Solutions for mobile non-destructive Terahertz Testing

Most electrically non-conducting materials such as plastics, ceramics and foams are transparent to terahertz waves, making components made of such materials well suited for non-destructive testing with terahertz sensors.

Unlike ultrasonic testing, terahertz testing does not require a coupling medium and can be applied in contact-free measurement arrangements. In addition, terahertz radiation is non-ionizing and therefore harmless to humans. After many years of research and development, a variety of terahertz products are commercially available now. However, since most terahertz sensors allow only single-point measurements, they are often integrated into stationary scanning mechanics for single-line or raster-scanning investigations. These arrangements are usually not mobile and components to be examined must be brought to the terahertz test system, which is sometimes not feasible due to component sizes or desired in-place investigations.

The Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, Germany has therefore developed solutions for mobile non-destructive terahertz testing applications. In this process, a mobile terahertz sensor is brought to the component to be investigated and not vice versa. Thus achievinge greater flexibility with regard to the location of the measurements. In addition, the mobile terahertz sensors can be better adapted to a component’s geometry for great versatility of possible measurement arrangements.

Two device versions are currently available. The hand-held scanner (image 1) is equipped with a position encoder wheel and thus enables fast terahertz line scans in the direction of sensor movement. In this way, the interior of the examined component can be displayed by means of a B-scan.

When larger areas of components under test are to be examined, the compact scanner (image 2), which scans the area of a DIN A4 sheet (20 x 30 cm²) in less than a minute, is the ideal solution. In addition to the single-line B-scans, full 2D cross-sectional images (C-scans) are also recorded here, allowing a 3D volumetric investigation of components. The image shows the inspection of a rail vehicle as part of a vehicle maintenance procedure. Here, the quality of bondings on the outer side is being inspected.

Possible areas of application for mobile terahertz scanners are found typically in maintenance, service or repair, where in particular, components, which are difficult for direct access, can be inspected, e.g.

• Inspection of plastic and coated metal pipes during production
• Inspection of insulated and specially protected pipes (e.g. for district heating or marine pipelines) during production, assembly and service
• Inspection of fiber composite assemblies in automotive and aviation such as radomes, train attachments or other lightweight construction during production, service and repair

In principle all sectors are addressed where non-metallic (more generally non-conducting) materials are used, in particular, in manufacturing and processing of plastics or polymers, fiber composites and ceramics in industries such as automotive, rail vehicles or pipe manufacturing, and also service providers.