Sensitivity to Axial Stress of Electro-Mechanical Impedance Measurements

In-situ stress determination in structures without a reference value is a challenging experimental mechanics task, especially if it requires a non-destructive approach. One potential application of this task is the management of longitudinal loads in railroad structures. This paper examines the potential of the Electro-Mechanical Impedance (EMI) method to provide an estimation of axial stress in bar-like structures. The EMI method is completely non-destructive, as it simply involves bonding a piezoelectric element on the host structure and measuring the electrical admittance signature of the PZT-structure assembly in selected frequency bands. Analytical models are proposed to relate such admittance signatures to uniaxial applied stress, as a function of relevant parameters of the EMI monitoring system. These models compare favorably to experimental results obtained from an aluminum bar and a steel bar, where appreciable sensitivities of the EMI admittance terms are found under varying stress levels.