Mechanical Behaviors and Failure Mechanisms of Z-pinned Joints for 2D C/SiC Composite Prepared by Chemical Vapor Infiltration
Supervisor:zhang li tong
Continuous fiber reinforced Silicon Carbide Ceramic Matrix Composites(SiC-CMC,such as C/SiC and SiC/SiC)are novel and critical materials in the aeronautic and the aerospace fields.And the manufacturing technique of the SiC-CMC complex components becomes crucial to promote the applications of the SiC-CMC composites.The Science and Technology on Thermostructural Composite Materials Laboratory invented an online z-pinning technique for manufacturing the C/SiC complex components based on the chemical vapor infiltration process(CVI).The technique solves the difficulites in the manufacturing of such C/SiC components.It then realizes the integration of their structural design and manufacturing.And it has been successfully applied to the thermal protection systems of aeronautic or aerospace vehicles.However,compared with the metallic bolted or riveted joints,the C/SiC pin is porous and its matrix has many microcracks,so that the design of the CVI-C/SiC z-pinned joints is special and then different from the metallic joints.In this paper,the 2D C/SiC z-pinned joints with a single pin or multiple pins were prepared by the online z-pinning method.The tensile behaviors and the failure mechanisms of the z-pinned joints were investigated from the mesomechanics view,and then we build the relationships among 2D C/SiC constituent properties,2D C/SiC basic mechanical properties,and the tensile properties of 2D C/SiC z-pinned joint with a single pin.From the damage mechanics view,a nonlinear finite element model was established to model the joint tensile behaviors and the shear-off failure of z-pin.Because 2D C/SiC z-pinned joints were used in the oxidation environments,the oxidation effects on the joint tensile properties were also investigated.The thesis lays the foundation of the design guidelines of SiC-CMC z-pinned joint,and contributes to the development of thin-wall SiC-CMC structures with complex configuration.The main conclusions are listed as follows:(1)The effects of total porosity of 2D C/SiC z-pin were studied on the tensile behaviors and the failure mechanisms of 2D C/SiC z-pinned joint with a single pin.Results showed that:(a)When the joint is under tension,the 2D C/SiC z-pin is sheared off under the coupled shear and bending stress.And the interface sliding and fiber bridging mechanisms control the fracture process of z-pin.(b)As the total porosity of 2D C/SiC z-pin increases,the joint shear strength decreases according to a power law.A modified rigid body sliding model is proposed to quantitatively characterize the relationship between the total porosity of 2D C/SiC z-pin and the shear strength of 2D C/SiC z-pinned joint.It shows that the shear strength of 2D C/SiC z-pinned joint equals the in-plane shear strength of 2D C/SiC composite plus the bending stress component of the fiber bridging stress.(c)A nonlinear finite element model for 2D C/SiC z-pinned joint is developed to accurately predict its nonlinear tensile behaviors and the shear rupture process of 2D C/SiC z-pin.The prediction shows that the joint nonlinear tensile behaviors are controlled by the coupled shear and bending failure process of 2D C/SiC z-pin.Hence,the calculated results verify the modified rigid body sliding model.(2)The effects of oxidation damages were studied on the mechanical behaviors of 2D C/SiC composite and the tensile behaviors of 2D C/SiC z-pinned joint based on the modified rigid body sliding model.Results showed that:(a)After oxidation,the tensile,the in-plane shear and the interlaminar shear strengths of 2D C/SiC composite decrease linearly,and the joint shear strength decreases according to a power law as the corresponding weight loss increases.These degradations can be characterized by the oxidation degradations of the constituent properties of 2D C/SiC composite.(b)Oxidation damages to the carbon phases decrease the interface sliding stress and the matrix fracture energy,and the load transfer efficiency along interface.And then the crack deflection mechanism changes from doubly deflection before and after 700 ~oC oxidation to singly deflection after 1000 ~oC and 1300 ~oC oxidation.(3)The load distributions of 2D C/SiC z-pinned joint with multiple 2D C/SiC z-pin were studied via their load-displacement curves and corresponding strain-displacement curves.Results showed that:(a)The joints with multiple pins in longitudinal array and the joint with two pins in transverse array shows uniform load distribution ability due to the same shear strength as the joint with a single pin.(b)The joint with three pins in transverse array,the joint with three pins in triangle array and the joint with four pins in rectangle array show nonuniform load distribution among their z-pins due to their shear strengths are lower than the joint with a single pin.The nonuniform load distribution phenomenon may be attributable to the nonuniform microstructures such as the densities of z-pins at the joining region.(c)Oxidation damages aggravate the non-uniform load distribution among pins in transverse array,while does not change the uniform load distribution among pins in longitudinal array.It shows that the large-scale fiber bridging mechanism,which occurred after oxidation,benefits the uniform load distribution among pins in transverse array.(4)The shear properties of 2D C/SiC z-pinned joint were optimized based on the microstructural and the geometrical features of the joint.Results showed that:(a)The 2D C/SiC z-pinned joint has nonuniform and random microstructural features,such as non-uniform bonding between the pin and the plates or between the plates,and the matrix density gradient from the pin/hole interface into the plates,and the randomly distributed pores.(b)The bonding between the pin and the plates influences the joint tensile properties,that is,the joint shear strength increases as the increase of the frictional coefficient.(c)The critical diameters for the z-pin rupture are experimentally determined.It is 4.3 mm for z-pinned joint with 2D C/SiC z-pin,5.5 mm for z-pinned joint with 2D SiC/SiC z-pin,and 5.1 mm for z-pinned joint with 3DN C/SiC z-pin.(d)When their diameters are equal,the shear strength of z-pinned joint with 2D SiC/SiC z-pin is the largest than the joint with 2D C/SiC or 3DN C/SiC z-pin.