  A layer of dielectric elastomer can be voltage actuated to behave as
actuators, but needs to avoid the electromechanical instability of excessively
thin down accompanied with electric breakdown. We develop a true method to
analyze the electromechanical stability of dielectric elastomers by adopting
the positive definiteness of the true tangential modulus matrix, and
demonstrate that the previous method is only valid for the special case of zero
prestress. Our new method is applicable for arbitrary prestress cases including
zero prestress, with predictions consistent with available experimental
measurements. Our theoretical results demonstrate the significant effects of
prestress on critical voltage and critical actuation stretches.
