Ultrasound for Structural Safety: EMATs, Liquid Crystals, and Future Vision at Warwick

Overview

Rachel Edwards introduces the Warwick ultrasound group and explains how high frequency ultrasound is used in engineering and applied physics to inspect structures without damaging them.

She highlights non-destructive testing as a key safety area for rail, power plants, aircraft, and pipelines and outlines current methods and future directions.

The talk blends demonstrations, practical challenges, and forward-looking ideas such as eliminating couplant and using novel sensing materials to visualize sound and defects.

Overview and context

Rachel Edwards presents the ultrasound group at Warwick, describing ultrasound as very high frequency sound waves above 20 kilohertz. She notes how ultrasound is largely an engineering and applied physics pursuit within a physics department and touches on public perceptions and the everyday images associated with ultrasound.

Non-destructive testing and safety imperative

The discussion centers on non-destructive testing as a means to detect hidden defects in critical infrastructure such as railway tracks, power plants and aircraft wings. The talk emphasizes safety and disaster prevention, citing Hatfield as a cautionary example and noting environmental risks from damaged pipelines.

Imaging options: radiography, MRI and ultrasound

The talk contrasts X-ray radiography as a gold standard that can be hazardous and surface EM techniques like MRI with limitations, arguing that ultrasound offers a practical balance of information and safety for many industrial scenarios. Time-of-flight measurements are introduced as a core concept, linking speed, distance and time to inspect material thickness and locate defects.

Live demonstrations and the physics behind them

A railway head analogue is used to demonstrate time-of-flight logic. A transducer emits ultrasound that travels through a material, and echoes are detected at progressively shorter times as the sensor moves, revealing a hole. The demonstration illustrates how a 2D scan builds a picture of the internal structure and potential defects.

Transducers, coupling, and industrial limits

The discussion moves to the hardware: piezoelectric transducers convert electrical energy to sound and back. Coupling gel is essential to minimize air gaps that reflect sound. Industrial environments pose challenges, including high temperatures and the need to inspect large structures quickly. Wheel probes for rails are explained, along with limitations such as shadow regions and the speed constraints of sound through metal.

EMATs: a couplant-free approach

Electromagnetic acoustic transducers (EMATs) use the Lorentz force to generate ultrasound without a couplant. A current pulse in a coil near a magnet induces motion in the metal via magnetic fields, effectively turning the metal into a loudspeaker for ultrasound. EMATs enable high temperature work and avoid coupling issues, though they are typically less energy efficient than piezoelectric transducers. Surface waves and focusing strategies using EMATs are discussed, including designing coils and magnetic geometries to steer waves along rails or into defects.

Future visions: from lasers to paint on sensors

The talk surveys future directions such as full-field wave-field mapping with laser interferometers, which provide detailed maps of surface vibrations but are expensive and slow. A central idea is to replace or complement lasers with absorbing layers and a visual sensor based on thermochromic liquid crystals. These crystals change color with temperature, enabling a medium that visually records ultrasound energy as heat patterns. Early experiments use parking-sensor transducers and painted, UV-curable layers on metal to produce colorful, real-time indications of vibration and defects. The ultimate aim is a simple, paint-on coating that, when paired with a phone camera, could deliver an instant defect map across large industrial components.

Liquid crystals and practical sensing

The discussion explains polymer dispersed liquid crystals PDLC and the principle of refractive index matching to achieve milky versus clear states. The team demonstrates how electrical or acoustic fields can modulate the crystal orientation, producing visible changes. A real-time demonstration shows the sensor responding to different frequencies by changing color, mapping out vibrational modes and energy absorption. The work emphasizes ongoing material exploration, high temperature operation, and the potential to integrate such sensors into industrial workflows for rapid, cost-effective inspection.

Current status and collaboration

The researchers acknowledge that the approach is at an early stage, with many variables to optimize across liquid crystal types, temperatures, and frequency ranges. Collaborations with liquid crystal specialists and other researchers are highlighted, and the talk closes with reflections on the balance between fundamental research and industry needs, and thanks to contributors.