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Thermographic inspection of semifinished leather products

Results from preliminary investigations, prospects for in-line applications
: Plinke, B.; Schlüter, F.; Meyer, M.; Nowara, P.

Forschungsinstitut für Leder- und Kunstledertechnologie -FILK-, Freiberg:
4. Freiberger Kollagensymposium = 4th Freiberg Collagen symposium : September 11-12, 2008
Freiberg: FILK, 2008
6 pp.
Freiberger Kollagensymposium <4, 2008, Freiberg>
Freiberg Collagen Symposium <4, 2008, Freiberg>
Conference Paper
Fraunhofer WKI ()

Several imaging techniques are used for material inspection such as X-ray radiography, ultrasound imaging or 'normal' image acquisition in the range of visible light. Thermography uses cameras and sensors for the range of 'therma' infrared radiation delivering thermal images which represent the radiation density emitted from a surface. A surface of a homogeneous solid body cooling down from a state of homogeneous temperature shows homogeneous thermal images. Any inhomogeneities in density, moisture content, thermal conductivity, surface properties or also internal delaminations etc. have an influence on the local heat flux. They appear in the thermal images as cold spots, hot spots or as a pattern corresponding to internal structures.
If the samples come homogeneously heated from an industrial process then "passive" thermographic inspection can be performed. Examples are
· board products leaving a coating press - detection of foil coating defects
· particle streams leaving a dryer - detection of overdried particles for fire protection
· raw particle boards - monitoring the density distribution
"Active" thermography means that the samples receive a heat pulse before passing the IR camera. This heat pulse may be short (from flashlights) or long (from an IR line source or using hot steam or heating plates) but it has to be more or less homogeneous. The heat flux from the warm surface goes into the depth of the material as well as (by radiation and convection) to the surrounding. Especially delaminations can be detected like e.g.
· Bonding defects in laminate floor panels of composite products made from several layers
· Internal defects in structural parts like rotor blades of wind power plants
Thermography is an imaging technique and can be used for a quick overview as well as for deeper inspections where several other techniques do no work for certain reasons. Cameras available today are so sensitive that even small temperature differences in the range of 15 mK can be detected. A specific evaluation technique is pulse phase thermography (PPT): An images series of the cooling phase is acquired so that the intensity as a function of time is obtained for every pixel. The image series is then transferred from the time domain to the frequency domain by pixelwise Fourier transformation. In such a way the contributions from objects in different depths of the material to the surface temperature pattern can be distinguished.
WKI has gathered experiences with thermographic material inspection since approx. 10 years. Together with the Research Institute of Leather and Plastic Sheeting (FILK), Freiberg, and the Society for the Promotion of Applied Computer Science (GFaI), Berlin, WKI works in a research project with the objective to develop methods to localize hidden defects in natural materials. Leather and wood may show growth features which are not all visible from the surface and where inspection techniques using infrared radiation and visible light can be combined. Especially for leather it is important to detect defects at the beginning of the value added chain to reject defective material. Therefore, initial investigations were focussed on leather in the "wet " state.
Leather samples in different states (raw; wet blue; wet blue but dried; crust) were investigated with active thermography using several methods:
· Excitation of stationary samples with a short thermal pulse from a flashlight,
· excitation of the samples moving on a conveyor with a continuous line IR source (quartz or carbon lamp),
· contact heating with hot plates in a laboratory press.
Leather has good thermal insulation properties. Therefore, the surface temperatures of the samples after excitation are in the range of 30 °C L 80 °C, and the cooling periods to be observed are 10 s .. 100 s, depending on the method of excitation and the sample properties. Thermal image series were acquired using a high-performance or a medium-performance infrared camera and evaluated with the software "FTS Viewer" developed by WKI.
Thermal images of wet-blue or dry leather samples acquired during the cooling period show hidden structures which are scarcely or not at all visible by the eye. Depending on the method of excitation, structures closer to the surface (using a flash light) or deeper in the cross section (using a continuous IR source or contact heating) are reflected in the thermal images, but also folds and borders due to the angle of incidence of the thermal radiation. More excitation techniques (contact heating in press devices, steaming) have been tested, new results will be reported in the presentation. For moist leather better results cannot be expected because thermal equilibrium is reached very soon after excitation and therefore no contrasts caused by material anomalies appear in thermal images. For dry leather products, however, the conditions are much better. Further investigations are scheduled for finished products. More findings regarding on-line measurements in a later project phase can be expected if the detection results are correlated with visual defect.