Detection of bolt load loss in hybrid composite/metal bolted connections

Hybrid composite/metal connections are susceptible to bolt load loss due to viscoelastic creep and/or environmental effects. Accordingly, the focus of this research is on experimentally quantifying changes in bolt load of composite/metal hybrid connections. A proof-of-concept model was created consisting of a fiber reinforced composite panel bolted to a steel frame. A piezoelectric actuator bonded to the center of the composite panel was used to provide controlled vibration input. The response of the plate was measured using either shear accelerometers or dynamic strain sensors located at the four corners of the composite panel. The load on an instrumented bolt was decreased and three different monitoring techniques were used to detect bolt load loss, including (a) low frequency modal analysis, (b) high frequency transfer functions between the actuator and sensors and (c) high frequency transmittance functions between pairs of sensors. Experiments demonstrate that the transmittance function approach shows the most promise and was able to reliably detect a single bolt loosening. A damage index based on change in transmittance function is very sensitive to changes in bolt load.