Abstract
In this work we challenge some earlier theoretical ideas on the strength of lightweight fiber materials by analyzing an extensive set of foam-formed fiber networks. The experimental samples included various different material densities and different types of natural and regenerated cellulose fibers. Characterization of the samples was performed by macroscopic mechanical testing, supported by simultaneous high-speed imaging of local deformations inside a fiber network. The imaging showed extremely heterogeneous deformation behavior inside a sample, with both rapidly proceeding deformation fronts and comparatively still regions. Moreover, image correlation analysis revealed frequent local fiber dislocations throughout the compression cycle, not only for low or moderate compressive strains. A new buckling theory including a statistical distribution of free-span lengths is proposed and tested against the experimental data. The theory predicts universal ratios between stresses at different compression levels for low-density random fiber networks. The mean ratio of stresses at 50% and 10% compression levels measured over 57 different trial points, 5.42 ± 0.43, agrees very well with the theoretical value of 5.374. Moreover, the model predicts well the effect of material density, and can be used in developing the properties of lightweight materials in novel applications.
Highlights
Fibrous materials are bulk media that consist of high-aspect ratio fibers of any type
The current study focuses on highly porous fiber materials prepared with foam forming without added matrix polymers
In this paper we developed a new theoretical model for the compression strength of low-density random fiber networks assuming that buckling of fiber segments dominates the strength behavior throughout the compression curve
Summary
Fibrous materials are bulk media that consist of high-aspect ratio fibers of any type. The current study focuses on highly porous fiber materials prepared with foam forming without added matrix polymers. Foam forming is a new technology which enables the production of ultra-light fiber networks with a density as low as 5 kg/m3 [1]. The strength behavior for very low densities has been analyzed in several papers [1,2,3]. The intermediate density range of 20 kg/m3 to 100 kg/m3 , perhaps the most relevant for practical applications, has been rather poorly understood
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