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The Influence of Stress on Electro-mechanical Impedance Measurements in Rail Steel

Piezoelectric transducers have been used in a wide range of structural health monitoring and nondestructive testing techniques. Lead zirconate titanate (PZT) is the most commonly used piezoelectric. PZTs are a ceramic that can take on many forms, such as square and circular patches, which can be tailored to a specific application. For the most part, PZTs have been used for the detection of damage to a structural element. In recent history, the evaluation of the electro-mechanical response of the PZT in detecting damage and stresses on a structural element has been explored. The technique, known as electromechanical impedance (EMI), measures the variation of the PZT’s electronic signature due to the onset of damage or changes in the stress state. In this paper, the influence of stresses on the EMI signature for a structural element was tested. The specimen tested was a rectangular section of rail steel milled from a 136RE rail. Rail steel is a high strength steel with a very high stiffness that will have very high mechanical impedance, making it a difficult material for the EMI. In this experimental study, three measurements of the EMI response, in both the real and imaginary domain, of the PZT were compared in order to see which had the greatest response to the difference in the element’s stress state. The impact on the change of stress was tested through the root-mean-square deviation and piezo efficiency factor, the two most commonly used parameters in EMI measurements. Additionally, in order to get a perspective of the global change in the EMI signatures, the slope of the conductance and susceptance of the admittance was explored.

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