Tensile Performance Of 3D-C/SiC Composites

Three-dimensional ceramic matrix composite C/SiC has much potential as structure material in elevated temperature environment. Its tensile performance is different from that of ID, 2D and 2.5D composites for its unique SD-fabricated texture. So in this thesis 3D-C/SiC tensile mechanical behaviors were investigated in different conditions.The testing 3D-C/SiC specimens produced by SD-woven PAN matrix carbon fiber prefabricate and SiC matrix with CVI method. Fiber volume fraction is about 40%. The thickness of pyrocarbon interface is 200 nm and porosity is 17%. Testing temperature range is from room temperature to1500℃. The specimens' displacement velocity is from 0.008mm/min to 5.82mm/min. Tensile experiments carry out under these conditions in vacuum. Typical parameters of tensile performance were calculated by using modified mesocosmic model deduced from unidirectional fiber reinforced composites. Affect mode of temperature and displacement velocity variance on 3D-C/SIC mechanical properties was discussed in detail. The specimens' structure and fracture morphology were analyzed through SEM and TEM and damage was studied in tensile process by electrical resistance method. The main research findings are as follows:At the same displacement velocity 3D-C/SiC tensile fracture stress increases with testing temperature increasing, when temperature reach material producing temperature 1100℃ the fracture stress reach maximum, then it decreases with the temperature increasing. As temperature increasing fracture strain decreases at first and reach minimum at 1100℃ then increases. Initial modulus doesn't change as temperature changing. Matrix cracking stress has the same changing tendency with fracture stress, but it reach maximum at 1300℃ 3D-C/SiC fracture stress increases with the increment of displacement velocity at room temperature, but it decreases at 1500℃. At producing temperature it is constant. Fracture strain decreases and initial modulus increases with the increment of displacement velocity at different temperature.Take advantage of mechanical property results which educed based on unidirectional fiber reinforced ceramic matrix composites model and simply modify by multiplying a 3Dstructure modified factor, them can be used to predict typical mechanical parameters. The predicted fracture stress matrix cracking stress and crack spacing just have error less than 20% with the corresponding experimental value. But in predicting modulus, predict value according the modified mixture rule of modulus has great error.Temperature and displacement velocity affect 3D-C/SiC performance by changing the status of interface microcosmic constitute and applied force, residual stress in fiber and matrix, strain ratio relativity of constitute, carbon fiber tropism and crack in matrix.Electrical resistance variance of 3D-C/SiC in tensile process was simply studied and found electrical resistance vary curves have the same changing tendency with stress-strain curves and the result show that the electrical resistance variance can be used to characterize the material damage. Damage of 3D-C/SiC is stem from pores formed by 3D weaving. The main damage mechanism is matrix cracking between fibers and between fiber bundles, crack extending in interface of fiber bundles, fiber and fiber bundles pull-out, attrition between crossing fiber bundles and fiber fracture.