The critical role of fracture in determining the adhesion strength of electroadhesives

The force capacity of an electroadhesive is typically calculated as the integration of the Maxwell electrostatic stress over the contact area. However, deformation of the electroadhesive and/or the substrate can lead to stress concentrations at the interface, and the detachment can happen in a progressive manner due to crack propagation. This paper studies the role of fracture in the analysis of electroadhesives. Crack propagation and detachment are investigated for a cylindrical, homogeneous electroadhesive electrostatically adhered to a rigid conductive substrate. For a given friction coefficient and Poisson's ratio, the ratio of the effective adhesion strength of the electroadhesive to its intrinsic strength predicted by electrostatics alone is controlled by a nondimensional parameter xi that is a function of material properties, electroadhesive geometry, and the applied electric potential. The electroadhesive only achieves its intrinsic adhesion strength when xi is smaller than a critical value xi(c), which depends on the friction coefficient (mu) and weakly on Poisson's ratio (nu). The strength is lower than the intrinsic adhesion strength when xi > xi(c). For v = 0.48, xi(c) = 5 when mu = 1, xi(c) = 7 when mu = 0.5, and xi(c) = 37 when mu = 0.2. This study highlights the importance of considering a fracturetype failure when designing electroadhesives, and is relevant to applications of electroadhesives in robotic grasping, haptics, and microtransfer printing.