Keywords: Lamb waves, time reversal, bolted joint, loosened bolt, piezoelectric ceramic materials, finite element method, metallic plates.

This paper proposes a time reversal Lamb wave technique which uses smart piezoelectric ceramic (PZT) materials as sensors or actuators to assess the problem of bolt load detection in metallic joints. The main idea, here, is that, since the classical time reversal method is based on the linear reciprocity of the system, for a linear elastic structure, any re-emission of a time reversed signal back to the original source location generates the reemitted waves such that the signal reconstructed at the source location should be qualitatively identical to the original signal emitted. When nonlinearities are introduced into the signal path, the linear reciprocity of the system breaks down and the time reversal fails. However, the repetitive collisions between contacted surfaces in a loosened bolt joint which are non-perfectly touching, generate a distortion of the probing elastic Lamb wave propagating through the bolted joint. In this case, the bolt is free to vibrate within the hole letting the contact surfaces vibrate relatively; Because of those free vibrations, the proposed bolt loosening detection technique is based on the premise, that if the bolt is loosened the symmetry of the reconstructed signal would break down.

To quantify the deviation of the symmetry condition of the main mode of the reconstructed signal from the original input signal an index, called the SYM index, based on the required symmetry of the reconstructed signal is defined and investigated at each driving excitation frequency. After numerically computing of the SYM index for the case of the plate model loaded without and without any bolt torque (cases of a loosened or tightened bolt) and taking into account their respective dynamic response one can observe that the time reversibility breaks down significantly when in the way of the reversed field are any nonlinearity defect which leads to a drastic change of the convergence of the individual wave rays.

Thus, this paper numerically investigates the performance of the Lamb wave time reversal imaging technique for bolt load loss detection using finite element models of critical joints in metallic structural components. Different conditions were imposed to simulate real-time bolt load loss and prove the efficiency of the present time reversal imaging method in tracking and monitoring the integrity of critical metallic bolted joints.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £130 +P&P)