Numerical Simulation Of ? And ? Type Fracture Of Parallel Strand Bamboo Based On XFEM

Parallel strand bamboo is a new type of Engineering bamboo composite material,which is composed of bamboo bundles and is formed by dissolving glue,forming embryos and pressing.However,in the complex production process of recombinant bamboo,due to the different diameter and size of bamboo bundles,the non-uniform distribution of phenolic glue and the incomplete directional arrangement of fibers,the micro cracks along the grain direction inevitably appear in the interior of recombinant bamboo.When the parallel strand bamboo is used in the stressed members,the stress concentration at the crack tip may lead to the crack propagation until the member breaks.The existence of cracks reduces the macro strength of the reconstructed bamboo.When it is used in building structures,there are engineering hidden dangers.In the engineering application,the internal cracks of the parallel strand bamboo components usually appear in the form of composite cracks,and the stress state is complex.Single I / II mode crack is the basis of studying composite mode crack.In order to prevent the occurrence of fracture failure,it is necessary to study the crack initiation and propagation laws of pure I and pure II cracks in composite materials,which is of great significance for engineering design and application guidance.In addition to analytical method and experimental method,numerical simulation method is also an effective method to study crack growth.With the continuous development of simulation technology,fracture mechanics is widely used in finite element software.In this paper,based on ABAQUS finite element software,the cohesive zone model(CZM)and the extended finite element method(XFEM)are used to simulate the I-type fracture and the II type fracture of the end notch of the double cantilever beam respectively(1)The 3D DCB and ENF models of the parallel strand bamboo can describe the crack initiation and propagation by the cohesive behavior based on the traction separation law,and the visual results of the whole process of crack propagation can be obtained.At the same time,the important grid size and the damage stability coefficient are parameterized,and the appropriate grid size and the damage stability coefficient are obtained,which makes the simulation results reliable.(2)The dynamic crack growth of DCB and ENF samples shows different leading edge shape of crack surface,and the analysis results show that the difference is reasonable.The length of FPZ under I and II mode is estimated respectively for the simulated visible crack surface,and the estimated results are consistent with the theoretical results.It is proved that the method of XFEM combined with CZM is feasible and effective.(3)In this paper,the numerical simulation of crack growth of different sizes(thickness,initial crack length)of bamboo DCB and ENF is carried out,and the external load crack opening displacement curve(P-? curve)is obtained by post-processing,and the rationality of considering the difference of peak load is compared between simulation curve and test curve.The results show that the elastic stage,toughening stage and softening stage of the load displacement curve in the simulation results are consistent with the test results.Then the modified beam theory is used to deal with the load displacement curve,and the crack growth resistance curve(R curve)is obtained.Compared with the R curve,the validity of the numerical method is verified again,which provides a reference for the practical application of the project,and the size effect analysis of the fracture toughness of the reconstructed bamboo is carried out according to this.(4)Considering the simulation results of type I and type II,the release rate of fracture energy of bamboo with different thickness and initial crack length was compared,and the effects of two size parameters,thickness and initial crack length,on the fracture toughness of bamboo were analyzed.The results show that the thin specimen has a large fracture toughness.With the increase of thickness,the component changes from plane stress state to plane strain state,and the energy release rate of the reconstructed bamboo tends to a constant low value.Among them,when the thickness is less than 40 mm,the plane stress accounts for a large proportion,and the size of FPZ has an impact on the determination of fracture toughness;when the thickness is more than 40 mm,the boundary constraints on both sides of the specimen are small,and the specimen is in plane strain state as a whole,so the measurement accuracy of fracture toughness of reconstructed bamboo is high within the thickness range.For the specimens under plane strain,the initial crack length has little effect on the simulation results of fracture toughness.For the same size of DCB and ENF,the fracture toughness of type II is greater than that of type I.