Detecting loosening/tightening of clamped structures using nonlinear vibrationtechniques

Abstract

Ultrasonic waves are useful tools to characterize the contact forces between components innon-destructive and non-invasive manners. It has been shown that the transmission andreflection coefficients of the ultrasonic wave are sensitive to the contact pressure or othercontact parameters. Theoretically, the normal and tangential stiffnesses of thecontact interface govern the transmission/reflection coefficients and can be used asparameters to characterize the contact condition. However, weak and incompleteinterfaces, formed by rough surfaces in partial contact, show a highly nonlinearbehaviour also when they are excited under free vibrations. In particular, theamplitude of the second harmonic is a relevant index of the contact stiffness, and thenonlinear response is strongly influenced by the nominal contact pressure applied tothe boundaries. In this study a new theoretical model of the nonlinear interfacestiffness was developed where the stiffness of the contact interface was described asa function of the nominal contact pressure. The developed theoretical contactpressure function of the second harmonic generation at the contact interface wasfound to agree with good accuracy with the experimental data. Moreover, thispaper presents also a theoretical and experimental study aimed at developing anintegrity index capable of assessing the stiffness of the contact interface betweenstructures when excited by free vibration or under controlled vibration excitation.