Imperial College London

Supervisors: Dr Bo Lan & Prof Mike Lowe

This project aims to characterise critical microstructures within engineering metals using ultrasound. Most metals are polycrystalline, and their critical performances (e.g. strength, fatigue life, and corrosion resistance) are determined by the grain microstructures (e.g. size, shape and clusters). However, there is a lack of effective characterisation methods for these volumetric properties, with current standard measurement restricted to destructive, two-dimensional section surfaces of sacrificial samples, which remains laborious, costly and inaccurate. Ultrasound has been pursued as an alternative, but it is subject to convoluted effects from the microstructures and preferred crystallographic orientations (texture), and there still lacks model-supported quantitative linkages to enable the characterisation.  

The NDE group at Imperial College has developed a breakthrough methodology to extract volumetric texture from the speeds of compressional ultrasound. It has been validated on a conventional water-bath system, which, in addition, has produced experimental evidences that the shear waves propagating through a metal sample are strongly affected by the microstructures. The amplitudes of the shear waves correlate with the grain sizes in the propagation direction, and they vary significantly when the waves propagate in different directions in 3D, which indicates the exciting possibility to simultaneously evaluate the texture and the volumetric morphologies of the microstructures non-destructively from the same ultrasound setup.  

This doctorate project aims to understand the physics of the wave behaviors and develop the method for industrial application. The student will firstly quantify the attenuation of the shear waves from experimental data from different propagation directions. Then a computational study, which incorporates the microstructures obtained from destructive tests, will be carried out to understand how the material affects the polarization and attenuation of the shear waves. The simulation models, once validated against experiments, will be able to supplementary data to the latter for further analysis. Finally, a theoretical model will be developed to describe the physical relationship between the wave propagation and the microstructures. It will form the foundation of the experimental characterisation and will be validated on further samples. The student will be trained comprehensively on the experimental, computational and theoretical aspects, with strong industrial connections, thus providing a head start for his/her future career.