Monitoring of the preload in bolted connections with piezoelectric wafer active sensors

Piezoelectric wafer active sensors are very sensitive to changes of loads in structures, which makes them useful for the load monitoring. Unfortunately, these transducers are also sensitive to environmental effects such as a change of the ambient temperature. These effects can mask the information about the load in the recorded measurement data of the piezoelectric wafer active sensors. Therefore, the effect of the load needs to be distinguished from these masking effects of the environmental influences in order to avoid misleading information about the occurring loads of the monitoring system. In this work, two monitoring procedures, which are applicable with piezoelectric wafer active sensors, shall be applied in order to have two different sources of information and be able to distinguish between the load information and the temperature influence. One approach is based on electro-mechanical impedance measurements and the other one is based on acousto-ultrasonics. The strategy of combining these two information shall be used to monitor the preload in bolted joints. In an experimental setup, piezoelectric wafer active sensors have been attached to the bolt’s head and the face side of its shaft. Then, different preload stages were set. At every load stage, acousto-ultrasonics measurements and also electro-mechanical impedance measurements were carried out. First experiments show very promising results.