Ketoja Jukka A, Paunonen Sara, Jetsu Petri, Pääkkönen Elina
VTT Technical Research Centre of Finland Ltd, Solutions for Natural Resources and Environment, P. O. Box 1000, FI-02044 VTT Espoo, Finland.
Materials (Basel). 2019 Jan 26;12(3):384. doi: 10.3390/ma12030384.
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.
在这项工作中，我们通过分析大量泡沫成型纤维网络，对一些早期关于轻质纤维材料强度的理论观点提出了挑战。实验样品包括各种不同的材料密度以及不同类型的天然和再生纤维素纤维。通过宏观力学测试对样品进行表征，并辅以纤维网络内部局部变形的同步高速成像。成像显示样品内部的变形行为极不均匀，既有快速推进的变形前沿，也有相对静止的区域。此外，图像相关性分析表明，在整个压缩循环过程中，不仅在低或中等压缩应变下，局部纤维位错频繁发生。我们提出了一种新的屈曲理论，该理论包含自由跨度长度的统计分布，并根据实验数据进行了验证。该理论预测了低密度随机纤维网络在不同压缩水平下应力之间的通用比率。在57个不同试验点测量的50%和10%压缩水平下应力的平均比率为5.42±0.43，与理论值5.374非常吻合。此外，该模型很好地预测了材料密度的影响，可用于开发新型应用中轻质材料的性能。
