A Novel Approach for Preload Monitoring in Bolted Connections Using Electro-Mechanical Impedance Spectra

The functionality of bolted connections depends significantly on the conservation of the bolt’s preload, which entails the necessity to monitor these components. Traditional monitoring techniques using electro-mechanical impedance spectra are based on the observation of the structural resonances and antiresonances. The change of structural resonances is clearly mirrored for example in the resistance spectrum. This paper introduces a novel approach for such a monitoring process utilizing the susceptance. The susceptance is the imaginary part of the impedance’s reciprocal. In an experimental setup, where piezoelectric wafer active sensors have been attached to the bolt’s head, different preload stages of these components were set. It has been observed that the elastic mechanical deformation of the bolt’s head, which is caused by the preload, affects the piezoelectric wafer active sensor. As the properties of the sensor are well pronounced especially in the susceptance spectrum, a condition index based on the susceptance is proposed. Usually varying ambient temperature can occur in practical applications of bolted connections. It needs to be differentiated from the changes of the preload conditions in order to avoid false alarm of the monitoring system. Therefore, the proposed condition index has been additionally investigated in dependence on varying ambient temperature. The experiments show very promising results.