title: Nonlinear ultrasonic wave mixing for the non-destructive evaluation of materials' properties
Creators: name: Yambangyang, Pracha
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Nonlinear Ultrasonic wave Mixing, Non-destructive Evaluation.

description: Nonlinear ultrasonic wave mixing has been shown to be a powerful method to detect and characterise damage or defects in materials. This technique has been used in a wide range of applications such as the non-destructive evaluation of material properties due to its higher sensitivity when compared to conventional, linear, ultrasonic techniques. This technique is based on an investigation of nonlinear behaviours in a material by using harmonic generation or nonlinear resonance, which are able to detect small scale defects or structural degradation, such as cracks, micro cracks, and material characterisation in the wider sense. Within the context of medicine, the detection of deep tissue injury, such as a pressure ulcer, could benefit from the capabilities of nonlinear ultrasonic wave mixing methods. These wounds are currently detected late, leading to difficult treatments. Earlier detection over smaller areas based on difference, in properties relative to neighboring tissue, means nonlinear ultrasound techniques could have a significant impact on patient recovery. The purpose of this thesis is to develop a platform which can detect nonlinearities of materials using nonlinear ultrasonic wave mixing techniques. In this study, two kinds of non-linear wave mixing techniques are introduced for the detection of small particles distributed in a hydrogel phantom, a proxy for an injury area in the early stages of the development of a wound. Wave mixing was also applied to measure an accumulated change to material properties also known as physical aging. The results demonstrated an improved ability of non-linear wave mixing method over linear techniques to distinguish minuscule particles and to be able to monitor a slow rate of change in material properties, which would be used to monitor deep wounds under the skin such as pressure ulcers. The results clearly show that the summed frequency of a nonlinear wave signal can detect a range of microparticle sizes (70m, 100m and 150m) with higher sensitivity and resolution as compared to the linear echo ultrasound. This is a factor of more than 10 increase in the resolution of defect detection as compared to linear methods, allowing for earlier wound identification. In addition, the response of the nonlinear summed frequency interaction was captured with a 30 s sampling rate at 6.25 MHz.This system showed that the nonlinear ultrasonic technique was suitable to detect the physical aging of amorphous polymers at the annealing temperature. When the polymer structure underwent structural relaxation, the nonlinear wave mixing energy gradually increased due to the non-equilibrium state and then continually developed due to physical aging. The system is not only designed to detect the nonlinearities of soft material properties using nonlinear ultrasonic techniques, but also to enable its application to evaluate the accumulated change of material properties. These experimental results suggest that nonlinear ultrasonic wave mixing technique may be useful for detecting small scale (early onset of) pressure ulcers developing deep in the skin. The ability to observe physical aging in polymers, and the analogous behaviour of polymer physical aging and ulcer development, suggest it may be possible to monitor pressure ulcer development with nonlinear techniques.