Microwave method for the measurement of lateral strain in insulating materials

A rapid, non-contact and fairly accurate means for measuring lateral strain in insulating materials is presented. A cavity resonator operating at microwave frequencies serves as the sensing device. The rod-shaped sample under investigation extends across the cavity and the change in its cross-sectional area (under the effect of a tensile force) induces a shift in the resonant frequency of the cavity. From this frequency shift the change in the sample´s transverse dimension is obtained by applying the perturbation theory. The effect of a possible variation of the sample´s permittivity with the applied force is eliminated by carrying out the measurements with two different resonant modes. The frequency shift is determined either directly, from the recorded resonant curves, or by applying the transmission method. Experimental results for the dependence of the lateral contraction on the applied tensile force are presented for samples of polypropylene, nylon and plexiglas.