Measuring the direct piezoelectric charge coefficient for polymer matrix composites

To realize proper measurements of the piezoelectric charge constant d33 of materials a certain mechanical load is required. Usually, this implies using a dynamic load (to evaluate the material's response) superimposed on a static preload (to achieve good mechanical contact). As the value of d33 depends on the applied stress, this preload can lead to load dependence of the experimentally determined value of d33. For stiff materials, the effect should be negligible but for compliant materials, such as composites using a low modulus matrix, a considerable contribution can arise, and thus the experimental data should be corrected accordingly. In order to characterise d33 accurately, the load dependence of d33 for several compliant polymer matrix composites was determined. The results show that for these composites correction to zero load is a prerequisite for accurate d33 evaluation.