Structural damage identification using dynamic numerical models

In this paper a review of two main groups of structural damage identification methods by dynamic tests is presented. The first group is concerned with metallic thin structures damages or imperfections and the second one with reinforced concrete beam structures damages. 1. The first group is addressed to the detection of potential imperfections, fissures and cracks appearing in industrial machines, aeronautical structures and motor engines. They are typically metallic structures and the tests are carried under controlled environment conditions, such as in a laboratory. The application of body waves, and more often, guided waves, as Rayleigh and Lamb waves, as dynamic excitation in order to detect the damage, is described. The studied imperfections have been divided into three classes. • Cracks, related to the structural safety. They are penetrating a significant part of the plate thickness. • The second class of imperfections are small cracks or fissures, and they can be called partially penetrating ones because they are extended only to a small part of the plate thickness. Imperfections of this class are difficult to detect, because sometimes they can not be observed on the plate surfaces. • Finally, the third class of imperfections are the superficial cracks and they are more related to the durability of the structure than to its safety. These imperfections are more connected to structural protection to the environment, i.e. to protective painting and coating. Dynamic models used to detect the first class of imperfections have been Kirchhoff or Reissner bending thin plate. The crack detection can be achieved quite accurately by comparison between the first spatial derivatives of the mode shapes of the uncracked and cracked plates. Partially penetrating and superficial cracks have been identified by application as dynamic input of Lamb and Rayleigh waves respectively. The use of these guide waves seems to be a very promising technique for imperfection detection. However, computational problems appear. They are related to the small time step and the large number of the finite elements needed in order to reach a suitable accuracy level. 2. The second part of the paper treats a different group of dynamic identification and location of damage in civil and building structures. In particular the damage in reinforced concrete beams,typically used in bridge and building structures is studied. Detection procedures in this part differ of the first ones, because the existing structure is tested in the field and reinforced concrete is rather heterogenous material in comparison to metallic material. Normally, potential cracks are detected, during the free vibrations of the structure, by estimation of the changes either of its natural frequencies, or in its mode shapes or in the measure of its dynamic flexibility. However,in general, the differences of these values between uncracked and cracked beams are small and in some cases they can not be distinguished from the inherent measurement errors occurring during the tests. After reviewing several different models applied to crack detection, one based on the linear elasticity has been developed. In this model the cracks are assumed to remain open and the rest of the structure to behave elastically, Using this model a sensitivity analysis of the presence of cracks,depth and location, respect to the variation of the structure natural frequencies and modes shapes can be carried out. Using this approach a crack identification methodology is proposed. Finally,some possible modifications of the proposed methodology aimed to improve the accuracy and reliability of the obtained results are discussed.