The Structural Origins of Wood Cell Wall Toughness

Abstract

arrangements. A widely accepted model [3] describes a helical arrangement within the cell wall with the S1, S2, and S3 layers of the cell wall distinguishing themselves by the angle of the microfibrils (MFA) relative to the cell axis (Figure 1c). However, further details of the ultrastructure are frequently controversially discussed, including MFA variations within a given cell wall layer, the existence of a radial component to the microfibril direction, and the structure at the interfaces between the secondary cell wall layers. [4,5] The fracture behavior of wood has been the subject of many studies [1,[6][7][8][9][10][11][12][13][14][15] and is influenced by whether the cracks propagate across the grain, thereby deforming and rupturing the wood cells, or along the grain. The energy of fracture or toughness for cracks that propagate across the grain is quite high, ≈10 kJ m −2 , [16] which is comparable to engineering Al alloys or medium C steels. The main toughening mechanism is believed to be plastic buckling of the cell walls under tension due to the helical arrangement of the cellulose microfibrils. [16] In contrast, the energy of splitting fracture along the grain is roughly 10 times smaller. [1] Although mode I splitting cracks typically grow unstably, showing brittle behavior, investigations of their morphology reveal toughening from microcracking and crack bridging involving several to many wood cells, which contribute to the modest toughness. [17,18] A closer look shows that splitting cracks either connect up the lumen by propagating through the cell walls between the lumen (transwall fracture), or propagate entirely within the cell walls (intrawall fracture or intercell fracture). [19,20] Although transwall fracture would clearly be energetically favorable if the cell walls were isotropic material, wood frequently exhibits intrawall fracture. For example, pine wood regularly splits by intrawall fracture at the S1/S2 interface, as well as in the CML or the S1 layer. [1,20,21] Although the anisotropic arrangement of microfibrils in the cell wall is clearly important in determining the crack path and the toughness, [7] its detailed role in splitting fracture has received little attention to date.In this work, we perform in situ electron microscopy studies of the propagation of axial splitting cracks in the cell walls of pine latewood. The goal is to understand how cell-wall level toughening mechanisms and crack path selection depend on local microfibril arrangements and account for the splitting fracture toughness of wood. Pine wood is chosen because of its relatively simple cell structure and because of its tendency for intrawall fracture. [1] Specimens for ultrastructure characterization by transmission electron microscopy (TEM) were prepared by microtome and focused ion beam (FIB) machining (Figure 1d, for details The remarkable mechanical stability of wood is primarily attributed to the hierarchical fibrous arrangement of the polymeric components. While the mechanisms by which fibrous cell structure an...

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