Damage Mechanisms Analysis Of 3D Angle-interlock Woven Composite Under Subzero Temperature Field And Transverse Impact Loadings

Three-dimensional(3D)woven structural composites have the advantages of light weight,high specific strength,high specific stiffness,high-performance near net shape manufacturing,good designability,with potential applications in the fields of structural engineering materials.Recently,3D woven composites have been used to manufacture turbine blades of aeroengine,landing gear,etc.In a complex and extreme serving environment,such as at typical airliner cruise altitudes,the temperature is around-50? to-60?.Besides low or extreme low temperatures,occasional hail,birds-strike and drop of maintenance tools are also risks associated with applications of composites.Thus,understanding dynamic impact responses and thermo-mechanical coupling mechanism of 3D woven composites at subzero temperatures is of vital importance for composite structures design and personal security,and also for the promotion and application of 3D woven composites.The specific objective of this thesis is to study the damage mechanisms,structural effects and temperature effects of 3D angleinterlock woven composites at subzero temperature fields under transverse impacts,by establishing thermo-mechanical coupling model and comparing simulation results with testing results.The main contents are as follows:(i)Multi-scale structural geometry models and thermo-mechanical coupling constitutive model of 3D angle-interlock woven composites were established based on homogenization method,bridging model,considering strain rate effects and adiabatic temperature rise.User-defined material subroutine(VUMAT)was written in Fortran90 for finite element analyse in Abaqus.(ii)Geometry models for transverse impact finite element simulation at subzero temperature fields were established in Abaqus.Transverse impact responses were calculated and thermo-mechanical coupling damage mechanisms were discussed.(iii)Quasi-static transverse loading tests of 3D angle-interlock woven composite were carried out under different temperatures(20?,-20?,-50? and-80?)with MTS810.23 materials tester combined with a self-made low temperature device.Loading responses,temperature effects and specimen direction differences were studied.(iv)Transverse impact loading tests of 3D angle-interlock woven composite were carried out under different temperatures(20?,-20?,-50? and-80?)and different impact pressures(0.2 MPa,0.3 MPa and 0.4 MPa)with a modified Hopkinson bar system combined with the self-made low temperature device.A high-speed camera was used to record its impact processes.Micro computed X-ray tomography(micro-CT)technology was used to investigate internal damages due to transverse impact under subzero temperatures.Effects of temperature,impact pressures and directions on the impact responses of the composites were analyzed.Major findings are as follows:(i)Subzero temperature environment has an effect on quasi-static transverse loading responses of 3D angle-interlock woven composites for both warp and weft directional specimens.Failure load,flexural strength and flexural modulus increase with temperatures decreasing.Under the same temperature,failure load,failure displacement,flexural strength and flexural modulus of weft directional specimens are all larger than those of warp directional specimens,respectively.Flexural modulus is more sensitive to temperature compared with flexural strength.Warp directional specimens are more sensitive to temperature compared with weft ones because of many small resin-rich areas on the upper surface of warp directional specimens.Epoxy resin tends to be more brittle under lower temperatures.Cracks are expanded along curved warp yarns.For weft directional specimens,failure modes include debonding,fragmentation of resin,breakage of weft yarns.(ii)Subzero temperature has an effect on dynamic transverse impact responses of 3D angle-interlock woven composites for both warp and weft directional specimens.Load peaks increase with temperature decreasing.Under the same temperature,load peaks of weft directional specimens are larger than those of warp directional specimens.With temperature decreasing,load peaks of weft directional specimens increase less than those of warp directional specimens.For warp directional specimens,failure modes include resin fragmentation increases and damage area on the upper surface decreases.For weft directional specimens,failure modes include resin fragmentation,debonding on the back and shear damage of weft yarns.Weft directional specimens consist of straight weft yarns who mainly bear the impact load.The increase of modulus of resin under low temperatures makes a stronger weft directional specimen as a whole,thus higher impact resistance.(iii)Impact pressure has an effect on dynamic transverse impact responses of 3D angleinterlock woven composites for both warp and weft directional specimens.Load peaks,displacement and energy absorption increase with pressure increasing.Under the same pressure,load peaks of weft directional specimens are larger than those of warp directional specimens.With multi-impact,load peaks of warp directional specimens decrease more than those of weft directional specimens.With pressure increasing,load peaks decrease more.For warp directional specimens,stress distribution along thick direction tends to be Y-shaped,including shear stress distribution along upper curved warp yarns and direct tensile failure for lower warp yarns.With pressure increasing,shear area decreases until none when impact pressure reaches 0.4 MPa.For weft directional specimens,shear stress distribution is shown.With pressure increasing,more resin fragmentations appear on the upper surface and damage area expands.(iv)The thermo-mechanical coupling constitutive model in this thesis makes the simulation of 3D woven composite under low temperatures and impact loadings efficiently.Stress concentrated under the impact point which reaches stress peak during the first impact.Temperature rises where stress increases;and stress decreases where temperature rises.Thermo-mechanical coupling effects are analyzed.For warp directional specimens,stress and temperature rise distribution are along weft direction;for weft directional specimens,they are concentrated on the impact point forming a diamond or hexagon shape with a wider zone than that of warp directional specimens.“ ? ” pattern shown in the stress distribution along out-plane direction of weft directional specimen tested at-80? and 0.3 MPa impact pressure.Stress and temperature rise for both specimens increase with temperature field decreasing.Warp directional specimens tend to be more sensitive to low temperatures than weft ones.This thesis established a thermo-mechanical coupling constitutive model for 3D angleinterlock woven composites which was validified by transverse impact tests under different impact pressures and different temperatures.This finite element method can largely reduce time,personnel and resources cost,providing strength prediction and feasibility study for thermo-mechanical coupling analyses and applications of various composite structures in extreme environments.