Abstract
Biological materials possess a variety of artful interfaces whose size and properties are adapted to their hierarchical levels and functional requirements. Bone, nacre, and wood exhibit an impressive fracture resistance based mainly on small crystallite size, interface organic adhesives and hierarchical microstructure. Currently, little is known about mechanical concepts in macroscopic biological interfaces like the branch-stem junction with estimated 1014 instances on earth and sizes up to few meters. Here we demonstrate that the crack growth in the upper region of the branch-stem interface of conifer trees proceeds along a narrow predefined region of transversally loaded tracheids, denoted as sacrificial tissue, which fail upon critical bending moments on the branch. The specific arrangement of the tracheids allows disconnecting the overloaded branch from the stem in a controlled way by maintaining the stem integrity. The interface microstructure based on the sharply adjusted cell orientation and cell helical angle secures a zig-zag crack propagation path, mechanical interlock closing after the bending moment is removed, crack gap bridging and self-repairing by resin deposition. The multi-scale synergetic concepts allows for a controllable crack growth between stiff stem and flexible branch, as well as mechanical tree integrity, intact physiological functions and recovery after the cracking.
Highlights
Direction of the helical windings of cellulose fibrils in the secondary cell wall and the cell longitudinal axis[23,24]
The wood tracheids within the region containing the sacrificial tissue are aligned perpendicular to the expected crack propagation direction enabling controlled cracking by loading the cells transversally
Since the crack propagation is accompanied by a tissue shear deformation, especially at the vertex of the embedded branch in the stem, bundles of tracheids reinforced with perpendicularly oriented wood rays are formed in the crack gap
Summary
Direction of the helical windings of cellulose fibrils in the secondary cell wall and the cell longitudinal axis[23,24]. It was observed that branch tissue is embedded in stem collars overgrowing the branch[25] where MFA magnitude in the stem envelope was found to be adapted to the environmental and functional requirements[21]. There were no pronounced strain concentrations measured at the branch-stem cross-section during branch bending[22]. MFA distribution in the vicinity of branch-stem junction, local microstructure as well as interface mechanical behaviour have not been studied yet in detail. Since the tree possesses a complex hierarchical microstructure, the interface optimization is expected to take place at multiple length scales
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