Ultrasonic detection of a plastic hinge in bolted timber connections

Connections between structural members are critical elements that typically govern the performance of structural systems; hence, techniques for monito ring the condition of connections are needed to provide early warning of st ructural damage. Plastic hinge formation in fasteners frequently occurs in timber connections when the yield capacity is exceeded. An innovative pulse echo testing technique was developed for detecting the formation of a plas tic hinge in bolted timber connections and estimating the associated magnit ude of connection displacement. A shift in overall signal centroid proved t o be the best predictor of plastic hinge formation, with a coefficient of d etermination (R-2) of 0.9. As the plastic hinge angle increased, the signal centroid shifted to the right since a higher proportion of pulse energy wa s forced to undergo multiple transverse wave reflections caused by the defo rmed geometry of the bolt. Because the determination of a shift in signal c entroid requires the availability of prior test information for the initial ly undeformed fastener, an alternate linear relationship between echo ampli tude ratios and plastic hinge formation was also proposed with an adjusted R-2 of 0.87. This three parameter regression equation had the advantages of requiring no prior testing information and eliminating ambiguity in signal analysis associated with selection of echo start and aid points. Plastic h inge formation was correlated with connection ductility, magnitude of conne ction overload and energy based measures of connection damage to assess res idual connection capacity.