Papers by Author: Li Cheng

Abstract: There has been increasing awareness of the use of intuitional imaging techniques to describe a damage event in the engineered structures. A Lamb wave-based diagnostic imaging approach was developed in this study, by fusing the prior probabilities established by the sensors of an active sensor network at different spatial positions of the structure under inspection. Rather than pinpointing the damage location and shape with definitive parameters, such an approach was intended to probabilistically predict the occurrence of a damage event, which is in nature more consistent with the implication of ‘estimating’ damage in SHM than traditional approaches. As validation, the approach was employed to detect mono- and dual-delamination in CF/EP laminates, and the results were represented in probability contour diagrams, where the structural damage became intuitional. Other major benefits of the approach include the independence of its effectiveness on the number of damage and enhanced tolerance to noise/uncertainties.

Abstract: A quantitative evaluation approach for multi-delamination in fibre-reinforced composite laminates was developed, in virtue of a built-in active sensor network. The approach, conducted hierarchically by activating different sensors in a sensor network, fused time-of-flight (ToF) extracted from signals at different levels, whereby to provide an overall consensus as to all possible instance(s) of damage in the laminates. Benefiting from the network, the dependence of evaluation on a specific sensor or signal was furthest minimised, and the need for interpreting complex signal involving scattering by multi-damage was avoided. The approach was validated by assessing the locations and shapes of dual-delamination in a CF/EP woven laminate.

Abstract: A method based on entropy-based criteria is present to choose the optimal decomposition of Wavelet Packets Analysis (WPA) for damage detection in composite materials. The structural damage indexes constructed based on energy spectrum variation of the structural vibration responses decomposed using WPA before and after the occurrence of structural damage usually generate a complete binary tree to calculate its elements. Date mining is carried out in this paper by adoption entropy as the criteria to choose the optimal decomposition tree. In the decomposition process, only the sub-signals which contain main information of the original signal are decomposed to generate next level sub-signals. New damage index is constructed based on the optimal decomposition. Then the dimension of the damage index is reduced while still keeping its sensitive to damage. Whether Artificial Neural Network (ANN) or genetic algorithm (GA) is used in the further process of telling structural damage status from damage index, this reduction will make remarkable time saving.

Abstract: 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.

Abstract: A comparison of strain measurement results, from an embedded fibre-optic Bragg grating (FBG) sensor and surface mounted strain gauge, at different vibration frequency ranges and using a clamped-clamped glass fibre composite beam, is presented. It is shown that the FBG sensor is able to precisely measure the peaks at the first-two natural frequency modes compared with the spectrum captured from the strain gauge. The results also demonstrate that the strains measured from the FBG sensor agreed well with the strain gauge at frequencies below 100 Hz. Beyond this value, the actual strain on the beam surface was less than 3µe, and the data extracted from the strain gauge are no longer valid. For a clamped-clamped structure, the longitudinal strain of the beam correlates to its vibration amplitude and excitation frequency. Increasing the frequency results in decreasing the longitudinal strain of the beam and erroneous measurements from the strain gauge resulted. This study provides important information on the feasibility of using embedded FBG sensors as vibration monitoring devices to measure mechanical performance of composite structures.