Determination of plastic strains from compression tests including the effect of change of Young's modulus

Under compressive loading piezoelectric material shows an interesting change of Young's modulus with increasing compressive strain. This effect is of importance for the determination of plastic strain contributions of stress-strain curves in compression tests. Two different possibilities are considered to define plastic strain contributions. To obtain Young's modulus, partial unloading tests were per-formed on a soft PZT material. The applied load was measured with a DC load cell and the related strains were obtained by a pair of strain gauges. In addition to the continuously increased external load small pulse-shaped partial unloadings of about I MPa were superimposed. Young's modulus was found to be variable during the mechanical tests and increased from 40-70 MPa (depending on the poling state) up to about 130-150 GPa.