  We performed dynamic pressure buckling experiments on defect-seeded spherical
shells made of a common silicone elastomer. Unlike in quasi-static experiments,
shells buckled at ostensibly subcritical pressures (i.e. below the
experimentally-determined load at which buckling occurs elastically), often
following a significant time delay. While emphasizing the close connections to
elastic shell buckling, we rely on viscoelasticity to explain our observations.
In particular, we demonstrate that the lower critical load may be determined
from the material properties, which is rationalized by a simple analogy to
elastic spherical shell buckling. We then introduce a model centered on
empirical quantities to show that viscoelastic creep deformation lowers the
critical load in the same predictable, quantifiable way that a growing defect
would in an elastic shell. This allows us to capture how both the critical
deflection and the delay time depend on the applied pressure, material
properties, and defect geometry. These quantities are straightforward to
measure in experiments. Thus, our work not only provides intuition for
viscoelastic behavior from an elastic shell buckling perspective, but also
offers an accessible pathway to introduce tunable, time-controlled actuation to
existing mechanical actuators, e.g. pneumatic grippers.
