The Study On Fracture Toughness And Toughening Mechanisms Of Moso Bamboo

Bamboo,with its excellent mechanical properties,is widely used as engineering materials.However,as a natural heterogeneous material,bamboo has some weak interfaces inside,for example,the cell gaps and pits.These defects would become micro cracks with some load,and then the bamboo would become weakly and easily to destroy.Thus,using the fracture theory and method to analyze the change of the defects and the propagate of the cracks was very important for the safety and service life of bamboo.It is also the key and the foundation to expand the application of bamboo in the engineering field.Therefore,the moso bamboo(Phyllostachys edulis)was used in this study.Based on the cellular structure of moso bamboo,we first studied the influence of fiber cell(or parenchyma cell)ratios on the mode I interlaminar fracture toughness of bamboo.The intrinsic and extrinsic toughening mechanisms of bamboo were analyzed at the cellular level by in situ scanning electron microscopy(In situ SEM).Then,the relationships between the toughness mechanisms of bamboo and its cellular structure were deeply analyzed from the angles of bending flexibility and weak interface toughness.From the perspective of bending flexibility,the main cell type that resulted in high flexibility of bamboo was analyzed.To analyze the bending mechanisms of these cells,the bending deformations of different cells were observed and calculated by in situ synchrotron radiation micro-tomography(μ-CT).From the perspective of weak interfaces(weak layers),SEM was used to determine the weak layers at the cellular and sub-cellular scales.The Nanoindentation and Raman spectroscopy were used to quantitatively characterize the mechanical properties and chemical components distribution of these weak interfaces.Finally,after understanding the basic toughening mechanisms of bamboo(intrinsic toughening mechanism,extrinsic toughening mechanism,bending flexibility,and weak interface toughening),the influences of moisture(relative humidity)on fracture toughness and toughening mechanisms of bamboo were discussed.The main conclusions are as follows:(1)The initial fracture toughness(crack initiation energy)and growth fracture toughness(crack growth energy)of the outer part of bamboo(bamboo green)were the lowest,and the growth fracture toughness of the middle part(bamboo timber)was the highest.The intrinsic toughening mechanism of bamboo was related to the size of the plastic deformation zone and the tortuous degree of the crack path.Both the size of the plastic zone and the tortuousness of the crack were determined by the content of parenchyma cells.As the content of parenchyma cells is the lowest in bamboo green,the initial fracture toughness of bamboo green was the lowest.The extrinsic toughening mechanism of bamboo was related to the fiber bridging.The effect of the fiber bridging was related to the length,number and capacity of fibers that participated in the bridging.The fibers in bamboo timber were the most and longest,and thus bamboo timber had the highest growth fracture toughness.Generally speaking,the intrisic toughening mechanism was the main toughening mechanism for bamboo mode I interlaminar fracture toughness.However,when the bamboo was thin and with radial-longitudinal cracks,the intrinsic and extrisic mechanisms were both very important to its fracture toughness.(2)The force to bend the bamboo green(high fiber cell content)was greater than the force to bend the bamboo yellow(high parenchymal cell content).This is because the density of fiber cells is greater than that of parenchyma cells.The bending displacement of the bamboo green was larger than that of the bamboo yellow when cracks occured(the crack length was 0 mm),which indicates that the bamboo green has better bending flexibility.The growth rate of the cracks in the bamboo yellow were 4.0 times that in the bamboo green.The bamboo green strips bent like an archer,while the bamboo yellow strips bent as a triangular.This is because the change of the unit area of the parenchyma cell on the cross section(-8.9%)was much greater than that of the fiber cell(-0.6%)during bending.To absorb the macro bending deformation,parenchyma cells increased the ratio of longitudinal and transverse areas;vessel cells reduced the cell cavity area or bent themselves;and fiber cells bent single fiber cell or slid between cells(or cell walls).(3)The observations on the fracture surfaces indicate that on the cellular scale,vessel cells were the easiest to destroy,while the fiber cells were the hardest to destroy.The weak interfaces at the subcellular scale were the compound middle lamella(CML)of the cell,the thin layer of the cell wall,and the pits.The average storage modulus values of these areas:CML was 13.7GPa,pits were 17.0 GPa,thin layer was 20.6 GPa,and thick layer was 25.3 GPa.Compared with the thick layer of the cell wall,the cellulose content of CML reduced by 51%,and the thin layer reduced by 41%.CML was the preferred path for crack propagation at the subcellular structure since it has the lowest cellulose content.The hardness of the pits was lower than the hardness of the adjacent non-pitted areas.(4)Moisture in bamboo increased its fracture toughness and bending flexibility.The results of mode I interlaminar fracture toughness show that the initial toughness of bamboo was not affected by relative humidity,which was approximately 520 J/m~2.However,the growth toughness increased linearly with the relative humidity.The growth toughness increased by 7.6J/m~2 when the relative humidity increased every 1%.In addition,the speed of crack propagation in the wet sample was slower than that in the dry sample.Therefore,for the same crack length,the wet sample has a greater degree of opening and a larger value of fracture toughness.The results of the bending flexibility of bamboo showed that with the increase of relative humidity,the cracks occurred later and slower.Water in bamboo would combine with the amorphous area of the cell wall,and then swelled the amorphous area and soften the cell wall.Besides,a water layer on the surface of the cell wall would form,and then reduce the binding force of the cell wall and increase the relaxation of the cell wall.Therefore,the wet cell wall can absorb greater deformation and rearrange to avoid stress concentration or crack propagation,which ultimately leaded a better bending flexibility and fracture toughness.In general,the fracture toughness of biomass materials(bone,wood,bamboo)increased with the moisture content,but the fracture toughness of non-biomass materials(stone,soil,clay)reduced with the moisture content.