Fraunhofer Institute for Wood Research We develop and implement measurement procedures for process and quality control in the woodworking industry. Of particular importance thereby are non-destructive testing procedures such as infrared thermography, image processing methods and optical measuring techniques. The developed measurement procedures are not limited to wood or wood-based materials. They are suitable for all industrial sectors in which composites and bulk materials are produced and processed.
Measurement Technology
Research focus
Heat-flow thermography
Heat-flow thermography is a non-destructive testing method and is based on the fact that all objects emit infrared radiation which is characteristic for the respective surface temperature. This can be detected using special cameras and presented as an image of the surface temperature.
In its most important variant, the object being examined is subjected to a thermal impulse (active thermography). The diffusion rate of the resulting heat front, which penetrates into the object, depends on the thermal conductivity, the specific heat capacity and the density of the material below the surface. Patterns therefore form in the surface temperature which enable conclusions to be drawn concerning the condition of the measurement object below the surface.
In this way, defects can be detected which lie below the surface and which are not externally visible. The procedure provides images, is fast and can be relatively easily automated. Fields of application are primarily the detection of bonding and adhesion defects, delamination, blisters and cavities, as well as the detection of foreign bodies, for example in food items.
Non-destructive inspection of rotor blades
One focus of our research is the application of heat-flow thermography for the non-destructive testing of the rotor blades of wind turbines. The rotor blades are subjected to extreme loads during operation. In order to avoid breakage of the rotor blade, it must be regularly inspected both prior to installation and during subsequent operation. With the aid of heat-flow thermography, we detect production and design flaws such as cracks, faulty bonding and air pockets as well as damage caused through wear and material fatigue – all within a few minutes per square meter of surface area. Heat-flow thermography is, in most cases, more reliable - and therefore more economical - than conventional procedures such as visual inspection or the percussion method.
Measurement of wood particle size
If wood particles (fibers, shavings, strands) are to be used to produce board materials, their size must be determined and the frequency distribution of size parameters such as length and width must be represented and characterized. In manufacturing, such characteristic values are difficult to determine and are often not accessible via sieve analysis. With image processing procedures which are tailored to suit the respective material, automated laboratory and online measurements are already possible: for example, wood species-specific chip shapes can be distinguished and particle sizes and shapes can also be determined through refiner fibers, pressed surface layer chips or strands in the fleece.
Near-infrared spectroscopy
Near-infrared spectroscopy is an established measurement technique. Additional knowledge concerning the spatial distribution of moisture or foreign matter in the fleece or the adhesive on wood particles can be obtained if the technique is applied not only with a simple sensor, but instead with a near-infrared camera and subsequent multivariate data analysis. In the recycling of waste wood, this technique can be applied in the sorting/rejection of wood which is contaminated through organic wood preservatives or PVC surfaces.
Computed tomography
Materials and components can be tested nondestructively using computed tomography (CT): The surface and the internal topology of the complete sample is mapped with a resolution down to the micrometer scale and imaged in three dimensions. A CT evaluation can comprise several qualitative and quantitative analyses, e. g. the particle structure, the density distribution and the dimensions. For example particle boards, MDF, OSB, WPC, plywood, insulation materials and wood foams can be measured. Samples with a size sufficient to evaluate local variations of the material quality (A4 and bigger) can be scanned with the equipment at Fraunhofer WKI.
Laser triangulation
With laser triangulation, distances can be measured. Furthermore, with an arrangement consisting of a line laser and an area scan camera, a 3D topography can be recorded on moving surfaces and important properties such as ripple, pore size and roughness can thereby be evaluated.
Transient Hot-Bridge (THB) procedure
With the Transient Hot-Bridge (THB) procedure, the thermal conductivity can be measured within minutes, even on very small samples. This procedure is suitable for both the determination of the heat dissipation of heat-conducting oils and the verification of thermal insulation materials used in buildings.