Narrow Results

This paper presents a method for the identification of damage in plates based on the post-processing with wavelets of modal rotation fields. These modal rotation fields are obtained by use of a numerical model of shearography, which includes the simulation of noise in the data generated. The discrete wavelet transform was chosen because of its high sensitivity to perturbations in the modal rotations. Distinct damage scenarios, defined by regions where the thickness of a plate is reduced, are considered in this paper. A study on the differences in the natural frequencies and the changes in modal rotation fields due to the damage is carried out. The order of the B-spline wavelets used in the post-processing of the modal rotation fields is discussed. The damage detectability in terms of its intensity, the selected mode, and the type of rotation field and wavelet coefficient is also studied. Finally, a scheme for the damage detectability enhancement, in particular for multiple damage scenarios, is proposed.

This chapter describes the application of shearography, which is a non-contact, full-field, and high-resolution optical method, for the measurement of modal response of beams and plates and subsequent damage localization. A review of literature on shearography and related interferometric techniques for vibration analysis is also presented. Since shearography is based on speckle interferometry, the physical and mathematical bases of speckle phenomenon and wave interference are described. The main techniques available to evaluate the phase maps obtained from the speckle patterns are considered and illustrated. Different processes of filtering and unwrapping the phase maps, which must be carried out in order to obtain the gradients of the modal displacement fields and thus the modal rotation field, are also presented in this chapter. Case studies of damage localization using measurements of modal response with two distinct shearography systems are also reported and discussed. Both single and multiple damages in aluminum beams are created using a saw or a milling machine. In order to test the accuracy and effectiveness of the damage localization approach, several amounts of damages are considered. It is found out that very small damage can be localized using higher order derivatives of the mode shapes, namely the fourth order.