New technologies drive development

One of these relatively new technologies is terahertz (THz) technology. This technology has been established for a long time, for example in people screening at airports. Terahertz systems are used there to search for hidden weapons or explosives. In industry, too, this technology is increasingly being used as an alternative to ultrasound or thermography. Terahertz waves - their wavelength of 0.1 THz (100 GHz) to 10 THz lies between that of microwave and infrared radiation - can penetrate foams, plastics, but also ceramics and other non-conductive materials. They are therefore suitable for investigating surfaces and interfaces. They ignore cavities and allow real-time measurements of layer thicknesses up to 500 mm. Compared to X-ray technology, terahertz waves have the advantage that they do not damage biological material. THz technology is not suitable for all conductive materials, i.e. metals. Structures of less than 0.1 mm in size also set limits to this technology. And water is also impenetrable for terahertz waves. Terahertz waves are generated either by laser or fully electronically.

Making quality defects visible with terahertz waves

At the 10th special show on non-contact measurement technology during Control 2014, organized by the Fraunhofer Vision Alliance, Recendt GmbH from Linz (Austria) presented a fiber-coupled measurement system. It can be used to perform both spectroscopy and total coating thickness measurements. Since the measuring head and measuring system can be spatially separated from each other by several meters, this opens up new possibilities in terms of agility. Becker Photonik GmbH from Porta Westfalica (Germany) is also playing the mobility card. With the SynViewCompact system, the company presented a mobile 3-D terahertz scanner at the special show. The waves are generated fully electronically and not by the comparatively complex method using lasers. The device can be used for non-destructive testing of plastics and ceramics, with a penetration depth of up to 100 mm, depending on the frequency and material. Concrete examples of applications include the examination of plastic welds or bonds.

Testing of failure-critical components

Defects in components can have fatal consequences. Time and again, accidents occur whose causes are to be found in faulty material. Material fatigue can also have serious, sometimes catastrophic, consequences. One example of this is the train accident in Eschede, Germany, on June 3, 1998, when an ICE train derailed at 200 km/h due to a broken wheel tire, killing 101 passengers. The testing of so-called failure-critical components helps to avoid such catastrophes. The resonance test method, for example, is used here. Here, the structure-borne sound, i.e. the sound that propagates in a solid body, is recorded and the vibration resonances are analyzed. The measured results can then be compared with reference values. Such tests are also carried out during the production process. However, process variations can now lead to a shift in resonance patterns. Therefore, these variations must be compensated for so that a reliable distinction can be made between pseudo defects and real defects. This is exactly what the Quasar PCRT (Process Compensated Resonance Testing) from Quasar Europe does. This device combines resonance ultrasound spectroscopy (allows statements to be made about the elasticity of a body) with the patented Vibration Pattern Recognition VIPR (allows statements to be made about defects that lead to functional impairments). According to the manufacturer, a central element is the sorting module. This contains a definition of resonance patterns of the IO test objects. The measured patterns are compared with these so that sorting into IO and NIO parts can take place for the subsequent production steps.

Need for optimization at automotive suppliers?

There is an increasing demand for this test method from the automotive industry. "The most important Tier 1 or component suppliers in the automotive industry are Quasar users," reports Dipl.-Ing. Thomas Köhler, Managing Director of Hesselmann & Köhler Prozessautomation GmbH in Limburg, which distributes Quasar PCRT. One reason for this is the standardization according to ASTM E2534-10. Another, probably more important reason, is to be found in the recent waves of recalls that have shaken the automotive industry. According to Hesselmann & Köhler, this increases the pressure to equip internal test benches with technologically more sophisticated processes. The results of studies by the Center of Automotive Management (CAM) also confirm this trend. In many quality assurance processes of automotive suppliers, there is a need for action for optimization.

Non-contact measurement of layer thicknesses

Many products today are wet or powder coated. A homogeneous and uniform coating thickness is a quality feature. Winterthur Instruments AG has developed the CoatMaster for the non-contact measurement of coating thicknesses. This device measures the coating thickness of wet, tacky, powdery and solid coatings with a thickness of 1 μm to 1 mm without contact and non-destructively immediately after application. Regardless of the color of the coating or the shape and material of the substrate, the device provides accurate test results. It works according to the principle of thermal coating testing. In thermal coating testing, the coating to be tested is briefly heated by exposure to light, while infrared sensors record the resulting curve of the surface temperature. The temperature decays with a characteristic dynamic that depends on the thermal properties of the coating. Thus, coating parameters such as coating thickness, but also porosity, thermal sheet resistance, thermal conductivity and thermal diffusivity can be determined quickly and reproducibly. The new version of the instrument visualizes the measuring range by means of LED technology and now also offers single point marking with a diameter between 1 and 50 mm. Layer thicknesses can be determined from a distance of 5 to 25 cm. The device can also be used inline for the fully automated control of coating systems. The company J. Wagner GmbH from Markdorf on Lake Constance, for example, has integrated the CoatMaster into a powder coating system and measures the layer thickness inline directly after the paint application.

Information and training

The above examples are only a small selection of the many applications of non-destructive testing with image processing. The technologies are constantly developing and opening up new fields of application. This was demonstrated at Control 2014, and a special show on "Non-Contact Measurement Technology" is again planned for Control 2015 under the leadership of the Fraunhofer Vision Alliance. It is not always easy for new potential users to decide whether one of the new technologies is suitable for their own application. In addition, most image processing systems are not "off the shelf" products, but rather devices developed specifically for the application. It is therefore worthwhile to keep up to date with the innovations presented by the suppliers.