Probability-based Diagnostic Imaging Techniques Using Lamb Waves

In recent years, diagnostic imaging techniques in conjunction with the use of Lamb waves have attracted increasing attention in NDE and SHM applications because such techniques generate intuitional and easy interpretation2D and3D images, capable of intuitively indicating the structural damage details and even overall health state of the structure under inspection. But as envisaged, traditional diagnostic imaging techniques such as tomography and phased array need a large number of transducers for image reconstruction, narrowing their potential toward SHM and NDE. To enhance the practicability of Lamb-wave-based damage identification with use of imaging techniques, probability diagnostic imaging (PDI) techniques such as damage index-based and TOF-based have been explored using less transducers and quicker image reconstruction algorithms. A PDI method attempts to describe a damage event using a binary image. PDI methods make use of signal features to construct diagnostic images. To this end, the monitoring area is meshed with a uniformly distributed grid. Each grid point is linked with the probability of damage presence at the spatial point of the inspected structure that exclusively corresponds to this grid point. Therefore, PDI techniques present damage in terms of its presence probability.Although damage index-based PDI techniques do not need any signal interpretation, such techniques can only highlight damages located among transducers. On the other hand, TOF-based PDI techniques can monitor damages anywhere in the structure nevertheless it is quite difficult to extract TOF due to the complex mechanism of lamb wave propagation in engineering structures. As a result, in this thesis firstly a PDI technique based on error functions is developed to highlight damages located anywhere in the structure without any need to the interpretation of the captured signals. Error functions were introduced based on the energy of the damage scatter signals. The efficiency of the developed technique was demonstrated by the identification of representative damages including through-thickness hole and crack through finite element analysis and experiment in aluminum plates.The common PDI techniques can only provide information about the existence and the location of damages in the structures and such techniques are unable to quantify damage which is vital for further investigation about remaining life or residual strength of the damaged structure. Therefore, secondly, a damage size characterization algorithm based on TOF-based PDI technique is presented to characterize the damage features including its shape and size. Briefly, damage size characterization algorithm consists of four main steps including; damage location identification, damage boundaries estimation, data processing and finally damage size and shape determination. The effectiveness of the damage size characterization algorithm was demonstrated by estimating hexagonal and triangular through-thickness hole with different sizes through FE simulation and corrosion through experiment in aluminum plates.Nondestructive inspection of structures for the identification of hidden defects has always been of great industrial interest. This work is always done in parallel with the development of nondestructive techniques. Point-scan nondestructive tests such as ultrasonic testing provide diagnostic image of the test-piece which highlights the damages in detail. However, the need to scan the specimen in two dimensions makes ultrasonic inspection a time consuming procedure. Therefore, thirdly, lamb-wave-based mobile sensing approach was presented. Unlike previous Lamb-wave-based damage identification methods which utilize the sensor network permanently attached to the structure in order to generate and collect Lamb waves into the structure, mobile sensing approach makes use of a hand-made mobile transducer set to generate and collect signals at different locations of the structure. The mobile sensing approach was introduced by developing an area-scan nondestructive technique. The area-scan nondestructive technique makes use of damage presence probability index (DPPI) defined based on the energy of the current signal, to highlight a region in the structure with the maximum probability of damage presence. The efficiency of the area-scan nondestructive technique was demonstrated by defining the areas with representative damages including through-thickness hole and crack through experiment in aluminum plates.Finally, a PDI technique based on mobile sensing approach was presented aims to highlight the most probable location of single and multi damage in the structures in an easily interpretable diagnostic image. Such technique uses a baseline-free damage scatter wave separation process to define the damage scatter signal. The diagnostic image was then constructed based on the energy envelope of all the damage scatter signals. The satisfactory results from different selected damage cases, including through-thickness hole and crack in the aluminum plates and single and multi added masses in the composite plates have demonstrated the effectiveness of the developed PDI technique in highlighting the most probable locations of damages. Since the developed PDI technique was developed based on mobile sensing approach, it can provide an area-scan inspection rather than a point-scan which is required in the most of conventional NDE techniques and in this way minimize the time and cost of the inspection. Conventional point-scan NDE techniques, such as ultrasonic C-scan and X-ray, can then be used once damage is found to characterize the damage in detail.