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Material compression causes axial stress in fibres in addition to their bending. By assuming that fibre segments longer than a0s(e) (a0 is the mean segment length) undergo a buckling failure at strain e, the compressive stress σ becomes 
σ(e)=σ1/[s(e)]2, with s satisfying [s(e)+1]exp[−s(e)]=e.
The theory was applied to fibre materials produced with laboratory foam forming process, which uses aqueous foam as transfer medium to deposit fibres into a connected structure. The achieved low density (20−100 kg/m3) of the dried material allowed for individual fibres to bend without contacting the neighbouring fibres. The used raw materials in our experiments were chemical, mechanical and regenerated cellulose fibres of varied dimensions.
The above simple mean-field theory described the experimental stress-strain behaviour surprisingly well at moderate, from 10% to 50%, compression levels. Moreover, high-speed imaging during compression showed abrupt local dislocations, interpreted as buckling failures of heterogeneous fibres under axial stress. In cyclic measurements, we observed significant acoustic emission only when the compressive strain exceeded the previous strains. This suggested a failure source other than fibre bending. Beyond c.a. 50% compression, the number of acoustic events grew rapidly suggesting a crossover to collective phenomena. At the same time, the compression-stress behaviour began to deviate from the mean-field prediction.
 J. A. Ketoja, S. Paunonen, P. Jetsu, E. Pääkkönen, Compression strength mechanisms of low-density fibrous materials. Materials, Vol. 12, 384, 2019.
|Publication status||Published - 11 Jan 2021|
|MoE publication type||Not Eligible|
|Event||14th WCCM & ECCOMAS Congress 2020: Virtual Congress - |
Duration: 11 Jan 2021 → 15 Jan 2021
|Conference||14th WCCM & ECCOMAS Congress 2020|
|Period||11/01/21 → 15/01/21|
- fibre network
- mean-field theory
- image analysis
- acoustic emission
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- 1 Finished
Kiiskinen, H., Lehto, J., Pääkkönen, E., Heikkilä, P., Asikainen, J., Salminen, K., Prakash, B., Forsström, U., Jaiswal, A., Jetsu, P., Jäsberg, A., Kamppuri, T., Keränen, J., Keränen, J. T., Koponen, A., Kouko, J., Lappalainen, T., Lehmonen, J., Pöhler, T., Pihko, R., Seppänen, T., Sorsamäki, L., Timofeev, O., Viitala, J. & Ketoja, J.
1/06/17 → 31/05/20
Project: EU project