A Lamb wave signal processing method for damage diagnosis in the plate structure

In this work, a damage location method for plate-like structures is developed based on Lamb wave signal processing using matching pursuit method, which is employed to decompose Lamb wave signals into a linear expansion of several chirplet atoms using a fast realization algorithm. The relationship between Lamb wave dispersion and the chirplet chirp rate is established, which can be used to identify the modes of Lamb waves. Then a method for damage location is developed based on the difference between the baseline and the damaged signals. The effectiveness and accuracy of the proposed method in identifying the modes and in locating defects are demonstrated through experimental tests on the isotropic plate structure and honeycomb sandwich composite structure

Nonlinear ultrasonic test using PZT transducer for crack detection in metallic component

A crack detection technique based on nonlinear ultrasonic is developed in this study. Acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by a sensing PZT wafer. Unlike an undamaged medium, a cracked medium exhibits high acoustic nonlinearity, which is manifested as harmonics in the power spectrum of the received signal. Experimental results also indicate that the harmonic components increase nonlinearly in magnitude with increasing amplitude of the input signal. The proposed technique identifies the presence of cracks by looking at the two aforementioned features: harmonics and their nonlinear relationship to the input amplitude. The effectiveness of the technique has been tested on aluminum specimens

Combination of time reversal process and ultrasonic tomography approaches for baseline-free damage diagnosis

Lamb wave time reversal method is a new and promising baseline-free damage detection technique for structural health monitoring. With this method, damage can be detected without baseline data. In the paper an online damage detection and identification method is presented using time reversal Lamb waves method and ultrasonic tomography for damage diagnosis of composites. The principle and features of the time reversal lamb waves in a composite plate have been introduced firstly. Then the time reversal method has been adapted to detect the local defects in composite plate structures by using active sensing system mounted on a composite plate to excite and receive Lamb waves. This method can identify the location and size of the damage in a composite plate quickly without relying on past baseline date. The image that indicates the damage can be obtained by the ultrasonic tomography algorithm. Experimental results demonstrate the applicability and effectiveness of the proposed method

Anti-Drug Campaigns and State Building : China's Experiences in the 1950s

China's anti-drug crusade in the 1950s successfully eliminated the centuries-long epidemic of opium abuse. This paper, using newly available materials, reconstructs a brief history of the campaign and answers the question that why the crusade was kept invisible to the outside world until the mid 1990s. By focusing on the ways the crusade was carried out in the historical contexts of the early People's Republic, it demonstrates that the crusade was a presentation of the Communists' desire for a "new China" identity and was an integral part of the state-building process. The second part of this paper challenges the common assumption that China succeeded in eradicating its drug problem in the early 1950s, and examines how and why prolonged drug suppression campaigns were conducted differently in the Yi and Tibetan areas in the mid to late 1950s. The paper argues that the application of different policies was consistent with the CCP's state-building efforts at that time

A Lamb wave signal processing method for damage diagnosis in the plate structure

In this work, a damage location method for plate-like structures is developed based on Lamb wave signal processing using matching pursuit method, which is employed to decompose Lamb wave signals into a linear expansion of several chirplet atoms using a fast realization algorithm. The relationship between Lamb wave dispersion and the chirplet chirp rate is established, which can be used to identify the modes of Lamb waves. Then a method for damage location is developed based on the difference between the baseline and the damaged signals. The effectiveness and accuracy of the proposed method in identifying the modes and in locating defects are demonstrated through experimental tests on the isotropic plate structure and honeycomb sandwich composite structure

Multi-damage detection in composite structure

In this paper a pre-stack reverse-time migration concept of signal processing techniques is developed and adapted to guided-wave propagation in composite structure for multi-damage imaging by experimental studies. An anisotropic laminated composite plate with a surface-mounted linear piezoelectric ceramic (PZT) disk array is studied as an example. At first, Mindlin Plate Theory is used to model Lamb waves propagating in laminates. The group velocities of flexural waves are also derived from dispersion relations and validated by experiments. Then reconstruct the response wave fields with reflected data collected by the linear PZT array. Reverse-time migration technique is then performed to back-propagate the reflected energy to the damages using a two-dimensional explicit finite difference algorithm and damages are imaged. Stacking these images together gets the final image of multiple damages. The results show that the pre-stack migration method is hopeful for damage detection in composite structures