The Study On The Interlaminar Fracture Toughness Of Bamboo

As a typical unidirectional long fiber reinforced biocomposite, after billion years’ evolution, the fine structure-"internode+node" of bamboo is formed and the existence of node plays an essential role in resisting the bending fracture caused by wind and snow for bamboo.Bamboo node consists of nodal ridge, sheath scar and diaphragm. When vascular bundles pass through node from internode, they all bend in different extent and most of them pass though node directly. While some of them change their directions in node:the vascular bundles closed to the inner wall trend to the outer wall; the vascular bundles closed to the outer wall trend to the inner wall oppositely; meanwhile there are some vascular bundles enter into diaphragm circuitously which incline procumbent circumferentially or interweave together through diaphragm to the opposite wall. And the density of bamboo node is a little higher than that of the adjacent internode.Bamboo is also a kind of extremely anisotropic material and its tensile strength along axial direction is as high as 150-300MPa while the transverse tensile strength and the shear strength along grain are only 1/85 time and 1/20 time respectively of the tensile strength along axial direction. Thus, in the process of application, cracks along grain always occur in different section and these behaviors are also called interlaminar cracking. What’s more, once cracks along grain happen, the propagation of interlaminar crack is controlled by interlaminar fracture toughness instead of strength. So it is very important to study on the interlaminar fracture of bamboo and to test the interlaminar fracture toughness for the structure strength design of bamboo and its practical application. But there are few studies or tests on this aspect and there’s no report or study on the behavior mechanism and toughness contribution of bamboo node in the progress of bamboo interlaminar fracture. And the Experiment parameter substitution method, Timoshenko beam theory method and Compliance calibration method are the methods those used to evaluate the Mode Ⅱ interlaminar fracture toughness of composites, while it still remains unknown which method is the proper one to evaluate the Mode Ⅱ interlaminar fracture toughness of bamboo.In this article, firstly, the double cantilever beam (DCB) method was applied to test the Mode Ⅰ interlaminar fracture toughness of Moso bamboo internode specimens and specimens with node to study the difference of the Mode Ⅰ interlaminar fracture toughness between Moso bamboo internode specimens and specimens with node; then the end notched flexure (ENF) beam specimen was used to test the mode Ⅱ interlaminar fracture toughness along grain of meso bamboo internode and the Experiment parameter substitution method, Timoshenko beam theory method and Compliance calibration method were used to calculate the fracture toughness Guc, and the fracture surface was analyzed. The results were, as follows:(1) The Mode Ⅰ interlaminar fracture toughness of Moso bamboo internode specimens was GIC-Internode=498.48J/m2 (SD=64.52 J/m2); the Mode Ⅰ interlaminar fracture toughness of Moso bamboo specimens with node was GIC-Node=1431.45 J/m2 (SD=198.01 J/m2).(2) Mode Ⅰ interlaminar fracture toughness of bamboo specimens with node was higher than that of bamboo internode specimens and the toughness contribution of node to bamboo Mode Ⅰ interlaminar fracture toughness was 1.87 times. Conclusion was obtained that bamboo node can contribute a lot to hinder the interlaminar fracture of bamboo.(3) The mode Ⅱ interlaminar fracture toughness was calculated by the Experiment parameter substitution method, Timoshenko beam theory method and Compliance calibration method. The Experiment parameter substitution method was more accurate and the value was 1303.18J/m2 (Coefficient of variation=8.96%) and the influence of bamboo height and specimen size on Mode Ⅱ interlaminar fracture was small.(4) The crack propagation of Mode Ⅱ interlaminar fracture was mainly self-similar cracking, but the fracture surface was rougher. Ground tissue in the zone of Mode Ⅱ crack propagation was characterized by hackle shearing deformation. The SEM photos showed that ground tissue separated from fiber along middle lamella under shear stress and as the increasing of the dislocation of upper and lower layer, the thin-walled ground tissue would fracture transversely by tension, while to thick-walled fiber cell, only middle lamella and primary wall were torn then debonded, fragments remained.(5)Mode Ⅱ interlaminar fracture toughness was about three times higher than the value of Mode Ⅰ interlaminar fracture toughness, which was corresponding with the phenomenon that the fracture surface of Mode Ⅱ was rougher than that of Mode Ⅰ.