Damage Assessment of Structures VIII

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Authors: Jin Ji Gao, Wei Min Wang, Li Dong He, Hong Xu
Abstract: Animals possess the function of self-recovery for adapting the change in their living conditions. Can machinery, by bionic design, just like animals, also possess the function of self-recovery for making itself free of fault, or for eliminating fault automatically during operation? In this paper, a new theory of Engineering Self-recovery is discussed, which could provide theoretical basis and new methodology for creating a new generation of machinery with fault self-recovery function. The engineering self-recovery theory is different from engineering cybernetics. Engineering cybernetics endows machinery with purposive behavior which originally is one of animal’s common characteristics, while engineering self-recovery theory endows machinery with the new function of fault self-recovery which originally is another one of animal’s common characteristics. Under the guidance of engineering cybernetics, the automation of machinery has come true. While under the guidance of engineering self-recovery theory, a new generation of machinery with the new function of fault self-recovery could also be realized. The self-recovery system, which can provide fault self-recovery force to restrain fault force, is investigated in detail. Based on fault mechanism and risk analysis, and by bionic design, a fault self-recovery system, which is a dynamic system to store, supplement and transfer the self-recovery force, is endowed to a machine with the ability to maintain the machine in a health state. Also, the research in engineering application of the new concept, fault self-recovery, is on the way. As an example, the new concept centrifugal compressor with fault self-recovery function is discussed here with axial displacement and flow induced vibration fault self-recovery as examples to show the steps of fault self-recovery system construction such as fault mechanism, necessity, determination, possibility, fault tolerance, limitation and execution.
Authors: Karen M. Holford
Abstract: Structural Health Monitoring (SHM) is of paramount importance in an increasing number of applications, not only to ensure safety and reliability, but also to reduce NDT costs and to ensure timely maintenance of critical components. This paper overviews the modern applications of acoustic emission (AE), which has become established as a very powerful technique for monitoring damage in a variety of structures, and the new approaches that have enabled the successful application of the technique, leading to automated crack detection. Examples are drawn from a variety of industries to provide an insight into the current role of AE in structural health monitoring.
Authors: Bing Li, Jie Zhuo, Zheng Jia He
Abstract: This paper presents the application of the wavelet finite element methods (WFEM) and neural network to crack parameters estimation and discusses the accuracy and efficiency of this method. The crack is presented by a rotational massless spring, and the natural frequencies for various crack parameters (location and depth) are obtained through WFEM. The neural network is then applied to establish the mapping relationship between the natural frequencies and the crack parameters, which uses feed-forward multiplayer neural networks trained by back-propagation, error-driven supervised training. With this trained neural network, the crack location and depth is estimated through using the measured natural frequencies as the input. The results of a cantilever beam experiment indicate that the estimating error of crack location is less than 3%, and the error of crack depth is less than 2%.
Authors: Luis Baeza, Hua Jiang Ouyang
Abstract: This paper presents a method for the vibration of a beam with a breathing crack under harmonic excitation. The infinitely thin crack is characterised by a parameter that takes into account the shape and the depth of the crack. The closed- and open-crack states are both modelled by a modal approach: two sets of equations of motion cast in the modal coordinates of their individual mode shapes. The state change (from closed to open or vice versa) involves the calculation of the modal coordinates associated with the new state from the modal coordinates of the previous state. By imposing the continuity of displacement and velocity the beam at the instant of the state change, the matrix that transforms the modal coordinates from one state to the other is determined and proved to be the Modal Scale Factor matrix. This analytical approach takes advantage of exact nature and mathematical convenience of beam modes and is time-efficient. Forced vibration at various values of crack parameter is determined. It is found that as decreases (crack length increases) the vibration becomes increasingly erratic and finally chaotic.
Authors: Xin Long Chang, Xiang Yong He, Bin Jian, Zheng Liang Li
Abstract: The fiber reinforced composite materials were widely used for aerospace aircrafts and missile weapons, and the delamination was a major problem which reduced the structural integrity and reliability of the solid rocket motor (SRM) composite shell seriously. In order to locate the delamination damage, the fiber Bragg grating (FBG) strain sensors network was embedded in SRM shell, and a multi-step approach of delamination damage location based on strain energy was performed: the strain field was measured in a scatter grid by the FBG strain sensors network in hydraulic testing; then the continuous displacement and strain field was reconstructed with relative sensors data using a moving least square (MLS) mesh-free fitting method; the strain energy of each subregions was calculated from the reconstructed data; finally the damaged subregions was identified successfully by singularity value of strain energy. The results of simulation and experiment indicated that the damage identification and location only required the measure of strain field of the SRM shell, and the presented approach achieved higher accuracy.
Authors: Pawel Kudela, Wiesław M. Ostachowicz, Arkadiusz Zak
Abstract: The aim of this paper is development of an algorithm for damage localisation in composite plates based on wave propagation signals registered by sensors. It is proposed to distribute the sensors uniformly over the area of a plate-like structure performing triangulation. Next the registered signals are processed and a damage influence map is created in each triangle separately in order to avoid problems connected with reflections from boundaries of the structure. The proposed procedure has been verified on numerical signals as well as experimental signals.
Authors: Dan Sheng Wang, Dan Yan Shen, Hong Ping Zhu
Abstract: Structural crack identification has been received considerable attention in recent decade. A lot of different techniques like acoustic emission, ultrasonic, or X-ray, etc have been used for structural crack detection. However, it is still difficult to identify the small crack in structures. A new method for identification of small crack in beam structures using the first anti-resonant frequency curve is proposed in this paper. The method makes use of the driving-point mechanical impedance characteristics of beam structures and a simplified rotational spring model to model the edge crack of beam. After the first anti-resonant frequency curve is obtained, signal process based on wavelet transformation will be carried out and the small crack in beam structures can be explored. The proposed method is validated by a numerical example of cracked beam with pinned-pinned or fixed-free boundary. It is concluded that not only the location of beam crack can be determined, but also the extent of crack damage can be identified qualitatively based on the first anti-resonant frequency curve and wavelet analysis.
Authors: Xiao Qiang Chen, Hong Ping Zhu, Dan Sheng Wang
Abstract: In this paper, a new time-domain method for detecting structural local damage has been developed, which is based on the measured strain signals. The “pseudo strain energy density (PSED)” is defined and used to build two major damage indexes, the “average pseudo strain energy density” (APSED) and the “average pseudo strain energy density changing rate” (APSEDR). A probability and mathematical statistics technique is utilized to derive a standardized damage index. Afterwards, these indexes are used to establish the damage identification strategies for beam structures and plate structures respectively. Furthermore, the wavelet packet transform is used to pre-process the measured dynamic strain signals. Then, the effectivity of the new damage identification method is confirmed by numerical simulations. Finally, a laboratory beam model experiment is conducted to verify this method examine the feasibility and applicability of the new method.

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