A phase shifting polarizing speckle interferometer is presented and applied to the analysis of the displacement field of composite sandwich materials submitted to indentation tests. The phase shift is obtained by a pockels cell and we present a calibration method based on a least squares fitting. The systematic error induced on the phase measurement by both the random fluctuations of the calibration parameter and the polarizing beamsplitter is investigated. It is shown that this systematic error depends sinusoidally on the random reference phase of the speckle field and on the phase to be measured and is closely related to both the statistics of the calibration parameter and those of the extinction ratio of the polarizing beamsplitter. The in-plane displacement field of a PVC foam core is measured over a total range of 1 mm. The experimental results obtained with the speckle technique are then compared to the model developed from a finite element analysis.