| Structural health monitoring systems for identifying deterioration and damage in many different kinds of engineering systems are receiving a considerable amount of attention in industry, academia and government laboratories. The envisioned system will help achieve condition-based maintenance and automated diagnostic and prognostic of civil infrastructures (e.g. bridges, buildings), transportation systems (e.g. aircraft, trains) and defense-related systems (e.g. rotorcraft, multi-functional armor).; In the past several decades, modal-based damage detection algorithms have been developed. By monitoring changes in measured structural vibration responses (natural frequencies and mode shapes), one can trace the changes in the structural parameters if structural damage occurs. Some of these methods have been successfully used on complex structural systems. However, several inherent drawbacks of modal damage detection methods have been pointed out. For instance, the modal parameters have low sensitivity to structure's global damage, especially incipient damage. Also, modal parameters are structure's global characteristics. Changes in these parameters may simply result from changes in boundary conditions, instead of structural damage.; An alternative approach, based on the concept of dereverberation is developed to improve the performances of vibration diagnostics. In this approach, a structure can be modeled as a network of traveling wave guides. Spectral representation is adopted to represent structural dynamics. By solving eigenvalue problems of transformation matrix, state vectors containing generalized forces and displacements are decomposed into local wave components. By considering force equilibrium and displacement compatibility conditions and choosing an appropriate control law, virtual controllers can be obtained to achieve matched impedance between elements or cancellation of wave components at elemental interfaces. Virtual Controllers can be applied in equation of motion to obtain the dereverberated transfer function response.; As poles and zeros being removed from conventional reverberated transfer functions, dereverberated transfer functions can be used to uncover the direct energy paths from a specific actuator to various sensors. The ratio of dereverberated transfer functions at sequential degree of freedoms represents structural local dynamics. Structural local damage tends to affect dereverberated transfer function ratio both in magnitude and phase angle. Studies show that, phase angle is more sensitive to damage than magnitude. A phase damage index method is formulated upon which the location, type and extent of damage can be determined. Experimental study on a scale-building model shows that this approach can locate as small as 1% stiffness loss. The ability of this approach to detect multiple types/locations of damage in 1-dimensional discrete structures has also been illustrated.; Sensitivity analysis shows that, the Dereverberated Transfer function based damage detection methodology has advantages because by introducing phase damage index, the sensitivity to damage is not only dependent upon several discrete natural frequencies as most, modal-based methods do, but on an accumulated effect over a broad frequency range. As compared to three model based modal methods (MRPT, SB-EBE, EADL), the dereverberated transfer function based method tends to be more sensitive to incipient local damage, more robust to measurement noises and loss dependent on a structure's physical model. |
