Modern multilayer piezoelectric actuators (MPAs) consist of stacks of piezoceramic layers with interdigitated metallic electrodes in between. The combination of thermal, electrical and mechanical loads during service may affect the structural and functional integrity of such electro-mechanical converters. Therefore, a deep knowledge of the coupling phenomena among the field-type quantities such as mechanical stress, electrical field strength and temperature is absolutely necessary. Theoretical simulations are the only way to obtain these (in detail unknown) physical relevant field quantities within MPAs. In this work, the constitutive laws describing the highly non-linear piezoelectric material behaviour are parameterised by experiments, in order to be implemented into FEA tools.