Laser ultrasound nondestructive evaluation of complex materials and geometries

Abstract

Ultrasonic nondestructive evaluation (NDE) is widely used to detect and characterise internal
defects in safety-critical components across industries such as aerospace, energy, and transport.
However, conventional ultrasonic methods often require physical contact and coupling media,
limiting their applicability to components with complex shapes or limited accessibility, especially
in scenarios such as in-situ inspection. With the increase use of carbon fibre reinforced polymer
(CFRP) composites, particularly in aerospace structures, there is a growing demand for inspection
techniques that can handle complex material with elastic anisotropy, layered construction, and geometrical curvatures. These materials are challenging to inspect due to unknown wave directivity,
high structural noise giving rise to imaging artefacts and low signal-to-noise ratio.
Laser ultrasound offers a promising solution by enabling remote, contactless and couplant-free
inspection. It uses a pulsed laser to generate broadband ultrasound via thermoelastic mechanisms
and an optical interferometer to detect surface displacement. These capabilities make laser
ultrasound well-suited for the inspection of CFRP components with complex geometries, such as
aeroplane fuselages and fan blades.
This thesis aims to develop and validate a laser-induced phased array (LIPA) framework
to detect and characterise defects in CFRP components while minimising scan time. Three key
objectives are addressed:
1. establishing a forward model for laser-generated ultrasound in anisotropic, layered CFRP
materials;
2. evaluating the performance of the total focusing method (TFM) and reverse time migration
(RTM) imaging algorithms;
3. optimising scan strategies to reduce data acquisition time without compromising image
quality.
The main contributions include finite element and analytical models for ultrasound directivity
in multi-layered anisotropic media, implementation and comparison of wave-based imaging
algorithms, and experimental LIPA scanning of a CFRP fan blade with flat bottom holes. The
results demonstrate that incorporating laser ultrasound directivity into the imaging algorithm
significantly improves the signal-to-noise ratio. The continuous scan strategy helps to quantify the
scan efficiency and the expected imaging performance.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jie Zhang (Supervisor), Alberto M Gambaruto (Supervisor) & Paul D Wilcox (Supervisor)