Aerospace, automotive and airplane construction rely on lightweight construction. But, to make sure that lightening the load does not come at the expense of safety, German researchers from the biggest European applied research orga- nisation, Fraunhofer, are working on new assurance systems for materials testing.

At the Composites Europe trade show, which took place from October 27 to 29, in Stuttgart, Germany, the researchers demonstrated a new kind of nondestructive diagnostic procedure.

Shearography and Thermography
Boats, airplanes and spacecraft are becoming lighter and lighter. Lightweight structures help save fuel and lower carbon dioxide emissions. “To ensure that police and marine emergency services can rely on their lifeboats, even in high seas, the lightweight components undergo comprehensive quality assurance before being put to actual use,” affirms a spokesperson for the Department of Testing Systems at the Institute for Manufacturing Engineering and Automation.

Dr Tina Wilhelm, group manager for measurement and testing technology at the department, colleague Wolfgang Schmidt and other experts are hard at work on quality assurance systems for lightweight construction. At the Composites Europe trade show, the researchers presented a combination of two testing systems: shearography and thermography.

Shearography is a laser optical proce- dure that allows measurement of material deformations in the triple-digit nanometre range.

Thermography allows one to measure temperature differences on the surface of materials with accuracy to just a few milli- kelvins. If an air pocket is enclosed on the adhesive side of a component, then the thermographic camera will measure different heat values than those of properly seamed locations.

“With shearography, the material expands differently at the site where a defect occurs,” says Wilhelm.

For both procedures, the material must be simulated mechanically, thermally or elec- trically, using force, ultrasound or light.

The experts measure twice: once before and once after the stimulus. The measurement values clearly indicate where delaminations, that is defects in the bonded connection, have occurred.

“Depending on the procedure, various physical effects are used for the testing. What is important is that you can reliably detect defects with the two technologies, both on the surface and in the interior of the component.

At the trade show, the Fraun-hofer researchers demonstrated the testing technologies on a clutch disc.
The researchers are not currently working on the combination of both measurement principles only. An essential focal point of the working group is to ascertain the materials for which shearography is best suited in order to find hidden delaminations, and to determine when it is better to use thermography.

“There is still a considerable amount of research needed,” says Wilhelm.

High Tech for Bicycles
Carbon-fibre composite materials (CFRPs) not only make cars and airplanes lightweight, but also benefit the lightweight constructions for valuable bicycle concepts. At the Composites Europe trade show, in Stuttgart, German researchers presented a spring-loaded seat post made of CFRPs. The seat post was designed at the Institute for Chemical Technology, in Pfinztal, as part of Fraunhofer’s innovation cluster known as Technologies for Hybrid Lightweight Construction.

The component is based on two CFRP profiles that work like laminated springs. The prototype of the CFRP seat post was manufactured through a resin-transfer moulding process. The new product gives more ride comfort at 30% less weight.