent of adhesion.
The impact strength of a viscoelastic adhesive is affected by the rate
of which the impact occurs. Often it is very difficult to achieve very
high rates of impact with conventional laboratory testing. One ex-
ample of this is an adhesive system used to bond shock mounting pads
to electrical equipment aboard submarines. Impact testing of speci-
mens done in a laboratory using pendulum or drop-weight impact fix-
tures showed that the adhesive would not fail under the loads ex-
pected. However, the proof test was to place the electrical equipment,
with shock mounting attached, aboard a barge and then set-off explo-
sive charges at various depths under the barge. The proof test showed
that the viscoelastic adhesive failed when explosive-induced high rates
of impact were experienced. The adhesive acts like a brittle polymer
at high rates of loading because the molecular chains within the ma-