Ultrasonic Non-Destructive Evaluation: Impact Point Prediction and Simulation of Ultrasonic Fields

Abstract

This work has two parts. The first part of the work (in Chapters II, III, IV and V) presents a method for locating the point of impact using acoustic emission techniques.The second part of the work is modeling the ultrasonic fields generated by one and two spherical cavities placed in front of a point focused acoustic lens using the semi-analytical distributed point source method (DPSM).Acoustic emission (AE) refers to the generation of transient elastic waves during the rapid release of energy from localized sources within a material.In this work the acoustic emission has been used for locating the point of impact on anisotropic and homogeneous or non-homogenous flat plates and cylindrical structures. In these cases the wave speed is a function of the angle of propagation. An optimization function is introduced and minimized to get the location of the impact point.This method has been used on a flat (fiber reinforced polymer) plate. The proposed new objective function reduces the amount of time needed for solving the problem and improves the accuracy of prediction. The method is extended to cylindrical structures for which the objective function is written in cylindrical coordinates and the method is tested on a FRP shell.In Chapter IV an alternative method is introduced called the near-field acoustic emission (AE) beamforming method. It has been used to estimate the source locations by using a small array of sensors closely placed in a local region. To validate the effectiveness of the AE beamforming method a series of experiments on a FRP shell are conducted. The experimental results demonstrate that the proposed method can correctly predict the point of impact.The semi-analytical mesh-free technique DPSM is then used to model the ultrasonic field in front of a point focused acoustic lens; anomalies such as cavities are introduced in the medium in front of the acoustic lens and the effect of those cavities are studied. Solution of this problem is necessary to get an idea about when two cavities placed in close proximity can be distinguished by an acoustic lens and when it is not possible.