Microstructure And Thermo-mechanical Coupling Mechanisms On Impact-induced Deformation And Damages Of 3-D Angle Interlock Woven Composites

Three-dimensional(3-D)woven composites are widely used in various engineering fields due to their high impact damage tolerance,structural design flexibility and lowcost manufacturing of textile preforms by using simple modified traditional looms.Structural composites are inevitably subjected to dynamic loadings and the surrounding environment during service or maintenance.The microstructure and thermalmechanical coupling mechanisms on impact-induced deformation and damages of 3-D woven composites are critical issues that need to be studied in engineering design and application.The objectives of this research were to:(i)reveal the key role of throughthickness reinforcement in improving the delamination resistance of woven composites;(ii)study the effects of impact velocity,warp/weft directional loading and ambient temperature on the mechanical response and damage of 3-D angle interlock woven composites;(iii)develop a validated finite element model and combine tests results to investigate the directional effect and microstructure mechanisms of deformation and damages of 3-D woven composites;(iv)study the temperature effect and thermomechanical coupling mechanism of 3-D woven composites under transverse impacts based on the numerical simulations.The research is expected to provide theoretical support for performance evaluation,structural design and optimization of 3-D woven composites for specific application scenarios.The primary research contents of this paper were as follows:(i)The transverse impact tests on carbon/epoxy woven laminates and 3-D angle interlock woven composites were conducted by Hopkinson pressure bar to obtain displacement-time and load-displacement curves,a high-speed camera was employed to visualize the transient deformation and damage process;(ii)The mechanical response of warp and weft directional 3-D angle interlock woven composites under different impact velocities(6 m/s,9 m/s and 12 m/s)were tested at room and high temperature(90°C and 130°C),the changes of impact-induced displacement and peak load,as well as the differences in progressive damage process between different specimens were expounded;(iii)The constitutive equations and failure criteria of each component were established,a mesoscale finite element model considering resin cracking,yarn fracture and yarn/resin interface damage was built to simulate the impact-induced deformation,damage initiation and evolution process of 3-D angle interlock woven composites;(iv)The thermal-mechanical coupling algorithm and dynamic mechanical properties of the epoxy matrix and impregnated fiber tows were implemented into finite element model.The plastic strain was calculated based on yield response and backward Euler method,and the inelastic thermal transformation relationship was incorporated into the adiabatic temperature rise equation.The thermal-mechanical coupling finite element model was developed and the impact behavior of 3-D angle interlock woven composites under the glass transition temperature was calculated.The following conclusions were drawn from tests and finite element analysis:(i)The interlaminar fracture toughness of 2-D woven composites was low for lack of through-thickness reinforcement,the fibers were fractured under transverse impacts,the interlaminar crack evolved into severe delamination,and the laminates lost its bearing capacity rapidly.In 3-D angle interlock woven composites,a set of warp yarns in the thickness direction were crimped and moved from one weft yarns layer to the adjacent layer to form an integral structure.The warp yarns bridged the delamination and suppressed macroscopic crack,which endow the 3-D woven composites with higher impact damage tolerance and delamination resistance;(ii)The transverse impact response of 3-D woven composites was affected by impact velocity,ambient temperature and textile architecture.The peak load and displacement of the 3-D angle interlock woven composites increased with increasing impact velocity;at the same impact velocity,the peak load of weft directional specimen was higher while the impact displacement and strength attenuation were lower than that of warp directional specimen;the thermo-mechanical properties of resin matrix affected the mechanical responses of the composites under high temperatures.The resin modulus dropped sharply at glass transition temperature,and obvious plastic deformation occurred under impact,which led to a decrease of peak load while an increase of impact displacement;the mechanical behavior of warp directional specimen was more sensitive to temperature,the change of peak load and displacement were greater than that of weft directional specimen;(iii)The damage of the 3-D woven composites initiated as matrix cracking and interface debonding,then evolved into matrix fragmentation and fiber breakage.The buckling instability of warp yarn accelerated the yarn debonding and matrix cracking to form local macroscopic crack in the warp direction,and finally fractured in shear along the inclined warp yarns.The damage of the weft directional specimen was mainly yarn breakage,while matrix cracking and yarn debonding were less.The warp yarn was susceptible to deform and fracture than straight weft yarn,thus the warp directional specimen was more vulnerable to failure.Reducing yarn fluctuation helps lessen impact-induced damage and improve anti-deformability of 3-D woven composites.The toughness of matrix was enhanced at the glass transition temperature,the failure mode of the composites transformed from brittle to plastic,resin fragmentation and yarn debonding were not obvious,macroscopic crack in warp directional specimen was disappeared,the fracture of warp yarn was more concentrated and the extent of weft yarn fracture was weakened,the damage tolerance of the composites was increased;(iv)Finite element analysis revealed that the undulation of warp yarns had an adverse effect on stress wave propagation.The shear stress and fracture of warp yarns was concentrated in the connection region of the linear and curve section,while weft yarns were fractured under axial compression,which led to higher energy absorption of weft directional specimen.The shear stress of resin caused matrix cracking and interface damage,the continuous resin-rich region in weft directional specimen would benefit the distribution of shear stress,interface damage and matrix microcracking appeared in the connection area between the impact and support points of weft directional specimen,yarn debonding and failure occurred simultaneously;the warp yarn prevented the propagation of shear stress,interface damage and matrix cracking in warp directional sample were concentrated in the impact area,the yarn debonding that occurred before shear fracture was more serious than that of weft directional specimen.Although warp yarns in the thickness direction can improve the out-of-plane properties of the composites,the shear fracture of warp yarns affected the bearing capacity,this can be resolved by adding filling yarn along the warp direction in fabric structure to improve the in-plane mechanical properties of the composites;(v)The transient temperature rises of 3-D woven composites under high temperature was mainly distributed in the resin matrix and temperature increased with the impact process.Comparing warp and weft directional samples,it was found that the buckling of warp yarn resulted in plastic deformation of the resin matrix in warp directional specimen,the temperature rise area near the impact point was larger than that of weft directional specimen;the shear stress distribution in the resin-rich zone of the weft directional specimen resulted in a larger temperature rise area between the impact and support point.The temperature rise of the warp yarn changed along the yarn fluctuations,the concentration of shear stress and the interaction of warp and weft yarns caused a sudden temperature rise and yarn fracture.The temperature rise of weft yarn was concentrated below the impact point,where the compressive fracture occurred.The present research elucidated the microstructure and thermo-mechanical coupling mechanisms of the transverse impact response and dynamic damage process of 3-D angle interlock woven composites,which could provide theoretical guidance for the structural design and optimization of 3-D woven composites under impact loading and high-temperature environment.