An investigation of structural stress in active acoustic black holes

Acoustic Black Holes (ABHs) are structural features that enable a notable level of vibration control by introducing a spatial variation in either the thickness or material properites of a structure, leading to a significant reduction in the wave speed and resultingly an increase in the efficiency of an applied damping treatment. There are two key potential limitations associated with ABHs – firstly, due to the focusing of structural waves into the ABH feature they can increase the structural stresses; and secondly, their performance at low frequencies is limited by the size of the ABH feature. It has previously been shown that through the integration of active control it is possible to enhance the lowfrequency performance of ABHs, however, the effect that this has on the resulting stress has not been previously explored. This paper presents an investigation into the change in structural stress due to the integration of active control into an ABH terminated beam. A feedforward wave-based controller is utilised, which aims to minimise the wave reflected from the ABH termination using a piezoelectric patch actuator attached to the ABH feature. This demonstrates that a conventional active ABH introduces a significant stress enhancement and, therefore, a modified control strategy is explored in which the stress measured in the ABH is included in the cost function being minimised.