Investigation of output voltage, vibrations and dynamic characteristic of 2DOF nonlinear functionally graded piezoelectric energy harvesterMahdieh M. Zamani, Mohammad Abbasi and Fariborz ForouhandehEur. Phys. J. Plus, 135 3 (2020) 298DOI: https://doi.org/10.1140/epjp/s13360-020-00276-0