Layer-like fatigue is induced during mechanical pulping |
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Authors: | Email author" target="_blank">Ari?SalmiEmail author Erkki?Saharinen Edward?H?ggstr?m |
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Institution: | (1) VTT Technical Research Center Finland, P.O.B. 1000, 02044 VTT Espoo, Finland;(2) Electronics Research Laboratory, Department of Physics, Division of Materials Physics, University of Helsinki, P.O.B. 64, 00014 Helsinki, Finland |
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Abstract: | The question whether fatigue is induced during mechanical pulping was addressed experimentally. The grinding process was interrupted
to image partly ground spruce samples. The grinding was performed at five different feed velocities using two different grindstones.
This approach allowed creating an in situ snapshot of the developing grinding zone in the wood samples. The depth profiles
of the stiffness modulus and nm-scale pores, close to and within, the grinding zone were quantified by ultrasonic pitch-catch
measurements and thermoporosimetry. To perform these profiling measurements, wood material was iteratively removed layer-by-layer
with a microtome from the sample surface after taking the snapshot. The grinding-induced changes in cell morphology inside
the sample were imaged using microcomputed tomography, whereas the changes on the surface of the samples were imaged with
optical microscopy and SEM. A layer that penetrated 0.5–1.5 mm into the sample exhibiting up to 80% decreased stiffness modulus—compared
to the unaltered sample parts—was detected when the Wave-type grindstone was employed. The corresponding layer thickness was
0.3 mm with the conventional grindstone. The results match previously measured temperature profiles, and confirm the Atack-May
hypothesis that grinding induces a fatigue layer. Confirming this old, widely used hypothesis is significant for the field
of energy efficiency research related to mechanical pulping and may provide new opportunities for grinding research. |
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