Ultrasonic Lamb wave based monitoring of corrosion type of damage in plate using a circular array of piezoelectric transducers

In this paper we propose a concept and report experimental results based on a circular array of Piezoelectric Wafer Active Sensors (PWASs) for rapid localization and parametric identification of corrosion type damage in metallic plates. Implementation of this circular array of PWASs combines the use of ultrasonic Lamb wave propagation technique and an algorithm based on symmetry breaking in the signal pattern to locate and monitor the growth of a corrosion pit on a metallic plate. Wavelet time-frequency maps of the sensor signals are employed to obtain an insight regarding the effect of corrosion growth on the Lamb wave transmission in time-frequency scale. We present here a method to eliminate the time scale, which helps in identifying easily the signature of damage in the measured signals. The proposed method becomes useful in determining the approximate location of the damage with respect to the location of three neighboring sensors in the circular array. A cumulative damage index is computed from the wavelet coefficients for varying damage sizes and the results appear promising. Damage index is plotted against the damage parameters for frequency sweep of the excitation signal (a windowed sine signal). Results of corrosion damage are compared with circular holes of various sizes to demonstrate the applicability of present method to different types of damage. (C) 2011 Elsevier Ltd. All rights reserved.

Lamb wave based detection of damage in a stiffener bonded to a plate

Stiffener is one of the major components of aircraft structures to increase the load carrying capacity. Damage in the stiffener, mostly in the form of crack is an unavoidable problem in aerospace structures. Stiffener is bonded to the inner side of the aircraft panel which is not accessible for immediate inspection. A sensor-actuator network can be placed on the outer side of the panel that is accessible. Ultrasonic lamb waves are transmitted through stiffener using the sensoractuator network for detecting the presence of damages. The sensor-actuator network is placed on both halves of the stiffened section on the accessible surface of the plate. Detecting damage in stiffener by using this technique has significant potential for SHM technology. One of the major objectives of the present work is to determine the smallest detectable crack on the stiffener using the proposed technique. Wavelet based damage parameter correlation studies are carried out. In the proposed scheme, with increase in the damage size along the stiffener, it is found that the amplitude of the received signal decreases monotonically. The advantage of this technique is that the stiffened panels need not be disassembled in a realistic deployment of SHM system.

Lamb Wave Propagation in Laminated Composite Structures

Damage detection using guided Lamb waves is an important tool in Structural health Monitoring. In this paper, we outline a method of obtaining Lamb wave modes in composite structures using two dimensional Spectral Finite Elements. Using this approach, Lamb wave dispersion curves are obtained for laminated composite structures with different fibre orientation. These propagating Lamb wave modes are pictorially captured using tone burst signal.

Collecção das Leis do Imperio do Brasil de 1847

Parte 1 - Leis

连栽杉木(Cunninghamialancelata(Lamb)Hook)林中树桩分解过程中的化学组分变化趋势

采用以空间代替时间的方法,对连栽杉木(Cunninghamialancelata(Lamb)Hook)人工林中采伐剩余树桩的分解过程进行了研究.根据Olson的分解模型,计算出连栽杉木纯林中树桩的分解速率为0.02695,树桩在分解过程中的前2年是一个氮素增加积累过程,当杉木树桩的C/N在463.2±27.3时开始净氮释放.树桩磷含量的变化与氮变化模式相类似,但钾的表现不同,钾的含量在树桩分解过程中一直是单调降低.利用固体高分辨核磁共振技术结合魔角旋转(MAS)和交叉极化(CP)技术,研究了杉木树桩的腐殖化过程及碳结构的变化.树桩中由纤维素和半纤维素组成的多糖碳和乙缩醛被首先分解,而由蜡质和表皮素等化合物组成的烷基碳,由酚类、木质素、单宁和石蜡等化合物组成的芳香族碳,及由酯、有机酸、酮和醛等化合物组成的羧基碳的分解速度较多糖碳和乙缩醛慢.

A Sermon on the occasion of the death of Mrs. Mary Chapman Halsey

Memorial Sermon