Talieh Hajzargarbashi

Locating the acoustic source in an anisotropic plate

By analyzing the arrival times of guided waves, the acoustic source in a plate is predicted. Solving this problem is important for continuous health monitoring of structures. Several techniques based on the triangulation principle have been proposed for this purpose but they do not work for anisotropic plates. The popular triangulation technique assumes that the wave velocity is the same in all directions, which is not true for anisotropic plates. An alternative method based on the optimization scheme was proposed by Kundu et al. (Kundu T, Das S and Jata KV. Point of impact prediction in isotropic and anisotropic plates from the acoustic emission data. J Acoust Soc Am 2007; 122(4): 2057–2066; Kundu T, Das S, Martin SA and Jata KV. Locating point of impact in anisotropic fiber reinforced composite plates. Ultrasonics 2008; 48(3): 193–201; Kundu T, Das S and Jata KV. Health monitoring of a thermal protection system using lamb waves. Struct Health Monit: Int J 2009; 8(1): 29–45; Kundu T, Das S and Jata KV. Detection of point of impact on a stiffened plate by the acoustic emission technique. Smart Mater Struct 2009; 18(3): 1–9) to locate the point of impact in anisotropic plates by analyzing the times of arrival of the ultrasonic signals received by the passive sensors attached to the plate. Recently, Hajzargarbashi et al. (Hajzargarbashi T, Kundu T and Bland S. An improved algorithm for detecting point of impact in anisotropic inhomogeneous plates. Ultrasonics 2010; 51(3): 317–324.) improved that technique. Following their modification, in this article, the acoustic source point in an anisotropic plate is predicted from the acoustic emission data with some additional modifications. Experiments are carried out with a carbon–epoxy plate. A parallel pre-stressed actuator is used as the acoustic source and the acoustic signals at different locations are received by adhesively bonded acoustic sensors. The source point is then predicted and compared with its actual location. Related theory and computed results of wave propagation in anisotropic plate are also presented and the theoretical predictions experimentally verified at frequencies higher than what is typically generated by the simple impact phenomenon.

Locating Point of Impact on an Anisotropic Cylindrical Surface Using Acoustic Beamforming Technique

A beamforming array technique with four sensors is applied to a cylindrical geometry for detecting point of impact. A linear array of acoustic sensors attached to the plate record the waveforms of Lamb waves generated at the impact point with individual time delay. A beamforming technique in conjunction with an optimization scheme that incorporates the direction dependent guided Lamb wave speed in cylindrical plates is developed. The optimization is carried out using the experimentally obtained wave speed as a function of propagation direction. The maximum value in the beamforming plot corresponds to the predicted point of impact. The proposed technique is experimentally verified by comparing the predicted points with the exact points of impact on a cylindrical aluminum plate and a cylindrical composite shell. For randomly chosen points of impact the beamforming technique successfully predicts the location of the acoustic source.

Scattering of focused ultrasonic beams by two spherical cavities in close proximity

Ultrasonic fields generated by one and two spherical cavities placed in front of a point-focused acoustic lens are modeled by the semi-analytical distributed point source method (DPSM). Results are generated by properly considering the interaction effect between two cavities placed in the focused ultrasonic field. The interaction effect between the two cavities prohibits the linear superposition of single cavity solutions to obtain the solution for two cavities placed in close proximity. Therefore, although some analytical and semi-analytical solutions are available for the single cavity in a focused ultrasonic field, those solutions cannot be simply superimposed for solving the two-cavity problem even for a linear elastic material. Solution of this problem is necessary to evaluate when two cavities placed in close proximity can be distinguished by an acoustic lens and when it is not possible. The comparison between the reflections of ultrasonic energy from two small cavities versus a single big cavity is also investigated.

Impact localization on a cylindrical plate by near-field beamforming analysis

A beamforming array technique with 4 sensors is applied to a cylindrical plate for detecting point of impact. Linear array of acoustic sensors attached to the plate record the waveforms of Lamb waves generated at the impact point with individual time delay. An optimization technique with an objective function is incorporated into the beamforming technique in order to deal with the direction dependent Lamb wave speeds in a cylindrical geometry. The optimization is carried out using the experimentally obtained wave speed as a function of propagation direction. The maximum point in beamforming plot with minimized objective function corresponds to the localized point of impact. The proposed technique is experimentally verified by comparing the predicted points with the exact points of impact on a cylindrical aluminum plate. For randomly chosen points of impact the beamforming technique successfully predicts location of the exact acoustic source.

A New Algorithm for Detecting Impact Point in Anisotropic Plates by the Acoustic Emission Technique

The wave speed in an anisotropic plate is dependent on the direction of propagation and therefore the conventional triangulation technique does not work for the prediction of the impact point. A method based on the optimization technique was proposed by Kundu et al. to detect the point of impact in an anisotropic plate. They defined an objective function that uses the time of flight information of the ultrasonic signals to the passive transducers attached to the plate and the wave propagation direction (θ) from the impact point to the receiving sensors. This function is very sensitive to the arrival times. A small variation in any one arrival time results in a significant change in the impact point prediction. This shortcoming is overcome here by modifying the objective function and following a new algorithm. Both old and new objective functions (denoted as functions 1 and 2) are used in the new algorithm. This algorithm uses different sets of transducers and identifies the common predictions from different sets. The proposed algorithm is less sensitive to the arrival time variation and thus is capable of predicting the impact point correctly even when the measured arrival time has some error. The objective function 2 is simpler, so the computer code run time is reduced and it is less likely to converge to the local minima when using the simplex or other optimization techniques. The theoretical predictions are compared with experimental results.

Detecting the point of impact on a cylindrical plate by the acoustic emission technique

An optimization based technique for detecting the impact point on isotropic and anisotropic flat plates developed by Kundu and his associates is extended here to the cylindrical geometry. An objective function is defined that uses the cylindrical coordinates of four sensors attached to the cylinder and four arrival times to locate the point of impact by minimizing the objective function that gives the least squares error. The proposed technique is experimentally verified by predicting the points of impact and comparing the predicted points with the actual points of impact.

Scattering of focused ultrasonic beams by cavities in a solid half-space

The ultrasonic field generated by a point focused acoustic lens placed in a fluid medium adjacent to a solid half-space, containing one or more spherical cavities, is modeled. The semi-analytical distributed point source method (DPSM) is followed for the modeling. This technique properly takes into account the interaction effect between the cavities placed in the focused ultrasonic field, fluid-solid interface and the lens surface. The approximate analytical solution that is available in the literature for the single cavity geometry is very restrictive and cannot handle multiple cavity problems. Finite element solutions for such problems are also prohibitively time consuming at high frequencies. Solution of this problem is necessary to predict when two cavities placed in close proximity inside a solid can be distinguished by an acoustic lens placed outside the solid medium and when such distinction is not possible.

Prediction of the point of impact in an anisotropic plate

Locating the point of impact of a foreign object in a plate is important for continuous health monitoring of structures. A new method based on an optimization scheme has been recently proposed to locate the point of impact in anisotropic plates by analyzing the times of arrival of the ultrasonic signals at the passive sensors attached to the plate. Following this optimization based technique, in this paper the impact point on an anisotropic plate is predicted from the acoustic emission data. Experiments are carried out with a carbon-epoxy plate where the impact point is modeled by an acoustic source. A Parallel Pre-stressed Actuator (PPA) is used as the acoustic source and the acoustic signals at different locations are received by adhesively bonded acoustic sensors. The source point is then predicted and compared with its actual location. Related theory is also presented in the paper.