Acoustic Emission Source Location on Composite Laminates

Composite materials are increasingly used for structural applications because of their characteristics of lightness and strength. However a major disadvantage is the difficult in visualizing on the outside the damage produced by a stress of exercise or an accidental event. Under this issue, the research has turned their eyes to new techniques for detection of damage. The investigative techniques used today, such as ultrasound and squid, are safe and effective but not practical for both equipment dimensions and not short investigation time. Recently, to overcome the limitations of these techniques, it is increasing the acoustic emission (AE) technique as a nondestructive investigation. The main advantages of this technique are: • ability to study and analyze real-time behavior of a material under a load • ease and convenience of inspection through the use of mobile probes The acoustic emission analyzes the ultrasonic pulses emitted by a defect when it occurs and spreads. The AE waves are mechanical ones and, as such, they follow the propagation of plane waves’ dynamics and laws. The AE analyse hasn’t been done on composite laminates due to the considerable difficulty in analyzing a signal from an anisotropic and not homogenous material: the signal, in fact, may depend on a number of parameters such as stratification, the orientation fibers in a single plate, the used materials and their coupling. Pencil lead break tests were carried out on symmetric CFRP laminates, and the results in terms of extensional velocity and amplitude were analysed. The aim was to investigate about the influence of the stacking sequence and direction on the acoustic response of the materials. The wave velocity was measured from the difference in arrival time of the signal between two successive Acoustic Emission sensors, and predicted by lamination theory in the different test conditions analysed. A general agreement was found between theory and experiments, even if the differences found in correspondence of some orientation confirmed previous results, indicating an influence of layer thickness on the wave velocity. The data were used to verify the possibility to correctly locate the acoustic source through the location analysis, yielding satisfactory results. A simple, exponential law was able to predict the dependence of signal amplitude on the distance from the source. Indeed, attenuation was scarcely affected by fibre orientation and lay-up. This suggests that a method based on attenuation, rather than arrival time, could be useful for source location in composite laminates. This paper proposes two alternative methods based on the optimization scheme to locate the point of impact in CFRP plates by analyzing the time of arrival of the ultrasonic signals received by three passive sensors attached to the plate in three different places: • using numerical methods in Matlab, the analytical solution for the location of acoustic source was found The acoustic emission events were produced by a lead break, and the response wave received by piezoelectric sensors bonded to the plate at three different locations. The impact point is predicted by the proposed method and compared with the actual location of impact; • using Artificial Neural Network (ANN) technology a very satisfactory correlation was found between the actual source locations and those predicted by the virtually trained ANNs. A simple empirical formula (relying on classical lamination and suggested by wave propagation theory) was able to accurately model the experimental trend. Based on this formula, virtual training and testing data sets were generated for the case of a plate monitored by three transducers, and adopted to select a potentially effective ANN architecture.