Quantitative ultrasonic characterisation of surface-breaking cracks

• Main Author: Saini Abhishek
• Other Authors: Fan Zheng, David
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering; Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/149030
• Institution: Nanyang Technological University

Ultrasonic array-based quantitative defect characterisation offers information about defect type, shape, size, and orientation. This approach facilitates high-accuracy assessment of structural integrity in safety-critical systems. Given the nature of current defect characterisation techniques, researchers face a key challenge in imaging of complex-shaped surface-breaking cracks (SBCs). This thesis explores the potential of exploiting ultrasonic array-based techniques for quantitative characterisation of complex SBCs growing from inaccessible surfaces. In this research, a ray-based method called the Half-Skip Total Focusing Method (HSTFM), and a waveform-based method called the Reverse Time Migration (RTM) with its modification called Multi-Mode RTM (MMRTM), are developed. The HSTFM is a synthetic focusing delay-and-sum based post-processing technique. This method has previously been used to size SBCs initiating from a horizontal backwall. However, limited research was performed to optimise the method rigorously and quantitatively, ascertaining the crack sizing performance. Given that the HSTFM is limited to characterising simple SBCs, an elastic RTM method was developed for accurate reconstruction and sizing of complex shape defects. The RTM image is generated by cross-correlating the forward and backward wavefield, accounting for mode conversions and multiple scattering. Furthermore, additional information about the defect can be manifest by multiple image views of the same region through exploiting geometric features and wave mode conversions. Therefore, the MMRTM was developed to perform imaging by using decoupled elastodynamic extrapolation. The MMRTM decomposes the elastic source and receiver wavefield into longitudinal (L) and shear (S) wave vectors (called wavefield separation) and then performing inner-product imaging to obtain the multi-mode images (LL, LS, SL, and SS). This thesis demonstrates the defect characterisation capability of the proposed imaging techniques for both 2D and 3D geometric configurations. For the 2D geometric configuration, a quantitative parametric study was performed to evaluate and improve the sizing capability of the HSTFM under different scenarios. These include the array position, the accuracy of acoustic velocity, the sample height, the angular variation of the back wall and the defect. Optimised results propose selecting the array position and its aperture based on industry acceptance-rejection criteria. Then, the recommendations were provided to select appropriate ray paths based on crack angular variation. To image and size a branched SBC and an embedded defect, the RTM method was applied and compared with the Total Focusing Method (TFM) quantitatively. The results demonstrate that the RTM method performs best in reconstructing the complete defect shape compared with the TFM. Sizing of defect was performed from the images and the RTM has been shown to be better at measuring complex cracks than the TFM. Furthermore, the MMRTM algorithm was implemented to generate multi-mode images (LL, LS, SL, and SS) of sub-branched SBC. Multi-mode images allow improved defect characterization by providing more useful information and minimal crosstalk artefacts from multiple wave-modes. The efficiency and accuracy of these methods have been demonstrated in detail through numerical simulations and experiments, recommends its practical employment. Finally, for the 3D configuration, a new hybrid imaging method was developed to reconstruct the SBCs by scanning a linear ultrasonic array. In contrast to conventional array with larger element length, smaller element length array was proposed for higher elevation resolution. Numerical and experimental results show a high-resolution crack reconstruction, along with an excellent sizing accuracy of SBCs in 3D. As array scanning is cumbersome, the 3D RTM was applied using a 2D ultrasonic matrix array and compared with the TFM. A GPGPU-based 3D RTM method was developed and implemented numerically. The 3D RTM solves the wave equation by back-propagating in-line and cross-line signals to reconstruct images without any dip limitations, which substantially improves defect characterisation. The simulation results show that the 3D RTM performs better than the TFM in the effective recovery and sizing of surface-breaking defects in 3D. The 2D and 3D imaging techniques developed in this thesis potentially offers enhanced interrogation and characterization of surface-breaking defects. The investigated results are encouraging and the techniques can be implemented in an actual application to yield richer information about the health state of the critical engineering structures.