, Volume 18, Issue 6, pp 1423–1432 | Cite as

Layer-like fatigue is induced during mechanical pulping

  • Ari SalmiEmail author
  • Erkki Saharinen
  • Edward Hæggström


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.


Ultrasound Thermoporosimetry μ-CT Mechanical pulping Grinding Fatigue 



We would like to thank Mrs. Raija Lahtinen for her valuable help with the microtome cutting. We are grateful for the valuable comments on the manuscript by Dr. Lauri Salminen. In addition, fruitful discussions with Mr. Mikael Lucander are gratefully acknowledged.


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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  • Ari Salmi
    • 1
    • 2
    Email author
  • Erkki Saharinen
    • 1
  • Edward Hæggström
    • 2
  1. 1.VTT Technical Research Center FinlandEspooFinland
  2. 2.Electronics Research Laboratory, Department of Physics, Division of Materials PhysicsUniversity of HelsinkiHelsinkiFinland

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