34 WAVEFORM BASED ANALYSIS TECHNIQUES FOR THE RELIABLE ACOUSTIC EMISSION TESTING OF COMPOSITE STRUCTURES M. SURGEON, C. BUELENS, M. WEVERS*, and P. DE MEESTER* METALogic n.v., Heverlee, Belgium, *KU Leuven, Leuven, Belgium ABSTRACT Although the acoustic emission (AE) technique has now aged for more than thirty years, the analysis techniques that are used in practical field testing are still very basic. The last decade has seen a growing awareness of the dangers involved in reducing an AE signal to its basic parameters and thus eliminating most of the information. This awareness has resulted in the development of practical AE analysis techniques that are based on the complete waveform rather than on the parameters. These techniques also make use of a growing insight into the theoretical principles of AE signal generation and propagation, which is evidenced by e.g. the use of the classical plate wave theory. This paper will lay out the principles of the classical plate wave theory and will demonstrate how this simple theory combined with a complete waveform acquisition can lead to a more reliable testing of composite materials. INTRODUCTION The acoustic emission technique (AE) has for many years been considered as the prime candidate for structural health and damage monitoring in loaded structures. It offers the user a number of inherent advantages, the main of which are its continuous and in situ monitoring capabilities and the possibility to examine the whole volume of a structure simultaneously with a limited number of sensors. The increasing use of composite materials in loaded structures and their complex damage development has created a need for an efficient and reliable NDT technique that can be used during the service life of these materials. AE clearly has the potential to serve as a continuous damage detection technique for composites and during the past decades many studies have been undertaken to develop the technique to higher levels of performance. Three main types of data analysis have been extensively explored so far. That is, AE activity analysis (focussing on the amount of signals that are detected during a test), AE parameter analysis (studying evolutions in the basic signal parameters like amplitude, duration or energy) and AE frequency analysis (analyzing the frequency content of AE signals). Although many applications can be envisaged in all of the areas where composite materials are being used or could be used, it is surprising to see that the number of practical applications exploiting the AE technique has remained relatively limited. The main reasons for this are the limitations of the analysis techniques discussed above, which make it difficult to extend laboratory results to industrial structures. Commonly encountered problems are the large amounts of gathered data, the difficult separation of noise from real damage signals, the material anisotropy, the large wave propagation paths, etc. Summarizing these observations, one can state that AE has remained mainly a qualitative technique and that its further acceptance and practical use require a more quantitative approach based on theoretical concepts, which can reliably take into account source phenomena and wave propagation effects. Another attempt at providing a better theoretical background for AE testing is now known as modal acoustic emission (MAE) or waveform based acoustic emission. MAE starts from the observation that AE waves are mechanical in nature and should therefore be treated as such. Following the general