Nonlinear electromechanical impedance spectroscopy: A powerful tool for studying amplitude dependent internal frictions of solids.

Strain amplitude dependent effects of materials/structures are very important in the field of material science and engineering and have been found to be extremely sensitive to defects or damage. In this work, a nonlinear electromechanical impedance spectroscopy (N-EMIS) technique is proposed to characterize the amplitude dependent internal frictions (ADIFs) and modulus defects (or resonance shift) of materials. First, a new experimental scheme called the on/off parallel resistor capacitor circuit is proposed to measure the N-EMIS of a piezoelectric transducer (PZT)-specimen composite system under high driving levels. Second, based on the N-EMIS, the formulas for calculating the ADIF are derived and validated by vibration measurement using a laser vibrometer. To further enlarge the strain amplitude, a PZT-stepped horn-specimen three-component system is then introduced, with which the maximum strain amplitude can reach 10. Finally, ADIF tests are conducted on polycrystalline pure copper and 1045-steel. The results show that at high strain levels, the internal frictions of both materials can reach several times than those at low driving levels, while the modulus drops only slightly. The proposed N-EMIS technique can effectively assess the strain amplitude dependent properties of materials and is expected to be widely used in the near future for evaluation of plasticity, fatigue, and damage.