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.

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