Numerical Simulation Of Mixed-mode Ⅰ-Ⅱ Delamination Of Parallel Strand Bamboo Based On Cohesive Zone Model

Parallel Strand Bamboo(PSB)is a kind of engineering bamboo developed independently in China.It has the advantages of high utilization rate of original bamboo,excellent mechanical properties,good durability and customized size.The lack of design specifications for bamboo has hindered its application in the field of building structure and the sound development of related industrial chain.Therefore,the standardization of PSB structure is not only the actual industrial demand,but also the innovation of the application of new materials in engineering structure.PSB is made up of bamboo fiber bundles.There are usually microcracks caused by manufacturing defects and processing errors in the interface between each bamboo fiber bundle layer and substrate.These microcracks will expand with the effect of load,which has a great impact on the bearing capacity,durability and safety of PSB.Therefore,improving the research on mechanical properties of PSB is the basis of the standardization of manufacturing technology and structural design of engineering bamboo,especially the Ⅰ-Ⅱ mixed-mode fracture of PSB,which is the most common in practical engineering.The application of finite element method in the fracture numerical simulation of composite materials has been relatively mature,which can be used as a supplement and optimization for the experimental study of the fracture performance of PSB.It can not only avoid the waste of manpower and material resources caused by a large number of repeated tests,but also can further study the fracture behavior of PSB and evaluate the accuracy of existing theories and models through numerical simulation.In this paper,the mechanical behavior of Ⅰ-Ⅱ mixed-mode fracture of PSB was studied by using Double Cantilever Beam,End Notched Flexure and Mixed-Mode Bending tests.Based on the test results,Compliance-based Beam Method(CBBM)and Williams Correction Method were used to calculate the mode Ⅰ fracture energy release rate GⅠ and mode Ⅱ fracture energy release rate GⅡ respectively,and the crack propagation criterion curves(BK law and power law)were fitted based on them.The three types of specimens were simulated by ABAQUS,and the load-displacement curve and R curve were compared.The stress distribution of crack interface and the shape of crack front in the simulation results were analyzed,and the appropriate crack propagation criteria were determined.The effects of different specimen thickness B and initial crack length a0 on the fracture properties of MMB were studied by ABAQUS software.The conclusions are as follows:(1)By comparing the simulation results of the two crack growth criteria fitted by the calculation results of the two data reduction schemes,it can be concluded that the power law based on William correction method has the best agreement with the experimental results,and the corresponding parameters are GIc=1446.01 J/m2、GⅡc=3246.85 J/m2、α=1.14728.(2)The contour of the crack front of MMB specimen changes with the change of the respective proportions of mode Ⅰ and Ⅱ cracks.The pure mode Ⅰ crack shows that it is gradually protruding from both sides to the middle to the front of the crack,while the pure mode Ⅱ crack is opposite to the pure mode Ⅰ crack,which shows that it is gradually concave from both sides to the middle to the inside of the crack.At the same time,the distribution of normal stress σy and shear stress τxy at the crack front is consistent with the contour line,showing that σy is the highest in the middle and the lowest on both sides,while τxy is the highest in the middle and the lowest on both sides,and they are symmetrically distributed along the thickness direction about the middle.(3)When0.4<a0/L<0.7,with the growth of a0,the values of GⅠ,GⅡ and GT of MMB specimens gradually tends to a constant;when a0/L≥0.7,the support of fixture applying vertical mode Ⅱ fracture load in the span of specimens will exert a load to close the cracks;when the fixture applying vertical mode Ⅰ fracture load at the end which causes the crack surface to open,the specimens will be counteracted;and under the action of the clamp in the span,the crack will be closed.End crack surfaces will close.Mode Ⅱ cracks always dominate and the effect of mode Ⅰ cracks can be neglected.(4)In the range of B=40-120 mm,with the increase of B,the MMB specimens gradually change from plane stress state to plane strain state,corresponding values of GⅠ,GⅡand GT all tend to a constant value.