Mechanical Behavior And Failure Mechanism Of 2D C/SiC Beam

Thermal protection system is one of the key technical problems for the hypersonic flight vehicle,and the thermo-structure components have been its development trend.Due to the outstanding properties,such as excellent heat resistance,low density and oxidation resistance,etc,C/SiC composites have become the preferred material for thermo-structure thermal protection components.To realize the application of C/SiC in thermal protection structure,it is necessary to understand the mechanical behavior of material,typic structrual elements and complicated components,respectively.Beam structure is an basic structural element for the flight vehicle.To understand the mechanical behavior and failure mechanisms of C/SiC beam,the research work about the effect of material properties,structural characteristics,online riveting and thermal shock on the mechanical properties of C/SiC beam have been carried out with 2D C/SiC channel beam and I-beam as the research objects.The main contents and results included as following:1.The bending mechanical behavior of 2D C/SiC channel beam was investigated.Results showed:(1)There exists nonlinearity in the bending responses of 2D C/SiC channel beam.With load increasing the bending stiffness of beam section decreased which follows an approximate linear law,and meanwhile the neutral axis of beam section deviated to the compressive side.(2)Under bending load,there exist tensile damage,shear damage,tensile-shear damage and compressive-shear damage in 2D C/SiC channel beam.At different locations,the damage type was also different: tensile damage was initiated at the tensile flange edge,shear damage was initiated at web,tensile-shear damage and compressive-damage were initiated at the intersections of flange and web.(3)Under bending load,the final failure of 2D C/SiC channel beam was triggered at tensile flange,and this failure was controlled by tensile-shear damage.2.Based on the structural characteristics of 2D C/SiC I-beam,its mechanical behavior under bending load and out-plane pulling load were investigated.Results showed: 2D C/SiC I-beam was a kind of composite structure which comprised of channel beam and plate beam,and local defects could be formed at the assembly interface between the substructures.(2)Under bending load,the assembly interface at tensile flange was prone to be delaminated,and the delamination behavior was related to the mechanical properties of the assembly interface as well as the constraining stiffness of the boundary.The delamination was initiated at the inner side of the tensile flange,and then propagated toward to the transverse direction and longitudinal direciton of the beam.(3)Under out-plane pulling load,delamination was initiated at the assembly interface of 2D C/SiC I-beam,and the critical load of delamination failure as well as the failure mechanism were related to the matrix content of the interface.3.The bending response characteristics of 2D C/SiC riveted composite beam were studied.Results showed:(1)The bending response of 2D C/SiC riveted composite beam was related to the shear stiffness of assembly interface.With the interface stiffness increasing,the bending response of the riveted composite beam is close to that of the single beam with same dimensions.(2)The assembly interface shear stiffness was related to the properties and arrangement of 2D C/SiC pin,and also related to the assembly the interface bonding state.In this paper,the shear stiffness of assembly interface of 2D C/SiC riveted composite beam ranged from 7.5-10 GPa.(3)Under bending load,pin main undertook shear load and local bending moment which could be affectd by the arrangment of the pins,while the torque affected the value and the direction of shear force undertook by the pins.4.The stress and damage characteristics around the pin-hole of 2D C/SiC riveted composite beam under bending load were studied.Results showed:(1)Under bending load,the stresses around pin-hole were affected by the pin-hole bonding state as well as the hole diameter.(2)Under bending load,the most serious damgaes were triggered at the position where the hoop stress was maximum,and the damages were main affected by the hole diameter.(3)Under bending load,the failure of 2D C/SiC riveted composite beam was still initiated at tensile flange,but the ultimate load was 20% bigger than the ordinary beam.(4)The failure modes were related to the bonding state of hole-pin interface.For strong bonding state,failure crack could pass through the pin,and for weak bonding state the failure crack propagated along the interface.The strong bonding state of hole-pin interface contributed to the bending strength.(5)The predicting method of bending strength proposed in this paper is adoptable,and the error between the predicting load and the experimental results was within 17%.5.The effects of thermal shock on the mechanical behavior of 2D C/SiC channel beam were investigated.Results showed:(1)In atmosphere,the thermal shock damages of 2D C/Si C channel beam included matrix thermal stress damages,fiber oxidation damages and matrix oxidation damages.At 700 ℃,the fiber oxidation damages were predominant,and the damage degree was increased with the thermal shock cycle increasing.At 1000℃,the thermal shock damages were related to the number of thermal shock cycles.When the thermal shock cycles reached to 30,the matrix thermal stress damages were predominant,and a few fiber oxidation damages could be found.When the shock cycles reached to 50,besides the matrix thermal stress damages,both fiber oxidation damages and matrix oxidation damages were present.(2)The thermal shock damage affected the bending response of 2D C/SiC channel beam.After thermal shocking at 700℃,the compressive nonlinearity emerged at the later stage of loading,and this nonlinearity was related to the thermal shock cycles.After thermal shocking at 1000 ℃,the tensile nonlinearity of 2D C/SiC channel beam which resulted from the matrix damages was obvious.(3)The bending failure modes of 2D C/SiC channel beam were affected by the damage modes resulted from thermal shock.Carbon fiber oxidation could lead to compressive failure of the channel beam,and matrix damages could lead to tensile failure of the channel beam.