Mechanical Behaviour of Carbon Fiber/BMI Composites and Joints at Space Environment Temperature

Author:Yang Bai Feng

Supervisor:yue zhu feng


Degree Year:2018





Low earth orbit is the service environment of aerospace vehicles.Vehicles enter and go out of earth shade at service repeatedly and the environment temperature changes alter-nately.Carbon fiber reinforced resin matrix composites are anisotropic.Thermal stress field generate when temperature changes which may result in crack initiation due to mis-match of thermal expansion coefficient of multiple components.Meanwhile,extreme temperature may affect matrix properties.Therefore,it is important to study extreme tem-perature effects on composites and joints used in aerospace vehicles.In view of the above statements,ZT7H/QY9611 carbon fiber/BMI composites and joints were studied using multiscale research methods in this paper.Environment temperature effects on mechanical properties and failure modes were studied.Representative volume element(RVE)was developed to simulate crack initiation and propagation on mesoscopic scale.Thermal cycling effects on crack propagation and mechanical properties were in-vestigated.A progressive damage analysis method involving thermal effects was pro-posed to study temperature effects on mechanical behavior of composites mechanica l joints.The main research contents are listed as follows:Mechanical properties test matrix at extreme temperature was designed and mechanica l behavior at-120℃200℃ was tested.Temperature effects on failure mechanism was studied and failure envelope onσ22-τ12 plane based on Hashin criterion at extreme tem-perature was described.The results show that mechanical properties and failure modes related to matrix and interface rely heavily on environment temperature.Composites has better transverse compressive properties at low temperature.High and low temperature have negative effects on transverse tensile and shear properties.A RVE model was developed reasonably applied with periodic boundary condition.In-terfacial fracture energy and strength effects on mechanical properties were discussed.Thermal stress field in RVE model was studies at different temperature.Crack initiatio n,evolution and growth process were studied on mesoscopic scale.Transverse tensile,com-pression and shear properties of composite RVE model at high and low temperature which agreed well with experimental values were studied.Cyclic thermal loading was performed in the temperature range between-100℃and200℃ for up to 200 cycles.Crack propagation process was examined in cross section and side section during thermal cycles.Dynamic thermomechanical analysis was con-ducted to study thermal cycling aged effects.The material responses were characterized through an assessment of mechanical properties and fracture morphology.Transverse ten-sile and in-plane shear properties reduced obviously along with increasing number of thermal cycle.Fiber-matrix interface debonding generated during thermal cycling re-sulted to micro-crack in interface which changed failure modes.Composite mechanical joints analysis process based on progressive damage analysis method was built.Composite-metal and composite-composite joints were studied using finite element method using user subroutine USDFLD.The FEM results agreed well with experimental results.A progressive damage analysis method involving thermal effects was proposed to study mechanical behavior and failure modes of composite mechanica l joints at-60℃,room temperature and 175℃.Crack initiation,evolution and failure of typical layers during load process were analyzed.The FEM results were compared with experimental results and the validity of the method was verified.Research results in this paper will provide reliable experiments and technical supports for carbon fiber reinforced resin matrix composites structure design work in aerospace vehi-cles,which has important engineering application value.