Delayed pressure buckling of viscoelastic spherical shells*

With fast-switching devices in mind, we performed dynamic pressure buckling experiments on defect-seeded spherical shells made of a common silicone elastomer. However, unlike in quasi-static experiments, shells buckled at ostensibly subcritical pressures (i.e. below the experimentally-determined elastic critical load), often following a significant time delay.
Imperfections are known to “knock down” the critical pressure in shells, but with geometry accounted for, we examine the materials more closely. Although silicone elastomers have earned their popularity largely due to their elastic behavior, they are inherently viscoelastic.
We rely on this viscoelasticity to explain our observations. The lower critical pressure threshold is predictable directly from the material properties, which is rationalized by a simple analogy to elastic spherical shell buckling. Further, we show that viscoelastic creep deformation lowers the critical load in the same predictable, quantifiable way that a growing defect would in an elastic shell, which we use to explain the delay time before buckling.
This work demonstrates a simple pathway to introduce tunable, time-controlled actuation to existing mechanical actuators, e.g. pneumatic grippers.