Coercivity Based NDT: From Sensor Design to Mobile Robot Deployment

Abstract

This thesis focuses on practical problems that limit non-destructive testing (NDT) in practical applications: bulky probes cannot be used in narrow spaces, lift-off between the probe and the surface causes errors, and manual scanning is slow and hard to apply in difficult-to-reach areas. To address these issues, this work is built around a coercivity measurement system and aims to improve accuracy, efficiency, and coverage by miniaturising the probe, reducing the impact of lift-off, and developing a mobile NDT robot that automates data collection. The first part of this thesis addresses coercivity measurement in tiny samples and narrow spaces. Conventional probes are large to provide enough winding space for strong magnetisation, but this prevents their use near weld roots, corners, or inside narrow components. Here, a compact coercivity probe is designed to preserve the key magnetic behaviour of the original probe. Tests on different core materials and coil configurations show that high–energy density cores perform best, and the final probe, about 1/6 of the original size, achieves a measurement error of less than 10%. The second part focuses on lift-off caused by coatings, rust, or uneven surfaces. Small changes in lift-off are a major error source in coercivity measurement. This work first characterises how coercivity varies with lift-off and derives the coefficients of the coercivity–inductance relationship for different samples. Based on this, a correction method is proposed to predict the base coercivity (lift-off = 0) from measurements with unknown lift-off. Experiments show that the error caused by air gaps can be reduced from about 40% to less than 10%. Traditional NDT is constrained by manual sensor handling, accuracy, and accessible scenarios. To address these issues, the NDT measurement system is integrated with a wheeled robotic platform. Considering the payload and volume of the robot, a compact eddy-current (ECT) sensor is first used to validate locomotion/pose control, synchronized data acquisition, and short-range wireless telemetry; once qualified, the ECT module will be replaced by the miniaturized coercivity probe. The study therefore prioritizes platform design and ECT-based verification to establish the feasibility of robotic NDT.

Date of Award	1 May 2026
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Wuliang Yin (Main Supervisor) & Hujun Yin (Co Supervisor)