Acoustic emission localization methods for large structures based on beamforming and array techniques

The inspection of bridges is currently made by visual inspection or by wired sensor techniques, which are relatively expensive, vulnerable to damage, and time consuming to install. In contrast, wireless sensor networks are easy to deploy and flexible in application so that the network can adjust to the individual structure. Different sensing techniques can be used with such a network, but acoustic emission techniques have been rarely utilized. With the use of acoustic emission (AE) techniques it is possible to detect internal structural damage from cracks propagating during the routine use of a structure or the break of wires of prestressed elements for example. Most of the existing AE data analysis techniques are not appropriate for the requirements of a wireless network, especially regarding power consumption, memory storage or time synchronization. New algorithms have been developed using a new concept called Acoustic Emission Array Processing. As a first step, beamforming and source discrimination techniques were tested as well as a method based on a modified velocity spectral (VESPA) process. Traditional signalbased acoustic emission techniques include a two or three-dimensional localization of AE events based on the time difference of arrival (TDOA) technique using at least eight sensors that have to be placed well-distributed around the hypocenter, i.e. the AE source. The array concept allows for much simpler localization techniques with less hardand software efforts. The beamforming method calculates the azimuth of the waves impinging on a small array of sensors by steering the beam through a range of azimuths until the steered delays match the observed time delays. This formulation operates under a different set of assumptions than the TDOA method and does not require the determination of any P wave arrival times. In fact, it doesn’t require the detection of a P wave at all. The direction of arrival can be determined simply from the relative time delays of Rayleigh waves (R-waves).