| Due to the lack of high modulus fibers in the through-thickness direction,interlaminar properties of composite laminates are poor.Z-pinning is a kind of through-thickness reinforcement technique which inserts carbon pins into the laminate,and has been proved to be efficient in improving interfacial properties.In this thesis,experimental study and multi-scale FE analysis on Z-pinned composites were carried out.The main contents are as follows:(1)FE unit cell model was used to determine the influence of Z-pin insertion on the elastic properties of unidirectional composite laminates.The features of eye-shaped resin-rich region and fiber waviness were included in the model.Besides,periodic boundary conditions were applied to the unit cell.Benchmark tests were carried out to verify the reliability of the model.Based on this,parametic analysis was conducted to examine the influence of Z-pin parameters including Z-pin diameter,spacing,and insertion angle.(2)Based on the unit cell model regarding elastic properties,a new model was proposed to study the effect of Z-pin on the strength properties of unidirectional laminates.In this new model,both the two sharp tips of the eye-shaped region were elimated,and a modified 3D Hashin criterion was used to predict the damage of the laminate and the Z-pin while maximum stress criterion was used to analyze the failure of the resin-rich region.By comparing the FE results with the experimental data,it was found that the results from the new model were in better agreement with the test results than the conventional model.After the validation of the proposed model,parametric analysis on the influence of Z-pin parameters was conducted.(3)A 3D micro-mechanical FE model based on cohesive zones was proposed to investigate the behavior of a single Z-pin under mixed-mode bridging load.A zero-thickness cohesive element was inserted into every pair of adjacent elements in the Z-pin and resin-rich region to predict the splitting of the Z-pin,fracture of the Z-pin,debonding of the Z-pin from the laminate and the snubbing damage in the resin-rich region.This model could characterize the full stage of Z-pin bridging including elastic deformation,dedonding of the Z-pin from the laminate,frictional pull-out,Z-pin splitting/fracture.The predicted bridging laws and damage modes under mode I,30° and 60° mixed mode,mode II loading were all agreed well with the experimental results.(4)Experimental and FE methods were utilized to study the load carrying capacities of Z-pinned composite T-joints under tensile,bending,and shearing load.In the FE model,nonlinear springs were used to model the bridging of Z-pin,in which the properties of the springs used to model the suppression of crack opening were from mode I Z-pin bridging tests while the properties of the springs used to model the suppression of crack sliding were from the micro-mechanical FE method in the study.Cohesive behavior was defined in the interface of flange/flange,flange/filler,skin/filler,and flange/skin to predict the delamination in the interface.Besides,a zero-thickness cohesive element was embedded into every pair of adjacent elements in the filler to model the non-directional crack in the filler under tensile and bending load.The predicted damage modes of the model agreed well with the experimental results and included all the damage patterns observed in the tests.(5)Based on the tensile tests of T-joints with different flange/skin thickness ratios,FE parametric analysis on the effect of thickness ratio on the reinforcement of Z-pin was carried out.Results indicated that the reinforcing effect of Z-pin on the tensile load capacity was greatly influenced by the flange/skin thickness ratio.(6)Tensile and shearing carrying capacities of K964 stitched and Z-pinned T-joints were compared experimentally and numerically.Results showed that the reinforcing effect of stich on the tensile capacity was superior to the Z-pin,while neither stitch nor Z-pin could improve the shearing load capacity of the T-joint. |
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