Experimental Study On Mode ? Fracture Energy Of Parallel Strand Bamboo

Parallel Strand Bamboo(PSB)is a kind of oriented bamboo-based composite material which is formed by cutting,drying(or carbonizing)the original bamboo,crushing it into bamboo tow and hot pressing after dipping.The stiffness degradation and the loss of bearing capacity caused by crack propagation are the main factors for the failure of PSB.The failure mode of the PSB is the fracture failure.Therefore,using the research method of fracture mechanics to analyze the damage of PSB structural members,and to establish the relationship between fracture parameters and macroscopic mechanical response of components,it is of great significance for the selection,design and structural safety analysis of PSB structures.In this paper,a double cantilever beam(DCB)specimen suitable for PSB mode I fracture is designed.The mode I fracture process and failure mode of the PSB are studied by experiments.The curve of the fracture process of the specimen is used to solve the crack length according to the flexibility method.According to the classical beam theory and Timoshenko beam theory,the fracture energy of the material during the crack propagation process is calculated,and the curve of J-? and J-?*of PSB are drawn.The fracture energy and the crack length are discussed for different specimen sizes and configurations.The relationship between fracture energy and crack opening displacement and the influence of fiber bridging on fracture energy are discussed.Based on the relationship between cohesive stress and fracture energy,the nonlinear softening cohesive model of mode I fracture of PSB was derived and the following conclusions were obtained:(1)The mode I fracture failure mode of the double cantilever beam of PSB can be divided into the following two types:1)The crack propagates stably along the initial direction.The crack extends from the initial expansion to the end of the specimen,and its expansion direction does not change,and it always extends along the main crack from front to back.This failure mode is an ideal mode for mode I crack propagation of PSB.2)The crack does not develop along the main crack at the later stage of the fracture.The crack propagates along the main crack in the initial stage of expansion,but as the crack deepens,one or more secondary cracks appear around the main crack.This failure mode is caused by the serious fiber bridging of the internal fibers of PSB.Mode I fracture failure of PSB is plastic failure.(2)The influence of the critical load of the specimen in the mode I fracture test is on the width,the initial crack length and the height of the specimen.As the width of the spesimen increases,the average critical load increases gradually.As the crack length increases,the critical load decreases gradually.As the height increases,the average critical load also increases.The initial crack length of the specimen have the following effects on the displacement corresponding to the critical load.As the initial crack length increases,the opening displacement increases.(3)The transverse shear deformation of the cantilever beam during the process of cracking has little effect on the calculation of the fracture energy by the DCB test.Therefore,it is accurate to calculate the fracture energy using the classical beam theory.(4)The development of fracture energy of PSB is divided into two stages.The first stage:as the crack expands,the curve shows an upward trend.This is due to the existence of a common fiber bridging inside the material.As the specimen breaks,the fiber peels off in turn,the crack begins to develop,and the bridging area gradually forms,leading to crack propagation resistance.With the development of the bridging area,the fracture energy continues to rise;the second stage:the curve maintains the level,the fracture energy no longer rises and reaches a stable value,because the cracks continue to expand when the bridging area reaches its critical length.The area of the bridging area is no longer developing,and the resistance to crack propagation remains unchanged.The development of the fracture energy and its stability value are related to the configuration of the specimen.The curve of J-? no longer characterizes the fracture property of the material due to the occurrence of the fiber bridging of the material inside.(5)The relationship between fracture and crack opening displacement obtained by the DCB fracture test is related to the width of the specimen and the initial crack length,while the height of the specimen has little effect on the curve.Compared with the curve of J-?,the curve of J-?*of the material is less sensitive to the configuration of the specimen.(6)Based on the relationship between fracture energy J and crack opening displacement?*,the basic form of cohesive model can be determined according to the test.The J-? curve of solving the non-linear softening cohesive model is basically consistent with the test value,and the selection of configuration parameters has little effect on the fracture energy JS required for crack instability propagation.