Temperature dependences of the strengths of a carbon fiber and a three-dimensional reinforced carbon-carbon composite

At a fixed tension rate, the ultimate tensile strength of a carbon fiber decreases nonlinearly with increasing temperature T. This nonlinearity is caused by a change in the statistics of atomic vibrations from quantum (at T < 2250 K) to classical (at T > 2250 K) statistics. To take into account the quantum statistics, quantum function F _q is introduced into Zhurkov’s equation instead of temperature; the value of this function is calculated from the temperature dependence of the specific heat of carbon. This equation gives the values of the fracture activation energy (≈16 eV) and parameter γ (≈0.15 nm³). The strength of the three-dimensional reinforced carbon-carbon composite decreases up to ≈1800 K and increases as the temperature grows further. The decrease in the strength is explained by an increase in the rate of fiber and matrix fracture with increasing temperature, and the increase in the strength is explained by a decrease in the strength of the fiber-matrix adhesion bonds at high temperatures. As a result of this decrease, fibers begin to move with respect to each other under load, and the stresses applied to them level off. Although the fiber strength continues to decrease with increasing temperature, this effect increases the composite strength.