Laser Ultrasonic Testing - Advanced NDT

Laser-ultrasonic testing is a non-contact, non-destructive testing (NDT) technique that uses one laser to generate an ultrasound and another to detect it, allowing the for high-speed inspection of complex geometries and materials in harsh environments. As a method for non-destructive detection, laser ultrasonic testing provides detailed, high-resolution mapping of defects like cracks, delamination, and voids without needing a couplant or immersion, making it suitable for the inspection of hot, moving, or irregular surfaces.

As a high-speed testing technique that is up to 10 times faster than traditional single-element ultrasonic testing, this method is capable of detecting surface-breaking microcracks (<0.1 mm) and characterising material properties.

Laser ultrasonic testing can be used with metals, composites, ceramics, and semiconductors, and has found applications in industries including aerospace, automotive, rail, and nuclear.

The process can be broken down into three steps, as follows:

Generation: A pulsed laser is absorbed by the material surface to create a rapid thermal expansion that generates ultrasonic waves
Detection: A second laser (interferometer) then measures the surface motion caused by the returning ultrasonic waves
Analysis: Both signals are analysed to detect, locate, and characterise any internal flaws.

Core Research Programme (CRP) and Joint Industry Projects (JIP)

Core Research

Each year the TWI Core Research Programme (CRP) addresses challenges on behalf of our Industrial Members as well as developing specific technologies and processes. Each of the projects under the CRP is focussed on engineering, materials or manufacturing technologies.

Joint Industry Projects

TWI also conducts Joint Industry Projects (JIPs) that bring together groups of Industrial Members to share the cost of research activities in areas of mutual industrial interest, gaining exclusive access to the outcomes. These projects cover a broad range of topics.

Laser Ultrasonic Testing at TWI

TWI has years of experience in developing laser ultrasonic testing techniques as a cutting edge non-destructive testing (NDT) method in which lasers are used to generate, and then measure, ultrasonic waves in a material.Laser ultrasonics are able to remotely generate ultrasound in materials without contact, leaving a very small footprint so that it can be applied to irregular geometries, allowing access to restricted areas via fibre optics.

Laser ultrasonic testing offers a number of benefits for industry, including:

Non-contact and remote, allowing inspection of samples at high temperature, e.g. during welding with restricted access

Small and adjustable footprint

Enables inspection of small and complex geometries

High frequency capable of detecting very small flaws

Laser beam scanning method for full coverage of inspection samples

Applications for the process include:

Industrial in-process measurements on hot, hazardous, remote samples which may be moving at high speeds

High-resolution measurement of small parts

Inspection of complex structures

TWI is pursuing various laser ultrasonics projects involving the inspection of friction stir welds, laser material depositing, and composite materials

Our work with the technology

Intensive research on the generation and properties of ultrasound has been carried out since the 1980s. A technology readiness level of between six and eight has been reached by certain laser-ultrasonic applications, such as thickness measurements for steel tubes coming out from the mill and damage detection of composite aircraft components.

Our experts have carried out a number of research projects that have further developed laser-ultrasonic testing as an inspection tool for different welding processes and continue to investigate the technology’s potential.

Broadly speaking, the mechanisms of generating ultrasound using this method are laser thermal shock and laser ablation. They both deposit a rapid laser pulse on the material surface, inducing a sudden stress in the sample. The stress then propagates in the material as ultrasound, while broadband, multi-mode waves are generated simultaneously.