Piezoelectric Response of Polyvinylidene Fluoride to Combined Compression and Shear

摘要

In this project we performed inclined-impact wave propagation experiments to assess the shear-stress sensitivity of the piezoelectric polyvinylidene fluoride (PVDF) sensing element of the National Bureau of Standards (NBS) soil stress gage. No sensitivity to shear stress was expected. However, because of deficiencies in the general understanding of the piezoelectric response of PVDF, especially under large stresses and strains, direct measurements were necessary to confirm this expectation. In a set of four plate-impact experiments, we investigated one combination of stress wave amplitudes: a 4.5-kbar (450 MPa) compressive stress and a 0.22-kbar (22 MPa) shear stress. The results are ambiguous and inconclusive. However, we could detect no piezoelectric sensitivity of the PVDF to in-plane shear stress. From this observation, we make a tentative estimate that the piezoelectric sensitivity of the NBS PVDF sensing element to in-plane shear stress is less than 0.27 times its sensitivity to hydrostatic compression. We discuss the uncertainties in the experiments and review the outstanding questions concerning the piezoelectric response of PVDF. We suggest two experimental methods that could usefully augment the observations that can be made from inclined-impact experiments.