Müller Ulrich, Gindl-Altmutter Wolfgang, Konnerth Johannes, Maier Günther A, Keckes Jozef
Institute of Wood Technology and Renewable Materials, University of Natural Resources and Life Science, Vienna, Austria.
Materials Center Leoben, Leoben, Austria.
Sci Rep. 2015 Sep 29;5:14522. doi: 10.1038/srep14522.
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 10(14) 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.
生物材料具有各种巧妙的界面，其尺寸和特性与其层次结构水平和功能要求相适应。骨骼、珍珠层和木材主要基于微晶尺寸、界面有机粘合剂和层次微观结构而展现出令人印象深刻的抗断裂能力。目前，对于宏观生物界面中的力学概念，如地球上估计有10（14）个实例且尺寸可达数米的树枝-树干连接处，人们了解甚少。在此，我们证明针叶树树枝-树干界面上部区域的裂纹扩展沿着横向加载的管胞的狭窄预定义区域进行，该区域被称为牺牲组织，在树枝受到临界弯矩时会失效。管胞的特定排列方式能够通过保持树干的完整性，以可控的方式将过载的树枝与树干断开连接。基于急剧调整的细胞取向和细胞螺旋角的界面微观结构确保了裂纹呈锯齿状扩展路径，在弯矩消除后机械互锁闭合，裂纹间隙桥接以及通过树脂沉积进行自我修复。这种多尺度协同概念允许在坚硬的树干和柔韧的树枝之间实现可控的裂纹扩展，以及树木的机械完整性、完整的生理功能和裂纹后的恢复。
