Assessing the quality of rivets by evaluating the complex valued electrical impedance of a piezoelectric ultrasonic transducer

摘要

A rivet 10 is inspected by sending an ultrasonic continuous wave to the rivet from a transducer probe 1. The probe 1 comprises a piezoelectric transducer element and a buffer rod (waveguide) which is acoustically coupled to the rivet head. A reference piezoelectric transducer element may also be provided. The rivet quality is evaluated by monitoring 13 variations in the complex valued electrical impedance or admittance of the transducer, which may be measured using a bridge circuit 12. The complex valued impedance, being a phasor in the impedance plane, can be rotated 14 and displayed at a two dimensional display 15 and a signal corresponding to one component of the phasor presented in the display can be fed to a threshold detector 16.. The method may be used to inspect self piercing rivets which join metal sheets on vehicles or aircraft.