Dynamic Mechanical Analysis (DMA) is not very common in wood research. For testing wood, no standards are given. Therefore, researchers make up their own test set-ups which makes a comparison of different measurements difficult if not impossible. In this study the development of a method to manufacture the specimens was one goal that had to be achieved. The following results are representative for tensile tests on a DMA with a load cell of ±25 N.
One of the biggest problems of DMA measurements when testing in tensile modus is the clamping system. The clamps face is smaller than the area necessary to hold the specimens in place because wood can´t withstand the pressure due to a low stability across the grain. With this small load cell one can only measure cross-sections with about 1 mm². Specimens for tensile tests should be as long as possible.
In this study, carried out in two research institutes, two ways of getting precise specimens were obtained: first method was to sand down a board of 3 mm thickness to 0.3 mm by gluing it partially to a MDF panel. The width of 3 mm is than cut by a router to prevent any saw marks and the result is a square cross-section. The advantage is a straight and even stripe having cells which have never been mechanically stressed. A big disadvantage is the need to enforce the ends of the stripes in order to prevent crushing the cells and creating a pull linkage. The second method used micro veneers with a thickness of 100 – 200 µm which were produced with a microtome similar to microscopy specimens. The width is then made by tearing the veneer along the grain. The advantage is a truly orientated strand but no squared cross-section. In general high numbers of growth rings are essential in the radial section. This could be better achieved by using the microtome method. On the other hand an undamaged wood cell is needed and it is for further investigations if the micro veneer has any micro cracks due to the cutting process. In every case specimens have to be climatized during the tests because of potential moisture evaporation during the measurement. The influence of preconditioning of wood samples and the control of relative humidity during measurements will be investigated in future trials.
Keywords: clamping, sample preparation, DMA, DMTA, tensile test, wood
Authors
Stenzel Mirko
Eberswalde University of Sustainable Development, Faculty of Wood Engineering, Eberswalde, Germany
Sanne Moritz
Eberswalde University of Sustainable Development, Faculty of Wood Engineering, Eberswalde, Germany
Larnøy Erik
Norwegian Institute of Bioeconomy Research, Department Wood Technology, Norway
Treu Andreas
Norwegian Institute of Bioeconomy Research, Department Wood Technology, Norway
Herold Nadine
Eberswalde University of Sustainable Development, Faculty of Wood Engineering, Eberswalde, Germany
Pfriem Alexander
Eberswalde University of Sustainable Development, Faculty of Wood Engineering, Eberswalde, Germany
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