Application of the electro-mechanical impedance method for the identification of in-situ stress in structures

Abstract

In the beginning, the electro-mechanical (EM) impedance method for structural health monitoring was recognized as a means of structural in-situ stress monitoring and measurement. Consequently, theoretical analysis based on the EM impedance method as a tool for in-situ stress identification in structural members was presented. A dynamic impedance model derived from the Euler-Bernoulli beam theory was developed to investigate the influence of in-situ stress on the dynamic and electro-mechanical response of a smart beam interrogated by a pair of symmetrically bounded, surfacebonded piezoceramic (PZT) transducers. Numerical simulation was performed for a laboratory sized smart beam subjected to a multitude of axial loads at the ends. It was found that natural frequency shifts takes place in the presence of in-situ stress. Furthermore, these shifts, which are linearly related to the magnitude of applied load, is directly reflected in the point-wise dynamic stiffness response. However, in terms of the electro-mechanical response, which can be measured directly, the shift of peaks of the EM admittance signature is not directly indicative of the natural frequency shifts. This arises as an inverse problem in engineering, which cannot be deciphered using direct approach. Back calculation of the in-situ stress using genetic algorithm (GA) was proposed.

Influence of loading on structures actuated with piezoceramic transducers

“…These shifts are predominantly right-ward. Ong et al (2002) observed such similar trend in the numerical simulation of an axially loaded beam specimen. In their simulation, the beam specimen was axially loaded such that the surface bonded with the PZT transducer experienced a tensile effect.…”

“…Therefore, the observations of this study coincide with their study. Ong et al (2002) also noted that there is a shift in the natural frequencies of the smart system when the system undergoes in-situ stress. The shifts observed by them were proportionate to the magnitude of stress levels and are independent of the excitation frequencies.…”

“…When the magnitude of applied load increases, the magnitudes of the peaks/valleys in the susceptance signatures are "diminishing" ( Figure 5). An explanation for this phenomenon is that upon loading, the structural impedance increases and thus, under the same level of external excitation, lesser energy is dissipated through the various structural modes, resulting in lower average resonant amplitude (Ong et al 2002).…”

“…Experiments were carried out and in-situ tensile stress was successfully identified. Ong et al (2002) investigated the effects of in-situ stresses on the frequency response functions of beam and plate structures. In their study, the case of pure bending actuation which was achieved by a pair of symmetrically surface bonded PZT transducers was considered.…”

Bolt Looseness Detection Based on Piezoelectric Impedance Frequency Shift

“…Equation (1) shows that the output admittance signature is affected by both the structural and sensor dynamics, since the admittance is the response of the structure to the harmonic actuation of the piezoelectric transducer and the applied load will influence the response of the structure in the presence of mechanical impedance. The stress changes at a specific location on the structure will cause resonant frequency shift, and vibration mode change of the structure, which will be reflected in changes in the admittance signature [37,38].…”