Key Engineering Materials Vol. 518

Abstract: Acoustic Emission (AE) techniques are used for the structural health monitoring (SHM) of civil, aeronautic and aerospace structures. In order to depart from the traditional reliance on parameter based analysis, AE diagnostic techniques require the analysis of wave propagation phenomena and the use of predictive modelling tools to improve the monitoring capabilities and provide reliable health monitoring. Additionally, modal based techniques offer potential for optimization of sensor networks in terms of sensor placement and number of sensors, increased source location accuracy and to get an insight into the source mechanisms. If the modes of propagation can be recognised in the received AE signals, then it would be possible to discriminate between damage types. On that account, the present paper develops two methodologies that are useful tools for the investigation and design of wave propagation based SHM systems established upon modal analysis. Firstly, a higher order plate theory for modelling disperse solutions in elastic and viscoelastic fibre-reinforced composites is proposed in order to investigate the radiation and attenuation of Lamb waves in anisotropic media. Second, spectral flat shell elements are used for the simulation of guided waves in shell structures. Numerical simulations and experiments validate the models and demonstrate that material anisotropy has a strong influence on the velocities, attenuation and acoustic energy for the different modes of propagation. It is expected that the presented methodologies may contribute to offer a higher computational efficiency and simplicity in comparison to traditional methods, and enable the design shortening time and cost of development of Lamb wave based damage detection systems for a rapid transfer from laboratory to in-service structures.

Abstract: Light rail systems have now their great return in many European cities carrying an increasing number of people every year. This increasing trend requires suitable operation and maintenance standards for both vehicle and track. Furthermore, in order to make a public transport competitive to private transport, its very important to increase safety and ride comfort for passengers. The aim of the presented work was to determine the suitable vibration-based symptoms for the identification of a light rail vehicle technical state, as well as the development of appropriate methodology to use the information contained therein. Both simulation and experimental phase are described. The present analysis is focused mainly on the suspension state monitoring, but some others failures were also considered.

Abstract: The paper investigates practical aspects of vibrothermographic testing of structures. Two application cases of vibrothermography are presented and discussed. The examples are weld test specimens and a military aircraft skin. Measurements have been performed with use of an in-house vibrothermographic testing system. In case of welded specimens a series of carbon steel test samples with different flaw types have been investigated. In case of aircraft testing, field measurements have been performed on a wing and fuselage sections in order to assess their structural integrity. The paper presents thermal image processing technique that allows to increase the quality and readability of the results coming from field measurements with poor ultrasonic excitation and with low thermal response from the structure. The paper is concluded with the discussion on applications of vibrothermographic testing in Structural Health Monitoring.

Abstract: The paper presents some results of research on the Weight-in-motion (WIM) system. The device is used for identification of loads on the road surface generated by traveling vehicles. The proposed approach utilizes the piezoelectric measurement techniques to monitor strain development in a deformable body and eventually these measurements are used for tire-pavement load identification. An advantage of the proposed concept is that no additional limitation for a vehicle velocity and direction is required in order to make the measurement feasible. The device allow to identify many parameters which can be stored for statistical and planning purposes. When an overload or an exceed in speed limit is detected the data can be sent for penalization purposes. The research includes a computer simulation of the bending plate detector using the Finite Element Method (FEM). Its objective is to validate the concept as well as to test some factors which are important with respect to the proposed load identification methodology. An experimental research involved field tests on the WIM system using a bending plate detector and inductive loops to detect a vehicle.

Abstract: Condition Monitoring of bearings used in Wind Turbines (WT) is an important issue. In general, bearings diagnostics is well-recognized field; however it is not the case for machines working under non-stationary load. An additional difficulty is that the Main Bearing (MB) discussed here, it is used to support low speed shaft, so dynamic response of MB is not clear as for a high-speed shaft. In the case of varying load/speed a vibration signal acquired from bearings is affected by operation and makes the diagnosis difficult. These difficulties come from the variation of diagnostic features caused mostly by load/speed variation, low energy of sought features and high noise levels. In the paper a novel diagnostic approach is proposed for main rotor bearings used in wind turbines. From a commercial diagnostic system two kind of information have been acquired: peak-to-peak vibration acceleration and generator power that is related to the operating conditions. The received data cover the period of several months, when the bearing has been replaced due to its failure and the new one has been installed. Due to serious variability of the mentioned data, a decision-making regarding the condition of bearings is pretty difficult. Application of classical statistical pattern recognition for data from the period A (bad condition) and the period B (after replacement, good condition) is not sufficient because the probability density functions of features overlap each other (pdf of peak-to-peak feature for bad and good conditions). Proposed approach is based on an idea proposed earlier for planetary gearboxes, i.e. to analyse data for bad/good conditions in two-dimensional space, feature - load. It is shown that the final data presentation is a good basis to the very successful classification of data (i.e. recognition of damaged and undamaged bearings).

Abstract: A project investigating the possibility to transmit electrical power (several mW) along the structure of an aircraft by using an intermediate medium of ultrasonic Lamb waves is being carried out at Cardiff University in conjunction with Airbus. This power supply method is aimed at wireless, surface-bonded sensor packages, primarily for the aircraft structural health monitoring (SHM) applications. It is expected to replace conventional batteries or energy harvesting devices. This paper presents methods of piezoelectric transducer characterisation, electric power measurement and electric circuit simulation that were developed in support of the ultrasonic power transmission project. The unique combination of low power and a high AC frequency range (up to 200 kHz) precluded the use of conventional power measurement instruments and called for a tailored system and software to be developed. Two approaches were developed: one relying on the measurement of the ultrasonic transducers impedance characteristics and their subsequent use in a circuit simulation; and another relying on the direct measurement of voltage waveforms in the power transmission setup. The two methods were found to be capable of producing closely matching results up to 300 kHz. Results of early power transmission trials are also presented. The optimum approach resulted in 1 mW of power transmission over a distance of 74 cm in a 1.5 mm thick aluminium plate.