Research On Microscopic Damage And Fracture Behavior Of The Continuous Fiber Reinforced Aluminum Alloy Composites Subjected To Quasi-static Compress Loading

Continuous graphite fiber reinforced aluminum alloy composites(CF/Al composites)take full advantage of the high strength and modulus of graphite fibers as well as the high toughness of aluminum alloys.It possesses excellent comprehensive physical properties such as low density,high specific strength and modulus,and low thermal expansion coefficient.At present,the research on CF/Al composites mainly focuses on the fabrication process,microstructure,interface formation mechanism and control.However,there is rarely any investigation on the influence of progressive damage of component materials and interface on the macroscopic mechanical behavior of CF/Al composites during load-bearing process.It is vital to understand the microscopic damage and failure mechanism of CF/Al composites so as to provide a theoretical basis for the design of components and interface properties and for the optimization of fabrication process.Meanwhile,This issue is a urgent scientific problem to be solved in order to promote the application of CF/Al composites in practical engineering.In this paper,CF/Al composites are fabricated by vacuum assisted pressure infiltration method.The damage evolution and failure behavior of the composites undergoing axial and transverse quasi-static compression are investigated utilizing micromechanical and experimental method.And the mechanism of its influence on the macroscopic fracture behavior of the composites is discussed.On this basis,the effects of interface and fiber properties,and fiber volume fraction on the axial and transverse compressive mechanical properties were further analyzed in order to provide theoretical basis for the design of components and for the optimization of fabrication process.The main research contents and results are as follows:(1)Vacuum assisted pressure infiltration method was used to fabricate CF/Al composites.Three representative volume element(RVE)models with different fiber arrangement were established according to the characteristics of microstructure and fiber distribution.The mechanical properties of the matrix alloy was tested to establish its ductile damage evolution and failure model.According to the mechanical properties of fiber,a transversely isotropic failure model based on the maximum stress failure criterion was constructed and a corresponding user material subroutine isdeveloped to describe the failure behavior of fiber.Thereafter,the micromechanical RVE model utilizing cohesive interface was established.The validity of the micromechanical RVE models based on different fiber arrangement were further inspected by the experimental results.(2)The axial and transverse compression behaviors of CF/Al composites at room temperature were tested by an electronic universal testing machine,and the macroscopic stress-strain curves were obtained.The mechanical behavior curves calculated by the micromechanical RVE model with regular diagonal quadrilateral fiber arrangement agree well with the experimental results.The calculation errors of axial compressive modulus,ultimate strength and fracture strain are-8.16%,1.73%and 7.69%,respectively.The calculation errors of transverse compressive modulus,ultimate strength and fracture strain are 13.73%,4.36% and 3.90%,respectively.(3)According to the results of micromechanical numerical simulation and experiments,the microscopic damage and fracture behavior of CF/Al composites during axial compression are investigated.The results show that the interface between fibers/matrix alloys occurs damage at the initial deformation stage.With the increase of compressive strain,the damage degree of interface increases gradually,which induces the local damage of the matrix alloys near interface.The damage accumulation of matrix alloy lead to the sudden failure of fibers in the later stage of compression,which eventually results in the axial compression failure of the composites.The fracture surface of the composites samples take on the characteristic of uneven fiber pull out and local fiber fracture with 45 °surface relative to axial direction.(4)According to the results of micromechanical numerical simulation and experiments,the microscopic damage and fracture behavior of the composites during transverse compression are investigated.The results show that the interface damage initialed at the first stage of transverse compression.With the increase of compressive strain,the damage degree of interface increases gradually,inducing the local interface failure of the composites.At the middle stage,the interface failure causes the local damage of the matrix alloy,which accumulates gradually with the increase of the compressive strain.In the last stage of compression,the fibers begin to fail due to the increase of compressive load,and the composites lose transverse bearing capacity.The transverse compression fracture of the composites exhibits the characteristic of interfacial debonding and local fiber fracture.(5)The effects of interfacial and fiber properties on the axial and transverse mechanical properties of CF/Al composites were analyzed based on the micromechanics FEA model.The results show that the influence of interfacial bonding strength and stiffness on the axial mechanical behavior was unconspicuous,while the fiber properties determine the axial mechanical properties of the composites.In the case of transverse compression,the ultimate strength of the composites increases with the increase of the interface bonding strength.At the same interface property,the ultimate strength can be enhanced by improving the fiber property.The transverse mechanical properties of the composites are mainly affected by the interfacial bonding strength.(6)Utilizing the micromechanics FEA model,the axial and transverse mechanical properties of the composites are calculated and predicted at different fiber volume fraction condition.At the same condition of components property and interface property,the results show that the axial elastic modulus and ultimate strength of the composites increase with the increase of fiber volume fraction,while the variation of axial fracture strain is unobvious.In the case of transverse compression,however,the fracture strain increase with the increase of fiber volume fraction,while the elastic modulus and ultimate strength decrease significantly with an increase in fiber volume fraction.