Fracture And Damage Of Wood And Bamboo

Wood and Bamboo are very only two types of plant which capable of structural materials in all plant kingdom, and is a bio-composites with obvious meso-structure and could be research on multi-scales. Because of the properties of inhomogeneous, anisotropic and some defects or fractures existent in microcosmic even in macroscopic levels, the irregular evolvement behavior of these defects or fractures when under load determined the macroscopic mechanics behavior of wood and bamboo. In this paper, some kinds of wood and moso bamboo were selected as testing materials to research the mechanics character, fracture rupture behavior and mechanism of strong toughness. The main works were as follow:1 .First the particularity of applying the principles of linear-elastic fracture mechanics (LEFM) to wood was expounded in theory, it had confirmed that when the crack self-similar propagate along the direction of fiber, the fracture behavior could be described with LEFM, the fracture toughness parallel to grain could be tested with the same experiment methods in isotropic materials, and the value could be adopted from compact tensile samples, which not influenced by the dimension and the length of crack. Base on this research, the relationship between the stress intensity factor parallel to grain and the rate of energy release were also analyzed.The fractal character of fracture surface parallel to grain were researched by applying fractal theory, then the fractal dimension of fracture surface of 5 kinds of wood were measured, based on these work the relationship between the fractal dimension of fracture surface parallel to grain and fracture toughness parallel to grain were established for the first time, the relationship would help reveal the internal relations between meso-structure and anti-fracture capability parallel to grain of wood.The experiment equipment of fracture toughness in model III were designed, then for the first time we test the resistance to crack expanded parallel to grain in model III of spruce.2. Application of finite element method analyzed the stress field characteristics near the crack tip of the wood components texture with crack perpendicular to grain. "Tangential normal stress ratio criterion" were adopted to research the starting direction of crack perpendicular to grain, the factor of critical stress intensity were also test at the beginning of crack. The study result indicate that the critical stress intensity factor in lateral crack of the wood components texture with crack perpendicular to grain didn't influenced by the dimension of specimens, as an nominal fracture toughness could be seemed as essential property of wood, so the K-criterion established on these couldn't become the strength design criterion of the wood components. Because of the structure of cell and the cell wall enhanced by fiber, wood could resist the fracture perpendicular to grain strongly. The testing of fracture in bending strength perpendicular to grain revealed that: after the crack perpendicular to grain crack along the grain, the wood components still presented high carrying capacity , this account for wood could resist the fracture perpendicular to grain strongly, the wood components with model I crack perpendicular to grain couldn't be fractured by stress singularity the low working stress because of the oddity in crack tip, this conclusion had be fully proved through the contrast test of bending, toughness and tensile in many species of wood(spruce, larch, poplar and prick). So, when the wood components with model I crack perpendicular to grain were carried through safety evaluation and intensity design, it was suggested adopting the remained dimension exclude crack in general intensity. Otherwise if the design criterion followed the critical load and set K criterion as its criterion it could cause waste in intensity margin. Consequently it will bring the waste of so, the exclusion of the general strength of crack under dressed size is suggested, i.e.For model I crack parallel to grain, the LEFM criterion (K criterion or G criterion) is adapted.For model I crack perpendicular to grain, the net section stress criterion is adapted.Although the fracture perpendicular to grain didn't influence the conventional strength under net section of wood, the crack in the side direction was still an attention destructive model. So, the formula to calculate the strain energy release rate in the process of model I interlaminar crack parallel to grain were derived by analytic method in the research. This formula could used to analyze the delamination damage when bending in plywood.Finally the mechanism of strong toughness of wood resist fracture perpendicular to grain were expatiated through the point of interface weakening and the multilayer cell wall meso-structure.3. The concept of meso-damage basic elements of wood had been proposed. and the basic elements in wood damage were defined initially. Based on these, by using acoustic emission technology the initiation and development of different kinds of damage and fracture of wood components under loading were distinguished for the first time. The results showed that the different mechanism of damage and fracture in the internal of materials under loading could cause different degree of energy release, and then abundant AE signal produced. The occurrence of acoustic emission event under load in non-defect specimens developed very slowly, and the amplitude of AE signals most were low, the abundance AE signals with high amplitude occurred near the peak value of load and fracture process. When the process of fracture under bending load of specimens with crack was monitored by acoustic emission, the different stage of the beginning and the expansion of the crack tip could be obviously distinguished. The character of acoustic emission was related with the scathe of wood, the AE character of cell wall fracture were high amplitude, high energy and long duration, the AE character of cell wall interface damage, interlaminar crack and cell wall yield and collapse damage were low amplitude , low energy and short duration. When wood under repeating load, it presented Kaiser effect under low load, it presented Felicity effect under high load, and by using the acoustic emission the Felicity effect could reflect the damage degree of wood preferably.4. The mechanics character of bamboo structure were analyzed and expatiated, through predigest parallel connection model established on cell wall between internodes, by applying the mixing rule of meso-mechanics of composite material to test the intensity and stiffness of bamboo fasciculi and basic tissue. And the intensity of bamboo fiber bundle was test after peeled of, and modulu of elasticity of the bamboo fiber bundle was test by shear-lag theory.5. The opining and shear interlaminar fracture behaviors of Moso bamboo have been studied in this paper. Because bamboo was seemed as single axial and long fiber compound material, and was intense anisotropism in intensity and stiffness, it was easy to initiated the beginning of interlaminar fracture parallel to grain under the transverse tensile force and cut force which caused by outer force and the change of environment condition, then the spread in divided layer didn't controlled by the intensity perpendicular to grain, but by interlaminar fracture toughness. Based on the energy concept, double-cantilever beam (BCD) sample and end notched flexure (ENF) sample were applied to measure the Model I and Model II interlaminar fracture toughness of Moso bamboo, and the experiment method and influencing factor were explored. The morphologies of rupture surface of Model I and Model II interlaminar fracture were analyzed by using SEM. then the mechanism of interlaminar fracture of bamboo have been expatiated.6. As an bio-composite materials which were enhanced by fiber, with intense structure character ,there were many energy -absorb mechanism in bamboo in different stage or process of fracture perpendicular to grain, because of the different mechanism of bamboo fiber bundle in the stress transfer and fracture in basic tissue. So the meso-mechanics model were established according to the different disable manner in the processes offracture perpendicular to grain to calculate the contribution of enhancing toughness ofdifferent disable model, and the morphologies of rupture surface were analyzed by SEM toexpatiate the mechanism of strong toughness in resisting fracture perpendicular to grain ofbamboo.