Mechanical Behaviour Of 3D Braided Composites Containing An Open-hole

The excellent damage tolerance of three-dimensional braided composites attracts the interest in aerospace field.The ability to tolerate openings is one of the important damage tolerance design criteria that needs to be considered in engineering design,especially in aircraft design.Fatigue failure is a very common form of failure in engineering,as candidate for future aerospace material,it has great engineering application value to study the effect of opening defects on component serving life.In this paper,based on open-hole tensile,compression and tensile/tensile fatigue tests,the static and fatigue life of open-hole three-dimensional braided composites under unidirectional and cycling loads are theoretically studied.Based on unit cell of 3D braided composite,the stiffness,strength and fatigue model were built to predict mechanical properties and fatigue life of open-hole 3D braided composites.The work of this thesis mainly includes the following parts:(1)The static mechanical properties of the three-dimensional braided composite with two braided structures were tested and evaluated.One is pure braided structure,other is braided structure with 42% of axial yarns.The effect of opening on the mechanical properties and the failure behavior of the two structures under tensile and compressive loading were studied.The 3D-DIC non-contact measuring technology were used to measure the stress concentration factor of the opening specimen,as well as the strain redistribution and failure mechanis.The study shows that the pure braided structure has better mechanical propertie.(2)Geometric model of the three-dimensional braided unit cells using the four-step braiding process was carried out to study the 3D braided in-plane and spatial motion trajectories.The specific unit cell model for the studied braided structure was established.The built unit cell model was periodically meshed on the Hypermesh platform,and a Python script was written to apply periodic boundary conditions to the unit cell on the ABAQUS platform,the unit cell was then homogenized.The results show that the mesoscale model can be used to predict the macroscopic mechanical properties of 3D braided composites.(3)Based on the ABAQUS / Standard finite element platform and UMAT user subroutine,Tsai-Wu strength criterion and material degradation criterion were involved,cohesive bilinear model and B-K interface failure criterion were introduced to simulate the interface behavior of the unit cell.The axial damage and shear damage model were built to study the damage evolution mechanism of the material matrix,yarn and interface in the unit cell.Studies have shown that the meso-and mesoscale model can predict the unidirectional and shear strength of 3D braided composites accurately.(4)Based on the macro-and meso-scale method,a model for open-hole 3D braided composites was established to study the tensile damage evolution and strength prediction.For macroscale method,the material performance parameters of the macro-scale model are from the parameters of 3D braided composite unit cell predicted by the meso-scale model.Based on the ABAQUS / Standard finite element platform and the UMAT user subroutine,the Larco2 and other strength criteria and material degradation criteria were introduced.The damage mechanism and tensile strength of the open-hole three-dimensional braided composites were investigated;the mesoscale model was based on the ABAQUS / Explict platform and the VUMAT user subroutine,strength criteria such as zerothickness interface phases,Hashin and material degradation models were introduced in the subroutine.The damage evolution law of yarn,matrix and interfacial phase of open-hole three-dimensional braided composites was observed and the material strength was predicted using mesoscale model.The results show that both models can reasonably predict the strength and failure mechanism of open-hole 3D braided composites.(5)Based on Lekhnitskii's orthogonal anisotropy theory,the stress distribution and stress concentration factor around the hole of the open-hole 3D braided composite material were analyzed.The theoretical analysis results were compared with the finite element and DIC test results;based on Whitney traditional open-hole plate PSC and ASC strength prediction model,the strength of openhole 3D braided composite material was studied.The results show that the numerical model is in good agreement with the experimental results,the PSC strength criterion can predict the strength of openhole 3D braided composites more accurately than ASC criterion.(6)The fatigue life of three-dimensional braided composites with and without openings were studied experimentally,and the fracture morphology of fatigued specimens was observed.The test shows that the pure braided structure has better opening and non-opening fatigue performance,it has less damage area and fracture crack under fatigue loading.The fatigue life model of the interface phase under cyclic load was introduced,the multiaxial fatigue strength criterion was used as the criterion for the failure criterion of yarn and matrix under fatigue load.The fatigue life method based on the mesoscale 3D braided unit cell was established then.Using the fatigue life and material performance parameters of the three-dimensional braided composite predicted at the mesoscale model,according to the dynamic fracture theory,a macro-scale open-hole 3D braided composite fatigue life prediction model was established.It is found the established macro-scale model can be used to predict the fatigue life of open-hole 3D braided composites.