Multi-scale Tensile Mechanical Behavior And Toughening Mechanism For Bamboo

Profound mechanical principle must be contained in bamboo, basing on its perfect mechanicalproperties, high strength, high toughness and high flexural ductility. Investigating themechanical behavior of bamboo, not only helps understand of the biomechanics of bamboogrowth mechanism, but also guids the development of bio-inspired materials. In the field ofcomposites, bamboo could be regarded as a kind of composite with parenchymatous groundtissues and fiber sheaths, which respectively function as matrix and reinforcement in bamboo.Besides, in the field of biological material, bamboo is a typical natural material withhierarchical structures from macroscopic to nanometer scale and the corresponding interfaces.It is necessary to obtain the mechanical properties of bamboo at different scale to understandthe mechanisms of high strength, high toughness, high ductility of bamboo.4years old Moso bamboo was studied in this report. By a PALM-Micro Beam,, the mainproblem on the mechanical properties study of bamboo that preparing the tensile samples ofbamboo fiber sheath with regular shape and little damage, was successfully resolved. Theparenchymatous ground tissues without fibers were also cut from soaked bamboo. Both makethe study at different scale available. In the second part of this report, the tensile mechanicalproperties for bamboo at different scale were tested and the mechanical variation of singlebamboo fibers with respect to age and locations in the culms, which aim to reveal the effects ofthe interface between single fibers and between the fiber sheath and the ground tissue on thestrength and modulus at macroscopic. In the third part of the report, the nominal fracturetoughness, work of fracture and the specific work of fracture of the bamboo and its componentswere obtained at different scale, which aim to reveal the effects of the interface between singlefibers and that between the fiber sheath and the ground tissue on the fracture toughness atmacroscopic. In the fourth part of the report, some innovative study were made. The bendingand compression behaviors were obtained for the first time and brought the key reason of the excellent flexural ductility for bamboo light.The major results are summarized as follows:1. The tensile mechanical properties of bamboo at different scale are obtained. The averagetensile modulus of Moso bamboo is10.22GPa,151MPa for tensile strength,2.34%forelongation. The average tensile modulus for the fiber sheath is47.33GPa,729.25MPa fortensile strength,2.04%for elongation. The average tensile strength for parenchymatous groundtissues is40.02MPa, tensile modulus of1.7MPa, elongation3.16%; The average tensilemodulus of Moso is30.83GPa,1110MPa for tensile strength,6.7%for elongation.2. This paper revealed the influence of different scale interfaces on the strength and modulus atmacroscopic. The excellent strength and modulus of single bamboo fiber is the basement forthe high strength and stiffness of bamboo. The interface between the single bamboo fibers cansignificantly reduce the tensile strength and elongation at break of the fiber sheath, but there isa certain enhancement effect on the tensile modulus. Interface between the ground tissue andfiber sheath is very special, which could enhance the tensile strength and elongation at breakfor the bamboo, but slightly reduce its macro-stiffness.3. The nominal fracture toughness, work of fracture and the specific work of fracture of thebamboo and its constituent units are obtained at different scales.The average nominal fracture toughness of Moso bamboo is6.40MPa·m^1/2,2.45×10⁶J/m3forwork of fracture,3.06×10⁶J/g for specific work of fracture. The average nominal fracturetoughness of bamboo sheath is2.53MPa·m^1/2,4.2×10⁶J/m3for work of fracture,2.8×10⁶J/g for specific work of fracture. The average nominal fracture toughness of bamboo is0.23MPa·m^1/2,0.67×10⁶J/m3for work of fracture,1.37×10⁶J/g for specific work of fracture. Theaverage nominal fracture toughness of single bamboo fiber is4.09MP·m^1/2,23.85×10⁶J/m3for work of fracture,15.9×10⁶J/g for specific work of fracture.4. The effects of different-scale interfaces on the fracture toughness at macroscopic. Bamboofiber has high fracture toughness, but the ground tissue is a kind of brittle materials. Theperfect toughness for bamboo mainly originated from the single bamboo fiber. The interfacebetween single fibers can significantly reduce the fracture toughness of the fiber sheath, but that between ground tissue and fiber sheath enhances bamboo toughness.5. The key mechanism of high flexural ductility for bamboo was also revealed. The loadingmode of the outer stretched and the inner compressed, resulting from the the decreasing of thefiber volume for bamboo from outer to inner, mainly contributes to the high bending ductility.The relative curvature would be reduced without the outer or the inner. Besides, theoutstanding resistance to compression is also important to raise the bending behavior ofbamboo.