Multi-frequency nonlinear ultrasonic diffuse imaging for material and defect characterisation

Abstract

Advances in ultrasonic nonlinear imaging techniques have increased the detectability of fatigue cracks during earlier stages of formation. This thesis explores the novel nonlinear ultrasonic diffuse energy imaging (NUI) technique used for non-destructive testing (NDT). In contrast to current linear methods the NUI method is able to image tightly closed crack tips and quantify the degree of nonlinearity present in the defect without the need for mechanical scanning or complicated bespoke set-ups. More specifically, unlike nonlinear coherent methods the NUI method is able to resolve nonlinear closed crack tips without the need for coherent reflections from damage, meaning the orientation of the crack tips is not required and the method can be extended to classical nonlinearity induced by lattice anharmonicity and dislocations.

A sensitivity analysis of the NUI technique is first explored to establish the sensitivity profile of the method and further understand the detectability of nonlinear features in materials. This is achieved through experimental and simulated data, and explores the influence of attenuation and transmission frequency on detectability of nonlinear features. Consistent sensitivity over the frequency transmission range investigated is observed and is attributed to the competing effects of increased attenuation and nonlinearity as frequency increases. The knowledge of the sensitivity profile is then used to investigate a multi-view diffuse method to allow optimisation of the position of the sensitivity profile used to detect the nonlinear feature. This is only effective at 5 MHz where a nonlinear feature is detected very close to the array which is otherwise undetectable using the existing single-view NUI technique.

Further optimisations of the NUI technique are investigated. Firstly, a multi-frequency adaptation is explored and compares the sensitivity of the NUI technique using varying transmission frequencies and receiving at different frequency components (low, sub-harmonic, fundamental and second harmonic). The most effective configuration is transmitting at 5 MHz and receiving at the low frequency and the second harmonic component. A phase metric is also investigated in an attempt to reduce the diffuse field dependency of this method. The results show some reduction in sensitivity to the diffuse field, suggesting there is some benefit to computing the proposed phase metric over the current energy metric. The optimisation of the diffuse time parameters is investigated and a phase coherence metric is established to process full matrix capture (FMC) data and determine the time in which ultrasonic energy has homogenised in the material. The results suggest the metric converges when the field is diffuse and is tested with the experimental performance of the NUI technique in imaging a crack tip.

Finally, using the optimisations proposed in this thesis the detection of classical nonlinearity (lattice anharmonicity and dislocations) is investigated on dog-bone samples subjected to loading from a 4-point bending jig. This study concludes that the NUI method requires further optimisation to detect lower-level classical nonlinearity.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Bruce W Drinkwater (Supervisor) & Anthony J Croxford (Supervisor)