When a dielectric material is subjected to an electric field, charges of opposite nature are induced on the two opposite surfaces of the material and hence a force of attraction is developed and the specimen is subjected to electrostatic compressive forces and when these forces exceed the mechanical withstands strength of the material, the material collapses. If the initial thickness of the material is d0 and is compressed to a thickness d under the applied voltage V then the compressive stress developed due to electric field is
where εr is the relative permittivity of the specimen. If γ is the Young’s modulus, the mechanical compressive strength is
Equating the two under equilibrium condition, we have
For any real value of voltage V, the reduction in thickness of the specimen cannot be more than 40%. If the ratio V/d at this value of V is less than the intrinsic strength of the specimen, a further increase in V shall make the thickness unstable and the specimen collapses.
The highest apparent strength is then obtained by substituting d = 0.6 d0 in the above expressions.
The above equation is approximate only as γ depends upon the mechanical stress. The possibility of instability occurring for lower average field is ignored i.e., the effect of stress concentration at irregularities is not taken into account.
