The Tension And Creep Behavior Of Notched 2D-C/SiC

2D-C/SiC composite is a new kind of promising thermo-structural material in aviation and aerospace field. There are certain investigation to the tensile performance and high-temperature creep performance of smooth 2D C/SiC composites, but there has little knowledge about tensile performance and creep behavior of notched 2D-C/SiC composites. The present work will intend to further study tensile and creep behavior of notched 2D-C/SiC composites. Tensile and creep specimens of 2D-C/SiC have double-edge arc notches, the radius of arc notch being 2mm and the depth of notch 0.5 mm. Tensile tests were carried out from room to 1500℃ under vacuum. Tensile creep tests were carried out at 1100℃, 1300℃ and 1500℃ in vacuum, respectively. ANSYS software was used to simulate initial longitudinal stress distribution of notched specimen, and counting its stress concentration factor. At different creep time, the specimen was taken out from the testing machine, and the surface matrix cracks and resonance frequency were examined. The electrical resistance method, relative change of elastic modulus and fractal dimension were used to evaluate the creep damage of 2D-C/SiC composites. The main conclusions were as follows:Stress concentration factor of double-edge arc notches 2D-C/SiC evaluated by finite element method is 1.53. In a high temperature range of 900℃ ~1500℃, the stress ratio of tensile strength and notched strength for 2D-C/SiC is situated between 1 and 1.53. Namely, under this paper notched specimen geometry size condition, 2D-C/SiC composite has certain notch sensitivity, but it is not the complete notch plastic material, also is not the complete notch brittle material. At 20℃, it is complete notch brittle material.The electrical resistance method can evaluate the tensile damage of 2D-C/SiC composites. The damage value D can be evaluated by △R/R0.When damage value is situated between 4% and 12%, the un-notched 2D-C/SiC will fracture. When damage value is about 0.5%, notched 2D-C/SiC will fracture. During tensile process, the damage of the smooth specimen takes place in the all area, but the damage of notched specimen mainly concentrates on near notch field.At 1100℃, no matter how much stress value might be, the creep strain of both smooth and notched specimen is concentrated on transient creep stage, and steadycreep rate is nearly zero, whereas at 1500℃, steady creep rate of notched specimen (creep stress 95MPa) and the smooth specimen (creep stress 170MPa) are in the same magnitude (10-5/h). Within creep time range (<100h), tertiary creep stage has not been observed. The temperature is the main factor to cause creep strain.Quantity of microcracks vs. time curves and microcrack width vs. time curve are extremely similar to creep curves, and these curves include a rapid increase stage at the beginning creep and a slow increase stage follows. During 0-10 hours, microcracks develop fast. The growth rate of the microcracks of the near notch field is faster than that of the far notch field within 0-2h, whereas growth rate of microcracks of the far notch field is faster than that of the near notch field within 2-10 hours. These reveal that stress would redistribute during creep course.Creep damage mainly concentrates on the near notch area. Microcracks are apt to appear on the near notch field and weaving cross points of fiber bundles, and the longitudinal fibers of the near notch field easily fracture.The electrical resistance method, relative change of elastic modulus and fractal dimension can evaluate the creep damage of 2D-C/SiC composites. The resistance method can evaluate creep damage uninterrupted creep test, the resistance curve is increases in overall, but in the partial domain, its change is the disorder. The relative change of elastic modulus and fractal dimension can evaluate the creep damage of 2D-C/SiC composites, and their damage curves development trend is basically unanimous. Both damage parameter D vs. creep time and fractal dimension vs. creep time curves include two stages, namely, the fast developing stage of creep damage and the slow change stage followed. When the creep damage evaluates by the relative change of elastic modulus, damage curves present decline to a certain degree at initial slow change stage, but its rise again with the increase of creep time, and during this period, fractal dimension monotone increases constantly. Regard matrix cracks as the main creep damage form, the variation of fractal dimension mainly reflects the local damage of creep specimen, and the variation of relative elastic modulus reflects the whole damage of creep specimen.