High-temperature treatment of carbon-carbon composite materials (7)

Blanks of this production series in the stage of carbon-reinforced plastic production have been prepared by geodesic winding of sheet and tape of high-modulus polyacrylonitrile carbon fiber, followed by autoclave hardening. A multifiber scheme was used during winding for the purpose of providing the least shape change due to reciprocal interweaving of direct and reverse turns. In the concluding stage of carbon-reinforced plastic preparation deviation from normal diameter has a mean statistical nature with a mathematical expectation of their average value around zero deviation from a nominal value. The average size of the diameter of carbon-reinforced plastic blanks almost equals the diameter of the metal mandrel, on which the blank was formed during winding and hardening in an autoclave. After completion of carbonization the average value observed of diameter deviation from the nominal value becomes positive. This means that the average diameter increases probably as a result of internal stress relaxation, accumulated during resin hardening. In the HTT stage the change in geometric shape of a blank increases markedly as there is an increase in maixmum process temperature, starting from about the level of 1900°C. the possible practical application of this component is determined by its purpose and design document tolerances.

As noted above, in the concluding stage of HTT (1800-2200°C) there is reduction in the increase in porosity with relatively small weight loss and a resumption in shrinkage, particularly in the plane of a layer. In this temperature range there is also the next stage of a reduction in the absolute level of carbonized material strength. With an increase in treatment temperature above 2000°C for materials based on both high and low modulus carbon threads the ultimate strength in bending is reduced by more than 30-35%.

In order to prevent irreversible blank shape changes it is desirable to set the maximum level of production treatment less than the measurement error. In order to increase production reliability to 0.997 it is desirable to introduce a limitation

Tproduction ≤ (Tstart of shape change – 3σ)

where σ is mean square deviation of local temperature from its mathematical expectation, i.e., the average nominal value.

From the results of executing production regimes presented in fig.2 it is easy to find that for the first regime σ=26.6°C, and for the second 30.7°C. In order to provide a treatment temperature not exceeding 1900°C and thereby total exclusion of diatortion it is necessary through the control sight glass to establish a temperature of 1810°C.

In carrying out high-temperature treatment for CCCM there are two temperature ranges for a stable state of the carbon substance: 1270-1450 and 1810-1900°C.

In industrial execution of the production operation for high-temperature treatment in order to exclude inhomogeneity of structural transformations and blank distortion throughout the volume of a charge a temperature is established through the control sight glass guaranteeing production reliability of a process of 0.997 (known as the 3σ rule)


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