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Damage Kinetics at the Sub-micrometric Scale in Bast Fibers Using Finite Element Simulation and High-Resolution X-Ray Micro-Tomography

Published

2 years ago

March 9, 2019

Republished by Plato

. 2019 Feb 21;10:194.

doi: 10.3389/fpls.2019.00194. eCollection 2019.

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Sofiane Guessasma et al. Front Plant Sci. 2019.

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Abstract

This study combines experimental testing and computation analysis to reveal the role of defects and sub-micrometric microstructure in tensile behavior of hemp bast fibers. In particular, these structural defects represent the footprint of the processes to which the fibers elements are subject along the whole transformation chain from the plant to the end use product. Tensile experiments performed on elementary fibers and bundles in a wide diameter range (40-200 μm) are simultaneously conducted with X-ray micro-tomography observation. 3D images of ultra-fine resolution (voxel size of 280 nm) are achieved at different deformation magnitudes up to the complete failure thanks to the use of synchrotron radiation (ESRF, Grenoble, France). A Finite element (FE) model is implemented based on the conversion of the tomograms into 3D meshes. High performance computing is used to simulate the tensile response of the hemp bast fibers. In particular, the effects of notching and sub-micrometric structure of the fibers are explored. Results show the presence of different types of diffuse damage kinetics, which are related to the variability in the fiber size, surface defects and the presence of the lumen space. The damage behavior is found to be sensitive to the type of stress criterion implemented in the FE computation. The predictive analysis demonstrates the relevance of using embedded microstructure simulations to reveal the extent of stress localization and predict the failure properties in bast fibers for innovative composite manufacturing for instance.

Keywords: X-ray micro-tomography; bast fiber; damage kinetics; finite element simulation; microstructure; tensile properties.

Figures

**FIGURE 1**

Optical images illustrating four tested hemp fibers: (left top) V-notched elementary, (left bottom) V-notched bundle, (right top) U-notched elementary, and (right bottom) U-notched bundle. Notching is achieved using laser micro-dissection and the geometrical shape (triangle or ellipse) expected are superimposed on the micrograph in green.

**FIGURE 2**

Experimental setup at synchrotron radiation facility at ESRF Grenoble, France showing the tensile testing equipment and the X-ray micro-tomography imaging system.

**FIGURE 3**

Magnified view close to a U-notch on digitalised hemp fiber showing 3D regular meshing using voxel-to-element conversion.

**FIGURE 4**

X-ray micro-tomography and corresponding optical images of hemp bundles and elementary fibers under various conditions: **(a)** bundle with V-notch acquired in a typical volume of 456 × 465 × 2048 voxels; **(b)** elementary fiber with a V-notch (typical volume 798 × 340 × 1846 voxels). Multiple elementary fibers in several bundles acquired in typical volumes **(c)** 1389 × 1722 × 1756 voxels and **(d)** 1004 × 970 × 2048 voxels. The voxel size is 280 nm in each case.

**FIGURE 5**

3D imaging of defects illustrated in hemp bast fibers: **(a)** tubular lumen space revealed using corner flooding, and 3D labeling technique on bundle FXU5, **(b)** longitudinal cross-section view near a collection of kink bands along the radial dimension of the bundle (white frame).

**FIGURE 6**

Statistical analysis on lumen structural attributes (cross-section area and aspect ratio) in the transverse dimensions.

**FIGURE 7**

Finite element (FE) computation results showing the displacement component UX and stress component σ_XX for a typical tensile loading by 8% with respect to the original length in X-direction of a hemp bundle with a U-notch (FXU4).

**FIGURE 8**

Finite element computation results showing the stress distribution evolution as a function of the load level for different damage criteria: S_I, S₁, and σ_ZZ (U-notch bundle FXU4). The load level label is indicated for each sequence.

**FIGURE 9**

Evolution of the damage ratio as a function of the load level for different damage stress criteria S_I, S₁, and S_ZZ (U-notch (FXU4). The damage ratio is labeled for each sequence. The snapshots are extracted based on a regular stepping between the initial load step (image on the top) and the displacement at break (image on the bottom).

**FIGURE 10**

Predicted damage evolution for different stress criteria for a hemp bundle with a U-notch (FXU4).

**FIGURE 11**

Finite element computation results showing the **(a)** principal stress distribution and **(b)** damage evolutions for the damage criterion S1 under tensile loading by 8% of an elementary hemp fiber with a U-notch (FU). Computations are performed with up to 36.37 × 10⁶ dof.

**FIGURE 12**

Predicted damage evolution for different stress criteria for an elementary hemp fiber with a U-notch (FU).

**FIGURE 13**

Predicted stress intensity and damage ratio for a hemp elementary fiber with **(a)** a V-notch (FIV15) and **(b)** a U-notch (FU19) using a stress intensity damage criterion.

**FIGURE 14**

Predicted damage evolution for different stress criteria for a hemp elementary fiber with **(A)** a V-notch (FIV15) and **(B)** a U-notch (FU19).

Cited by 1 article

The Middle Lamella of Plant Fibers Used as Composite Reinforcement: Investigation by Atomic Force Microscopy.

Melelli A, Arnould O, Beaugrand J, Bourmaud A. Melelli A, et al. Molecules. 2020 Feb 1;25(3):632. doi: 10.3390/molecules25030632. Molecules. 2020. PMID: 32024088 Free PMC article.

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