L.H. Yam

Sensitivity studies of parameters for damage detection of plate-like structures using static and dynamic approaches

Abstract In this paper, the sensitivities of static and dynamic parameters to damage occurring in plate-like structures are investigated systematically, and corresponding damage indices are proposed to analyse their identification capabilities. For static analyses, damage indices are formulated using the out-of-plane deflection, and its slope and curvature based on a finite element model; for dynamic analyses, two damage indices related to the curvature mode shape (CMS) and the strain frequency response function (SFRF) are proposed. Compared with the existing results, the effects of the numbers of the selected modes and defective areas on the CMS index and the influence of the “natural frequency shift” on the SFRF index are investigated. Numerical simulations and experimental tests are performed to verify the identification capability of the proposed indices, and guidance on selecting the proper parameters for damage detection is given.

Identification of damage locations for plate-like structures using damage sensitive indices: strain modal approach

Abstract This paper addresses the problem of identification of damage locations for plate-like structures using strain mode technique. Based on the Rayleigh–Ritz approach, the strain modal analysis of a damaged plate is performed and strain mode shapes are consequently obtained. In light of the continuity condition and the residual strain mode shape technique, two novel damage sensitive parameters are proposed to determine the locations of damage, and corresponding damage indices are constructed. Compared with the conventional indices, they are simple and intuitive, and easy to be used in practice. Numerical simulations and experiments are carried out. Results show a good consistency and a strong identification capability of the proposed indices.

Vibration analysis of annular-like plates

Abstract The existence of eccentricity of the central hole for an annular plate results in a significant change in the natural frequencies and mode shapes of the structure. In this paper, the vibration analysis of annular-like plates is presented based on numerical and experimental approaches. Using the finite element analysis code Nastran , the effects of the eccentricity, hole size and boundary condition on vibration modes are investigated systematically through both global and local analyses. The results show that analyses for perfect symmetric conditions can still roughly predict the mode shapes of “recessive” modes of the plate with a slightly eccentric hole. They will, however, lead to erroneous results for “dominant” modes. In addition, the residual displacement mode shape is verified as an effective parameter for identifying damage occurring in plate-like structures. Experimental modal analysis on a clamped–free annular-like plate is performed, and the results obtained reveal good agreement with those obtained by numerical analysis. This study provides guidance on modal analysis, vibration measurement and damage detection of plate-like structures.

Nondestructive Detection of Internal Delamination by Vibration-based Method for Composite Plates

A method for locating internal delamination in multilayer composite plates is presented. A finite element method for composite plates is developed to investigate the relationship between delamination and change of modal parameters. The mechanism of the mode-dependent energy dissipation of composite plates is revealed by numerical analysis of delamination-induced variations of modal parameters. Experiments are carried out using piezoelectric actuator and accelerometer to measure the dynamic response to sinusoidal sweep excitation for several cantilever composite plates. The results show that the region of internal delamination in composite plates can be predicted according to the measured acceleration response and the computed modal strain energy.

Robust vibration control of uncertain systems using variable parameter feedback and model-based fuzzy strategies

Abstract In this paper, two control strategies are investigated for robust vibration control of parameter uncertain systems. Firstly, for the variable parameter feedback control, the region of the adjustable parameter r is determined rigorously and accurately so as to improve the robustness of the controller. Then, for the model-based fuzzy control (MBFC), the robustness can be achieved by updating the coefficient matrix Θ ( α ( k )) on-line. Compared with the conventional design, the derived formulae for the MBFC are for systems with multi-output rather than single output. Finally, some experiments on vibration control of a thin-plate system were carried out.

Identification of complex crack damage for honeycomb sandwich plate using wavelet analysis and neural networks

In this study, crack damage detection for a honeycomb sandwich plate is studied using the energy spectrum of dynamic response decomposed by wavelet transform and the artificial neural network (NN). The results show that taking the energy spectrum of the decomposed wavelet signals of dynamic responses as the inputs of the NN can simplify the NN design for structural damage detection and it possesses a high sensitivity to small damage. Experimental results also show that the NN designed in this study can accurately detect multiple damage parameters or give some significant reference range of the damage parameters.

Damage Detection of Composite Structures Using Dynamic Analysis

A study on the use of modal parameter analysis for damage detection of structures made of composites is conducted. The damage-induced variations of modal parameters are investigated both numerically and experimentally. An appropriate finite element model is proposed to analyze the dynamic characteristics of different types of structures made of composites, such as honeycomb sandwich plates and multi-layer composite plates, with internal cracks and delamination. The numerical results are in good agreement with experimental results available in the literature. Natural frequencies, modal displacements, strains and energy are analyzed for the determination of damage severity and location. Vibration measurements are carried out using piezoelectric patch actuators and sensors for comparison and verification of the FEM model proposed in this study. Energy spectrum for wavelet packets decomposition of structural dynamic responses is used to highlight the features of damaged samples. The mechanism of mode-dependent energy dissipation of composite plates due to delamination is revealed for the first time. Experimental results clearly show the dependence of changes of modal parameters on damage size and location. The results obtained in this study show that the measured modal damping change combined with the computed modal strain energy distribution can be used to determine the location of delamination in composite structures. Both numerical and experimental findings in this study are significant to the establishment of guideline for size and location identification of damage in composite structures.

Use of ambient response measurements to determine dynamic characteristics of slender structures

This paper deals with the experimental studies for determining the dynamic characteristics of slender structures by using the measured structural responses to ambient excitations e.g. traffic excitation or wind load. The theoretical background for random response of linear structures, the experimental method, data acquisition and analysis techniques for identifying the characteristics of the structures are expounded. On-site measurements of a 30 m tall lamp post structure under ambient ground motion and the excitation of wind of Beaufort force 6 were carried out. Natural frequencies and the correspondent modal shapes are obtained.

Numerical modeling of a damaged plate with piezoelectric actuation

In this paper, the numerical modeling of a damaged plate with piezoelectric actuation is presented. Unlike in previous studies, the effects of the defect are considered and embodied by mass and stiffness reductions in the equation of motion. The model is validated using indices related to frequency variation and energy change in both frequency and time domains. A comparison of the tendencies of the measured indices with those of the simulated ones shows a reasonably good agreement. Results show that the energy index is more sensitive to damage than the frequency index. Both square-wave and pseudo-random excitations can be used to assess the energy index variation. For the latter, however, an averaging over time signals is needed to provide more reliable results.

Vibration-Based Non Destructive Structural Damage Detection

Proceedings of the 11th Asian Pacific Conference on Nondestructive Testing, Jeju Island, 3-7 November 2003