Detection of Damage in Lightning Masts and Loosening of Bolted Connections in Structures Using Changes in Natural Frequencies
As a global structural damage detection method, a vibration-based method that uses changes in natural frequencies of a structure to detect the locations and extent of damage can in principle detect various types of structural damage with minimum test data, including damage that occurs at the joints and boundaries of the structure. However, it is a challenging task to practically detect damage in engineering structures such as space frames using the vibration-based method. The major challenges can arise from the forward problem, i.e., the creation of an accurate physics-based model for both the undamaged and damaged states of a structure, and the inverse problem, i.e., the development of a robust iterative algorithm to search for the locations and extent of damage. With the recent development of the modeling techniques for fillets and bolted joints by the authors, space frames with filleted thin-walled beams and bolted joints can be accurately modeled with a reasonable model size for both the undamaged and damaged states. The damage detection algorithm developed earlier for under-determined systems is improved here by introducing a logistic function transformation to convert the constrained optimization problem into an unconstrained one and a trust-region search method to solve the nonlinear least-square problem. The new iterative algorithm is ensured to converge to a stationary point of the objective function, the convergence speed is significantly increased, and the robustness of the algorithm is improved. A real lightning mast at an electric substation and a scale mast with machined sections in the laboratory can be accurately modeled using mainly shell and beam elements, respectively, and numerical and experimental damage detection is performed using the new damage detection algorithm. The modeling techniques for fillets and bolted joints and the new damage detection algorithm are used to numerically and experimentally detect loosening of bolted connections in a component of a space frame, which consists of two Lshaped beams with a bolted joint. The exact locations and extent of damage can be detected in the numerical simulation for both types of structures. With the errors between the measured and calculated natural frequencies within 2%, the locations and extent of both types of damage can also be successfully detected in the experiment.
KeywordsDamage Detection Space Frame Stiffness Reduction Modal Assurance Criterion Underdetermined System
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